The Phanerozoic Tectonic and Sedimentary Evolution of North America Andrew D. Miall⁎, Ronald C. Blakey† ⁎
Department of Earth Sciences, University of Toronto, Toronto, ON, Canada, †Colorado Plateau Geosystems, Carlsbad, CA, United States
Chapter Outline Introduction The Major Phases of Tectonic Development Phase One: The Construction of Pangea Plate-Tectonic Evolution Sedimentary Evolution of The Interior and Western Continental Margin Sedimentary Evolution of The Eastern Continental Margin Sedimentary Evolution of The Southern Margin Sedimentary Evolution of The Arctic Margin Phase Two: Development of The Southern Midcontinent and Ancestral Rockies Plate-Tectonic Evolution Sedimentary Evolution of The Mid-Continent and Ancestral Rockies
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Sedimentary Evolution of the Western and Northern Continental Margin Phase Three: Breakup of Pangea and Formation of the Cordilleran Orogen Plate-Tectonic Evolution Sedimentary Evolution of The Western Margin Sedimentary Evolution of The Western Interior Sedimentary Evolution of The Arctic Margin Sedimentary Evolution of The Atlantic and Gulf Margins Mid-late Cenozoic Tectonism of the Western Margin Late Cenozoic Modifications Acknowledgments References
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ABBREVIATIONS ADD ANB ARM BAT BIB BSB BUU CALIP CCT CHO CIA DEB EKT EST GRA GVB
Adirondack Dome Anadarko Basin Ancestral Rocky Mtns Blue Mtns and related terranes Bisbee Basin Bird Spring Basin Boothia Uplift Central Atlantic Large Igneous Province Cache Creek Terrane Chortis Terrane Cincinnati Arch Delaware Basin East Klamath Terrane East Sierra Terrane Grampian arc Great Valley Basin
The Sedimentary Basins of the United States and Canada. https://doi.org/10.1016/B978-0-444-63895-3.00001-2 © 2019 Elsevier B.V. All rights reserved.
2 The sedimentary basins of the United States and Canada
HBB Hudson Bay Basin ILB Illinois Basin MDB Midland Basin MIB Michigan Basin NSB North Sea Basin NST North Sierra Terrane OQB Oquirrh Basin OUX Oaxacan Terrane OZD Ozark Dome PB Paradox Basin PEB Pedregosa Basin PET Pearya Terrane QUT Quesnell Terrane SFL South Florida Block SJSB San Joaquin-Sacramento Basin STT Stikine Terrane SVB Sverdrup Basin TAA Taconic Arc TOB Tobosa Basin WIA Wisconsin Arch WIB Williston Basin YTT Yukon-Tanana Terrane YUC Yucatan
INTRODUCTION The purpose of this chapter is to offer a succinct review of the Phanerozoic history of the North American continent. This will provide a framework within which to evaluate the details of the individual chapters that comprise the remainder of the book. An attempt has been made to develop a coherent narrative of the complex kinematic evolution of the continent, in which subsidence and sedimentation in one province can be understood with respect to deformation and uplift elsewhere, as the plate of which North America is a part underwent drift and rotation and was subjected to plate-margin and intraplate tectonism. This introduction draws on and refers to the constituent chapters in this book (Fig. 1) and builds on the excellent summaries of the tectonic setting and basin history of North America prepared for the Decade of North America project, starting with Volume A (Bally et al., 1989; Bally, 1989), plus many other sources, as noted throughout the text.
