Evolution of the Northern Gulf of Mexico Sedimentary Basin Thomas E. Ewing*, William E. Galloway† *
Bexar Geological Surveys, San Antonio, TX, United States, †University of Texas Institute for Geophysics (Emeritus), Austin, TX, United States
Chapter Outline Introduction: Present-Day Topography and Bathymetry Crustal Structure and Origin of the Gulf of Mexico Basin Crustal Structure and Provinces History of Extension and Ocean Opening Postrift Subsidence and Basin Modification Structural Framework of the Northern Gulf of Mexico Basin Basement Structures Gravity Tectonic Structural Styles Gravity Tectonics and Shelf Margin Progradation: Structural Domains Structural Growth History Depositional Framework of the Northern Gulf of Mexico Basin Overview of Depositional Episodes and Sequences Late Triassic–Middle Jurassic (Norian-Callovian) Depositional Episodes Middle Jurassic–Earliest Cretaceous (CallovianBerriasian) Depositional Episodes Early Cretaceous (Valanginian-Cenomanian) Depositional Episodes
627 630 630 632 635 636 636 636 638 641 642 642 644 644 651
Late Cretaceous (Cenomanian-Maastrichtian) Depositional Episodes 655 Laramide Depositional Episodes (Paleocene-Eocene) 659 Middle Cenozoic (Eocene–Oligocene) Volcanism and Related Depositional Episodes 662 Miocene Depositional Episodes 667 Pliocene–Quaternary Depositional Episodes 671 Patterns and Generalizations in Gulf Depositional History 673 Sediment Supply and Transport: “Source to Sink” 673 Climate and Oceanography 677 Evolution of Siliciclastic Shelf Margins; Progradation and Retrogradation 678 The Question of Cenozoic Marine Transgressions 680 Energy and Mineral Resources of the Northern Gulf of Mexico Basin 680 Hydrocarbon Source Rocks 681 Hydrocarbon Migration Pathways, Reservoirs, and Seals 682 Energy Minerals and Other Mineral Resources 684 Acknowledgments 685 References 685
INTRODUCTION: PRESENT-DAY TOPOGRAPHY AND BATHYMETRY The Gulf of Mexico is a small ocean basin lying between the North American plate and the Yucatan block. In its northern and northwestern sectors, it contains a succession of Jurassic through Holocene strata that is as much as 20 km thick. Sediment supply from the North American continent has filled nearly one-half of the basin since its inception, primarily by offlap of the northern and northwestern margins. This review will focus on the origin of the northern and northwestern portions of the Gulf of Mexico Basin (the Northern Gulf of Mexico Basin), and the history of its filling. The fundamental geologic principle that “the present is the key to the past” has found wide application and success in the Gulf of Mexico basin, both offshore and onshore. The modern Gulf of Mexico (Fig. 16.1) has a central Sigsbee abyssal plain that generally lies at >3 km depth (Bryant et al., 1991). The eastern part of the abyssal plain is dominated by the morphology of the late Quaternary Mississippi fan; the western abyssal plain is deeper and featureless. The continental slope of the northern Gulf of Mexico margin (Texas–Louisiana slope) displays a bathymetrically complex morphology that terminates abruptly in the Sigsbee escarpment in the central Gulf, and is filled in by the head of the Mississippi fan to the east (Steffens et al., 2003). The hallmark of the central Gulf continental slope is the presence of numerous closed to partially closed, equidimensional, slope “minibasins,” each some tens of kilometers in diameter. In the eastern part of
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628 The Sedimentary Basins of the United States and Canada
FIG. 16.1 Principal physiographic elements of the Gulf of Mexico basin and adjacent North America. Base image from Google Earth. Esc, Escarpment; TX-LA, Texas–Louisiana; B-CΔ, Brazos-Colorado Delta; RGΔ, Rio Grande (Rio Bravo) Delta. Topographic and bathymetric base courtesy of Google Earth.
the basin, the Florida platform forms a broad ramp and terrace that terminates at depth into the steep Florida escarpment, with seabed slopes up to 45 degrees (Bryant et al., 1991). The western basin margin is narrower, but also is bathymetrically complex. On this margin, numerous contour-parallel ridges and swales dominate the middle to lower slope morphology. The modern shelf edge, as reflected by a well-defined increase in basinward gradient, generally lies at a depth of 100 to 120 m. Landward, the northwestern, northern, and eastern Gulf of Mexico is bounded by broad, low-gradient shelves that range from 100 to 300 km in width (Fig. 16.1). Today, as throughout its history, the Florida and Yucatan platforms, which bound the basin on the east and south, persist as sites of carbonate deposition. Onshore, the northern and northwestern Gulf margin displays a broad coastal plain (Fig. 16.1). The lower Texas coastal plain, a flat, low-relief surface, is underlain by Neogene and Quaternary strata. Three major deltas form conspicuous headlands (the Mississippi, Rio Grande (Rio Bravo), and Brazos/Colorado deltas), with intervening shoreline, marsh/lagoon, and coastal strandplain environments. The upper Texas coastal plain displays modest relief of generally <100 m, created by gentle Neogene and Quaternary uplift and incision into older Neogene, Paleogene, and late Cretaceous strata by numerous large and small rivers. East of the Mississippi River valley, the coastal plain is undivided. The basin is bounded by a variety of Cenozoic, Mesozoic, and remnant Paleozoic uplands (Fig. 16.1), including the high mountains of the Sierra Madre Oriental of Mexico (a foldbelt of Cenozoic age), the Lower Cretaceous limestone-capped Edwards Plateau (gently uplifted in the Neogene, with a faulted margin called the Balcones fault zone), the Ouachita Mountains of southern Arkansas (a late Paleozoic foldbelt; Chapter 8), and the Cumberland Plateau and southern Appalachian Mountains of northern Mississippi and Alabama (Paleozoic foreland basin, and fold and thrust belt; Chapter 4). The northeastern margin of the Gulf of Mexico basin merges into the southern
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Atlantic coastal plain across northern Florida (Chapter 15); however, the basin boundary is generally placed somewhat west of the center of the Florida peninsula. The subject of this Chapter is the Northern Gulf of Mexico basin, defined as the part of the greater basin that lies northwest of the abyssal plain. It extends from Tampico in the southwest to Arkansas in the north and the west coast of Florida in the east (Fig. 16.2). The basin has a number of regions, some of which are distinct subbasins. The northeastern flank of the basin contains a series of basins and intervening structural highs; several basins contain salt structures. The west end of this series of basins and arches is formed by the East Texas subbasin, bounded on the east by the Sabine Uplift. The northwestern margin of the basin through central Texas can be called the “Northwest Shelf,” passing southwestward into the broader Rio Grande Embayment. The western margins were originally another province of basins and arches, but these have been extensively reworked by Laramide compression and Neogene uplift and tilting. The Central Basin is the region with the thickest sediment accumulation and the thickest and most complexly deformed salt. The geology of the Gulf of Mexico has been reviewed by numerous authors. Two syntheses stand out. Grover Murray’s 1961 Geology of the Atlantic and Gulf Coastal Province of North America summarized classic mid-20th century stratigraphic and structural understanding of the basin. The Geological Society of America’s 1991 Geology of North America series, volume J, The Gulf of Mexico Basin, edited by Amos Salvador, provided a synthesis of all facets of basin geology and resources integrated through the initial applications of modern concepts of crustal tectonics, depositional systems, genetic stratigraphy, deep marine studies, and gravity tectonics. The objective of this paper is to incorporate the wealth of new ideas and information that has been published in the two decades since the Salvador volume into a succinct description of the structural and stratigraphic framework and the depositional history of the northern Gulf of Mexico basin, from the upland basin margins to the abyssal plain.
FIG. 16.2 Limits and subregions of the Northern Gulf of Mexico Basin. RGE, Rio Grande Embayment; BW Basin, Black Warrior Basin; T-M Basin, Tampico-Misantla Basin; Sab. Bsn., Sabinas Basin; FTB, Fold and Thrust Belt. Topographic and bathymetric base courtesy of Google Earth.
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CRUSTAL STRUCTURE AND ORIGIN OF THE GULF OF MEXICO BASIN The Gulf of Mexico basin was created by an episode of crustal extension and seafloor spreading during the Mesozoic breakup of Pangea (Salvador, 1987; Sawyer et al., 1991; Buffler and Thomas, 1994; Pindell and Kennan 2001, 2009; Jacques and Clegg, 2002; Harry and Londono, 2004; Hudec et al., 2013a). The extensional origin of the basin is reflected in the distribution and nature of the basement crust (Figs. 16.3A and 16.3B). Since the cessation of seafloor spreading, the entire basin and its margins have formed part of the North American plate. Subsequent tectonic modification has been extensive only on the western margin, where convergent tectonics created mountains and uplifted plateaus in the Cenozoic.
Crustal Structure and Provinces The Gulf of Mexico basin is surrounded to the northwest and northeast by Proterozoic continental crust (with normal thickness of 40–45 km) of the Laurentian craton, to the south and east by Paleozoic (?) continental crust of the Yucatan Block and Florida Platform (of similar thickness, of Gondwanan origin but now part of the North American plate), and to the southwest and west by a collage of mostly continental blocks welded to North America during the Mesozoic and modified by orogeny and volcanism. Three-quarters of the basin is underlain by “transitional crust”: that is, continental crust that was stretched, thinned, and modified by Middle to Late Jurassic rifting. Two types of transitional crust can be differentiated (Figs. 16.3A and 16.3B).
FIG. 16.3A Crustal types, generalized depth to basement (km), and original distribution of Jurassic Louann premarine evaporite beneath the Gulf of Mexico basin (in gray). Depth to basement contours modified from Sawyer et al. (1991) and Peel et al. (1995); oceanic crust (in rose) boundaries constructed from Sandwell et al. (2014). Note that some modern reconstructions suggest depth to basement of >20 km beneath the north-central Gulf depocenter. Red line is the boundary of the Cuban/Caribbean crustal province, established in the Cenozoic. Abbreviations: Emb., mbayment, FTB, fold and thrust belt.
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FIG. 16.3B Crustal types and inferred tectonic features related to Gulf of Mexico extension and seafloor spreading. Spreading center interpreted from data in Sandwell et al. (2014); pole of rotation after Pindell and Kennan (2001, 2009); Phase I NW-trending linears (fault zones) from Ewing (2011) and Hudec et al. (2013a,b).
The basin margins are underlain by a zone of thick transitional crust, which displays modest thinning of <50% (crustal thickness 20 km or greater), and typically lies at depths between 2 and 12 km subsea depth (Sawyer et al., 1991). The broad area of thick transitional crust was thinned heterogeneously; it contains blocks of continental crust of nearly normal thickness separated by areas of thinner stretched crust. The thinner crust areas have subsided more deeply than the thicker blocks due to isostasy. The result is a chain of arches and intervening basins around the northeastern and western peripheries of the Gulf of Mexico Basin (Fig. 16.3B). In some areas, discrete grabens or half-grabens filled with nonmarine sediments of Late Triassic and Early Jurassic age can be recognized (the Eagle Mills Formation in the East Texas–Arkansas area, the fill of the South Georgia Graben, and the Chittim and Huayococotla anticlines in southwest Texas and eastern Mexico). Nearly all of the present lower coastal plain, shelf, and continental slope of Texas and Louisiana is underlain by thin transitional crust, which has been highly extended and thinned >50% to yield a present crustal thickness of <20 km; this crust is buried to depths of over 15 km below sea level. Some reconstructions of deep seismic traverses (Peel et al., 1995) indicate that basement lies below 20 km in the central depocenter beneath the south Louisiana coastal plain and adjacent continental shelf. The zone of thin continental crust includes areas of hyperextended crust (crustal thickness <10 km), which may include areas of mantle exhumed by slow, amagmatic spreading (Van Avendonk et al., 2015); it also may contain zones of abundant volcanism, forming volcanic-rich margins in the eastern basin (Imbert, 2005) and possibly in the northwestern part of the central basin (Mickus et al., 2009). The deep abyssal plain and lower continental slope are underlain by basaltic oceanic crust that was formed during late Jurassic through early Cretaceous seafloor spreading. Gulf of Mexico oceanic crust lacks the magnetic reversal signature typical of more recent oceanic crust, as it was created during a magnetic quiet period; that and the >10 km thick sedimentary
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cover have delayed recognition of typical oceanic features in the basin (Imbert and Phillippe, 2005). Recently, direct imaging of the oceanic crust has been achieved with long-offset seismic data (Snedden et al., 2014) and gravity gradient data (Sandwell et al., 2014) have imaged spreading centers and transform faults. The crust appears to be of normal oceanic thickness (7 km) and formed by rotational opening about a pole of rotation near present-day Cuba (Pindell and Kennan, 2009; see later). The location of the continent-ocean boundary beneath the depocenter of the northern Gulf of Mexico Basin is not well imaged and is somewhat controversial; Figs. 16.3A and 16.3B is one reconstruction.
History of Extension and Ocean Opening The broad history of plate tectonic movements that culminated in the Gulf basin is generally understood (Marton and Buffler, 1999; Pindell and Kennan, 2001, 2009; Jacques and Clegg, 2002; Harry and Londono, 2004; Bird et al., 2005; Chapter 1), if not fully agreed upon in detail. The Gulf of Mexico opened in two phases during the separation of the North American plate from Pangea. Initial rifting spread southward along the Central Atlantic spreading ridge, through a possible volcanic center in southern Florida and the Bahamas (Pindell and Kennan, 2009) and into the Gulf of Mexico. Phase I extension of Late Triassic through Middle Jurassic age (Fig. 16.4A) seems to have been oriented NW-SE, roughly parallel to mid-Atlantic spreading directions. It created a complex series of grabens and half grabens filled with terrestrial redbeds and volcanics separated by blocks of thick crust (Salvador, 1991b). Northwest-trending intracontinental transforms partitioned extension and transferred strain. These transforms are particularly marked along the northeastern margin (Mississippi-Alabama-Florida; MacRae and Watkins, 1996, Harry and Londono, 2004) and in northern Mexico (Ewing, 2011). Recognition of potential seaward-dipping reflectors in the northeastern Gulf suggests an early phase of subaerial volcanism during the initial spreading phase (Imbert, 2005; Eddy et al., 2014). It has been proposed on potential field evidence that volcanism formed a ridge or margin in the northwestern basin as well (Mickus et al., 2009). Phase I grabens in Mexico and adjacent Texas have a northwesterly trend, suggesting southwestward extension of Mexican terranes related to Pacific tectonics (Rueda-Gaxiola, 2003). Continued Phase I
FIG. 16.4 Maps showing the formation of the Greater Gulf of Mexico Basin. (A) The closed configuration of North and South America circa 225 Ma (Late Triassic). Sites of Late Triassic and Early Jurassic grabens are shown by red shading. (B) Configuration at close of Phase I extension (~162 Ma, Callovian). Extent of Louann Salt and equivalents shown in green; thick salt in dark green. Site of future seafloor spreading shown as dashed black line.
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FIG. 16.4 Cont’d (C) Configuration at close of Phase II seafloor spreading (~144 Ma, Valanginian). Figures from Ewing (2016a), data from Pindell and Kennan (2001, 2009), Dickinson and Lawton (2001).
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stretching and hyperextension of continental crust during the Early and Middle Jurassic created a broad sag basin, probably well below sea level, separated by sills from both the central Atlantic and proto-Pacific oceans. Sedimentary filling of this basin may have occurred before salt deposition in a manner similar to the Brazilian margin; these sediments have been interpreted to overlie thinned crust and exposed mantle based on seismic refraction data (Van Avendonk et al., 2015), but no such deposits have yet been imaged or penetrated. In Middle Jurassic time, marine incursions from Mexico led to widespread deposition of thick Louann Salt and associated evaporites, a defining event for the later structural evolution of the Gulf sedimentary fill (Fig. 16.4B). Salt thickness was greatest (up to 3–5 km) above the areas of greater crustal extension, including several of the interior basins and, more regionally, in the central sag basin built on thin transitional crust (Figs. 16.3A and 16.3B). The regional unconformity beneath the evaporite layer separates localized synrift deposits and possible sag-basin deposits from blanket postrift deposits and is widely taken as the base of the Gulf of Mexico sedimentary basin fill (Sawyer et al., 1991; Buffler and Thomas, 1994). The age of the salt is not well determined; it has generally been thought to be Callovian (ca. 162 Ma) based on stratigraphic relations in Mexico (Salvador, 1987), but recent dating using strontium isotopes (Snedden et al., 2018b) suggests an older Bajocian age (ca. 170 Ma). Salt deposition may be synchronous with collapse of the outer continental margin (Pindell et al., 2014). Rifting may have continued after salt deposition until seafloor spreading began, opening Phase II (Hudec et al., 2013a). Phase II consisted of rotational seafloor spreading and generation of oceanic crust (Fig. 16.4C) along a generally east–west trend with transform offsets concentric about a pole centered east of Yucatan; this spreading continued through the late Jurassic into the early Cretaceous (Pindell and Kennan, 2001, 2009). The Gulf of Mexico ocean basin was formed by approximately 100–500 km of extension due to counter-clockwise rotation of the rigid Yucatan block by about 42 degrees to its present position, accompanied by large amounts of NNW–SSE dextral shear along the western flank of the basin (Marton and Buffler, 1999; Pindell and Kennan, 2001, 2009; Jacques and Clegg, 2002; Nguyen and Mann, 2016). Recent gravity-gradient mapping reported in Sandwell et al. (2014) has disclosed traces of the oceanic spreading center and numerous transform faults that confirm the rotational model. Crustal rupture and emplacement of basaltic crust began probably by the Oxfordian and continued until the termination of spreading in the latest Berriasian or early Valanginian (approximately 160–144 Ma). Salt deposition ended nearly contemporaneously with the onset of seafloor spreading; because of ocean crust formation, the Louann salt basin was split into northern and southern Gulf segments (Fig. 16.4). Jacques and Clegg (2002) suggest two phases of rotation during Phase II about differing poles. Subsidence of the broad areas of transitional crust began once seafloor spreading commenced, and extension was focused on the new spreading center. In Valanginian time, Gulf of Mexico seafloor spreading ceased, and plate motions were transferred into the protoCaribbean basin; thereafter, cooling and subsidence of the Gulf of Mexico oceanic crust and its stretched continental margins dominated basin development. By the end of the Early Cretaceous, the combination of regional subsidence and deposition of rimming carbonate platforms had created the modern outline and morphology of the basin (Winker and Buffler, 1988). Late Cretaceous and, especially, Cenozoic history was dominated by load-induced subsidence beneath basinward-prograding delta and shorezone systems, complicated by intrabasinal gravity tectonics (salt and shale) and extrabasinal tectonics on the western margin. The history of Gulf of Mexico extension and seafloor spreading created four distinctly different types of basin margin. The northwestern margin is a relatively simple divergent margin with a broad zone of stretched continental crust separating oceanic and continental crust. The southern margin is also a divergent margin but has only a thin rim of highly extended crust rimming the Yucatan block of thick transitional crust. This pronounced asymmetry has suggested a simple-shear model for Phase I extension (Marton and Buffler, 1993; Watkins et al., 1995). The Mexico (western) and Mississippi-Florida (northeastern) margins primarily reflect displacement of crustal blocks along a series of transfer faults. To the west, the continental margin is characterized by an elongate gravity high and a narrow zone of primarily late Cenozoic growth faults and folds overlying the Late Jurassic transform margin that juxtaposes oceanic and continental crust (Bird et al., 2005; Ambrose et al., 2005). On the northeast, the margin was formed by a series of crustal blocks of varying degrees of internal extension that rotated between two parallel intracontinental transform faults, the Pearl River (“Florida-Bahamas”) and Tunica (“Cuban”) zones. These faults follow similar trajectories to Atlantic fracture zones (Klitgord et al., 1984; MacRae and Watkins, 1996), but also parallel older Paleozoic structures. The family of basement arches (underlain by thick, less extended continental crust) and basins (underlain by thin, more extended transitional crust) that extends from the Mississippi Salt Basin southeast to the Sarasota Arch (Fig. 16.3A) were produced in this transtensional domain (Watkins et al., 1995; Marton and Buffler, 1999; Pindell and Kennan, 2001; Stephens, 2009). The Sabine block and the adjoining East Texas and North Louisiana basins form part of this complex, although they lie southwest of the inferred Tunica fault zone. Similar complexes of arches and sags formed in south Texas and northeastern Mexico during Phase I extension (Ewing, 2011).