THE MAJOR PHASES OF TECTONIC DEVELOPMENT The ancient core of North America, Laurentia, constitutes the Canadian Shield, which crops out across the northern half of the continent, underlies the cratonic sedimentary cover of the continent’s vast interior plains, and extends in attenuated and metamorphosed form deep beneath the orogens on the continental margins (Fig. 2). The Shield was built in three broad stages, which are thought to represent cycles of supercontinent assembly (Hoffman, 1988, 1989; areas 1, 2, and 3 of Fig. 2). Around the margins of North America, on the east, south, west, and north, this basement, with its cratonic cover, is stretched, thinned, and buried beneath the complexly deformed rocks of the Appalachian, Ouachitan, Cordilleran, and Innuitian orogens, respectively. These belts are in part accretionary; that is, contained within the orogen are small to large fragments, ranging from tectonic slivers to microcontinents, derived from other, non-North American sources, or from pericratonic sources of probable North American affinity (e.g., Kootenay Terrane in Cordillera). The convergent tectonism that formed the orogens began in the east, during the Ordovician, and continued there until the Permian (area 4 in Fig. 2). On the western margin, localized episodes of convergent-margin tectonism have been documented from as far back as the Devonian (Antler orogeny), but the main phase of Cordilleran orogen development began during the Jurassic. From a continental perspective, orogeny and all its consequences (earthquakes, igneous intrusion, volcanism, metamorphism, deformation) have been virtually continuous on the western continental margin since then, with active orogeny extending from Alaska to Mexico (area 5 in Fig. 2).
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FIG. 1 Basins covered by the area chapters in this book.
It is convenient to commence the narrative in the late Precambrian, because this is when the ancient and structurally stable central shield achieved the composition and outline shape that it exhibits at the present day. As documented in detail by Hoffman (1988, 1989), the Canadian Shield is the product of a series of collisional orogens that took place between Archean and late Proterozoic time, the last and one of the most important being the Grenville orogeny, which climaxed at about 1 Ga. This was the final stage in the assembly of the supercontinent now named Rodinia (Dalziel, 1991; Hoffman, 1991). Breakup of Rodinia commenced with widespread rifting at about 800 Ma, culminating in the isolation of North America as a separate continent by about 600 Ma (Hoffman, 1988, 1989). During the period when the plate was undergoing rifting and separation from the rest of Rodinia and leading up to its involvement in the assembly of Pangea, the continent is referred to as Laurentia. By the end of the Precambrian, Laurentia was almost entirely rimmed by extensional margins (Fig. 3), much as are Africa and Antarctic today following the breakup of Pangea (Bally, 1989), and for much the same reason—the continent was surrounded by sea-floor spreading ridges from which other continents rotated away (Hoffman, 1988, 1989, 1991). Following the breakup of Rodinia, the newly separated continent, Laurentia, was subjected to profound erosion and peneplanation. The Grenville orogeny had created a major mountain belt in eastern North America about a billion years ago, but by the time of the major early Paleozoic transgressions, this had almost completely been eroded. Studies of detrital zircon and other evidence suggest that the thick Neoproterozoic clastic wedges on the western and northern margins of Laurentia may have been derived in part by peneplanation of the Grenville mountains (Rainbird et al., 1997; Rivers et al., 2012). Tracing the present erosional edge of the Phanerozoic cover around the margins of the Canadian Shield, a geologist cannot help but be struck by the lack of topographic relief on the unconformity surface. Basal Paleozoic strata contain a few sandstone layers and scattered boulders, but in many places, a few meters above the unconformity, the section passes up into platform carbonates, evidence for the development of giant, detritus-free, interior platform seas of a magnitude
FIG. 2 The assembly of the North America continent, in five broad stages: 1. The original North American continent, Arctica, which started to form about 2.5 Ga; 2. Area added during the formation of Nena, about 1.9 Ga; 3. Grenville orogen, added to complete the formation of Rodinia, between 1.3 and 1.0 Ga; 4. Appalachian orogen, added between 600 and 300 Ma; 5. Cordilleran orogen, added during the breakup of Pangea, commencing about 250 Ma. (From Eyles (2002).)
FIG. 3 The Laurentian plate at the end of the Precambrian. (Modified from Hoffman (1989).)
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nowhere replicated on the present-day Earth’s surface. Reuter and Watts (2004) described a buried paleochannel on the Precambrian surface in Ohio, and other evidence for late Precambrian Grenville-derived clastic deposits is summarized by Rivers et al. (2012). The development of this enormous peneplain suggests that the continent remained largely undisturbed by tectonism for at least 200 million years, from the end of the Grenville orogeny until the beginning of rifting at about 800 Ma (a further discussion of this great unconformity is provided by Miall, 2016). The continental interior continued to undergo subaerial erosion until the first Paleozoic transgressions, some 300 million years later. As noted elsewhere, dynamic topographic processes generated broad westward or eastward tilts to the continent at different times during the late Precambrian and Phanerozoic, which controlled fluvial erosion, sediment transport, and sediment delivery on a continental scale, and provided for the removal of enormous volumes of rock in the construction of the post-Precambrian peneplain and the major Phanerozoic sequence boundaries (see also Chapter 19). The late Precambrian to early Paleozoic history of Laurentia is characterized by thick sedimentary successions in rift basins and sedimentary wedges along the extensional-margins of the continent (Fig. 3). Several “failed rifts” extend into the continental interior, some of which became substantial basins during the late Precambrian and Paleozoic (e.g., Chapter 8). The crustal fragmentation that occurred during this period continued to provide the focus for intraplate earthquakes throughout the Phanerozoic. The Reelfoot Rift beneath the Mississippi valley, and the St. Lawrence Rift of eastern Canada, are particularly well known as the loci of modest to major earthquakes. The Phanerozoic history of North America can be divided into three broad phases, as described in the following paragraphs. (1) During the first phase, which lasted from the late Precambrian to the Permian, Pangea was under construction. The western continental margin was either a divergent (passive) margin, facing the paleo-Pacific Ocean (Panthalassa) (Chapter 5) or a backarc basin bordering that ocean, while the eastern margin, beginning in the Middle Ordovician, underwent convergent and collisional tectonism, with the generation of the Appalachian orogen (area 4 in Fig. 2). Baltica and Africa continued to move against eastern North America until the Permian, generating significant extensional and strike-slip displacements on the eastern margin in some places (e.g., parts of Atlantic Canada; Chapter 6), but also significant fold-and-thrust belt development, as in the Appalachian Basin and Ouachita foreland basin (Chapters 4 and 8). The North American craton was marked by broad, persistent regions of subsidence, the cratonic basins. (2) Phase two, which extended through the Pennsylvanian and Permian, continued into the Triassic, and overlapped in time with the first phase, saw the southwestern margin of the continent affected by oblique-slip displacement between Laurentia and Gondwana (including portions of what are now Mexico and the southwestern United States), with the development of an orogenic highland called the Ancestral Rockies (Chapter 7). This phase is defined primarily for the distinct history that the southwest part of the continent experienced because of the unique regional plate kinematics. (3) Phase three, commencing in the Late Triassic or Early Jurassic, corresponds to the Pangea breakup phase, during which North America drifted northwestwards (relative to a hot-spot reference frame: Engebretson et al., 1985). The eastern continental margin became the modern extensional Atlantic margin (Chapter 15), while the western margin underwent accretionary tectonism leading to the assembly of the Cordilleran orogen (area 5 in Fig. 2; Chapter 10), the development of the Western Interior Seaway (Chapter 9), and early Cenozoic Rocky Mountain (Laramide) basins (Chapter 13). Events on the northern (Arctic) margin of North America can be correlated to phases 1 and 3, as discussed in following sections (Chapter 14). A major contrast between phases 1 and 3 is that, until the Triassic, North America was undergoing convergence with Africa-Europe in the east, while a long-lived extensional margin faced Panthalassa in the west, and then from Late Triassic time to the present, relative motions reversed, with an extensional margin facing the widening Atlantic Ocean in the east, while a complex, convergent, accretionary orogen developed in the west. This evolution is encapsulated by two time charts (Figs. 4 and 5) and a series of cross-sections across Canada (Fig. 6). The purpose of the time charts and the cross-sections is to convey an overall impression of the Phanerozoic evolution of the continent and, in particular, to facilitate comparisons between events on opposite margins of the continent. The time scale used in the charts is from www.stratigraphy.org (June 2017 edition). Continual minor revisions of this scale are to be expected as an integral part of this online data source. The Sloss sequences, which were, of course, first defined by Sloss (1963), were assigned detailed stage ages by Sloss (1988, Chapter 2). The ages of these stages have been updated according to the www.stratigraphy.org website. The plate-tectonic evolution of the continent has been reconstructed in considerable detail by Ron Blakey, whose series of paleogeographic maps is used in this chapter and is posted on his website at http://cpgeosystems.com/paleomaps.html. The central column of the time chart, showing the changing orientation and latitude of North America, is based on these reconstructions.