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Postrift Subsidence and Basin Modification As with other passive margin basins, total subsidence of the crust of the Northern Gulf of Mexico Basin is the sum of crustal extension, subsequent cooling, and load-induced subsidence. Crustal thinning and heating during Phase I continental extension, followed by Phase II cooling and subsidence as the basin migrated away from the axial spreading center, followed further by basinwide subsidence after seafloor spreading ceased, resulted in a total tectonic subsidence of 5 to 7 km of the thin transitional crust in the central basin (Sawyer et al., 1991). In the central basin, this subsidence created a deepening starved basin. The marginal basins and shelves received significant sediment input and supported a prograding shelf margin, despite rapid subsidence. The marine basin probably reached maximum water depths in the middle and late Cretaceous, when the shelf margin reached a terminal point near the thick to thin transitional crust transition. Subsequent depositional loading of the crust in the central portion of the Northern Gulf of Mexico Basin began with a pulse in the middle Cretaceous and then continued steadily from Paleocene through the Holocene. This loading has further depressed the crust to its current depth of 10 to 20 km below sea level (Fig. 16.3A), while prograding the shelf margin to its present location. Cenozoic load-induced subsidence onto thick, mobile salt and shale substrates has created the distinctive character of the Northern Gulf of Mexico Basin. Additional Mesozoic and Cenozoic tectonic phases have further modified the subsidence history of the Gulf (Fig. 16.5). Several of the marginal highs, including the San Marcos arch, Sabine arch, and Monroe uplift display short pulses of uplift of as much as a few hundred meters, creating angular unconformities in strata ranging from middle Cretaceous to lower Eocene strata in age (Laubach and Jackson, 1990; Ewing, 2009a). The Cretaceous uplifts in East Texas, Louisiana, Arkansas, and Mississippi are dome-shaped and apparently related to thermal uplift and igneous activity (particularly in Arkansas and Mississippi). The San Marcos Arch and the Eocene phase of uplift on the Sabine arch appear to be related to Laramide thrusting and foredeep subsidence, which in turn were related to changing rates of plate convergence at the
FIG. 16.5 Map of postrift tectonic modifications of the Gulf of Mexico Basin. Purple diamonds—Cretaceous igneous features. Abbreviations: mid-K, middle Cretaceous; Ku, Upper Cretaceous (Tur-Turonian, Camp-Campanian, Mas-Maastrichtian); ME, Middle Eocene; Pg, Paleogene; Ng, Neogene.
636 The Sedimentary Basins of the United States and Canada
Pacific margin. In the Late Cretaceous, intrusive and extrusive volcanism occurred around the northern and northwestern periphery of the basin (Byerly, 1991; Stephens, 2001; Ewing, 2009a). Principal volcanic clusters lie around the inner edge of the central and south Texas coastal plain, and in southern Arkansas and the adjacent Monroe uplift of northern Louisiana and adjacent Mississippi. Igneous lithologies include basalt, nephelene syenite, phonolite, and peridotite. The igneous activity appears to be related to the large domal uplift of the north-central part of the basin (Southern Arkansas Uplift) and to the San Marcos Arch to the west. Extensive Neogene crustal uplift across northern Mexico and the southwestern United States including West Texas (Gray et al., 2001), in part associated with Basin and Range extension, tilted Mesozoic and Paleogene strata of the western Gulf, as far northward as central Texas. Cenozoic mobilization of thick bodies of salt into a wide variety of gravity-tectonic structures due to basin tilting and sedimentary loading has created great volumes for sediment accommodation in local to regional areas. It has created as much as 1–2 km of often rapid subsidence of the overlying outer shelf and upper slope sediments at various times along segments of the northern Gulf continental margin (Diegel et al., 1995; Galloway et al., 2000). The role of salt tectonics is discussed further in the following section.
STRUCTURAL FRAMEWORK OF THE NORTHERN GULF OF MEXICO BASIN The depositional history of the Northern Gulf of Mexico Basin is best understood in the context of both the basement structures, which influenced sediment supply and accumulation patterns, and gravity-tectonic structures, which reflect dynamic interactions among depositional loading, salt and shale mobilization, creation or loss of accommodation space, and deformation.
Basement Structures Basement structures and their influence on overlying stratigraphy are most readily apparent around the periphery of the basin underlain by thick transitional crust. They include the halo of marginal basins (including sags or “embayments” that open to the central Gulf) and intervening arches or uplifts (Ewing, 1991) (Figs. 16.3A and 16.3B). Most of the basins contain a significant thickness of Louann salt and thicker sequences of Late Jurassic and Early Cretaceous strata relative to the adjacent arches or uplifts. Salt-floored basins, including the East Texas basin, North Louisiana salt basin, Mississippi salt basin, and Apalachicola embayment (also known as the De Soto Canyon salt basin) contain well-described families of salt domes and related structures (e.g., Seni and Jackson, 1984). Deep crustal structures of the thin transitional and oceanic crustal domains of the central basin are less easily defined. Gravity and magnetic data, changes in basement topography and rates of subsidence, and salt distribution all suggest a family of NW-SE trending basement transfer (intracontinental transform) faults created during Phase I extension (Watkins et al., 1995; Huh et al., 1996; Stephens, 2001). Transform faults within the oceanic crustal domain trend SSW, forming part of the rotational opening of Yucatan relative to North America.
Gravity Tectonic Structural Styles The Northern Gulf of Mexico basin fill displays one of the best-described and most complex assemblages of gravity tectonic structures to be found in the world (Worrall and Snelson, 1989; Nelson, 1991; Diegel et al., 1995; Jackson, 1995; Peel et al., 1995; Watkins et al., 1996a; Jackson et al., 2003; Hudec and Jackson, 2011). The combination of a kilometers-thick, basin-flooring Jurassic Louann salt, rapid sediment loading, and offlap of a high-relief, continental-margin sediment prism has resulted in mass transfer of salt and overpressured mud up-section and basinward throughout the history of the Northern Gulf of Mexico. The resultant panoply of structures and related features includes (see Hudec and Jackson, 2011, for more detailed definitions and descriptions): 1. Growth fault families and related structures (Winker, 1982; Watkins et al., 1996b). Extensional growth faults, usually listric, have their main period of growth during active deposition at the continental margin. Here, extension results from basinward gravitational gliding or translation of the sediment wedge along a detachment zone, typically found within salt or overpressured deep-marine mud (Rowan et al., 2005). When the detachment is shallow, these systems merge into submarine slumps or slides; when it is deep, they root well into the sediment column or to its base. Extension creates a family of features, including primary synthetic growth faults, splay faults, antithetic faults, and rollover anticlines (Fig. 16.6A). 2. Allochthonous salt bodies, including salt canopies and salt sheets (Diegel et al., 1995; Fletcher et al., 1995; Peel et al., 1995; Jackson et al., 2003). Loading of the Louann salt has resulted in regional extrusion of salt basinward and up-section.
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Extension Splay faults
Translation Synthetic fault
Toe fold & Reverse faults
Salt pinch out
Salt decollement Ramp fault
Roho - floored & Transform faults
Diapir Flap fault
Salt evacuation surface Salt weld
Evacuated allochthonous salt canopy
FIG. 16.6 Typical intrabasinal gravity tectonic structural styles and features of the northern Gulf margin. (A) Linked salt- and shale-based detachments. (B) Salt-based detachment fault system, or Roho structure. (C) Salt-withdrawal minibasin. Modified from Shoup and Karlo (2000).
Salt canopies typically develop beneath the continental slope, where salt rises as a series of coalescing diapirs or as injected tongues. Salt may also be extruded to the surface, forming salt sheets (nappes) that move basinward much like salt glaciers. Canopies and sheets may subsequently be deflated, forming salt welds, Roho systems, minibasins, and diapirs. 3. Salt welds (Jackson and Cramez, 1989; Jackson et al., 1994). Welds (Fig. 16.6B and C) are surfaces that juxtapose discordant stratigraphic sections. They form where nearly complete expulsion of salt stock feeder dikes, salt tongues, or salt canopies has occurred. 4. Roho fault families (Schuster, 1995; Rowan, 1995; Jackson et al., 2003). Lateral salt tongue extension by gravity spreading creates a linked assemblage of extensional faults and compensating, down-slope compressional toe faults, anticlines, and salt injections in the overlying sedimentary cover (Fig. 16.6B). 5. Salt diapirs and their related withdrawal synclines and minibasins (Seni and Jackson, 1984; Rowan, 1995; Fletcher et al., 1995; Rowan and Weimer, 1998; Jackson et al., 2003). In the marginal basins, salt diapirs rise directly from the autochthonous Louann “mother” salt. In the Central Basin, depositional loading of allochthonous salt canopies and sheets beneath shelf and slope areas causes renewed evacuation into high-relief salt diapirs separated by depressions floored by salt welds (Fig. 16.6C). Progressive salt evacuation creates shifting, localized sites of extreme subsidence, and sediment accumulation. Resulting features include withdrawal synclines created by local evacuation of salt from diapir flanks, bathymetric depressions (minibasins) that form local depocenters, sediment-cored turtle structures, and local fault families including down-to-basin ramp faults, counterregional flap faults, and crestal faults above salt bodies. 6. Basin-floor compressional foldbelts (Weimer and Buffler, 1992; Fiduk et al., 1995; Trudgill et al., 1999; Hall et al., 2000). Basinward gravity spreading or gliding along a detachment zone (usually salt), and resultant updip extension, requires compensatory compression at the toe of the displaced sediment body, except for movement taken up by squeezing of salt diapirs. Compressional features include anticlinal toe folds and reverse faults (Fig. 16.6A). They commonly form at the base of the slope, but also can form on the basin plain where a step discontinuity in the basement occurs or the salt layer terminates. 7. Raft tectonics. Upon basinward tilting, large internally undeformed masses of sedimentary postsalt strata can move basinward as a block or raft on a salt detachment surface. At the updip end or between rafts, a trough is formed that may be filled by great thicknesses of younger sediments, often with complex faulting. Around many parts of the Northern Gulf of Mexico Basin, the updip limit of salt is marked by grabens and related faults that represent the updip ends of rafts that include most of the postsalt sedimentary basin fill.
638 The Sedimentary Basins of the United States and Canada
Gravity Tectonics and Shelf Margin Progradation: Structural Domains The most complex array of gravity-tectonic structures lies within the Texas and Louisiana shelf and slope (the Central Basin), which hosts the thick Cenozoic sedimentary wedge of the northern Gulf of Mexico basin (Fig. 16.7). Principal structural features include: the peripheral series of graben systems marking the edge of salt; a series of continuous, strikealigned growth fault families beneath the lower coastal plain of Texas and central Louisiana; complex and less continuous fault families beneath South Louisiana and its adjacent continental shelf; a broad zone of relatively shallow salt stocks and coalesced autochthonous canopies beneath the upper continental slope; a base-of-slope salt tongue or nappe, which forms the Sigsbee bathymetric escarpment; and several lower slope and basin floor compressional foldbelts, in part covered by salt tongues. Sediment loading of the allochthonous salt canopies has created a large number of largely filled minibasins on the shelf and unfilled minibasins with bathymetric relief on the continental slope. This mosaic of gravity tectonic features can be grouped into genetically related structural domains (Peel et al., 1995; Fig. 16.8). Each structural domain has a consistent set of structures and a finite time span of primary structural growth that can be associated with one or more successive episodes of siliciclastic sediment accumulation in the Gulf. Domains generally become younger basinward, beginning with the Jurassic Louann detachment domain in the marginal salt basins and the basin margins, and culminating in the Plio-Pleistocene minibasin and salt canopy domains of the continental slope. The Oligocene–Lower Miocene and later Miocene compressional domains are exceptions to this general pattern, as they occurred at the toe of the continental slope. The smaller basins of the northeastern and northwestern margins of the Northern Gulf of Mexico Basin host salt diapirs and related structures (pillows and turtle structures) of the South Texas, East Texas, North Louisiana, Mississippi, and De Soto Canyon salt basins, which lie around the northern basin periphery. Surrounding these basins and outlining the limit of Louann salt are a system of peripheral grabens and related fault zones, including the Mexia-Talco, State Line, and PickensGilbertown fault zones (Fig. 16.7). All of these features reflect diapiric growth of autochthonous Louann salt accompanied by basinward rafting of the overlying sediments. Growth of structures within these inboard domains occurred largely in Mesozoic time, although motion on the peripheral graben system continued episodically into the Cenozoic. To the south and seaward of the Cretaceous shelf margin lie a series of “detachment” domains exhibiting large, continuous growth fault systems associated with Cretaceous and Paleogene siliciclastic shelf margin progradation—the Tuscaloosa, Wilcox, Yegua, Vicksburg, and Frio fault systems (Fig. 16.7). These regions do not exhibit salt features; they may root into the autochthonous salt layer or in Mesozoic or Paleogene shale-based decollements. In South Texas, a large
FIG. 16.7 Gravity-tectonic structural features of the Northern Gulf of Mexico Basin. Compiled from Ewing and Flores-Lopez (1991) and Watkins et al. (1995). Abbreviations: FZ, fault zone; FB, foldbelt; Atasc FZ, Atascosa fault zone; K-W FB, Keathley-Walker Foldbelt; SC FF, South Cameron Fault Family; SEI FF, South Eugene Island Fault Family; ST-SS FF, South Timbalier–Ship Shoal Fault Family.
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FIG. 16.8 Structural domains of the northern Gulf of Mexico; age of detachment or canopy horizons in parentheses. Compiled from Ewing (1991), Diegel et al. (1995), Peel et al. (1995), and Jamieson et al. (2000). Abbreviations: EGSB, East Texas salt basin; BSB, Brazos salt basin; NLSB, North Louisiana salt basin; DSSB, De Soto salt basin; STSB, South Texas salt basin; HSB, Houston salt basin.
raft of Mesozoic sediments left behind a trough filled with Wilcox and later Eocene sediment, the Rosita trough (Diegel et al., 1995; Fiduk et al., 2004). Parts of the younger systems (Yegua and Frio) transition to continuous detachments that may outline Eocene salt canopies or discrete salt structures, especially in southeast Texas and south Louisiana. Cretaceous and early Paleogene fault extension was accommodated by detachment at the Louann salt; Oligocene-Recent extension typically detached on allochthonous salt canopies or in marine shales (Rowan et al., 2005). South and southeast of the detachment domains are a complex set of salt-related detachments, minibasins, and roho systems associated with the formation and deflation of salt canopies of Oligocene and Miocene age. On the shelf and onshore, many of the structures are concealed by shelf-margin progradation, and only isolated salt diapirs extend upward to shallow depths. On the present continental slope, minibasins and salt canopies have pronounced bathymetric expression and a complex Neogene structural and stratigraphic history. Farthest south are the deepwater compressional domains of the Perdido, Keathley Canyon, and Mississippi Fan foldbelts. These compressional folds of late Oligocene through Pliocene age are cored by Louann salt in its autochthonous position. Several parts of the foldbelt lie along the transition from highly extended continental to oceanic crust, which may be somewhat thicker. In many cases, the contraction is balanced by extension in the detachment domains farther landward, perhaps assisted by Miocene loading with sediment and allochthonous salt sheets (Radovich et al., 2011). The complex three-dimensional structural and stratigraphic architecture of the northern basin is illustrated by a regional north–south section across the north-central basin fill (Fig. 16.9). The boundary between thick and thin transitional crust is reflected by a subsidence hinge that became the focus for development and stabilization of the Cretaceous continental shelf margin, most clearly marked by an extensive Albian reef system. Basinward, the thick Cenozoic sedimentary prism overlies thin basinal Mesozoic strata and thin transitional crust, which has been depressed 16 to 20 km by sedimentary loading. The prism extends beneath the coastal plain and shelf, reaching its thickest point near the present continental margin. The continental slope extends basinward to the Sigsbee escarpment and foldbelts, near and somewhat south of the position of the transitional/oceanic crust boundary. Beneath this sediment prism, most of the autochthonous Louann salt has been expelled, forming a primary salt weld on the middle Jurassic unconformity.
640 The Sedimentary Basins of the United States and Canada
FIG. 16.9 North–south cross-section of the northern Gulf of Mexico continental margin. (A) Crustal types, generalized stratigraphy, and structural elements including major salt canopies and detachment zones. (B) Principal facies associations (J, Jurassic; K, undifferentiated basinal Cretaceous; LK, Lower Cretaceous; UK, Upper Cretaceous; P-E, Paleocene-Eocene; O, Oligocene; M, Miocene; Plio., Pliocene; Pleist., Pleistocene). For location see Fig. 16.8. Modified from Line 4 of Peel et al. (1995); 5× vertical exaggeration. Reprinted courtesy of AAPG.
Paleogene and Neogene deposits form a basinward-stepping series of sediment wedges marked by the advancing shelf margin. Paleocene through Miocene wedges are expanded and deformed by a succession of growth fault families included within the Wilcox and mixed upper Eocene and top salt detachment provinces. The off-stepping deposition acted as a giant rolling pin, pushing salt basinward and upward into three major salt canopies. The inboard canopy (of Eocene to Oligocene age) was loaded and largely evacuated by subsequent deposition, forming the vast central Gulf shelf domain of minibasins, roho fault systems and diapirs. Beneath the continental slope, a shallow salt canopy of Miocene and Pliocene age forms the slope minibasin and salt canopy domains, which terminate in the Sigsbee scarp. However, at the east end of the slope minibasin province, salt rose directly from the autochthonous level (Fig. 16.10A). The base of the canopy rises through flatlying basinal Cretaceous and Cenozoic strata to the final salt sheet of the Sigsbee nappe, which is intruded into Pleistocene strata. Transects through the northeastern and northwestern Gulf margins (Fig. 16.10) illustrate additional features of structural domains and general basin stratigraphy. In the northeastern Gulf (Fig. 16.10A), the total basin fill is relatively thin, depressing the crust only to depths between 7 and 11 km. The crustal boundary again pins the location of the Mesozoic shelf margin; here, Neogene deposition has built the margin only about 50 km farther basinward. Growth faulting is limited. A small number of salt stocks, which rise from the largely evacuated autochthonous Louann salt and Mesozoic salt canopies (Bouroullec et al., 2017), define the eastern margin of the slope minibasin domain. Jurassic rocks are rafted southwestward (out of the plane of this section) on the Louann salt toward the basin center. The basinal toe of the section illustrates compressional features of the east end of the Miocene contractional domain. The northwestern Gulf transect (Fig. 16.10B) illustrates the structure of the basin depocenter located beneath the continental shelf and slope on highly extended to hyperextended crust. Autochthonous Louann salt on highly extended crust has been evacuated upwards into the OligoceneMiocene canopy, which has largely deflated and formed salt stocks and growth-fault (roho) systems updip, as well as the compressional Port Isabel foldbelt on the slope. The Louann salt deposited on the hyperextended crust has been evacuated primarily basinward into a Neogene canopy (west end of the Sigsbee canopy), and also into salt-cored folds of the Perdido foldbelt at the edge of primary salt accumulation (Fiduk et al., 1995, Trudgill et al., 1999). In contrast to the central and northeastern Gulf, the northwestern Gulf displays broad, complex middle Cenozoic compressional domains, including the
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 641
FIG. 16.10 Dip cross-sections of the northeastern (A) and northwestern (B) Gulf of Mexico continental margins. For location see Fig. 16.7. Modified from lines 1 and 5 of Peel et al. (1995), with additions to B from Radovich et al. (2007) and Ewing (2016a); both at 3× vertical exaggeration. Reprinted courtesy of AAPG.
Perdido and Port Isabel foldbelts. The Port Isabel foldbelt is linked by a decollement within the Oligocene salt canopy to the Miocene Clemente–Thomas, Corsair, and Wanda fault zones of the Oligocene-Miocene detachment province (Figs. 16.7 and 16.8; Hall et al., 2000). Additional contraction was taken up within the compound salt canopy formed within Oligocene and Miocene section.
Structural Growth History Backstripping analysis of regional cross-sections (Fig. 16.11) reveals the dynamic interplay between deposition, wholesale mass transfer of salt, development of growth structures, and outbuilding of the Gulf margin that has characterized the basin’s Cenozoic history (Diegel et al., 1995; Peel et al., 1995; McBride, 1998). Late Jurassic accumulation of up to 4 km of Louann salt extended across the subsided thinned transitional crust. By the end of the Cretaceous, pelagic and limited siliciclastic deposition had expelled much of the landward part of the autochthonous salt basinward, beneath the paleocontinental slope toe and northern basin floor (Fig. 16.11B). Extension of the upper slope in Paleogene time was largely accommodated by compressional deformation at the slope toe. A remnant layer of autochthonous salt, referred to as parautochthonous salt (Hudec et al., 2013a,b) provided a detachment horizon for basinward gravity spreading of overlying strata. By the end of the Oligocene (Fig. 16.11C), successive pulses of Paleogene deposition had prograded the shelf margin over the Cretaceous slope, deflating the parautochthonous salt layer by intruding it into canopy complexes under the advancing continental slope as well as further inflating the abyssal salt sheet. The Oligocene Frio growth fault zone migrated basinward with the prograding continental margin; here detachment occurred within upper Eocene mudstone as well as in the deeper salt. The resultant continental slope at the end of the Paleogene was a mix of sediment and near-surface salt bodies. Miocene–Pliocene deposition loaded the salt canopies, triggering passive diapirism and further gravity spreading, creating roho fault systems and isolated salt stocks separated by welds (Fig. 16.10D). Thick minibasin fills separate the salt stocks. Loading also initiated extrusion of another salt sheet at the toe of the slope. Pleistocene deposition has filled the updip minibasins and built the continental slope onto the distal salt sheet, where incompletely filled minibasins dominate present slope topography (Fig. 16.10E).
642 The Sedimentary Basins of the United States and Canada
FIG. 16.11 Reconstruction of the regional north–south cross-section of the Gulf continental margin (Fig. 16.9) showing evolution of salt canopies and fault complexes. Modified from Peel et al. (1995). Reprinted courtesy of AAPG.
DEPOSITIONAL FRAMEWORK OF THE NORTHERN GULF OF MEXICO BASIN The stratigraphic architecture of the postrift northern Gulf of Mexico Basin displays many elements typical of divergent continental margins (Winker, 1982, 1984; Winker and Buffler, 1988). Above a breakup unconformity, initial sedimentation onlapped the subsiding basin margin. Following this onlap phase, sediment supply overcame subsidence, and margin aggradation accompanied by offlap became the principal style. A deep, sediment-starved basin center became separated from the marginal coastal plain and shelf by a clearly defined shelf edge and slope. Further deposition created a succession of thick, offlapping stratal units constructing a broad coastal plain and continental shelf. This nearly continuous depositional record, which covers >160 million years of geologic time and continues today, produced a succession of regionally correlative stratigraphic units that are separated by major marine flooding horizons, sediment-starvation surfaces, and erosional unconformities.