FIG. 4 A time chart for Canada.
FIG. 5 A time chart for the United States.
8 The sedimentary basins of the United States and Canada
FIG. 6 Sequential east-west cross-sections through Canada, summarizing the changes in the plate-tectonic regime during the Phanerozoic.
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Through much of the Phanerozoic, Laurentia lay in low latitudes and the equator transected the continent (see the column “Latitude and Orientation of North American Craton” in Figs. 4 and 5). Approximate latitudes for the western and eastern margins of the continent, as they changed through time, have been read off Blakey’s maps, and are shown below each of the small maps of the continent. The cities of Calgary and Quebec City for Canada, and Salt Lake City and New York for the United States, have been selected as locations situated within cratonic Laurentia, and their position can therefore be tracked from as far back as the Precambrian. Many coastal locations, such as Los Angeles in the west or Halifax in the east, did not, of course, exist in their present form until terrane-accretion events took place, whereas the cities selected serve as useful markers for mentally positioning the continent as it drifted and rotated through the Phanerozoic. The orientations of the small maps, and the latitude values, show how North America rotated anticlockwise and then, commencing in the Jurassic, drifted northward to its present high-latitude position. It is not generally appreciated that for most of Phanerozoic time the continent occupied tropical to warm-temperate regions, and these conditions persisted, in the Arctic Islands, until the Miocene. Major events in four regions are shown in each of the two time charts. These regions, and the events used to typify them, essentially constitute a transect of the geology across the continent along two broad cross-sections, the Trans-Canada Highway in Canada, and major east-west highways, such as I-70, in the United States. Alaska and the Canadian Arctic are not shown at all, and events in other northern regions, such as northern British Columbia and Yukon, are necessarily omitted. Needless to say, the tectonic history is highly simplified, and many readers will be aware of local details that imply considerably greater complexity. The ages of events have been rounded off to easily remembered whole numbers, and the equivalent standard Phanerozoic ages indicated in each case. The right-hand column on each chart shows the relative motion of North America in relation to Gondwana. The arrows define three broad periods, the three “phases” of this chapter, which are also indicated by the background coloring of the chart. Also shown on each chart, in the form of broad white arrows, is the changing flux of sediment transport across the continent. Detrital zircon studies first reported by Dickinson and Gehrels (2003) confirm a suggestion made first by Dickinson (1988) that much of the thick Paleozoic and early Mesozoic detrital succession present in the Colorado Plateau area (including the units exposed in the Grand Canyon) were derived by uplift and erosion of the eastern continental margin. Detrital zircon of Grenville and Appalachian provenance is abundant, indicating the presence of long-vanished river systems transporting detritus westward across the continental interior. This dispersal pattern may have originated during the Neoproterozoic (Rainbird et al., 1997). During much of the Jurassic and Cretaceous, a continental-scale drainage system flowed northward through the Western Interior Basin, fed by sources in the Cordilleran and the Appalachian orogens (Fig. 4; Chapter 9), and southward drainage, forming what became the Mississippi-Missouri-Ohio system developed following the late Sevier and Laramide orogenies in the Cenozoic (Fig. 5). A reversal of the westward continental transport pattern in Canada in the Cenozoic is indicated by the upper white arrow on the Canadian chart (Fig. 4). An east-flowing drainage system was suggested by McMillan (1973), who was speculating about the possible sources of the thick Cretaceous-Cenozoic sedimentary accumulations on the Labrador shelf. The concept has received support from Duk-Rodkin and Hughes (1994), who mapped remnant landforms and terraces in parts of northern Canada. This drainage pattern was disrupted by glacial erosion and glacial meltwater drainage channels during the late Cenozoic. Except for the last, glacially related drainage system, these continent-wide dispersal patterns required regional tilting of the continent. The westward tilt during the Paleozoic is a dynamic topographic effect related to heating of the crust near the center of Pangea. The eastward and southward tilt during the Cretaceous and Cenozoic reflects uplift associated with the development of the Cordilleran orogen. The northern drainage system was then disrupted by the southward spread of continental ice in the Neogene.
PHASE ONE: THE CONSTRUCTION OF PANGEA Plate-Tectonic Evolution During the Cambrian, Laurentia lay astride the equator, rotated about 90 degree clockwise, relative to its present-day orientation (Fig. 7). The Iapetus Ocean along Laurentia’s eastern margin functioned as a growing ocean for about 100 million years (this is less than half the duration of the present-day Atlantic Ocean) and is estimated to have been about 5000 km wide by the end of the Cambrian. The divergent margin was marked by a series of promontories and embayments, formed by transform offsets in the Iapetus spreading center. The largest of these are the Newfoundland promontory with the Quebec embayment immediately to the south, and the Alabama promontory and Ouachita embayment (Fig. 8; Thomas, 2006). These promontories had a pronounced effect on tectonism and the development of sedimentary basins during the subsequent series of Appalachian orogenies (Chapter 4).
FIG. 7 (A) Plate-tectonic setting of North America during the Middle Cambrian. These and subsequent maps have drawn extensively on the data and ideas contained in Cook and Bally (1975), Ziegler (1988), Scotese (1998), Scotese and Golonka (1992), and Stampfli and Borel (2002). Additional maps and references may be found at http://cpgeosystems.com/paleomaps.html. (B) Explanation of the colors used in the paleotectonic-facies maps.
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FIG. 8 The eastern North American margin, during the early Cambrian. At this time eastern North America was functioning as an extensional margin, bordering the Iapetus Ocean (Thomas, 2006).
As Cambrian seas transgressed the continent the Transcontinental arch materialized as a mostly positive area, parts of which were covered by thin sediment and other parts exposed as lowlands. The Illinois and Michigan basins have strata exceeding 1000 m in thickness, and parts of the Cordilleran miogeocline and Appalachian basin have several thousand meters of sediment. By mid-Cambrian time, closure of Iapetus Ocean east of the continent (this was south, in the Cambrian configuration) had commenced. One or more arc-microcontinent systems approached from the north and east (present directions) including the Grampian (GRA) and Taconic arcs (TAA) and the Dashwoods microcontinent (Fig. 7). The collision with and deformation of these arcs against Laurentia initiated the Taconic Orogeny. This was the first in a series of orogenies that rafted fragments of Gondwana against the eastern Laurentian margin, resulting in a complex orogenic collage that took some 20 years to fully comprehend (from Williams, 1978 to van Staal et al., 1998). Traditionally, the Taconic orogeny has been interpreted to have begun in the Middle Ordovician; however, some evidence (see Chapter 3) indicates that it may locally have commenced in the latest Cambrian. The Iapetus Ocean separated these elements from Baltica and peri-Gondwanan terranes. Western Laurentia faced the vast Panthalassa Ocean (Chapter 5). From the Late Cambrian to Late Ordovician, the east- and north-fringing arcs and microcontinents were thrust over the shelf-slope-rise on the eastern margin of Laurentia and generated the Grampian and Taconic orogens. The number of arcs and their geometry is highly debated, though clearly Laurentia was the lower plate. The Taconic orogen affected the entire eastern margin of the continent as far south as the Carolinas and Georgia (Hatcher, 2010; Fig. 9). The formation of the Appalachian foreland basin commenced at this time (Fig. 10; Chapters 3 and 4). At about this time a new ocean, variously called the Prototethys Ocean or Rheic Ocean, formed between Baltica and Africa (Figs. 11 and 12). Rifted fragments of Africa and South America were rafted by sea-floor spreading within this ocean, and eventually collided with
12 The sedimentary basins of the United States and Canada
FIG. 9 Plate tectonic setting of North America during the Ordovician.