Overview of Depositional Episodes and Sequences The stratigraphic framework, chronology, and nomenclature of the onshore portion of the Northern Gulf of Mexico Basin was established during the early to middle 20th century using conventional stratigraphic concepts (see Murray, 1961). The exposed, largely marine Cretaceous strata were subdivided using lithology and fossil content. The siliciclastic, largely
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nonmarine to marginal marine Paleogene section exposed in the Texas coastal plain was subdivided using the fossiliferous marine shale tongues that record regional transgressions across the northern shelf. Drilling for hydrocarbons extended the correlation of these shales into the subsurface, and identified additional fossiliferous shales that defined younger Oligocene and Neogene deltaic and shoreline units. Onshore drilling also found the buried marine and nonmarine Jurassic and Lower Cretaceous units, which were studied from the 1940s on. D.E. Frazier formalized the concept of transgression-bounded genetic units, based on the Cenozoic stratigraphic record, in a seminal paper (Frazier, 1974). Frazier argued that the Cenozoic fill recorded a succession of depositional episodes, each characterized by a foundation of progradational marine and coastal facies, overlain and replaced landward by aggradational coastal plain and fluvial facies. Both facies successions were capped by a relatively thin succession of transgressive or back-stepping coastal and marine shelf facies. The “Frazierian” genetic unit is bounded basinward by submarine starvation surfaces (condensed beds) created during and soon after transgressive retreat of coastal depositional systems. If relative or eustatic sea-level fall further punctuates the history of a depositional episode, the genetic unit will contain an internal subaerial unconformity within its updip strata. Using the “Frazierian” depositional model, Galloway (1989a) defined the genetic stratigraphic sequence as a fundamental unit of Gulf of Mexico Cenozoic stratigraphy (Fig. 16.12). The genetic sequence consists of all strata deposited during an episode of sediment influx and depositional offlap of the basin margin. It is bounded above and below by a family of surfaces of marine nondeposition and/or erosion created during transgressions that are generalized as the maximum flooding surface. This pattern is readily recognized in the Paleogene section, where transgressive marine shelf mudstone and glauconitic sandstone units extend to outcrop (Galloway, 1989b). It also applies in Neogene strata, where prominent transgressive markers record glacioeustatic sea-level rise events (Galloway et al., 2000). Thus, genetic sequences typically correspond closely to widely used stratigraphic nomenclature. The sequence stratigraphy paradigm, which uses subaerial erosion surfaces and their correlative conformities as sequence boundaries (Fig. 16.12), was developed in the 1970s (see Mitchum et al., 1977) and applied as an alternative to the traditional lithostratigraphic framework of the basin (e.g., Yurewicz et al., 1993; Mancini and Puckett, 1995; Lawless et al., 1997). The sequence paradigm has proven especially useful in late Neogene strata that are strongly influenced by
FIG. 16.12 General section showing difference between sequence-stratigraphic surfaces based on Mitchum et al. (1977) and subsequent authors, and genetic stratigraphic boundaries of Galloway (1989a), for (A) Type I sequences, where lowstands fall below the shelf edge, and (B) Type II sequences, where lowstands remain on the shelf. Modified from Galloway (1989a). Reprinted courtesy of AAPG.
644 The Sedimentary Basins of the United States and Canada
glacioeustasy (Weimer et al., 1998; Roesink et al., 2004). Depositional sequence models for carbonate and mixed successions, which are appropriate for the Mesozoic part of the basin fill, are summarized and illustrated by Handford and Loucks (1993). The synthesis of the depositional history of the Northern Gulf of Mexico presented here largely uses the traditional lithostratigraphic framework of the Mesozoic and Paleogene sections and the regional marine flooding horizons characterized by widely identified faunal markers within Neogene strata. Building upon the syntheses of Winker and Buffler (1988), Galloway (1989b), Morton and Ayers (1992), and Galloway et al. (2000), we use a genetic stratigraphic framework that groups strata into a succession of 29 principal Gulf of Mexico depositional episodes (Figs. 16.13–16.15). Each episode is a major genetic stratigraphic sequence that records a long-term (ca. 2–12 Ma) cycle of sedimentation in the basin, often accompanied by shelf margin offlap. Deposits of each episode are characterized by distinctive lithologies (sandstone, mudstone, carbonate, evaporite), vertical stacking of lithofacies and parasequences, and relative stability of sediment dispersal systems and consequent paleogeography. Almost all of the depositional episodes terminated with a phase of deepening and/ or basin-margin transgression (Figs. 16.14 and 16.15). Deposits of these episodes are bounded by prominent, widely recognized, and well-documented stratigraphic surfaces (Figs. 16.14 and 16.15). Bounding surfaces variously include marine starvation and condensed horizons, maximum flooding surfaces, marine and subaerial erosional unconformities, and faunal gaps that are described and interpreted by multiple authors. Such depositional episodes conform to the basic definition of a sequence as a contiguous suite of genetically related strata bounded in part by unconformities. In fact, most of the Mesozoic depositional episodes described here correspond to sequences identified by one or more authors (e.g., Yurewicz et al., 1993; Dobson and Buffler, 1997; Goldhammer and Johnson, 2001). They are widely recognized as fundamental stratigraphic building blocks of the Gulf of Mexico basin fill. At the same time, a depositional episode framework is sufficiently flexible and robust to accommodate stratigraphic units that were variously dominated by tectonic deformation, sediment supply and composition histories, or eustatic sea-level change. The stratigraphy, depositional system framework and paleogeographic evolution of the northern Gulf basin will be discussed in the context of the 29 depositional episodes. These episodes logically cluster into Late Jurassic and earliest Cretaceous (Oxfordian-Berriasian), Early Cretaceous (Valanginian-Cenomanian), Late Cretaceous (CenomanianMaastrichtian), Laramide (Paleocene-Eocene), Mid-Cenozoic (Eocene–Oligocene), and Neogene (Miocene-Recent) families. Each episode is recorded by a genetic sequence of strata that is constructed of a suite of carbonate and/or terrigenous siliciclastic depositional systems. These systems, in turn, record geologically long-lived paleogeographic features that constituted the physical geography of the northern Gulf of Mexico. The depositional system classifications (Fig. 16.16) follow those of Galloway and Hobday (1996) and Handford and Loucks (1993).
Late Triassic–Middle Jurassic (Norian-Callovian) Depositional Episodes As already stated, little is known of presalt sediment deposition in the Northern Gulf of Mexico Basin. As extension began in the Late Triassic, a series of half-grabens or full grabens were formed and filled with nonmarine redbed conglomerate, sandstone, and shale. In the northernmost part of the basin in southern Arkansas and East Texas, such rocks have been penetrated over a large area and this is called the Eagle Mills Formation. Similar rocks occupy a large graben complex centered in South Georgia and southern Alabama. By analogy with the better-known rocks of the Newark Supergroup in eastern North America, they are inferred to include both Late Triassic and Early Jurassic rocks and may include basaltic flows or sills. Similar redbeds are found in eastern Mexico and inferred in southwestern Texas, and red sediments are found in a number of wells in South Texas. Additional Triassic-Jurassic grabens are likely to occur in all of the extended basin areas. Early and Middle Jurassic (presalt) rocks are not known from the basin, although they are likely to exist in the more extended portions where they are buried deeply (see Van Avendonk et al., 2015). In other extensional basins, notably the Brazilian continental margin, presalt sediments include alluvial siliciclastics and lacustrine microbial carbonates (Guardado et al., 2000). Volcanic rocks may also be present in varying amounts. Such units could be thick and extensive, lying within a broad “sag basin” built on highly extended crust. The complete lack of subsalt penetrations in the central basin, combined with poor seismic resolution beneath complex salt structures, lead to our nearly complete ignorance of this phase of basin sedimentation.
Middle Jurassic–Earliest Cretaceous (Callovian-Berriasian) Depositional Episodes The Upper Jurassic and lowest Cretaceous Louann, Norphlet, Smackover-Haynesville, and Cotton Valley episodes form a tectonostratigraphic megasequence bounded below by the base of salt unconformity and above by a prominent
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FIG. 16.13 Generalized time-stratigraphic chart showing the stratigraphic succession and architecture of the Northern Gulf of Mexico Basin. Time scale from Gradstein et al. (2012). Modified from Winker and Buffler (1988). Organic-rich units are underlined. Abbreviations: MM, Middle Miocene; Vx, Vicksburg; Jx, Jackson; MWx, Middle Wilcox; San M, San Miguel; Tusc, Tuscaloosa; VUC, Valanginian unconformity.
intra-Valanginian unconformity, which may record the termination of seafloor spreading (Todd and Mitchum, 1977; Winker and Buffler, 1988; Wu et al., 1990; Salvador, 1991b; Dobson and Buffler, 1997; Marton and Buffler, 1999). By Bajocian time (about 170 Ma), Phase I continental extension had created a wide nonmarine basin that may have lain substantially below world sea level. In Bajocian time (according to recent Sr-isotope dating of the salt in Alabama by Snedden et al., 2018a,b), a marine connection was made, either with the Pacific Ocean across central Mexico
646 The Sedimentary Basins of the United States and Canada
FIG. 16.14 Mesozoic postrift depositional episodes as reflected by major phases of siliciclastic and carbonate sediment accumulation in the Northern Gulf of Mexico basin. Major stratigraphic surfaces include basin-margin unconformities, deepening events (D) and associated ravinement, and maximum flooding disconformities (MFS). Composite episodes reflect regionally concordant stratigraphic units bounded by major surfaces and a relatively stable paleogeography. VUC, Valanginian unconformity; MCU, Mid-Cretaceous unconformity; SAU, Sub-Austin unconformity; PGU, Pecan Gap unconformity; BU, Bigfoot unconformity.
(Salvador, 1987; Padilla Sanchez and Jose, 2016) or through an Atlantic connection (Martini and Ortega-Gutierrez, 2016). The resulting influx of marine water and intense evaporation led to widespread deposition of Louann salt and associated anhydrite that blanketed the transitional crust and any presalt deposits (Salvador, 1987, 1991a,b; Dobson and Buffler, 1997; Hudec et al., 2013a). As much as 4 km of nearly pure halite (with anhydrite around the margins and in the eastern part of the basin) buried the underlying topography and lapped northward onto the structural margin of the basin (Salvador, 1987; Figs. 16.9 and 16.10B). Salt accumulation was replaced, in the Callovian to early Oxfordian, by deposition of a relatively thin but widespread siliciclastic-dominated sequence that is known on the northern and
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 647
FIG. 16.15 Cenozoic depositional episodes as reflected by major phases of siliciclastic sediment accumulation in the Northern Gulf of Mexico basin. Major stratigraphic surfaces include basin-margin unconformities and maximum flooding disconformities. Composite episodes reflect regionally concordant stratigraphic units bounded by major surfaces and a relatively stable paleogeography. Neogene episodes incorporate multiple glacioeustatic cycles and their resultant high-frequency sequences. PETM, Paleocene-Eocene Thermal Maximum; MECO, Middle Eocene Climatic Optimum; MB, Moodys Branch.
n ortheastern basin margin as the Norphlet Formation. The boundary between the Louann and Norphlet sequences is poorly defined; deposition may have been either continuous or disconformable (Salvador, 1991a). In either case, the Norphlet deposits further onlapped the breakup unconformity, especially in the structural embayments of the northeastern basin margin in Alabama. There, several small alluvial fan and wadi systems are preserved in local depocenters up to 300 m thick. Thick eolian, sabkha, and playa deposits are abundant, including an extensive eolian sand sheet in southern Mississippi, Alabama, and the adjacent offshore waters (Hunt et al., 2017), indicating continued aridity. Basinward, siliciclastics grade into thin marine shale and limestone. Rapid marine transgression over the Norphlet sand sheet has preserved dune morphologies in some areas. Although we have differentiated the Louann and Norphlet as two episodes, based on the prominent lithologic change and evidence of a pulse of siliciclastic input, the Norphlet may best be considered the transgressive cap of a single, evaporite-dominated Louann sequence (Goldhammer and Johnson, 2001). Continued Oxfordian transgression onto the stable basin margin initiated the Smackover-Haynesville depositional episode, the first carbonate-dominated depositional episode of the Gulf. Together, the Smackover, Buckner, and Gilmer Formations record a 5-million-year cycle generally bounded above and below by transgressive flooding surfaces
648 The Sedimentary Basins of the United States and Canada
FIG. 16.16 Generalized paleogeographies of (A) carbonate-dominated and (B) siliciclastic-dominated episodes of deposition within the northern Gulf of Mexico. Principal depositional systems are distinguished using this format on the following paleogeographic maps.
(Budd and Loucks, 1981; Salvador, 1991b; Prather, 1992; Dobson and Buffler, 1997; Goldhammer and Johnson, 2001; Mancini and Puckett, 2005) (Fig. 16.13). Initial deposits consisted of fine-grained, dark, carbonate ramp sediments, which were succeeded by a heterogeneous assemblage of carbonates, including prominent ramp-edge grain shoals (Fig. 16.17A). These banks aggraded and coalesced to form a broad Kimmeridgian shoal system around the northwest and west-central Gulf (Fig. 16.17B) (Budd and Loucks, 1981; Moore, 1984). In the northeastern domain, grain shoals formed around the emergent basement arches. In midepisode, evaporites of the Buckner Formation accumulated on the shoal-restricted, shallow inner platform. Seaward, carbonate muds formed a broad carbonate ramp or slope. Siliciclastic influx was minor. Small to moderate delta systems with flanking
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FIG. 16.17 (A) Paleogeography and principal depositional systems of the Upper Jurassic (Oxfordian) Smackover depositional episode, earlier portion (Smackover). (B) Paleogeography and principal depositional systems of the Upper Jurassic (Kimmeridgian) Smackover depositional episode later portion (Gilmer—Haynesville).
shorezone systems (the Haynesville Formation of Mississippi) prograded onto the northeastern basin margin. The episode ended with terminal flooding and deposition of the transgressive Gilmer Limestone. Gilmer shoals also formed along the flanks of the Sabine block in East Texas. Organic mudstones were deposited in a broad area of the flooded shelf in north Louisiana and East Texas (the Haynesville Shale; Hammes et al., 2011). The pulse of siliciclastic sediment input along the northeastern basin margin (Fig. 16.17B), which coincided with the later part of the episode, limited transgressive Gilmer carbonate deposition on that margin to the outer ramp and basin (Cicero and Steinhoff, 2013).
650 The Sedimentary Basins of the United States and Canada
Siliciclastics of the Cotton Valley depositional episode of Tithonian-Berriasian age (Figs. 16.12 and 16.13) abruptly overrode the transgressive Gilmer and Haynesville strata and Bossier shale in the northern part of the basin (Salvador, 1991b; Prather, 1992; Dobson and Buffler, 1997; Goldhammer and Johnson, 2001; Klein and Chaivre, 2002). The dramatic change from carbonate-dominated to siliciclastic-dominated deposition across the entire northern Gulf basin reflects a combination of rejuvenated source areas, subsidence-induced breaks in the rift-shoulder highlands surrounding the basin, and possibly climate change. Large, sandy delta systems prograded from major fluvial axes centered in the northwestern East Texas basin, the northern Mississippi salt basin, and the Apalachicola embayment, with strike-fed shoreline systems between them (Fig. 16.18; Ewing, 2001). Suspended sediment spread basinward to form a broad, muddy, marine shelf platform (the Bossier shale, in part organic-rich) that built basinward into deeper water (Cicero and Steinhoff, 2013). As Cotton Valley deposition progressed, filling the marginal basins to sea level, a distinct shelf/slope break emerged at the southern or southwestern margins of those basins. This major episode of siliciclastic input and progradation lasted >10 million years, and deposited >300 m of sediment around much of the northern part of the basin. During Cotton Valley time, a large area in the Mobile area (offshore Mississippi and Alabama) was affected by raft tectonics, with blocks of Norphlet and Smackover units carried downdip to the southwest (Pilcher et al., 2014). Much of the Cotton Valley prograding wedge in the northern part of the basin may be Berriasian, according to some recent work (Staerker et al., 2013). It terminated with a relatively brief phase of carbonate accumulation initiating at the shelf margin, creating the back-stepping Knowles Limestone of earliest Cretaceous (Berriasian) age (Fig. 16.14). This carbonate blanket marks the terminal transgression of a siliciclastic-dominated episode; together, the Cotton Valley and Knowles form a major transgression-bounded sequence. Siliciclastic rocks offshore of Florida and Alabama have a distinctive source. Snedden et al. (2016a) used detrital zircon geochronology of Cotton Valley and other sandstones to establish a significant source of sediment in the Peninsular Arch (basement rock of Gondwanan affinity). Similar detrital zircons are also found in rocks from Norphlet to Hosston ages. Although the Cotton Valley depositional episode ended with the conventional record of transgression, its deposits are separated from strata of the overlying Hosston episode by a prominent unconformity throughout the northern Gulf of Mexico divergent margin (Salvador, 1991b; Goldhammer and Johnson, 2001; Fig. 16.13). Updip, this unconformity spans the entire Valanginian (about 5 million years); basinward, the span decreases until strata become concordant in the basin beyond the Cotton Valley progradational margin. There, Valanginian strata form a thick, sandy fore-shelf lowstand wedge with associated shelf-margin limestones, the Calvin and Winn limestones (Fig. 16.12; Loucks et al., 2017). The unconformity records subaerial exposure and erosion, which clearly reflect progressive uplift and basinward tilting of the basin margin. Coincidence of the unconformity with termination of seafloor spreading in the Gulf of Mexico and its medial location within a phase 25 million years long of coarse siliciclastic sedimentary influx to the northern basin suggests that
FIG. 16.18 Paleogeography and principal depositional systems of the Upper Jurassic (Tithonian) Cotton Valley depositional episode.
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 651
FIG. 16.19 Paleogeography and principal depositional systems of the Lower Cretaceous lower Hosston depositional episode.
its formation is a consequence of changes in the intraplate stress regime and resultant deformation of the North American plate. Together with the subsalt unconformity, the Valanginian unconformity bounds the postrift, syndrift strata of the basin.
Early Cretaceous (Valanginian-Cenomanian) Depositional Episodes Following termination of seafloor spreading in the Gulf of Mexico, a succession of six composite depositional episodes (Fig. 16.14, Lower Hosston–Washita) forms a megasequence that provides a record of diminishing continental source area relief and basin margin stabilization (Winker and Buffler, 1988; McFarlan and Menes, 1991; Scott, 1993; Yurewicz et al., 1993; Marton and Buffler, 1999; Goldhammer and Johnson, 2001; Kerans and Loucks, 2002; Badali’, 2002; Mancini and Puckett, 2005). The climatic setting remained tropical and arid. Siliciclastic input gradually decreased and carbonate deposition came to dominate most of the basin (Fig. 16.12). Two phases of regional progradation of a reef-rimmed carbonate margin, separated by a regional late Aptian to early Albian flooding event (Fig. 16.14), produced a well-defined shelf edge across the basin that separated open shelf and restricted platform depositional systems from slope and deep basin mudstone equivalents. Following this distinctive phase of early Cretaceous deposition, which lasted for nearly 40 million years, the intra-Cenomanian unconformity and subsequent mixed siliciclastic, carbonate, and organic deposition marked a basin-scale reorganization of regional depositional patterns. Siliciclastic sediments continued to be supplied to the Mississippi embayment and farther to the east, associated with continuing uplift in the southern Appalachian area. Siliciclastic sediment supply continued to dominate Valanginian and Hauterivian deposition. The conglomeratic, sandy Hosston Formation (called the Travis Peak Formation in Texas) records this siliciclastic influx. Hosston stratigraphy is internally complex and displays three depositional styles. Basal Hosston deposits form a shelf-margin lowstand prograding wedge coincident with coastal plain and shelf bypass across the Valanginian unconformity (Yurewicz et al., 1993). Beginning in late Valanginian, basal Hosston strata were buried by deeper-marine strata as subsidence of the basin margin and expansion of the depositional area of the basin resumed (Fig. 16.19). During the Hauterivian, lower Hosston strata onlapped the northern basin margin, aggraded the shelf, and prograded the shelf edge basinward of the Valanginian lowstand wedge. In the late Hauterivian, a minor episode of mid-Hosston deepening and transgressive flooding interrupted the depositional offlap. Together, the lowstand wedge and overlying aggradational and progradational lower Hosston deposits form the Lower Hosston compound depositional episode that was initiated by tectonically forced regression and terminated by transgression and onset of carbonate deposition on the outer shelf (Fig. 16.13; Goldhammer and Johnson, 2001). As in the Cotton Valley episode, four deltaic depocenters marked lower Hosston accumulation (McGowen and Harris, 1984; Dutton, 1987; McFarlan and Menes, 1991). Source areas located to the northeast and northwest coalesced into four sandy bedload fluvial systems that prograded marine-modified braidplains and deltas into the Apalachicola embayment
652 The Sedimentary Basins of the United States and Canada
(Florida delta), the Mississippi salt basin (Mississippi delta), the East Texas basin (Travis Peak delta), and slightly into the Rio Grande embayment. Extensive wave reworking of the sandy delta fronts nourished a series of interdeltaic strandplain and barrier/lagoon systems and associated shallow sandy shelves. Suspended sediment spread from deltas to form a muddy, prograding outer shelf and slope across the northern part of the basin. Carbonate production began in central Texas, on the interdeltaic shelf above the San Marcos arch. Following late Hauterivian minor flooding event, a mixed carbonate/clastic Sligo depositional episode inaugurated the reef-rimmed carbonate margin progradation that is the hallmark of the lower Cretaceous in the Greater Gulf of Mexico Basin (Fig. 16.20; Kauffman and Johnson, 1997). Landward, siliciclastic upper Hosston deposition resumed, but progressively decreased in geographic extent through the Barremian (McGowen and Harris, 1984; Bebout et al., 1981, McFarlan and Menes, 1991). At the same time, carbonate-forming open platform, reef, and grain shoal systems of the Sligo Formation expanded landward across the outer shelf and created a well-organized shelf-margin reef system that stretched from northern Mexico to the southern Florida platform (Bebout, 1977). Coral-like rudistid mollusks built these and later Lower Cretaceous reefs and banks. After modest progradation, the barrier reef-rimmed shelf margin stabilized and aggraded along a narrow belt located near the boundary between thin and thick transitional crust. This shelf margin, which persisted until the early Cenozoic, reflects a hinge line between the two crustal subsidence domains (Sawyer et al., 1991). By middle Aptian, the carbonate environments extended to the depositional limits of the basin, reducing siliciclastic facies to a thin, undifferentiated “ring” around the basin fringe (exposed as the Sycamore and Hammett Formations of Central Texas). The shallow shelf north of the reef margin formed a broad, open carbonate platform upon which local rudistid banks and grain shoals accumulated (Bebout and Loucks, 1974; Bebout et al., 1981). Extensive shoals, smaller patch reefs, and carbonate banks are particularly abundant over residual basement highs, such as the Sabine and San Marcos arches (McFarlan and Menes, 1991). After nearly 10 million years of carbonate platform growth and consolidation, abrupt late Aptian deepening terminated the Sligo depositional episode (Fig. 16.13, 16.14, and 16.20). The northern shelf of the basin was blanketed by the thin, widespread and locally organic-rich Pine Island Shale (Fig. 16.13). Temporary shoaling and rejuvenation of carbonateforming environments resulted in a brief (1–2 million year) and minor James depositional episode of carbonate platform
FIG. 16.20 Stratigraphic diagram oriented NW-SE through the Texas continental margin in Cretaceous time, showing the relationship of stratigraphic units, sequences, and depositional episodes. From Ewing (2016a), after Kerans and Loucks (2003); see also Phelps et al. (2013). Reprinted courtesy of Gulf Coast Section SEPM.
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 653
and margin deposition, forming the James or Cow Creek Limestone (Fig. 16.14). A second drowning event, recorded by the Bexar Shale, terminated the depositional episode. The shales and limestones of the James depositional episode are characteristically dark and fine-grained, interspersed with thin sandstones. Together, the Pine Island–James–Bexar interval, termed the Pearsall Group in Texas, constitutes a punctuated, retrogradational depositional episode that culminated with a basin-wide flooding surface. Following this flooding, the shelf margin that was initially reestablished in the early Albian was displaced far landward of the underlying Sligo shelf edge around much of the northern basin (Fig. 16.13). Shelf drowning was followed by slow reestablishment of regional carbonate platform and barrier reef systems during Albian time that became the defining features of the mid-Cretaceous Northern Gulf of Mexico basin. Reef progradation and aggradation reconstructed the Cretaceous shelf edge into a nearly continuous barrier rim, the Stuart City reef, extending from Mexico to south Florida. Rudistids continued as principal bank and reef builders, forming the barrier reef as well as patch-reef and bank complexes on the platform (Bebout and Loucks, 1974; Scott, 1990; Kauffman and Johnson, 1997). Corals, encrusting algae, and stromatoporoids also contributed to reef construction. Bathymetric contrast between the shallow carbonate platform and the Gulf of Mexico abyssal plain likely approached its maximum. Winker and Buffler (1988) calculated that water depth in the central Gulf of Mexico above oceanic crust was probably between 4.2 and 4.7 km in the Albian, over 1000 m deeper than the modern Gulf abyssal plain. Three deepening events and one episode of siliciclastic sediment input punctuated the 12-million-year history of the Albian rimmed platform, creating three depositional episodes named for the outcropping Glen Rose, Fredericksburg, and Washita groups that compose them (Fig. 16.14). Basal beds of the Glen Rose depositional episode onlap the underlying strata (Yurewicz et al., 1993), suggesting a local disconformity. Strata of the Glen Rose episode are characterized by sandy to argillaceous, oolitic and bioclastic lime mudstone, packstone, and grainstone. Siliclastics were introduced only around the basin margins and decreased with time (the Hensel Formation of central Texas; Mooringsport Formation of northern Louisiana). Glen Rose carbonate deposition extended farther onto the northwestern basin margin than previous units and rocks of this unit are widely exposed in central Texas. Contained within the middle of the Glen Rose limestones are evaporites and dolomites of the Ferry Lake Anhydrite, which accumulated in an internal salina behind the barrier reef. Detailed facies analysis of the Glen Rose in the northwestern basin margin indicates an internal flooding surface that might be used to further subdivide it into upper and lower episodes (Fig. 16.20; Kerans and Loucks, 2002). Terminal deepening followed by an updip unconformity, shoaling, and resurgent siliciclastic influx onto the inner shelf separate the Glen Rose from the overlying Fredericksburg depositional episode. The Fredericksburg genetic sequence consists of three principal lithostratigraphic components. The Paluxy (Texas) and Danzler (Mississippi and Alabama) Formations record initial, short-lived progradation of deltas and flanking shorezone systems onto the inner to middle shelf of East Texas and the northeastern basins (Fig. 16.21; Caughey, 1977; McFarlan and Menes, 1991). Shelf limestone and
FIG. 16.21 Paleogeography and principal depositional systems of the Lower Cretaceous Fredericksburg depositional episode.
654 The Sedimentary Basins of the United States and Canada
dolomite of the Edwards Group and its equivalents accumulated throughout the episode on the outer shelf and transgressed landward over Paluxy inner shelf, deltaic, and shorezone systems late in the episode. The shallow-water rimmed and restricted Edwards carbonate platform developed over the San Marcos Arch and central Texas, forming the Kainer Formation and Kirschberg evaporites (Rose, 1974). Intrashelf basins occurred to the northeast and southwest (the Tyler Basin and Maverick Basin), with McKnight evaporites deposited in the latter. On the northern margin of the Maverick Basin, the Devil’s River trend of carbonate shoals formed along the southern margin of the Edwards rimmed carbonate platform (Rose, 1974; Chenault and Lambert, 2005). As siliciclastic bypass to the slope decreased and carbonate systems dominated shelf margin and slope sedimentation, the declivity of the continental slope increased to angles exceeding 10 degrees (Corso et al., 1989). The resultant high-relief, steeply bounded carbonate margin around the northern Gulf set the stage for later development of the prominent mid-Cretaceous stratigraphic discontinuity. Throughout the episode, low-relief reefs flourished along the shelf margin, ultimately forming the widely recognized Stuart City reef (Fig. 16.21) (Bebout and Loucks, 1974; Scott, 1990). The reefal shelf margin prograded across the northern shelf of the basin, regaining and slightly prograding beyond the older Sligo margin, except along the segment lying south of the East Texas basin and Sabine arch where the Stuart City reef remained a few tens of kilometers landward of the underlying Sligo reef. In South Texas, the Stuart City reef axis diverted westward, across the Rio Grande embayment; its westward extent and character are uncertain. During the Fredericksburg depositional episode in the middle Albian, global sea-level rise and incipient development of the Western Interior Basin combined to extend marine deposition over most of Texas and occasionally to open a connection to the Western Interior seaway. This major flooding event on the craton reorganized continental drainage systems, terminating sand supply to the northwestern part of the Northern Gulf of Mexico basin. However, in the northeastern part of the basin, deltaic and shorezone siliciclastic systems fed by streams arising in the Appalachian uplands continued to accumulate sand and mud throughout the Fredericksburg episode. The Fredericksburg episode terminated with local exposure of the shallow-water Edwards platform (Rose, 1974), followed by widespread accumulation of dark, calcareous claystone and interbedded lime mudstone of the Kiamichi Formation and its equivalents in the Tyler basin, and the correlative “regional dense member” and related beds across the Edwards Platform (Fig. 16.9). The Kiamichi lithologies indicate regional deepening of the shelf in the Tyler basin. The Washita depositional episode bridged the Early to Late Cretaceous boundary (Albian-Cenomanian); however, its depositional style remained that of the Early Cretaceous. The episode was characterized by aggradational growth of the Stuart City reef. North of the reef trend, widespread accumulation of shallow shelf lime mud, bioclastic sand, marl, and calcareous mud dominated. The Edwards Platform was reestablished (Segovia Formation) but occupied a more limited area along the San Marcos Arch, which began a history of intermittent uplift. The southern margin of this platform began to rapidly subside across the Devils River shoal trend; to the south, the Maverick basin deepened and received open-marine lime mudstone (Salmon Peak Formation). Small deltas built into northeastern Texas, interfingering with carbonate shelf deposits. Although siliciclastic influx in the northwestern basin was significant, it was limited to fine, suspended load that was dispersed widely across the shallow marine shelf. Latest deposits of the Washita episode are widely distributed marine calcareous claystone and open-shelf carbonate mudstone, recording partial drowning of the carbonate platform and diminished carbonate formation within the deeper embayments. In the northeastern part of the basin, deltaic and shorezone environments continued as previously. The episode terminated with the formation of one of the major discontinuities in the Mesozoic record of the Gulf of Mexico, the mid-Cretaceous unconformity (MCU), which is widely used as a practical boundary between early and late Cretaceous rocks in the basin (Fig. 16.14). The mid-Cretaceous unconformity records a profound break in the depositional architecture of the northern Gulf of Mexico (Wu et al., 1990; Buffler, 1991; but was mistakenly used in the deep Gulf of Mexico for a horizon that is in fact the Top Cretaceous; see Dohmen, 2002). The broad carbonate-dominated shelf was replaced by mixed alluvial, deltaic, coastal and open-shelf carbonate depositional systems. The reefal Lower Cretaceous shelf margin, which had persisted for nearly 14 million years, was abandoned and overstepped in some areas by siliciclastic progradation. Upper Cretaceous marine strata blanketed the relict shelf edge, subduing its morphology and creating a pronounced ramp-like inflection across the buried reef complex. Along the Florida and western Gulf continental slopes, scours and channel cuts record the onset of active submarine erosion. The MCU has been widely attributed to the mid-Cenomanian sea-level fall shown on various sea-level charts (Buffler, 1991; Yurewicz et al., 1993). However, several attributes of the MCU indicate that global sea-level change was at best a minor factor in its formation. (1) Uplift and tilting of the Southern Arkansas Uplift and episodic erosion on the San Marcos arch removed varying amounts of the Lower Cretaceous section. Subaerial erosion cut as deeply as the Upper Jurassic Cotton Valley sandstones in northeast Louisiana, indicating uplift of as much as several hundred meters (Ewing, 2009a). Angular discordance across
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 655
the MCU in northern Louisiana clearly demonstrates the role of tectonic uplift in its origin. Changes in crustal stress regime, possibly associated with changing rates of Pacific and North American plate convergence and the Sevier orogeny of the western United States, may explain the basin flank deformation (Laubach and Jackson, 1990; Cao et al., 1993); but the domal nature of the Southern Arkansas Uplift and its association with igneous activity suggest a subcrustal warm upwelling, a hotspot or mantle plume (Ewing, 2009a). (2) Uplift in northern Louisiana and Arkansas and concomitant igneous activity was coincident with and closely followed by a nearly 8-million-year influx of sandy sediments into the northern portion of the basin, from fluvial systems draining rejuvenated or newly created upland sources. The timing and location of post-MCU siliciclastic depocenters is consistent with interpreted uplift of the Mississippi embayment by the Southern Arkansas Uplift, much of which now lies beneath the modern Mississippi Valley. The domal uplift has been connected with the passage of the Bermuda hotspot beneath thinned Paleozoic crust (Cox and Van Arsdale, 2002), but the lack of temporal progression in uplifts and related volcanism makes this identification unlikely. (3) The MCU can be traced onto the bounding continental slopes where it records a variable period of sediment starvation and separates Early Cretaceous basinal carbonates from Late Cretaceous or Cenozoic basinal mudstone (Buffler, 1991). However, its interpreted correlation with a prominent reflection horizon beneath the Gulf floor (Buffler, 1991) has been disproven by the results of drilling in deep water (Dohmen, 2002), where Lower to Upper Cretaceous strata do not show a faunal break. (4) The stratigraphic context shows that the shallow-water carbonate factory was progressively drowned by deepening of the shelf, as recorded in the later part of the Washita episode, then terminated as muddy siliciclastics (and anoxic waters) poured onto the shelf. The MCU thus can be seen as an excellent example of a drowning unconformity, albeit one enhanced by tectonic activity (Schlager and Camber, 1986; Wu et al., 1990). As regressive siliciclastic systems prograded in areas over the dying Stuart City reef, sedimentary bypass and slumping created complex onlap relations between the siliciclastic and carbonate slope wedges. Unlike a short-term sea-level fall, differential tectonic uplift of the basin margin, creation of a new upland source area, and long-term drowning and poisoning of the carbonate factory readily explain regional subaerial erosion, long-term rejuvenation of siliciclastic influx, carbonate suppression, and a permanent change in basin-wide depositional style across a composite unconformity surface.
Late Cretaceous (Cenomanian-Maastrichtian) Depositional Episodes The Upper Cretaceous strata overlying the MCU contain five composite depositional episodes, which vary in siliciclastic versus carbonate supply (Fig. 16.13 and 16.14) (Winker and Buffler, 1988; Wu et al., 1990; Sohl et al., 1991; Mancini and Puckett, 1995; Goldhammer and Johnson, 2001; Liu, 2004). Six transgressions associated either with regional flooding surfaces or with basin-margin disconformities bound and separate the episodes. Additional deepening or transgressive events appear to be present at least locally, and might be used to further subdivide these episodes in the future. The Tuscaloosa/Woodbine/Eagle Ford composite depositional episode consists of the Lower and Upper Tuscaloosa episodes of the Louisiana margin and the Woodbine and Eagle Ford episodes of the Texas margin (Snedden et al., 2016b). It is marked by two major deltaic systems, a larger one (Tuscaloosa) that prograded through Mississippi into Louisiana, and another (Woodbine) that prograded through the East Texas basin (Fig. 16.22). Ongoing uplift of the Sabine arch (the Rusk uplift of Ewing, 2009a) and the Southern Arkansas uplift separated the two siliciclastic depocenters and dispersal systems. The complex Tuscaloosa fluvial/deltaic system prograded rapidly across the Mississippi shelf and spilled over the relict Stuart City reef, to create a prograding shelf-margin wedge of delta and delta-fed slope apron sandstone and mudstone (Mancini et al., 1987). The prograding deltas constructed a new shelf edge slightly seaward of the foundered reef. Offlap of the siliciclastic wedge, which was >1 km thick, onto the steep carbonate slope initiated the first of the many growth fault families of the northern Gulf (Fig. 16.7). Sand was transported into the deep Gulf of Mexico to form a large submarine fan or apron of undetermined extent. Tuscaloosa deposition was interrupted by a marine transgression, creating lower fluvial/ deltaic and upper deltaic sandstone units separated by an organic-rich “marine Tuscaloosa.” Through latest Cenomanian and Turonian time, Tuscaloosa deltas backstepped as the Southern Arkansas thermal uplift began to collapse and sediment supply decreased. To the west, the Woodbine fluvial/deltaic system remained largely on the shelf (Fig. 16.22). Wave-dominated deltas prograded to the south and southwest during late Cenomanian time into the East Texas basin (Oliver, 1971; Turner and Conger, 1984). A shelf-margin delta was formed locally in southeast Texas. Elsewhere, distal suspended mud spread across the Cretaceous reef and built a muddy shelf margin that merges with the Tuscaloosa deltaic wedge south of the Sabine arch. The Woodbine sediment dispersal system records a single Cenomanian siliciclastic pulse, but was complicated by ongoing
656 The Sedimentary Basins of the United States and Canada
FIG. 16.22 Paleogeography and principal depositional systems of the Upper Cretaceous Tuscaloosa-Woodbine–Eagle Ford depositional episode.
uplift and subaerial exposure of the Sabine arch (Ambrose et al., 2009). Emergence of the uplift in Turonian time eroded and redeposited the Woodbine sands into a later Harris Delta (Kurten field) and created the angular truncation of Woodbine strata on the east flank of the East Texas basin (the trapping mechanism for the giant East Texas oil field; Hentz, 2010). Latest Cenomanian transgression of the Woodbine fluvio-deltaic plain, which lay more distant from the Southern Arkansas highland, led to widespread deposition of the Eagle Ford calcareous mudstones and muddy limestones across a broad east Texas shelf that was contemporaneous with renewed progradation of the Upper Tuscaloosa (Fig. 16.14). In the area southwest of the San Marcos Arch, organic-rich lime mudstones and marlstones of the Eagle Ford Formation accumulated; the most organic-rich units were Cenomanian and contemporaneous with the Woodbine deposition to the east, but preceding the global OAE 2 anoxic event (Denne and Breyer, 2016). These units reflect regional advance of anoxic waters onto the foundered shelf and shelf margin. Organic-rich shale units also occur in East Texas and may correlate with the Tuscaloosa marine shale in eastern Louisiana (Lowery et al., 2017). The composite episode is internally complex, with several merged unconformities representing generally slow sedimentation. It terminated with regional flooding and development of a latest Turonian condensed flooding horizon across the northern shelf, recording waning sediment supply and renewed subsidence and reworking (Fig. 16.13). Condensation and/or erosion are also suggested by contact relationships with the overlying Coniacian strata from south Texas to north Louisiana (Stephenson, 1929; Lundquist, 2000). Depositional style changed dramatically throughout the basin in the Coniacian. The Austin depositional episode (Fig. 16.13) is defined and named for the blanket of chalky limestone that covered the basin (Lundquist, 2000). The basin platform (north and west of the drowned Stuart City reef) was dominated by extensive deep carbonate shelves (Fig. 16.23) that extended to and beyond present outcrop and connected with the deposition of the Niobrara chalk in the Western Interior Basin (Chapter 9). Austin deposits are characterized by chalky lime mudstones created from deposition of globigerinid and coccolith oozes on an open, deep, siliciclastic-starved shelf. Pelecypod and echinoderm-rich grainstones, mudstones, marls, and calcareous shales are also widespread. More shell-rich molluscan limestone facies were deposited on the episodically uplifting San Marcos Arch (Grabowski, 1995). Mud cracks and intertidal features indicate local carbonate shorezone deposition in this area. Minor river systems continued to deliver sediment to southern Arkansas, forming the Tokio alluvial and delta system and associated mudstones east of Dallas. The northwestern part of the basin remained an open platform connecting to the Western Interior seaway (Fig. 16.23). Currents flowing across the connection between the two large oceanic basins may have played a role in creation of the distinctive intraformational scours, channels, and hard grounds that typify the Austin chalk in northeast Texas, including
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 657
FIG. 16.23 Paleogeography and principal depositional systems of the Upper Cretaceous Austin depositional episode.
the deep Waco Channel (Durham and Hall, 1991; Hovorka and Nance, 1994). Interchange of the boreal water mass of the Western Interior Seaway with the Tethyan water mass of the Gulf of Mexico is recorded by presence of mixed faunas in central Texas (Lundquist, 2000). A minor pulse of siliciclastic sediment supply built coastal deposits (Eutaw Formation) across the inner northeastern and central shelf, but these were a fraction of the earlier Woodbine and Tuscaloosa fluvial/deltaic systems. Most of the sandstones contain abundant glauconite and carbonate grains, reflecting extensive marine reworking in shorezone and shallow shelf environments associated with ongoing transgression during the Austin episode. The Lower Cretaceous shelf margin subsided and was blanketed by a ramp-like wedge of fine carbonate sediments. In contrast to the hundreds of meters of Austin strata found on the northern shelf and slope, the deep, central Gulf was largely sediment starved during this interval of regional highstand. Across the north central shelf, from the East Texas basin to the Monroe uplift, tuffs and bentonites record extrusive volcanism from a number of vents located in southern Arkansas and on the Monroe uplift (Byerly, 1991). Igneous activity was widespread elsewhere in the basin, including the volcanoes of the Balcones igneous province in Central and South Texas, the Jackson Dome volcano in central Mississippi, and isolated volcanic occurrences in southeastern Louisiana and in the deep basin (reviewed in Ewing, 2009a). The Austin depositional episode, although characterized by accumulation of open shelf carbonates across the northwestern Gulf, nonetheless is interpreted by Lundquist (2000) to record a shoaling cycle bounded by periods of relatively deep water. The upper part of the Austin chalky limestone is clearly early Campanian in age near Austin, and grades upward into marine mudstones (Sprinkle) and local sandstones (Wolfe City). At the top is a regional disconformity separating the lower Taylor from middle Campanian chalky marls and mudstones that begin the Upper Taylor depositional episode. In Arkansas, the Tokio sandstone and shale is overlain by marl (Brownstown), then a glauconitic transgressive sandstone and marine mudstone (Ozan). A deepening event there separates mud- and limestone-dominated lower Taylor from the chalky limestone (Annona Chalk) that begins the upper Taylor episode (Dolloff et al., 1967). The Upper Taylor depositional episode (Fig. 16.24), after initial deposition of the Pecan Gap chalky lime mudstone and marl and the Anacacho limestone shoal complex in south Texas, was characterized by renewed terrigenous sediment influx to the Gulf margin, this time to depocenters in the western part of the basin, in South Texas and in northern Mexico (Weise 1980; Tyler and Ambrose, 1986; Sohl et al., 1991). Rivers sourced in the growing North American Cordillera to the west and northwest filled in the southwestern part of the Western Interior seaway and spread muddy plumes into the western part of the basin. By the late Campanian, the wave-dominated San Miguel (Fig. 16.13) delta system spilled into the Rio Grande embayment and northern Mexico. This flux of Laramide-sourced siliciclastics created a depocenter that
658 The Sedimentary Basins of the United States and Canada
FIG. 16.24 Paleogeography and principal depositional systems of the Upper Cretaceous Upper Taylor depositional episode.