FIG. 10 Clastic wedges that developed as a result of the Appalachian orogenies. (Modified from Thomas (2006).)
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FIG. 11 Plate-tectonic setting of North America during the Silurian.
and sutured against Laurentia. A fragment interpreted as part of the continental margin of what is now Morocco—The Gander Terrane—collided with Laurentia during the Early Silurian, simultaneously all along the continental margin, from Newfoundland to Maine, causing what has been termed the Salinic Orogeny (Chapters 3 and 4). Meanwhile, a series of terranes and the continent Baltica approached eastern and northern Laurentia. Arctic terranes derived from eastern Baltica and Siberia, including Pearya, began docking in the Late Ordovician (Colpron et al., 2007). Domeier (2016) included several Paleozoic “orphan” terranes in his peri-Gondwanan (aka Avalonia s.l.) terrane including Carolina, Ganderia, West Avalonia, and Meguma. Eastern Newfoundland and parts of Nova Scotia, New Brunswick, and Maine (and parts of central England) originally constituted the Avalon terrane. The arrival and docking of this microcontinent during the Silurian was the cause of the Acadian Orogeny, which has been well documented in New Brunswick and Maine (Fig. 11). This was one of the last events in the long series of arc-continent and continent-continent collisions that ultimately brought Laurentia and Baltica together. European geologists call this collision the Caledonian Orogeny (Caledonia is the ancient name for Scotland), based on the evidence for Ordovician to Early Devonian tectonic episodes along what is now the suture between these two continents (Figs. 11 and 12; McKerrow et al., 2000). By the end of the Silurian and the beginning of the Devonian, Iapetus had closed everywhere. A major tectonic episode, termed the Scandian phase of the Caledonian Orogeny, occurred when Baltica collided with Greenland (which was then still an integral component of Laurentia) (Figs. 11 and 12). The reverberations of this episode were felt in the Arctic Islands as a series of small uplifts in the central and eastern Arctic. The oblique orientation of the Avalonian terranes coupled with presumed transform offsets helps explain the numerous suborogenies of the Acadian-Caledonian orogen. Carolina and Ganderia collided near the Ordovician-Silurian boundary (Salinic phase), West Avalonia in Late Silurian (Acadian phase), and Meguma in the Devonian (Neoacadian phase).
14 The sedimentary basins of the United States and Canada
FIG. 12 Plate-tectonic setting of North America during the Devonian.