dominates the otherwise thin Campanian sequence of the basin. Deltaic and shorezone deposition continued and expanded into the lower Maastrichtian with the Olmos Formation (Snedden and Kersey, 1982; Tyler and Ambrose, 1986). In northeast Texas, progradation of the minor Nacatoch delta and shorezone systems records a small but significant siliciclastic pulse (McGowen and Lopez, 1983). As deltas built from the west and north into the seaway outlet, the mixing of Tethyan and boreal water masses was gradually curtailed and the Gulf of Mexico was separated from the Western Interior Seaway. Volcanism continued from the late Austin into the upper Taylor episode in South Texas, providing local ash and siliciclastic sources. Igneous activity and crustal heating formed a local high (Uvalde uplift) on the south Texas shelf, allowing the formation of thick bioclastic grainstone shoals of the Anacacho Limestone (Ewing, 2004). Local shoals also formed around volcanic highs (Luttrell, 1977). Across the broad platform east of Texas, relative sea level remained high, submerging the basin margin throughout much of the Upper Taylor depositional episode. Deposition of chalky lime mudstone (Annona Chalk) occurred dominantly in shallow to deep open-shelf systems similar to earlier Austin deposition. Even the fringing terrigenous deposits, found along the present outcrop belt northward into Tennessee, largely record shallow shelf, shoreface, and transgressive marine settings. Thus the depositional sequence consists of a mosaic of marine sediments including calcareous claystone, fossiliferous mudstone, glauconitic and fossiliferous sand, marl, chalk, and impure limestone. In the northeastern part of the basin, shallow shelf sandstones, chalks, and marlstones bracket a lower to middle Maastrichtian shorezone sandstone containing one or more inner shelf disconformities (Skotnicki and King, 1989; Mancini and Puckett, 1995). Abundant lags of phosphorite, bored phosphatized mud clasts and fossil casts, turtle, shark, fish, and mosasaur teeth and bone fragments, and durable shell debris indicate nearshore to inner-shelf current erosion formed the disconformities, likely in response to relative sea-level fall. Collapse of the Mississippi embayment as the earlier thermal uplift cooled and subsided had, by this time, created a marine reentrant that extended northward along the modern Mississippi Valley. The last depositional episode of the Late Cretaceous is the minor Escondido depositional episode, of middle to late Maastrichtian age (Fig. 16.14). The base of the episode is a thin transgressive shelly marl (Corsicana Formation). In south Texas, a renewed uplift of the San Marcos arch created a significant unconformity below this transgression, the Bigfoot unconformity (Ewing, 2003). Above the Corsicana transgression, the episode contains a succession of strata that record a phase of siliciclastic-dominated progradation and shoaling bounded above and below by intervals of erosion, marine transgression, shelf starvation, and prominent flooding surfaces (Mancini and Puckett, 1995, 2005). In South Texas, the relatively thick Escondido Formation continues the depositional styles of the upper Taylor, with a mix of deltaic and shorezone environments extending across the Rio Grande embayment and sourced from Laramide
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u plands in northern Mexico (Snedden, 1991; Ewing, 2003). In north Texas, the equivalent strata are thin mudstones (Kemp clay); to the east they become chalky (Arkadelphia Chalk). The Cretaceous-Tertiary boundary strata of the Gulf of Mexico constitute a complex condensed horizon and unconformity, recording widespread sediment starvation and/or erosion throughout the area of preserved Cenozoic strata (Sanford et al., 2016). They record a cataclysm of global proportions, the Chicxulub meteorite impact event (Hildebrand et al., 1991). The Chicxulub crater, 140 km by 190 km in diameter, is located beneath the Yucatan Platform, in the southern Gulf of Mexico basin. The consequent seismic shock triggered submarine slides and mass flows along the continental slope (Bralower et al., 1998; Poag, 2017). An impact tsunami created a distinct event bed that is widely noted around the northern Gulf margin. This event is associated with significant erosion of the shelf and shelf margin and redeposition of a mixed “cocktail” of Upper Cretaceous sediments in the deep basin (Schulte et al., 2006; Denne et al., 2013; Sanford et al., 2016). Significant portions of the Escondido depositional episode may be missing from much of the basin or may be redeposited due to the Chicxulub event (Kinsland and Snedden, 2016).
Laramide Depositional Episodes (Paleocene-Eocene) The Cenozoic depositional history of the Northern Gulf of Mexico basin has been summarized by Galloway et al. (1991a, 2000). Galloway et al. (2000) identified 18 Cenozoic depositional episodes in the basin. Here, we group these into 13 episodes (Fig. 16.15) by combining some minor episodes and emphasizing the first-order changes in supply history and paleogeography. These episodes can be further grouped into four phases that record major developments in the adjacent North American drainage basins: (1) Paleocene–Middle Eocene episodes related to Laramide uplift and erosion; (2) Late Eocene–Oligocene episodes related to volcanism, crustal heating, and related uplift in the Southwestern United States and Mexico; (3) Miocene episodes related to erosional rejuvenation of eastern North American uplands and uplift in Central and West Texas; and (4) Early Pliocene–Quaternary episodes that record rejuvenation and integration of western interior drainage basins due to uplift, climate deterioration, glaciation, and high-amplitude, high-frequency glacioeustatic sealevel change. The boundaries of these phases are gradational, and individual episodes may show the effects of multiple phases. Deposits of each depositional episode are separated by regional transgressive marine shales that contain, at or near their base, a maximum flooding surface. These Cenozoic depositional episodes create the archetypal genetic stratigraphic sequences (Galloway, 1989b). Regional flooding of the Gulf margin to and beyond present outcrop characteristic of the Cretaceous persisted for the first few million years of the Paleocene. Widespread shelf muds and marls of the Midway Group and Porters Creek Formations blanketed the northern Gulf margin, becoming thin and condensed toward the basin center. Minor amounts of sand entered the far western parts of the basin in South Texas, but much less than in the previous Upper Cretaceous episodes. However, beginning in the Late Paleocene and extending through the early Eocene, depositional outbuilding of the coastal plain, spearheaded by delta systems built by large river systems flowing down the Houston, Mississippi, and Rio Grande axes, heralded the onset of successive waves of Cenozoic siliciclastic influx (Galloway et al., 2000). Four major depositional episodes dominate Paleocene through middle Eocene history throughout the northern Gulf of Mexico basin (Figs. 16.13 and 16.15). They record surges of siliciclastic supply, modulated by the pulses of Laramide uplift that began in the Central and Southern Rocky Mountains of the United States in the latest Cretaceous and spread progressively southward into the Sierra Madre Oriental of northern Mexico (Winker, 1982; Galloway, 2005b; Chapter 13). Laramide compressional crustal stress extended eastward into the basin by Middle Eocene time, as reflected by broad folding in the Rio Grande embayment, rejuvenation of the Sabine and related arches, and accentuated subsidence of the western Gulf abyssal plain (Laubach and Jackson, 1990; Cao et al., 1993; Feng et al., 1994). The Late Paleocene and Early Eocene Wilcox episodes significantly prograded the shelf margin and continental slope of the northern Gulf of Mexico from the relict Cretaceous reef margin (Fig. 16.9 and 16.10B). The Lower Wilcox depositional episode records the first major Cenozoic influx of sediment onto the northern Gulf continental margin. A broad fluvial-dominated delta system (Rockdale delta) prograded across East Texas and the Houston embayment and the San Marcos arch and onto the relict Cretaceous slope (Fig. 16.25). A second, smaller fluvial-dominated system (Holly Springs delta) built across the Mississippi salt basin to the shelf margin in Louisiana. Both form large late Paleocene depocenters. An extensive wave-built shorezone system extended across the San Marcos arch through south Texas into northern Mexico, with high rates of sediment influx and subsidence (Galloway et al., 2011). In Mexico, erosion fed into thick siliciclastic deposits in a narrow foredeep trough, the Magiscatzin basin, extending from Coahuila southsoutheast into the Chicontepec area southwest of Tampico. Little sediment made its way over the high to the east of this basin (the old Tamaulipas arch, also known as the Tampico-Misantla high) into the main part of the basin.
660 The Sedimentary Basins of the United States and Canada
FIG. 16.25 Paleogeography and principal depositional systems of the Late Paleocene Lower Wilcox depositional episode.
Rapid sediment loading mobilized the deepwater muds and the autochthonous Louann salt, initiating numerous extensional growth faults seaward of the Cretaceous shelf margin. These growth faults form the inboard elements of the Wilcox fault zone, which extends from northern Mexico to east-central Louisiana (Fig. 16.7). Loading also induced salt mobilization and expulsion from beneath the Wilcox basin depocenters toward the continental slope of that time, where salt canopies began to form. Contemporaneous Laramide compression uplifted and tilted the Cretaceous shelf deposits in the Rio Grande embayment. This tilting triggered the Lobo megaslide, centered east and southeast of Laredo in the southern Rio Grande embayment, which affected >10,000 sq. km of the Gulf margin (Long, 1986). Seaward of the Lobo slide, large rafts of Mesozoic strata began to move eastward on the deep autochthonous salt, leaving behind troughs that formed the active depocenters of the later Upper Wilcox episode (Fiduk et al., 2004) Ongoing seismicity associated with foreland deformation of the western Gulf of Mexico margin may have triggered frequent smaller slumps and slides that are observed along the prograding siliciclastic shelf margin from south Texas to central Louisiana. Several of these slumps near the San Marcos Arch and in Louisiana nucleated submarine canyons that excavated up to several hundreds of meters of older Wilcox strata (Galloway et al., 1991b). Large amounts of sand-rich sediment were carried into the deep Gulf of Mexico through these canyons and slumps, forming extensive submarine fans in the western part of the basin. The Lower Wilcox depositional episode terminated with backstepping of delta and shorezone facies and regional transgression. A second, latest Paleocene Middle Wilcox depositional episode followed, bracketed by two widely correlated, thin, marine shale horizons (Galloway et al., 2000). The younger of these, the Yoakum Shale, extends nearly to outcrop in many areas and is associated with the beginning of the pronounced carbon isotopic excursion genetically related to the Paleocene-Eocene Thermal Maximum (PETM; see Dickey and Yancey, 2010; Sluijs et al., 2014). The associated regional transgressions, recorded by the regional Yoakum Shale above and Big (Tilden) Shale below, punctuated the episode and are associated with the largest submarine canyons. The best known of these, the Yoakum Canyon, is located above the San Marcos arch in the central Texas coastal plain. It cut across the transgressive shelf >150 km landward from the shelf edge, and excavated as much as 1.5 km of underlying Lower Wilcox deltaic deposits (Galloway et al., 1991b). A canyon of this size was not seen again in the basin until the Pleistocene (Galloway, 2005a). Some have proposed that sea levels were drawn down at or near Yoakum shale time due to isolation of the Gulf of Mexico by Cuban collision (Cossey et al., 2016); however, this hypothesis is flawed, as there is a well-documented connection to the Atlantic Ocean through the Suwanee strait, as well as many deepwater wells with bathyal and abyssal fauna during the time in question (Umbarger and Snedden, 2016). The Middle Wilcox episode records a decline in siliciclastic supply from the Lower Wilcox in the shelf and shelfmargin areas, but produced large submarine fans extending far into the basin. It is capped by the fossiliferous, glauconitic Sabinetown Formation in Texas outcrops and the Yoakum Shale in the subsurface.
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After the PETM flooding event and Yoakum Shale deposition, rejuvenated and reorganized Early Eocene bedloaddominated fluvial systems entered on either side of and across the San Marcos arch, forming the Upper Wilcox depositional episode (Fig. 16.26). The fluvial systems deposited an amalgamated network of coarse, sandy channel fills across the central and south Texas coastal plain, creating the Carrizo Sandstone, one of the major aquifer systems of the basin (Hamlin, 1988). Basinward, these fluvial systems supported a family of wave-dominated deltas (Rosita delta system; Edwards, 1981) that prograded rapidly to and over the shelf margin in south Texas. Here, basinward rafting of the underlying Mesozoic section, likely begun in the Paleocene, had opened an arcuate, growth-fault-bounded “Rosita depotrough” that now collected highly expanded successions of delta front and slope apron sediment (Fiduk et al., 2004). Initial progradation into the head of the Yoakum canyon likely led to sediment bypass down the canyon and the final stage of a submarine fan system at the base of the central Texas continental slope; thin submarine fan sediments of Upper Wilcox age are reported in deepwater Lower Tertiary wells (McDonnell et al., 2008). Limited shelf-margin progradation also occurred in southeastern Texas. Tectonic realignment of continental drainage systems appears to have diverted most sediment supply away from the central Mississippi axis of the basin during the Upper Wilcox episode. There, limited sediment supply and extensive marine reworking created broad, sandy shorezone and shelf systems that trend northward along the flank of the deltaic coastal plain, which extended as far as east-central Louisiana, and into the Mississippi embayment (Fig. 16.26). The Sabine arch may have begun to provide a low relief upland source drained by minor fluvial tributaries. The northeastern shelf was largely a slowly subsiding shallow shelf with small fluvial shelf deltas and shorezone deposits and mixed siliciclastic shelf deposits. Carbonate sediment accumulated throughout the length of the Florida platform south of the Suwannee strait. Beginning in latest Cretaceous, and continuing into the Eocene, the broad, moderately deep Suwanee strait (Fig. 16.26) connected the northeastern corner of the Gulf of Mexico with the Atlantic Ocean (McKinney, 1984; Umbarger and Snedden, 2016). Strong marine currents were funneled through this strait, which effectively separated the northern siliciclastic and eastern carbonate shelf provinces. By the time of Upper Wilcox deposition, the northeastern basin shelf margin had evolved the compound dip profile that is still reflected today in the West Florida terrace and Florida escarpment (Fig. 16.1). A shallow, perched, prograding shelf break, located near the present Florida coastline, separated the shallow siliciclastic and carbonate shelf systems from a broad submarine ramp, which in turn was bounded seaward by the foundered, relict Cretaceous deep shelf and forereef slope. The regional Reklaw transgression terminated the Wilcox depositional episode. Meanwhile, steady erosion and burial of the Laramide southern Rocky Mountain uplands, which provided the principal source of sediment to the basin, resulted in diminished sediment supply into the Middle Eocene (Galloway and Williams, 1991; Galloway et al., 2011). The Queen
FIG. 16.26 Paleogeography and principal depositional systems of the Eocene Upper Wilcox depositional episode.
662 The Sedimentary Basins of the United States and Canada
City and Sparta episodes deposited sediment primarily on the Wilcox depositional platform (Figs. 16.14 and 16.15). The continental slope and abyssal plain remained sediment starved, particularly in the central and eastern Gulf of Mexico. During the Queen City depositional episode, paleogeography resembled that of the Upper Wilcox. Deposition of wavedominated deltas and thick barrier and strandplain systems was centered in the Rio Grande embayment; thin shoreline and shelf sediments extended across East Texas and an embayed marine shelf extended from there eastward across Louisiana and Mississippi and northward into the Mississippi embayment. This very broad, funnel-shaped embayment amplified the normally low tidal range of the Gulf of Mexico and created, in Queen City deposits, a unique assemblage of tide-dominated shorezone and shelf facies in the East Texas basin (Ramos and Galloway, 1990). Farther seaward in south-central Louisiana and Mississippi, the episode is highly condensed, forming the thin, calcareous Cane River shale. The extensive and long-lasting Weches transgression followed, which produced ferruginous marine deposits across most of the northern part of the basin. The overall low rate of sediment supply and extensive but shallow marine flooding of the northern Gulf margin created widespread fossiliferous marine shale and glauconite beds that extend to outcrop and record long periods of very slow sediment accumulation. The transgression records as much as one million years of time in its few meters of sediment. Following the Weches transgression, the minor Sparta depositional episode records a shift of continental fluvial drainage axes back into the central Gulf, as the Mississippi embayment was filled with deposits of a fluvial and shelf delta system that approached the shelf margin in central Louisiana. Shoreline and small shelf delta deposits occur in the western part of the basin. The Cook Mountain transgression, which terminated the Sparta episode, records up to 2 million years of marine inundation of the western and northern shelves of the basin.
Middle Cenozoic (Eocene–Oligocene) Volcanism and Related Depositional Episodes The latest Middle Eocene (Bartonian) saw a modestly rejuvenated sediment influx onto the northwestern and central Gulf margin. Some of this sediment in the Houston embayment may be derived from erosion of Wilcox sandy sediment on the Sabine arch and related features. Deposits of the Yegua (YAY-wah) depositional episode (Fig. 16.27) are also distinguished by the appearance of abundant volcanic ash beds. Initially, Yegua (Cockfield in Louisiana and east) fluvial-dominated deltas prograded across the shallow transgressive shelf that had submerged the Sparta delta and shorezone systems in the Houston embayment and Mississippi salt basin. As the actively prograding Yegua deltas in the Houston embayment approached the shelf margin, they first built across the perched Sparta and Queen City sand-rich delta platform margins. Progradation was then over the older muddy shelf developed on the old Wilcox delta plain.
FIG. 16.27 Paleogeography and principal depositional systems of the Eocene Yegua depositional episode.
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The combination of rapid sediment influx and renewed loading of the old, muddy continental margin in the Houston embayment triggered a succession of submarine slumps and growth faults that coalesced along the Yegua delta front to form a compound intraformational mass wasting surface that soles out within underlying muddy Eocene strata (Edwards, 1991). Slide scars extended as much as 20 km inland from the Yegua shelf margin, creating steep slopes and local depocenters that both initiated and collected further mass flows and turbidity currents. Following this early retrogradational phase, shelf margin deltas built across the slump complex, healing the embayed margin and prograding the shelf edge (the Liberty delta; Ewing and Vincent, 1997; Ewing, 2007). Sediment remobilized from the unstable shelf-margin delta front and prodelta formed a heterolithic slope apron. Yegua strata in many parts of the basin are well known for their well-developed incised channels or valleys that extend tens of kilometers from updip shelf delta lobes across a muddy outer shelf and terminate in small, low-stand, shelf-margin delta lobes (Ewing 2002, 2007). A series of basinward shifts in relative sea level created 5 to 10 (depending on location and author) significant progradational lowstand pulses that healed and prograded the shelf margin during the 2.5 million year Yegua episode (Fig. 16.28), each bounded by transgressions and flooding surfaces in part corresponding to updip deltaic deposition (Edwards, 1991; Meckel III and Galloway, 1996; Ewing, 2007). Detailed micropaleontological analysis (Fang, 2000) of mid- and downdip Yegua strata confirm that the maximum flooding surfaces found within the bounding transgressive marine shelf mudstones represent substantial time intervals. In contrast, despite clear evidence of basinward shifts in relative sea level and channel cutting across exposed shelf mudstones, no biostratigraphically significant hiatus can be documented across these “sequence boundary” surfaces within the Yegua episode. Significant amounts of sand made their way into the slope setting and into intraslope basins, but are not known from the basin floor. The episode ended with regional marine transgression and deposition of the Moodys Branch marl (east and central) or Caddell claystone (west). Additional fluvial-deltaic systems prograded into the Rio Grande embayment and across the Mississippi salt basin into south Louisiana (Fig. 16.28). In South Texas, deltaic sediment (Falcon delta) was reworked along strike to build thick, progradational, barrier and strandplain systems, which also show evidence of multiple forced regressions (Fig. 16.28B). Rivers in the Mississippi embayment deposited an extensive but thin set of shelf deltas in central Louisiana and southern Mississippi; some incised channels occur south of these deltas and may have reached the shelf margin, The Suwannee strait was in the process of filling in, but it continued to separate the carbonate-dominated shelf of the Florida platform from the siliciclastic shelf and shorezone systems of Mississippi and Alabama. Eocene deposition terminated with the minor, but economically significant Jackson depositional episode (Fig. 16.15). Deposition during this episode remained firmly on the shelf and sandy deltaic and shorezone systems were restricted to the northwestern margins of the basin in Texas (Galloway et al., 1991a). Small delta systems prograded into southeast Texas. Extensive and thick barrier island and strandplain systems extended across the central and south Texas coast in front of a large lagoon, with one basinward shift in relative sea level creating a seaward “lowstand” delta (Hockley delta) (Fig. 16.28B; Ewing, 2002). For the last time, the Mississippi embayment suffered marine inundation as the entire central and northeastern margin reverted to an extensive muddy shelf (Yazoo clay), which is biostratigraphically condensed and sediment-starved in basinward areas. Jackson strata in Texas contain common bentonite and vitric ash beds and abundant shelf muds, consistent with the inception of continental volcanism in the Big Bend of Texas and adjacent Mexico at about 36 Ma (Yancey et al., 2018). The Oligocene was a time of massive sediment influx into the western half of the Northern Gulf of Mexico Basin (Galloway and Williams, 1991; Galloway et al., 2011). This influx resulted from extensive crustal heating, uplift, and volcanism in source areas in northern Mexico and the southwestern United States, including westernmost Texas. This uplift and volcanism began in the Late Eocene, reached its climax during the Oligocene (35–24 Ma), and continued into the early Miocene. Uplift impinged on the western margin of the basin; Cretaceous and early Cenozoic foreland basin fill was elevated >3 km in Oligocene and Miocene time in northeastern Mexico (Gray et al., 2001). In the uplift area, numerous large calderas sourced immense felsic ash-flow sheets as well as large amounts of airfall ash. This explosive volcanism combined with uplift to create a long-lived outpouring of sediment including recycled sedimentary rocks, volcanic rocks, volcaniclastics, and large amounts of reworked, devitrified ash. The response in the western and central basin was the Frio-Vicksburg composite depositional episode, which lasted for >8 million years (Fig. 16.15). The composite episode includes the lower Oligocene Vicksburg Formation (or Group) and the later Frio Formation. The base of this depositional episode is unconformable in many parts of the basin margin. Uplift in Mexico and western Texas related to prolific volcanism created an unconformity overlain by early Oligocene volcanic-rich siliciclastic sediments in southern Texas (Yancey et al., 2018). This event affected the north-central margin as well, by rejuvenation of drainage basin hinterlands and pervasive deposition of easily reworked ash. Rapid arrival of this ash is documented by Oligocene zircon ages (Blum et al., 2017). The preceding transgression capping the Jackson coastal deposits was brief; it is most clearly recognizable in the shallow subsurface in the central Texas coastal plain (Galloway et al., 1994). At outcrop along the
664 The Sedimentary Basins of the United States and Canada
FIG. 16.28 Stratigraphic cross-sections of Yegua and associated episodes in Texas; (A) NW-SE section, southwest of Houston;
Evolution of the Northern Gulf of Mexico Sedimentary Basin Chapter | 16 665
FIG. 16.28 Cont’d(B) W-E section, west of Corpus Christi; from Ewing (2016a). Datum on the Moody Branch MFS (top of the Yegua depositional episode).
666 The Sedimentary Basins of the United States and Canada
n orthwestern basin margin, the boundary is variously manifested by the abrupt superposition of alluvial plain deposits on coastal Jackson facies, prominent mature paleosols, locally incised basal Vicksburg alluvial channel and valley fill successions, and low-angle discordance between Jackson and basal Oligocene deposits (Galloway, 1977; Galloway et al., 1979; Combes, 1993). This assemblage of features shows that relative base level rise and transgression of the Jackson fluvial and shorezone systems were contemporaneous with minor tilting and relative uplift along the updip margin of the basin. Indeed, the beginning of the Oligocene marks a change in the style of tectonic subsidence along the basin margin (see Galloway et al., 1991a, their Fig. 3; Hudec et al., 2013b; Dooley et al., 2013). Paleocene-Eocene subsidence involved minimal basinward tilting; sequences thicken only gradually until they reach the paleoshelf margin (see Fig. 16.28). In contrast, Oligocene and all younger sequences thin rapidly as they approach the outcrop, indicating that tilting subsidence along a basinward prograding hingeline between a rapidly subsiding sediment-loaded basin and a neutral or uplifting forebulge to the northwest has characterized the later Cenozoic (Hudec et al., 2013b). The combined influences of continental uplift, deposition of massive amounts of air fall ash, and sourcing of volcanicrich sands and gravels in the various fluvial drainage basins is reflected in the total load, sediment composition and texture, and the progressive growth of the four delta systems that were active during Vicksburg and Frio deposition (Galloway, 1977; Galloway et al., 1982b; Fig. 16.29). The primary Oligocene depocenter lay in the Rio Grande embayment and consisted of up to 5 km of deposits of the Norias wave-dominated delta system and associated fluvial and delta-fed apron systems (Galloway et al., 1982b). Norias deposition began with rapid progradation of Vicksburg-phase deltas onto a thick foundation of muddy Eocene shelf and slope deposits. The shelf margin here may have been further destabilized by seismicity related to uplift and tilting of the western basin margin. The immediate consequence was the formation of the Vicksburg detachment (Fig. 16.7), a shallow detachment system with a basal glide plane in the Jackson marine shale that extends >500 km along strike northward from the Burgos basin in northern Mexico (Diegel et al., 1995). Atop the detachment is the Vicksburg growth fault zone, containing elements of both margin progradation and rafting (Fig. 16.7). The shallow detachment resulted in basinward displacement of Vicksburg delta front and upper slope deposits of as much as 16 km horizontally (Diegel et al., 1995; Feragen et al., 2007). Following stabilization of this detachment, further Frio progradation built the continental margin 90 to 145 km beyond its Eocene position. Progradational loading of the continental slope initiated several lines of growth faults (Frio fault zone) that form the updip part of the Oligocene-Miocene detachment province of the northwestern basin (Figs. 16.7 and 16.8). The extension was compensated basinward by compressional faulting and folding in the Port Isabel foldbelt, which lay at the base of the Oligocene continental slope and salt canopy (Figs. 16.8 and 16.10B).
FIG. 16.29 Paleogeography and principal depositional systems of the Oligocene Frio depositional episode. HB, Hackberry embayment.
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To the south in Mexico, the Norma delta complex rapidly prograded into the basin. Here, however, most tilting and uplift followed the initial early Oligocene progradation. A prominent angular unconformity separates the Norma Conglomerate (fluvial channel axis) and the equivalent “Nonmarine Frio” from underlying Vicksburg and “Marine Frio.” As uplift of the Sierra Madre Oriental and its foredeep (Gray et al., 2001) migrated eastward, older Cenozoic strata deposited in the Paleocene and Eocene foreland basin were elevated and recycled. The third principal delta system, the Houston delta, is centered beneath the southeast Texas coastal plain (Fig. 16.29). Initial delta lobes of Vicksburg age are thin, and largely remained on the Eocene shelf platform; only their distal fringes reached the shelf margin and were affected by growth faulting. However, relative base-level fall along the basin margin is indicated by the incision of valley systems that extend from outcrop into the shallow subsurface (Galloway, 1977; Combes, 1993). As Frio deltas prograded into and across the Houston salt basin, loading of subjacent Louann salt, either autochthonous or formed into salt sheets in the Eocene, fostered a phase of active salt diapir growth and minibasin development (Diegel et al., 1995). Lower Frio deposition prograded the shelf margin and resulted in growth faulting, but of lesser magnitude than in South Texas. A fourth delta system, which was fed by a large, suspended load-rich “Ancestral Mississippi” fluvial system flowing south along the axis of the Mississippi embayment, prograded into the South Louisiana salt basin in the Lafayette area. Here, large-scale salt evacuation from beneath the shelf-margin delta and continental slope depocenter accommodated as much as 4 km of Oligocene strata (Fig. 16.9). However, arrival of sediment to the central basin axis was delayed to the late Oligocene; the Vicksburg strata of south Louisiana consist of thin shelf mudstone and marlstone. The Ancestral Mississippi fluvial system, unlike its sister systems of the northwestern and western basin, was less directly affected by uplift and erosion of its tributary drainage basin (except perhaps in Colorado). However, sediment influx was accelerated by the rapid recycling of altered volcanic ash, in the form of suspended mud, through this midcontinental drainage system. Thus, the resultant delta system was large, but mud-dominated and slow to develop. Between the delta systems, the Frio sequence contains thick successions of strandplain and barrier bar/lagoon complexes (Fig. 16.29). These wave-dominated, slightly progradational shorezone systems were nourished by longshore reworking of sediment from the prograding deltaic headlands. The central Texas barrier/lagoon complex contains as much as 1.5 km of stacked, amalgamated barrier, beach ridge, and shoreface sand (Galloway et al., 1982b). Together, the thick, prograding delta, shorezone, and slope apron systems initiated and perpetuated a succession of growth faults that extend almost continuously from northern Mexico to eastern Louisiana (Fig. 16.7). Between the Houston and Lafayette delta systems, particularly rapid mid-Oligocene salt withdrawal from beneath the shorezone, shelf, and upper slope systems combined with weak marine Jackson shales beneath, triggered a brief phase of shelf-margin collapse or foundering, and subsequent submarine erosion that retrograded the shelf margin. The resultant Hackberry embayment is one of the bestdescribed examples of the many destructional slope systems within the Gulf of Mexico Cenozoic section (Ewing and Reed, 1984; Cossey and Jacobs, 1992; Galloway, 1998a). The eastern portion of the basin remained starved of siliciclastic sediment. By Early Oligocene time, the Suwannee strait was filled in, allowing the Florida platform to merge with the northeast shelf. Carbonate deposition on the outer shelf expanded westward as far as Louisiana. Local latest Oligocene patch reefs, known as the Heterostegina Limestone, developed over active salt domes as far west as the Houston salt basin during deposition of the Anahuac transgressive shale, and represent the westernmost expansion of Floridian carbonate systems during the Cenozoic. Decreasing rate of sediment supply and accumulation in the late Oligocene (Galloway and Williams, 1991) terminated the Frio depositional episode. Long-term backstepping of delta and shorezone systems culminated in regional transgressive flooding and deposition of the Anahuac marine shale across the breadth of the shelf margin. This transgressive shale did not, however, reach as far landward as the Eocene shales and does not appear in outcrop; there, the equivalent horizon appears as an unconformity due to regional basinward tilting and subsequent incision of the uplifted margin.
Miocene Depositional Episodes Miocene basin fill consists of three multi-million-year depositional episodes that record the progressive shift of the primary locus of deposition in the northern Gulf of Mexico from the northwestern to the central and northeastern margins. To the northeast, the Appalachian and Cumberland Plateau uplands were uplifted and rejuvenated and supplied sediment to rivers emptying into the central and east-central parts of the basin. To the west, volcanism was greatly reduced, volcanic uplands that sourced basinward-draining fluvial systems were lowered, and the western mountains became increasingly arid. During the Miocene, Basin and Range tectonism formed interior drainages within the Rio Grande rift grabens. Distant effects of Basin-and-Range tectonism extended to the western margin of the basin, with activation of the Balcones fault system, uplift of the Edwards plateau to its west, and continued uplift in northeastern Mexico (Gray et al., 2001). Consequent long-term
668 The Sedimentary Basins of the United States and Canada
changes in the rate and location of sediment supply largely defined three episodes that are approximately coincident with the Early, Middle, and Late Miocene (Fig. 16.15). Concurrently, global climate was evolving toward the icehouse world of the late Cenozoic. Increasing amplitude and frequency of glacioeustatic sea-level fluctuations impacted stratigraphic and facies architecture, especially within the deposits of Miocene shorezone systems (Galloway, 1998b, 2002). The Miocene stratigraphy of the Northern Gulf of Mexico basin is characterized by extensive progradation of the siliciclastic continental margin, triggering growth faulting and salt tectonics. The dominant extrabasinal fluvial systems became established in positions that closely approximate Quaternary counterparts (Galloway et al., 2000; Galloway, 2005b). The Lower Miocene composite depositional episode, 8 million years in duration, resembles the Frio-Vicksburg composite episode in its configuration (Fig. 16.30; Galloway et al., 1986). An extended phase of high rates of sediment supply and continental margin outbuilding followed upon the Anahuac transgression. A transgressive interruption at about 18 Ma (the “Marg A shale”) was used by Galloway et al. (2000) to differentiate two lower Miocene episodes; this shale is best developed in the northwestern part of the basin. Afterwards, margin progradation was renewed, followed by a pronounced phase of retrogradation and transgression that terminated the episode. The widespread Amphistegina shale and its contained maximum flooding surface, which is named for the diagnostic Amphistegina B faunal datum, caps the lower Miocene depositional episode (Fig. 16.14). Following the Anahuac transgression, the bedload-dominated Rio Grande and Norma fluvial axes declined in relative importance, although they remained a major depocenter, feeding the North Padre and Norma deltas that prograded the shelf margin several tens of kilometers. Wave reworking and long-shore transport dominated the delta system, shifting the maximum sediment thickness northeast toward the laterally adjacent central Texas barrier-strandplain system. In the central Gulf, the Ancestral Mississippi fluvial axis continued to increase in relative importance. A new fluvial axis, coincident with the modern Trinity and Sabine rivers, but with a drainage basin and size more commensurate with those of the modern Red River (Xu et al., 2017), entered the basin near the Texas/Louisiana border, as the Sabine arch was no longer an active high, and formed the Calcasieu delta. Together, these two fluvial-dominated deltas prograded the continental margin 65 to 80 km basinward. At the onset of deposition, the Calcasieu and Mississippi deltas and their slope aprons experienced a second episode of Hackberry-like continental margin foundering and mass wasting. Numerous slump scars, fault-expanded shelfmargin deltas, and submarine canyon fills reflect the interplay of margin collapse, submarine erosion, and rapid deposition. The collapse of this “Planulina embayment” and concomitant development of the Planulina fault zone (Fig. 16.7) may have been a consequence of large-scale salt withdrawal from beneath coastal Louisiana in response to the eastward migration of depositional loading. Combined deflation of the shallow, underlying Oligocene canopy and extension along the
FIG. 16.30 Paleogeography and principal depositional systems of the lower Miocene depositional episode (Anahuac shale to Amhistegina shale). PE, Planulina embayment.
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Oligocene and autochthonous Louann detachment zones (Fig. 16.11, panels C and D; Diegel et al., 1995; Peel et al., 1995) accommodated nearly 7 km of lower Miocene sediment in the central Gulf depocenter. Thick, sandy turbidite successions began to spill down the continental slope in the east-central and northeastern parts of the basin. Despite the proximity of the paleo-Mississippi delta system, the northeastern Gulf margin initially remained a carbonate province. Much of the Alabama and Mississippi shelf was sediment starved, creating a prominent nonconformity and a narrow carbonate shelf. Later in the episode, the delta-fed shelf muds encroached eastward, terminating reef growth and restricting carbonate platform deposition to the Florida shelf. Depositional loading beneath the deltas and well-nourished interdeltaic shorezones, and offlap of thick, sandy slope apron systems created prominent structural features, including the Clemente-Tomas and Planulina fault zones (Fig. 16.7). Salt extruded from beneath the prograding margin spread southward, nucleating new canopy complexes beneath the lower paleo-slope and adjacent abyssal plain. In the western part of the basin, compression related to North Padre deltaic deposition continued within the Port Isabel foldbelt (Hall et al., 2000). Inland along present basin-margin outcrop belts in Texas, low-angle unconformities separate basal Miocene (Oakville Fm.) strata from underlying Oligocene (Catahoula Fm.) strata, and basal middle Miocene (Lagarto Fm.) strata from underlying lower Miocene (Fleming Fm.) strata. Similar unconformities occur throughout the Neogene (Fig. 16.15; Galloway et al., 1982a, 1986). These discordances record intermittent tilting or flexural subsidence generated by successive episodes of sediment supply and crustal loading (Hudec et al., 2013b). The middle Miocene depositional episode records a relatively brief (ca. 3 m.y.) episode that was terminated by a regional but short-lived Gulf margin transgression associated with the Textularia stapperi faunal top (“Tex W shale”). The paleogeography of this episode clearly documents the effects of early Neogene continental tectonics and source area rejuvenation (Fig. 16.31; Galloway et al., 2000; Combellas-Bigott and Galloway, 2006). A new fluvial system, named for the Tennessee River, which currently occupies the comparable headwaters area, made its appearance. This system drained uplands characterized by Paleozoic outcrops and, consequently, transported sandy, mineralogically mature sediment to the basin. Together the Ancestral Mississippi and paleo-Tennessee rivers created the dominant mid-Miocene depocenter and prograded the continental margin as much as 70 km. Initial margin progradation was interrupted, however, by a third pulse of salt evacuation and margin collapse, located beneath the southeast Louisiana coastal plain (Combellas-Bigott and Galloway, 2006). This “Harang embayment” and its bounding fault zone (Fig. 16.6) are the culmination of an Oligocene (Hackberry) to Miocene west-to-east wave of salt evacuation from beneath the prograding margin. Beneath the central Texas shelf, a newly consolidated Corsair fluvial-deltaic system prograded onto the continental slope (Morton et al., 1988; Galloway et al., 2000). Here, salt withdrawal and prolonged growth of the Corsair detachment fault zone created a depocenter that was filled
FIG. 16.31 Paleogeography and principal depositional systems of the middle Miocene depositional episode (Amphistegina Shale to “Tex W” shale).
670 The Sedimentary Basins of the United States and Canada
by wave-dominated delta and delta-fed apron deposits. Between deltaic headlands, extensive wave-dominated shorezone systems were fronted by narrow, muddy to sandy shelves and prograding, muddy, shelf-fed slope aprons. Notice that the Calcasieu and North Padre deltas have vanished as major elements in the middle Miocene episode. In the northeastern part of the basin, combined margin collapse, slope bypass, and alignment of a series of dip-elongate slope minibasins created a focused submarine transport pathway that diverted a large quantity of sediment from the paleoTennessee delta front to the adjacent slope toe and abyssal plain (Fig. 16.31; Combellas-Bigott and Galloway, 2006). The McAVLU submarine fan system (named for the three US deepwater protraction areas where it is located) was born. It persisted as a major depositional feature of the eastern basin floor until the end of the Miocene. This and subsequent Neogene fan systems are distinguished from slope aprons by (1) location of a depocenter at the base of the contemporaneous continental slope, on the abyssal plain, (2) aggradational, rather than offlap, stratigraphic architecture, and (3) development of a radial sediment dispersal pattern indicating focused down-slope transport from a point source rather than a line source. By these criteria, fan systems are unusual features of the Gulf deep water; slope aprons are much more common and volumetrically more important. The apex of Middle Miocene submarine fans are offset as much as 100 km eastward from their deltaic point sources, which Snedden et al. (2012) attributed to enhanced outer shelf and slope oceanographic currents during the progressive closure of the Central American seaway. Combined depositional loading and extension along the western shelf margin caused continued compression along the Port Isabel foldbelt and particularly in the Perdido foldbelt. Loading in the central shelf and shelf-margin triggered further shallow salt canopy inflation beneath the continental slope of Louisiana, and initiated the Keathley-Atwater and Mississippi Fan foldbelts at the basinward edge of autochthonous Jurassic salt on the northeastern abyssal plain. The Upper Miocene depositional episode (Fig. 16.32) records a period of 6 million years of relative paleogeographic stability and high sediment supply (Morton et al., 1988; Galloway et al., 2000; Wu and Galloway, 2002). Sediment input was dominated by the joint Ancestral Mississippi and paleo-Tennessee systems. A large, compound fluvial-dominated delta system prograded onto the slope in the central Gulf, where the shelf edge advanced 40 to 90 km. The McAVLU fan continued to expand and evolve until late in the episode. West of the major delta complex, the Corsair delta and surrounding shorezone systems decreased in importance as sediment repositories. Wave reworking created a broad strandplain, interrupted by several small wave-dominated deltas extending from northern Mexico to eastern Louisiana. Depositional loading of the basin margin in the north-central basin by up to 5 km of upper Miocene sediment continued to drive wholesale basinward salt displacement into salt sheets and masses beneath the paleo-continental slope and into the cores of folds of the Mississippi fan foldbelt beneath the abyssal plain. Along the curvilinear, wave-dominated northwestern
FIG. 16.32 Paleogeography and principal depositional systems of the Upper Miocene depositional episode (“Tex W” shale to “Big A” shale).
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margin, slow continental margin progradation onto muddy slope aprons built the shelf edge to or near its present position. Here, loading created a linear belt of growth faults known as the Wanda fault zone (Fig. 16.7). Compensatory contraction was focused along the northeastern segment of the Miocene compressional domain (Fig. 16.8. 16.10B). The upper Miocene episode terminated with regional marine flooding surfaces associated with the last occurrence of the benthic foraminifers Robulus E and/or Bigenerina A and the presence of Buliminella 1. The subsequent minor Buliminella 1 depositional episode bridges the latest Miocene to early Pliocene (Fig. 16.15). Although sediment supply rates remained high and siliciclastic input continued to be focused through the Mississippi and paleo-Tennessee rivers, accumulation shifted back onto the continental shelf and margin (Galloway et al., 2000). The thickest deposits occur within a combined fluvial-dominated delta system and upper slope delta-fed apron on the central Gulf margin. Upper slope minibasins captured the bulk of the sediment that spilled over the shelf edge. Continued remobilization of the subjacent salt canopy is recorded in the South Timbalier–Ship Shoal fault family, which is part of a larger roho domain (Figs. 16.7 and 16.8) (Schuster, 1995). The middle and deep slope was supported by the extensive Miocene Sigsbee salt canopy complex. The McAVLU fan system was completely abandoned. West of the major delta, a thick marine mudstone was deposited on the outer shelf and shelf margin (the Pliocene Shale). The southward movement of a basin forebulge resulted in extensive erosion in southeastern Texas that removed the entire Buliminella I episode and much of the underlying Miocene strata (Ewing, 2016b).