During the Devonian, orogenic belts wrapped around Laurentia from South Carolina to Arctic Alaska (which accreted to NW Laurentia in the Early to Middle Devonian) (Fig. 12). The accretion of Avalonia and related terranes was followed in the Late Devonian and Mississippian by another batch of peri-Gondwanan terranes—the southern group would eventually form parts of Mexico and Central America and the northern group comprised the Variscan terranes of Central and Southern Europe. Meanwhile, some Arctic terranes looped or rotated around Arctic Alaska and transformed along western Laurentia to accrete during the Kootenay-Antler orogeny in the Late Devonian and Mississippian (Colpron et al., 2007). The Alexander terrane continued migrating toward Panthalassa. Between the Mississippian and the Permian, North America underwent orogenic collision on three of its four margins (Figs. 12–14), and by the end of this period North America had become one of the larger components of the new supercontinent Pangea. During the Late Devonian or Mississippian, the east-facing Antler arc approached the western margin, and collided with the Cordilleran miogeocline (Chapter 11; Fig. 13). There is some evidence that the Antler Orogeny affected areas as far north as British Columbia (Chapter 5). Distal, passive-margin, deep-water deposits were thrust eastward over coeval carbonate shelf deposits as the Roberts Mountains Allochthon. The arc itself collided with partly rifted fragments off Western North America; the largest blocks became the nucleus for the Quesnell and Stikine terranes, which were accreted to North America in the Mesozoic. During the Carboniferous to Early Triassic, these terranes were approaching North America from the west. Active arcs, within which significant volumes of Panthalassa oceanic crust were being subducted, lay off the western continental margin (Figs. 14 and 15). These arcs and associated terranes began to accrete to the western margin in the Permian and Triassic (Sonoma orogeny of California-Nevada-Oregon: Chapter 11).
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FIG. 13 Plate-tectonic setting of North America during the Mississippian.
Until the Late Triassic, much of the continental margin off western Canada remained in an extensional-margin tectonic and sedimentary (miogeoclinal) regime, with the accumulation of craton-derived shallow- to deep-marine deposits, including turbidites (Chapter 5). In the east and south, the Gondwana margin (now comprising the west coast of Africa and the north coast of South America) underwent the final collision with North America, causing the Pennsylvanian-Permian Alleghanian orogeny from Alabama to the Atlantic provinces (Chapter 4) and the Marathon-Ouachita orogeny from Alabama to Texas and into northern Mexico (Figs. 14 and 15; Chapter 8). The Carboniferous Ouachita basin has been interpreted by Graham et al. (1975) and Ingersoll et al. (1995) as a remnant ocean basin between the approaching Laurentian and Gondwana continents. However, their original model of diachronous collision of Laurentia with Gondwana needs to be modified in light of modern data relating to the importance of terrane-accretion events. The Ouachitan Orogeny probably began as a terraneaccretion event in Alabama in the mid- to late Mississippian (Fig. 13), reaching Mexico in the early Permian. There would therefore have been a remnant ocean across southern Texas during much of the Pennsylvanian (Chapter 5). There may also have been a remnant ocean behind (south of) these terranes, corresponding to a much reduced proto-Tethys, as suggested in Fig. 13. As noted in the following text (in the section titled Phase Two), this collision was related to the uplift of the Ancestral Rocky Mountains in the southwest part of the continent (Chapter 7). The Alleghanian Orogeny represents the final collision between Gondwana and Laurentia, generating a new, larger continent termed Laurussia. It probably commenced in the Early Pennyslvanian, and may have proceeded from south to north, reflecting diachronous contact between
16 The sedimentary basins of the United States and Canada
FIG. 14 Plate-tectonic setting of North America during the Pennsylvanian.
irregular continental margins (Figs. 14 and 15; Chapter 4). Taconic, Acadian, and Alleghanian orogenies together accounted for 100–400 km of crustal shortening and overthrusting above the basal décollement beneath the Appalachian Mountains of the United States (Chapter 4). The final stage of Pangea construction, during the Pennsylvanian and Permian, involved strike-slip displacement between Gondwana and Laurussia, with the formation of linear, strike-slip, and extensional basins bordered by fault-bounded uplifts extending through the Canadian portion of the Appalachian orogen. Oblique convergence and diachronous terrane collision was accompanied by crustal delamination, heating of the overlying crust, the generation of large volumes of mafic magma, and also of large volumes of late- to post-orogenic felsic magma (Chapter 6). The cessation of this activity was followed by significant thermal subsidence during the Permian, which accounts for the final configuration of the Maritimes Basin and is probably largely responsible for the development of the present-day Gulf of St. Lawrence. The plate-tectonic evolution of the Arctic margin is not so well understood. Paleogeographic considerations suggest the presence of a land barrier and/or sediment source, called Crockerland by Embry and Beauchamp (Chapter 15), north of the present Arctic margin during the late Paleozoic, and possibly until the Early Jurassic. This land was probably a fragment of what became part of Siberia (Chukotka-Arctic Alaska), after the Canada Basin opened in the Cretaceous.