Pliocene–Quaternary Depositional Episodes Sediment influx and depositional patterns of the later Neogene continue to show the combined influence of the tectonic controls described for the Miocene deposits, but also show effects from pronounced global and North American climate change that led to continental glaciation and high-frequency, high-amplitude glacioeustatic sea-level fluctuations. As in the earlier Cenozoic, sediment accumulation was concentrated along the continental margin and slope where depositional loading and salt evacuation and migration produced a mosaic of minibasins and salt-cored highs. These minibasins have progressively been filled by advancing delta-fed slope aprons (Prather et al., 1998). Rapid, high-amplitude glacioeustatic sea-level changes are manifested in the Gulf stratigraphic record by development of multiple sequences of one to five hundred thousand years duration with well-defined subaerial exposure and flooding surfaces (Lawless et al., 1997; Weimer et al., 1998). Depositional paleogeography (Figs. 16.33 and 16.34) and supply rate suggest these can be grouped into two low-order genetic sequences each lasting about 2 million years (Fig. 16.15).
FIG. 16.33 Paleogeography and principal depositional systems of the Pliocene (Globoquadrina altispira interval) depositional episode.
672 The Sedimentary Basins of the United States and Canada
FIG. 16.34 Paleogeography and principal depositional systems of the Pleistocene (Trimosina A interval) depositional episode.
Following the brief, post-Buliminella 1 transgression, the pattern of deposition changed in several ways (Galloway et al., 2000, 2011; Galloway, 2005b; Snedden et al., 2018a): 1. The Calcasieu fluvial axis was rejuvenated. This may reflect a rejuvenation of the Red River drainage basin to epeirogenic uplift and eastward tilting of the western High Plains and Rocky Mountains, or an avulsion from a prior location that fed either the Mississippi or Corsair fluvial axes. Shelf-margin progradation occurred along the combined front of the Calcasieu/Red River and Mississippi delta systems. 2. Sediment supply through the paleo-Tennessee axis continued to decline. As a consequence, depocenters shifted westward into the north-central basin, and the northeastern continental slope again became relatively sediment starved. There, Pliocene strata are sandy, but thin. 3. Slope instability and mass wasting affected both the northeastern and northwestern shelf margins on either flank of the combined delta complex (Pulham, 1993). The northeastern upper slope and shelf edge locally retreated by combined subsidence and mass wasting, particularly in the mid Pliocene. On the western flank of the combined delta, a megaslide scar was developed; an apron of correlative debris is mapped on the western abyssal plain (Fig. 16.33). 4. Along the relatively steep northeast margin, turbidite channel complexes extended to the slope toe, initiating a new submarine fan system. This fan has been informally called the WRLU fan for its location beneath Walker Ridge and Lund protraction areas (Fig. 16.33). Deposition in this fan system continued for much of the Pliocene. 5. Across most of the central and northwestern slope, depositional loading of the shallow salt canopy began a process of molding the minibasin province that is reflected in the modern slope structure and topography (Figs. 16.1 and 16.7). Beneath the outer shelf, salt withdrawal caused active growth of the South Cameron fault family (Fig. 16.7). 6. Shorelines on the northwestern and western shelf remained wave-dominated. However, in the middle Pleistocene the integration of the Rio Grande drainage formed a significant delta that prograded over the shelf and formed small shelf-edge deltas during lowstands (Fig. 16.34). 7. To the north and northwest, the Pliocene and Pleistocene are represented by various allostratigraphic units, bounded by unconformities below and above, the Willis Formation representing the Pliocene episode and the younger terraces (Lissie and Beaumont being the major ones) representing the Pleistocene. Oxygen isotopic data indicate inflow of glacial meltwater into the Gulf by the latest Pliocene (Joyce et al., 1993). Development of the North American ice sheet profoundly altered drainage systems flowing into the Gulf. The Mississippi drainage basin was enlarged and reached its present extent as north-flowing streams were dammed and diverted south to
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form the Missouri River. Recurrent eustatic lowstands, deglaciation events, and consequent meltwater pulses began the process of excavation of the modern Mississippi Valley (Saucier, 1994; Fig. 16.1). As the valley was cut and backfilled by glacial outwash, the Red and Tennessee Rivers were intermittently and then permanently trapped (the latter via the Ohio tributary). The enlarged catchment of the Mississippi “Father of Waters” that now drains the middle of the United States was established by the late Pleistocene. Development of a singularly large river draining into the central Gulf of Mexico created an extensive fluvial-dominated delta and subjacent slope apron system (Fig. 16.34). At the same time, the high-amplitude, high-frequency glacioeustatic sea-level changes of the Pleistocene punctuated the stratigraphy. Rapid transgressions forced shorelines temporarily landward 150 to 250 km, creating broad flooded continental shelves during “highstand” conditions. Subsequent sea-level drawdowns to “lowstand” conditions forced streams to carve deep incised valleys across the exposed shelves and, together with the high rates of sediment supply, developed delta lobes at the shelf edge. Instabilities associated with rapid shelf-edge deposition, pulses of glacial outwash, and frequent changes in sea level triggered episodes of mass wasting and submarine canyon erosion and filling of a magnitude not previously seen in the basin. Canyon excavation was most active on the eastern flank of the delta system. The Quaternary Mississippi fan system, the third in the succession of Neogene abyssal fan systems, has been fed through these canyons. Smaller, relatively short-lived canyons have created smaller fans, such as the Bryant fan. Beneath the prograding slope apron, minibasins continued to subside and fill. Many delta-fed turbidite channel/lobe complexes and debris flows spilled down slope from the prograding shelf margin delta complex. Salt mobilization and loading beneath the outer shelf are recorded by growth of the South Cameron and South Eugene Island fault families (Fig. 16.7). The modern basin reflects, in its sediment distribution and morphology, the latest Pleistocene Wisconsinan (18 ka) lowstand and subsequent Holocene transgression. Much of the modern (highstand) shoreline in Texas and in the eastern basin is relatively stable, lying at or near the shoreline positions of previous Pleistocene interglacial highstands. However, the Louisiana coastal zone is a dynamic product of the extensive Holocene progradation accomplished by formation and abandonment of a succession of Mississippi delta lobes. Ongoing delta-plain subsidence and wetland loss largely reflect the natural instabilities of such a young deltaic coastline.
PATTERNS AND GENERALIZATIONS IN GULF DEPOSITIONAL HISTORY The depositional and structural history of the Northern Gulf of Mexico Basin as outlined in this review is long and complex. However, there are some common elements that deserve additional consideration. Some of these result from the high rate of sediment supply and accumulation, which has created an unusually complete record of nearly 160 Ma of North American geologic history.
Sediment Supply and Transport: “Source to Sink” The shifting paleogeography and deltaic depocenters of the northern Gulf margin are a reflection of the varying tectonic forces that shaped the southern portions of the North American continent. One way to appreciate these shifts is to examine the locations of major fluvial axes that brought sediment into the basin from basin-margin and extrabasinal sources from Jurassic through the Cenozoic (Fig. 16.35). Eight fluvial axes routed sediment into the basin, although not all were active at any one time. The San Juan and Rio Grande axes fed sediment through the area of the Rio Grande embayment in south Texas and northeastern Mexico from Late Cretaceous through Cenozoic time; a minor axis fed sediment from the north during the uplift of the San Marcos arch. The Brazos and Red River axes fed sediment into the East Texas embayment throughout the basin’s history (the Red River’s transfer to the Mississippi drainage being a more recent event). The Mississippi axis occupied the Mississippi embayment and, joined by the Tennessee-Alabama axis, fed into the southern Mississippi basin. To the east, the Apalachicola axis fed into the embayment of that name. In the past, the relative contributions of these axes varied greatly: In the Late Jurassic (Tithonian), major rivers breached the rift margins in East Texas and the northern Mississippi salt basin, as well as in the Alabama-Apalachicola area, forming major delta systems. These persisted until the Aptian in Texas and to the Santonian in Mississippi, with slowly declining sediment input. In the middle to Late Cretaceous, small to moderate deltas were fed by streams flowing south from the Ouachita area in Oklahoma within the Red River axis (Paluxy, Nacatoch). Larger deltas appeared briefly in the Middle Cenomanian (Woodbine/Tuscaloosa), associated with thermal uplift in Arkansas and adjoining areas, as well as an Appalachian-derived sediment influx.
674 The Sedimentary Basins of the United States and Canada
FIG. 16.35 Axes of fluvial input of siliciclastic sediment into the northern Gulf of Mexico basin, Jurassic–Recent, grouped into recurrent trends. Abbreviations shown on Figs. 16.14 and 16.15.
In the Late Cretaceous, fluvial axes built eastward from the Cordillera into the Rio Grande axis and along the San Marcos Arch (San Miguel, Olmos, Escondido). But elsewhere, little siliciclastic sediment was supplied to the muddy, chalk-rich basin. In the Paleocene, fluvial systems occupied the Red River–Brazos axis (Rockdale) and the Mississippi (Holly Springs) axis, and from the Rio Grande axis (Carrizo/Rosita) in the Early Eocene. Middle Eocene sediment-poor rivers fluctuated between Rio Grande and Mississippi sources, followed by the Yegua fluvial axes in all three main embayments (Falcon, Liberty, Concordia/Amite). Oligocene sedimentation, greatly influenced by volcanism and uplift in Mexico and the southwest, created major rivers in the Rio Grande and San Juan axes (Norias and Norma), the Red River axis (Houston), and the Mississippi axis (Lafayette). In the Neogene, the Rio Grande axis became insignificant by Middle Miocene time. The Red River fluvial axis migrated widely, from the Calcasieu delta (Texas–Louisiana border) southwest to the Corsair delta and back, ultimately being absorbed by the Mississippi in the Plio-Pleistocene. The Mississippi axis grew steadily in size and power. The TennesseeAlabama fluvial axis provided much sediment in Miocene time but decreased thereafter as its headwaters were captured by the Mississippi drainage. This much we can deduce from the basin stratigraphy based on studies of outcrops, subsurface well penetrations, and seismic data. Yet we know that the major changes in quantity and location of sediment delivered to the basin are a mirror of inland continental tectonics. What were the river systems that connected probable mountainous source areas to the depositional “sinks” in major deltas and deep-water slope and fan deposits? To answer these questions, until recently we could only use geometry and relative timing of events. However, U-Pb geochronology of detrital zircons has become a key tool for deciphering provenance and provenance changes through time. Here we can give an overview of how the river drainage evolution may have worked, based on numerous studies, well summarized in Xu et al. (2017), Blum et al. (2017), and Snedden et al. (2018a,b) (Figs. 16.36 and 16.37). During Jurassic rifting, the margins of the extending basin were probably highlands. Alluvial fan and fan-delta systems built out from these highlands a short distance into the main basin. In Late Triassic time, a throughgoing river flowed northwestward from the backside of the northwestern highlands to a delta in Utah (Chinle River; Riggs et al., 1996; Mickus et al., 2009). Immediately postsalt, rivers from the Appalachian highlands fed sandy sediment to the eastern part of the basin that was reworked to form the eolian Norphlet sand sheet. After marine flooding and Oxfordian-Kimmeridgian carbonate deposition, major rivers entered the basin from the northwest (Lone Oak river, debouching into the East Texas embayment) and the north (Ancestral Mississippi river, debouching into the Mississippi salt basin). Mississippi sources were probably in the Appalachian area (modern Ohio drainage); Lone
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FIG. 16.36 Source areas, inferred river systems, and deltaic and slope sediment “sinks” during Mesozoic and earliest Cenozoic deposition. (A) Late Jurassic and Early Cretaceous; (B) mid-Cretaceous (Cenomanian); (C) Late Cretaceous (early Maastrichtian); (D) Late Paleocene. Green shading represents the drainage system with most influence on the sedimentary record; blue shading represents persistent marine deposits. In part after Blum et al. (2017) and Snedden et al. (2018a).
Oak river sources were probably in the rift shoulders of far West Texas and southern New Mexico, formed along the Border Rift System active in the Late Jurassic (Fig. 16.36A). The Morrison Formation to the west represents a separate depositional basin, separated by a drainage divide. The Lone Oak river remained a stable feature, slowly dying off in volume to the Aptian. Sediment recycling due to the Valanginian unconformity (simultaneous with the cessation of seafloor spreading) enhanced early Hosston deposition. Ancestral Mississippi river connections remained strong through the Early Cretaceous. A first-order global eustatic highstand caused Aptian and later marine deposits to rise over the basin margin, ultimately to connect with the Western Interior Seaway and flooding the old river systems. The Cenomanian mantle-driven uplift of the Southern Arkansas Uplift, centered in the northern Mississippi embayment, as well as the Sabine and Monroe uplifts, provided new sediment sources that were tapped by fluvial systems of the Woodbine/Tuscaloosa episode (Fig. 16.36B). Renewed uplift in the southern Appalachians is recorded in detrital zircons of the Tuscaloosa system (Blum et al., 2017). Principal rivers flowed into the marginal basins in East Texas (Woodbine) and Mississippi (Tuscaloosa). In the Late Cretaceous, the Western Interior Seaway began to be filled in by rivers draining the rising mountains of the North American Cordillera. By Campanian time, the West Texas area was brought above sea level by advancing deltas; these advancing deltas reached South and Central Texas in the late Campanian and Maastrichtian (Fig. 16.36C). Detrital zircons in the correlative Difunta Group of northeastern Mexico show clear signals of derivation from the rising Mexican Cordillera (Lawton et al., 2009). Paleocene through middle Eocene pulses of Laramide uplift along the Central and Southern Rocky Mountains and in the Sierra Madre Oriental of eastern Mexico combined with a general eustatic fall in sea levels from their Cretaceous high to generate the early Cenozoic siliciclastic-dominated depositional episodes. The Rockdale river had sources in the Southern Rocky Mountains, perhaps draining intermontane basins as far west as Utah (Fig. 16.36D). The Mississippi drainage did not include today’s Missouri drainage, but may have acquired drainage from the Central Rocky Mountains (Sharman et al., 2017; Blum et al., 2017). Appalachian contributions were still significant. South
676 The Sedimentary Basins of the United States and Canada
FIG. 16.37 Source area, inferred river systems, and deltaic and slope sediment “sinks” during Cenozoic deposition. (A) Middle Eocene, (B) Oligocene, (C) Middle Miocene, (D) Pliocene, (E) Late Pleistocene to modern. VF, volcanic field; SMA, San Marcos Arch; SA, Sabine Arch. Paleogeography in part after Xu et al. (2017), Blum et al. (2017), and Snedden et al. (2018a).
Texas did not receive major siliciclastic input until the Early Eocene (Fig. 16.37A), probably as highlands rose in Mexico, intervening foreland basins that trapped Paleocene sediment were filled, and the Rosita/Falcon River system was integrated. This river continued to transfer sediment through the Middle Eocene, although it was somewhat disrupted by late Laramide folding. Middle Eocene arching in the Sabine and San Marcos areas generated additional sediment supply during the Yegua episode.
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Volcanism in Mexico (Sierra Madre Occidental), the Big Bend, and northward into Colorado began in the Late Eocene and continued through the Oligocene. This volcanism was associated with crustal heating, and consequent uplift and erosion of much of north-central Mexico and the southwestern United States. River systems carried coarse conglomerates into South Texas (Gueydan river) and sands into East Texas and eastward (Fig. 16.37B). Huge quantities of falling ash blanketed the landscape and seas and were reworked into abundant marine muds. Initiation of renewed uplift and erosion of the Cumberland Plateau and Appalachian Mountains in Early to Middle Miocene time reinvigorated sediment supply to the east-central Gulf basin via the Tennessee-Alabama and Ancestral Mississippi fluvial axes (Fig. 16.37C). At the same time, the Rocky Mountain uplands experienced continued regional uplift and exhumation, associated with early Basin and Range extension. The Rio Grande sediment flux was greatly reduced, probably due to a combination of drainage disruption by Basin and Range rifting that created closed basins, and by regionally drier climates. Climate changes may also have helped to increase sediment supply farther north, which enriched the Red and Mississippi/Arkansas drainage system. Late Miocene and Pliocene uplift of the western High Plains further rejuvenated the northwestern Rocky Mountain sources (Fig. 16.37D), and created a broad eastward slope that opposed the west-sloping alluvial apron of the eastern interior. Streams from east and west were combined and directed southward, forming the distinct Red, Mississippi, and Tennessee fluvial axes that dominated Middle Pliocene through Holocene sedimentation. The Pliocene course of the Mississippi River extended south from Minnesota through Illinois (Lumsden et al., 2016). High rates of Pleistocene sediment accumulation reflect rapid Quaternary climate cycling, and glacial erosion and runoff directly into the principal sediment transport systems. Only in the late Pleistocene was the Mississippi valley sufficiently enlarged that the Tennessee and Red Rivers became permanently trapped within its catchment (Saucier, 1994). The vast sediment-rich Missouri drainage was added during the Pleistocene (Fig. 16.37E), due to disruption by glaciation and glacial rebound; these areas previously drained northeastward to Hudson Bay. In the middle Pleistocene, the Rio Grande was successfully reintegrated, although continued aridity limited the streamflow and delta growth.
Climate and Oceanography The climatic setting of the Northern Gulf of Mexico basin has remained relatively constant throughout its history. The Gulf of Mexico region has generally lain within warm, subtropical climate zones. The Jurassic aridity of south-central North America is reflected in the widespread occurrence of evaporite, eolian, and sabkha deposits in the Louann and Smackover episodes. Evaporite deposits also occur in Lower Cretaceous strata from northern Mexico to the Florida platform, suggesting that hot, dry conditions continued. Late Cretaceous continental flooding likely led to a more equable climate across the northern Gulf, as suggested by abundant coals in the Rio Grande area; however, limited preservation of terrestrial strata may bias the preserved record of continental climate indicators. By early Cenozoic time, the climate of the northern Gulf margin was uniformly wet and subtropical. Lignite deposits occur widely within Paleocene and Eocene fluvial, deltaic, and shorezone systems (e.g., Snedden and Kersey, 1981; Warwick et al., 2011). A dramatic climate change occurred at the beginning of the Oligocene Frio episode. Lignite deposition ceased. Carbonate-bearing paleosols across the Texas coastal plain indicate the rapid development of a strong east–west climatic gradient from wet subtropical in Florida to arid in northern Mexico (Galloway, 1977). This strong east–west zonation persists today. The Gulf of Mexico evolved as an ocean basin with several changes of marine currents and connections to the global ocean. The earliest (Middle Jurassic) marine opening may have been to the Pacific Ocean across Mexico (Salvador, 1991a), although an Atlantic connection is equally plausible (Martini and Ortega-Gutierrez, 2016). However, by the onset of the Smackover episode, the Gulf had clearly opened to the central Atlantic and the Tethys. Through late Jurassic deposition, the basin evolved into a small, east–west-elongate ocean basin open to the Atlantic through broad straits between the Yucatan and Florida platforms (Marton and Buffler, 1999). The basin was probably connected to the Pacific through shallow-water straits, but paleogeographic reconstructions of Mexico during this time are uncertain. During the Albian, transgression and continental flooding temporarily established a shallow-water connection across northwest Texas with the southern end of the Cretaceous Western Interior seaway. The connection became permanent in the late Cenomanian and persisted until the Maastrichtian. Strong marine currents periodically flowed through this broad strait, leading to submarine canyon/channel development (Fig. 16.24). The connection also allowed episodic mixing of equatorial Tethyan faunas with cool-water boreal faunas of the seaway (Lundquist, 2000). Following the reemergence of the basin margin following the Late Cretaceous flooding, a new pathway to the Atlantic, the Suwannee strait, extended from the northeastern Gulf of Mexico across Georgia to the Atlantic Ocean (McKinney, 1984; Popenoe et al., 1987; Umbarger and Snedden, 2016). Initially, this strait formed a relatively deep trough that limited southward diffusion of terrigenous siliciclastic sediment southward onto the Florida platform (Fig. 16.28). Large erosional
678 The Sedimentary Basins of the United States and Canada
scours at the Atlantic end of the straits indicate that strong currents flowed through the trough. This marine connection would have prevented possible drawdown of the Gulf of Mexico during the Wilcox episodes, including the PETM. Later in the Eocene, the strait shoaled and filled. Bridging of the strait is recorded in the appearance of Oligocene siliciclastic beds sweeping south into the previously pure carbonate platform facies of south Florida (Missimer and Ginsburg, 1998; Guertin et al., 2000). In the middle Miocene, the first appearance of strong, deep marine currents flowing through the Florida and Yucatan straits (Gulf Stream) is recorded by erosion on the Florida escarpment and outer platform (Mullins et al., 1988; Guertin et al., 2000), the first appearance of contourite drift deposits in the western Gulf abyssal plain (Galloway et al., 2000), and the offset of Middle Miocene submarine fans from their fluvio-deltaic point sources (Snedden et al., 2012). This current system persists today as the Loop current and related systems. Glacial outflows from the North American Laurentide ice sheet first entered the Gulf in the latest Pliocene (about 2.2 Ma). The broad ocean fetch and prevailing south-to-north winds over the Gulf of Mexico, at least during the Cenozoic, is reflected by the dominance of strandplain, barrier island, and wave-dominated delta systems along the northwest Gulf margin (Figs. 16.26–16.36; Galloway et al., 2000). However, the volume of sediment actually preserved in shorezone systems relative to delta and slope systems shows a pronounced and long-term decrease beginning in the middle to late Miocene, coincident with increasing frequency and amplitude of glacioeustasy (Galloway, 2002). Tidal modification of northern Gulf of Mexico coastlines was minimal throughout much of the Cenozoic. Tidal facies formed in local environments, such as barrier inlets and bays that enhanced the usual tidal prism. Only at a few times and locations did an ideal combination of shelf width and embayed coastal geography lead to development of regionally tidedominated coasts (e.g., Queen City episode; Ramos and Galloway, 1990).
Evolution of Siliciclastic Shelf Margins; Progradation and Retrogradation The continental shelf margin of the northern Gulf of Mexico experienced five general phases in its development, resulting in the evolution of the shelf margin shown on Fig. 16.38 (Winker and Buffler, 1988). 1. Evolution from ramp to a prograding mixed siliciclastic‑carbonate shelf–slope break during the Smackover through Hosston depositional episodes. 2. Stabilization of a rimmed carbonate platform and accentuation of slope-to-basin relief during the early Cretaceous Sligo through Washita episodes.
FIG. 16.38 Mesozoic and Cenozoic shelf margins at maximum offlap of principal depositional episodes, and location of major shelf-margin deltaic depocenters of the Northern Gulf Basin. Modified from Galloway (2005a).
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3. Brief progradation of shelf-margin deltas and slope aprons during the Woodbine/Tuscaloosa episode. 4. Drowning and blanketing by deep shelf and ramp deposits of the late Cretaceous episodes. 5. Episodic to sustained Cenozoic offlap and partial filling of the basin, driven by delta and shorezone progradation and modified by retrogradational processes. Continental margin outbuilding has primarily been accomplished during siliciclastic-dominated episodes by progradation of large delta systems (shaded areas on Fig. 16.38) and their associated subjacent, sandy slope aprons (Prather 2000; Winker and Booth, 2000; Galloway, 2005a). Between the deltaic headlands, sediment was reworked along strike to form shorezone and shelf systems that also prograded and spilled over the shelf edge, creating subordinate, but also extensive, shelf-fed aprons, which consist largely of muddy sediment. In the northeastern part of the basin, Cretaceous shelf margins retreated up to several hundred kilometers landward in response to subsidence and Cretaceous sea-level rise. Subsequent Cenozoic margins have advanced through a combination of deposition and global sea-level fall. The episodic progradation of delta systems and shorezones loaded stratiform salt (either autochthonous Jurassic or allochthonous salt sheets formed previously), forcing salt out and up into salt sheets and a wide variety of diapiric structures. Although dominantly depositional in origin, the Cenozoic continental margins of the Northern Gulf of Mexico basin also record numerous phases of shelf edge and slope retreat and erosion (Edwards, 2000; Galloway et al., 2000) (Fig. 16.39). Such margins can be considered as destructional (Galloway, 1998a), in comparison to the offlapping, constructional continental margins that dominated most of the deposition in the basin. The destructional margins form three general groups, depending on their morphology and origin; these groups are on a spectrum and many features have intermediate characteristics. 1. Submarine canyons are dip-elongate, erosional troughs that are hundreds of meters deep. Canyons generally occur in middle to upper slope strata, but may extend tens of kilometers onto the shelf. They were excavated by combined processes of submarine mass wasting and erosive flow of turbidity currents. Large submarine canyons cluster along margins constructed by the Paleocene–early Eocene Wilcox depositional episodes (Fig. 16.39, features 4–7) and in the late Pliocene and Pleistocene margins (Fig. 16.39, features 18–21).
FIG. 16.39 Location of principal submarine canyons, slides, slump scars, and compound retrogradational slope complexes in the Northern Gulf of Mexico Basin. 1, Cretaceous canyon; 2, Top Cretaceous megaslide; 3, Lobo megaslide; 4, Lavaca/Smothers canyons; 5, Yoakum canyon; 6, Hardin canyon; 7, St. Landry canyon; 8, upper Wilcox slumps; 9, Queen City slumps; 10, lower Yegua retrogradational slope; 11, lower to middle Frio canyon; 12, Hackberry embayment/megaslide and retrogradational slope; 13, Planulina embayment/retrogradational slope; 14, Harang embayment/retrogradational slope; 15, Middle Miocene megaslide; 16, Globoquadrina altispira (middle Pliocene) retrogradational slope; 17, Globoquadrina altispira (middle Pliocene) megaslide; 18, late Pliocene slump and canyon cluster; 19, early Pleistocene canyon cluster, 20, late Pleistocene canyon cluster; 21, Bryant canyon; 22, DeSoto canyon; 23, Alabama scour trough; 24, terminal Cretaceous slumping and intense disruption (Poag, 2017).
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2. Megaslides are laterally extensive features that include extensive slump or slide deposits and an embayment within the upper slope and shelf edge bounded by a prominent slump scar and glide plane faults. Examples include features 2, 3, 15, and 17 (Fig. 16.39). They record brief, catastrophic failures of the continental margin due to tectonic tilting, salt withdrawal, or sedimentary loading. The latest Cretaceous megaslides (features 2, 24) are the most exotic, being triggered by the Chicxulub meteorite impact event. 3. Retrogradational slopes are the largest, longest lived, and most complex destructional slope type. Examples occur along the Yegua (feature 10), Frio (feature 12), Lower Miocene (feature 13), Middle Miocene (feature 14), and Pliocene (feature 16) margins. Such slopes exhibit complex mixtures of slump scars, canyons and erosional channels, growth faults, and thick sections of mixed shelf-margin delta, turbidite and debris flow facies. Most are distinguished by a muddy sediment wedge containing deepwater faunas and by concave reentrants in the shelf margin similar to (and grading into) megaslides. They frequently occur when sandy shelf-margin sediments prograde rapidly over unstable marine mudstones. Many appear to be related to pulses of evacuation of subjacent thick primary salt or salt canopies, causing subsidence and tilting and destabilization of a segment of the slope and outer shelf. Tilting led to oversteepening and repetitive slope failure. Forming and filling of the embayments commonly required more than a million years. Submarine canyons, slides, and retrogradational embayments in the Northern Gulf of Mexico basin, as in many basins, have frequently been related to falls in eustatic sea level. Such an attribution does not explain their localization along otherwise normally prograding margins, their diversity, nor their common paleogeographic location on the margins of major deltaic depocenters. On the other hand, correlations with structural elements and events are clear for many of the destructional features. It is possible for lowstands of sea level to trigger failure events, but that does not prohibit their occurrence during highstands.
The Question of Cenozoic Marine Transgressions An unanswered question concerns the origin of the episode-bounding marine transgressions, particularly during the Cenozoic. As described earlier, Eocene marine transgressions were long-distance inundations that went far past today’s erosional outcrop. Oligocene and Miocene transgressions, because of the evolving basin-margin flexure near the Oligocene outcrop belt (and subjacent Cretaceous shelf margin and crustal boundary), do not reach the outcrop, but still represent major inundations. It is hard to demonstrate updip deltaic equivalents of the downdip marine transgression; the maximum flooding surfaces seem to pass into unconformities beyond the marine limit toward the basin margin. These inundations affect the entire northwestern and central shelves and shelf margins of the basin, from northeastern Mexico to Mississippi. They punctuate the march of deltaic and shelf margin progradation every one million to four million years, and may last as long as two million years. What causes this episodic punctuation? A standard explanation would be a set of eustatic rises in sea level, which flood back over the extensive, low-gradient floodplains. However, these would be major events, higher than known highstand deposits in the Yegua and other formations. To our knowledge, other basins with Eocene (and Cenozoic) deposition do not show this sort of pronounced episodic marine transgression. This would suggest a North American cause. Since the major increases and decreases in progradation over 10- to 50-million-year timespans are caused by tectonic uplift and generation of source areas with integrated drainage systems, these extrabasinal tectonic factors might also be responsible for the 1- to 5-million-year cycles. But the highlands are still eroding and the streams still flowing, be it in the Carrizo or the Reklaw, the Frio or the Anahuac. Severe climatic shifts such as Sahara-like aridification might reduce erosion or stream transport, but we lack substantial evidence for climate shifts of this magnitude. One possibility is that sediment is trapped in updip alluvial basins that form during a periodic crustal depression. If there were episodic uplifts and depressions—crustal “waves”—between the Rocky Mountain/Cordilleran/volcanic source areas in the west and the Northern Gulf of Mexico Basin to the east, they could trap sediment and then release it during the next episode. One would like to see some erosional remnants of alluvial sediment of the appropriate ages in the Great Plains, but later uplift may have obliterated that evidence. More work is needed to explain these useful if puzzling features of the depositional framework of the basin.
ENERGY AND MINERAL RESOURCES OF THE NORTHERN GULF OF MEXICO BASIN The northern Gulf of Mexico basin is a world-class repository of hydrocarbons (petroleum and natural gas), and has also produced significant lignite, uranium, and certain minerals such as salt and sulfur (Nehring, 1991; Riggs et al., 1991). It has been actively explored for over 100 years, creating a three-dimensional well and reflection seismic database of unique abundance, extent, and diversity. Hydrocarbon exploration and development began onshore in both the interior basins and
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the coastal plain, moved to the offshore shelf in midcentury, progressed onto the continental slope in the 1980s, and by the 2000s onto the abyssal plain. As stated by Nehring (1991), “No other basin worldwide has even come close to producing so many discoveries for such a long period.” Because of this history, as well as the presence of the world oil centers of Houston and New Orleans and numerous leading educational institutions, the northern Gulf of Mexico basin has become a natural laboratory for understanding the sedimentary processes, facies, stratigraphy, and gravity tectonics of prograding continental margins.
Hydrocarbon Source Rocks The northern Gulf of Mexico Basin is one of the world’s great prolific “superbasins.” In large part this is due to the multiple high-quality organic source rocks, the complex basin structuring, and diverse reservoirs and seals. There are six episodes of source rock deposition, ranging from Late Jurassic to Eocene, four of which have been most effective in generating petroleum that has been trapped in reservoirs (Fig. 16.40; Hood et al., 2002; Cunningham et al., 2016): 1) Oxfordian organic-rich marls were deposited as part of the Smackover depositional episode over much of the basin. They are in the oil window and have sourced reservoirs around the northern and eastern flanks of the basin, especially Smackover carbonate reservoirs and Norphlet eolian gas reservoirs. They have also generated petroleum found in the deep lower slope and foldbelt plays of the deep Gulf of Mexico. 2) Kimmeridgian organic-rich mudstones were deposited in a shelf-basin embayment on the north-central part of the basin at the close of the Smackover episode, creating the Haynesville shale that is a major gas-productive resource play. Overlying Tithonian rocks of the Bossier shale (base of the Cotton Valley episode) are also gas-productive in the east Texas–north Louisiana area. Tithonian organic mudstones also occur throughout the basin and have generated much of the oil found in the lower slope and foldbelt plays in the deep Gulf of Mexico (Cunningham et al., 2016).
FIG. 16.40 Principal oil sources in the Northern Gulf of Mexico Basin, as determined from petroleum geochemistry. From Hood et al. (2002). Reprinted courtesy of AAPG.
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3) Minor source rocks occur at the late Aptian flooding surface overlying the Sligo depositional episode (Pearsall Group). Global OAE 1 deposition in the Berriasian–lower Aptian stages has not generated known source rocks, although some might occur in the deep basin. 4) Cenomanian–Turonian organic-rich calcareous mudstones were deposited across much of the basin, as part of the Woodbine/Eagle Ford episode. In South Texas (south of the San Marcos Arch) rich organic mudstones were deposited nearly free of dilution by clays, generating the world-class Eagle Ford shale resource play. The richest organic shales were deposited prior to the global OAE 2. In East Texas and the eastern basin margin, the organic contents are often diluted by prodelta and shelf muds from the Woodbine and post-Woodbine delta systems, but organic rich layers are being exploited (Eaglebine, Tuscaloosa Marine Shale). Hydrocarbons from these sources have sourced the Tuscaloosa and Woodbine reservoirs, as well as overlying Austin Chalk fractured reservoirs. 5) Minor source rocks may occur in the lower Austin Chalk (Santonian), and in overlying claystones (Campanian). 6) Paleogene source rocks, primarily of Paleocene and Eocene age, occur over much of the basin. Abundant terrestrial material in basin clays derived from prograding delta systems are preserved in the thick basinal mud settings (Wilcox, Carrizo and later episodes), and provide a diffuse gas-rich source for the shelf-margin trend fields of Texas and adjoining areas. To the east and southeast, marine (oil-generating) Paleogene source rocks are known or inferred; the mudstones surrounding the Sparta (middle Eocene) episode in Louisiana are particularly organic-rich. Oil-to-gas ratios increase from south Texas to south Louisiana, reflecting the relative proportions of terrestrial and marine organic material.
Hydrocarbon Migration Pathways, Reservoirs, and Seals Unlike many basins, where a limited volume of the stratigraphic section produces the bulk of reserves, major hydrocarbon plays are found throughout the basin fill in reservoirs nearly every depositional episode (Fig. 16.41). In roughly descending volumetric rank, hydrocarbons occur in Miocene, Paleocene-Eocene, Oligocene, Plio-Pleistocene, Upper Cretaceous, Lower Cretaceous, and Jurassic strata. Total production, proved, and probable reserves in the northern Gulf of Mexico Basin exceed 200 BBOE (billion barrels of oil equivalent) (Nehring, 1991; Burgess et al., 2016; IHS Markit, pers. comm.). From these references, this total amount is distributed between the onshore marginal basins (East Texas to Florida and Burgos) with 45 BBOE, the onshore part of the central basin with 97 BBOE, and the offshore shelf and slope with 58 BBOE. Production to date is roughly 174 BBOE. Of this total, oil and condensate aggregate about 70 BBO; natural gas volumes exceed 500 Tcf (trillion cubic feet). Ongoing exploration has discovered additional large reserves of oil within Paleogene and Miocene slope and basin fan reservoirs beneath the continental slope and deep basin. Large gas reserves have been
FIG. 16.41 Oil and gas fields, Northern Gulf of Mexico Basin.
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developed in low-permeability sandstone (“tight gas sand”) reservoir systems within Jurassic and early Cretaceous units (Cotton Valley, Hosston) of the north-central Gulf margin. Huge resources of oil and gas can be extracted from significant source-rock shales; the two largest plays are the Eagle Ford, 3.4 BBO and 20.8 Tcf (EIA, 2011), and the Haynesville, 52 Tcf (Browning et al., 2015; Tinker, 2017). Significant undiscovered resources and enhancements to existing producing areas are expected in the future that will significantly increase the recoverable oil and gas endowment. The magnitude and variety of resources in the basin continue to make the northern Gulf of Mexico basin one of the most active hydrocarbon exploration theaters in the world. Hydrocarbons have been generated from source rocks over most of the basin’s history depending on local burial history and heat flow, and have migrated vertically and (to a lesser extent) laterally into all sorts of reservoirs bounded by effective seals. Much Mesozoic-sourced oil is found in Cenozoic reservoirs. The present-day configuration of the oil and gas windows for generation is shown on a diagrammatic cross-section through East Texas southward into the deep Gulf of Mexico (Fig. 16.42). In the East Texas Basin, the Cretaceous source rocks are only occasionally in the oil window; source rocks of this age are productive on the lip of the main basin, and oil was carried northward by lateral migration in Woodbine sandstones. The deep Jurassic source rocks have provided most of the hydrocarbon charge in East Texas, except for the Woodbine and associated reservoirs; their present position ranges from the oil window to the gas window. To the east, these source rocks and the neighboring Cotton Valley reservoirs are in the gas window. In the main basin, the Jurassic and Cretaceous sources pass downward deep into the gas window (the Cretaceous sources being oil productive in a rim around the main basin). However, earlier in basin history they would have generated liquid hydrocarbons that may have migrated vertically into cooler sections and been preserved. Paleogene source rocks are most mature beneath the advancing shelf margins. The salt sheets of the deepwater Gulf of Mexico have a substantial effect on hydrocarbon maturation and preservation. Salt is an effective conductor of heat, and diapiric salt wicks heat up from its deep source areas and out into shallow horizons. The net effect in a partly diapiric salt sheet (only schematically shown on Fig. 16.42) is to cause an expansion and repetition of the oil and gas windows, which allows oil to be preserved in subsalt environments that would otherwise be too warm. The source horizons in this area are now in the gas window, but were oil-productive in the earlier Neogene. Toward the deep Gulf of Mexico, the oil window intersects the source rock horizons at the present day, sourcing the oil-rich foldbelt plays in the lower slope and basin. The varied depositional history of the Northern Gulf of Mexico basin has created a wide variety of reservoirs in shelf, shelf-margin, slope, and basin environments. Episode-bounding transgressions combine with lesser marine shales and salt features to provide effective seals. The structures that might trap hydrocarbons developed early in many areas, particularly salt rollers and early loading features, growth-faulted structures formed during shelf-margin progradation, and salt features on the slope. Other structures developed later, as turtle and inversion structures. Thus hydrocarbons could be trapped as they
FIG. 16.42 Schematic section showing maturation windows, source rocks, and other petroleum system elements in the central part of the basin. From Ewing (2016a).
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formed and migrated, leading to the massive hydrocarbon charge of the basin. This is despite the basin’s leakiness; natural gas and oil seeps are abundant (McDonald et al., 2015). The long and varied history of deposition and the immense volume of porous rock, combined with the equally long and complex history of gravity deformation and salt migration, have created a diversity of reservoir and trap combinations rarely matched in global petroleum mega-provinces or “superbasins.” Stratigraphic, structural, and combined trap types abound. The array of structures created by salt deformation and migration has played a particularly important role in trap development.
Energy Minerals and Other Mineral Resources In addition to its hydrocarbon wealth, the northern Gulf of Mexico basin contains large energy mineral resources and significant stores of certain other mineral resources (Fig. 16.43). The basin contains large reserves of bituminous coal and lignite, as much as 97 × 109 short tons (Riggs et al., 1991; Warwick et al., 2011). Lignite is extracted from strata in the lower Wilcox, Yegua, and Jackson episodes in Texas and Louisiana. Established reserves of bituminous coal occur in Upper Cretaceous strata (Olmos and related episodes) of northern Mexico and in south Texas near the Rio Grande border. Since the 1970s, lignite has been used in mine-mouth electrical generation and has been an important basis of the electrical system in the Gulf Coast states. The bituminous coal of the Mexican state of Coahuila is that country’s major coal district, mined for use in heavy industry in the Monterrey area. There is some potential for coalbed methane production (Warwick et al., 2011). Sedimentary uranium deposits occur in a province along the South Texas coastal plain in strata of the Upper Wilcox (Carrizo Sandstone), Jackson (Whitsett Sandstone), Frio/Vicksburg (Catahoula Formation), Lower Miocene (Oakville Sandstone), and Middle Miocene (Goliad Sandstone) depositional episodes (Riggs et al., 1991). Uranium ore occurs along the irregular boundaries between reduced and oxidized parts of aquifer sand bodies known as roll fronts because of their C-shaped cross-section. Uranium was leached from reworked air-fall ash associated with the middle Cenozoic (principally Oligocene) volcanogenic phase, transported by groundwater into the shallow fluvial and coastal sand aquifers, and trapped by reduction by detrital organic matter, deep groundwater upwelling along growth faults, and/or epigenetic sulfide minerals (Galloway, 1982). Ore has been mined both by open pit and in-situ solution methods; the latter is nearly universal at present. The South Texas uranium industry is not a major component in world supply, but ranks third in US mining districts that can supply the energy-rich metal.
FIG. 16.43 Energy mineral and nonfuel mineral resources of the Northern Gulf of Mexico Basin.
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Several nonfuel mineral products are (or have been) mined in the basin (Ewing, 2016a; Chapter 9). The most important of these (next to sand and gravel mainly from Quaternary alluvial deposits) is rock salt (halite). This product is sourced from diapiric salt stocks that rise to shallow levels within the onshore coastal plain and in interior basins. Major subsurface mines are found on five of these “salt domes” (Cote Blanche, Avery Island, and Weeks Island in Louisiana and Grand Saline and Hockley domes in Texas). They are significant contributors to the US salt market, forming 33% of the rock salt production capacity in 2015 (Bolen, 2017). Salt domes are also used across the basin as storage devices, with large bottle-shaped solution cavities being used to hold petroleum products, chemicals, and hazardous wastes. The highly impermeable and self-healing nature of rock salt forms an ideal material for the walls of these cavities. Sulfur has been mined in great quantities from the cap rock of various salt domes, formed by dissolution of rock salt, concentration of anhydrite impurities, and microbial reduction of sulfate to native sulfur. At present, sulfur needs are met by byproduct production from sour natural gas, but the resources are still there for the future. There is also significant production of fireclay and other clays and ash. Iron-rich glauconite in the Weches formation of East Texas supported an important iron ore industry until recent times. Heavy mineral resources (ilmenite, rutile, and zircon) occur in sandstones from Tennessee to northern Florida that were derived from the southern Appalachian highlands. There is major production of crushed limestone from Lower Cretaceous (mostly Edwards) carbonates in central and north Texas, and limestone from Upper Cretaceous (Austin) chalky limestone used for the manufacture of cement. Asphalt is mined in South Texas from a Campanian limestone shoal that lies at the center of a 6- to 8-billion-barrel ultraheavy oil province (Ewing, 2009b).
ACKNOWLEDGMENTS Jeffrey Horowitz drafted the figures of the first edition, which were modified for this publication. Co-workers Patricia Ganey-Curry, Timothy Whiteaker, and Lisa Bingham aided in preparation of select figures. Comments of reviewers John Snedden and Andrew Miall significantly improved the clarity and content.
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