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FIG. 15 Plate-tectonic setting of North America during the Permian.
Sedimentary Evolution of the Interior and Western Continental Margin The Cordilleran margin, from Alaska to California, is characterized by thick prisms of mid- to Upper Proterozoic sedimentary rocks, including the well-known Windermere, Miette, Belt, and Purcell supergroups of Montana-Idaho-Alberta-British Columbia (Hoffman, 1989; Fig. 16). Subsidence analysis of the Phanerozoic margin of southern British Columbia indicates that the extensional subsidence that initiated the opening of Panthalassa began at about 600 Ma (Bond and Kominz, 1984; Chapter 5), so these Proterozoic successions are thought to represent several discrete episodes of rifting that preceded the breakup of Rodinia. They are at least as old as 800 Ma, indicating that rifting lasted for some 200 m.y. during the breakup phase. This compares with the lengthy predrift rifting of the North Atlantic margins. For example, the rifting of East Greenland lasted for 170 m.y. before oceanic crust began to be generated between Greenland and Europe (Surlyk et al., 1981). Upper Proterozoic strata exceed 4 km in thickness in parts of British Columbia, and 7 km in Utah (Hoffman, 1989). They may have been derived in part from erosion of the mountains formed by the Grenville collision and transported across Laurentia by a continental drainage system (Rainbird et al., 1997). The strata consist primarily of thick successions of coarse clastics, including coarse, poorly sorted conglomerates, much of it interpreted as glacigenic in origin. Although Upper Proterozoic glacigenic strata have been interpreted in terms of a frozen earth, as part of the Snowball-Earth hypothesis (Hoffman et al., 1998), the presence of thick successions of debris-flow deposits (e.g., McMechan, 2000), while
18 The sedimentary basins of the United States and Canada
FIG. 16 Proterozoic strata of the Cordilleran margin. (Modified from Hoffman (1989, Fig. 48).)
consistent with deposition under temperate glacial, possibly Alpine-glacial conditions, indicates the presence of abundant liquid water and the predominance of aqueous environments, which is inconsistent with the frozen-ocean concept that is the central element of the Snowball-Earth hypothesis (see critique and alternative model for late Precambrian glaciation by Eyles and Januszczak, 2004). Strata of Cambrian to Early Jurassic in age constitute the oldest four of Sloss’s classic Phanerozoic sequences: the Sauk, Tippecanoe, Kaskaskia, and Absaroka (Sloss, 1963, 1988; Chapters 2 and 5; Fig. 17). The first three, at least, are widely distributed across the interior of the continent. It seems likely that during parts of the Ordovician and Silurian, exceptionally high sea levels caused transgressions to cover most, if not all, of the cratonic interior of Laurentia, including most of
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 19
FIG. 17 The six classic sequences of Sloss (1963).
the area now exposed as the Canadian Shield (Chapter 2). The heavy blue lines in Fig. 9 suggest a more conservative view regarding the extent of the seas. The thickest cratonic Ordovician deposits are in the Michigan and Illinois basins, with lesser accumulations in Hudson Bay, Williston, Anadarko, and Tobosa basins and parts of the Canadian Arctic. The term “Great American Bank” has been coined for the carbonate platform that extended across southern Laurentia during the Cambrian and Ordovician (Derby et al., 2012). Transgression and regression were caused by a combination of eustatic sealevel changes and epeirogenic tilting and warping driven by mantle thermal processes, the latter in part a consequence of collisional orogeny on the continental margins (Chapter 2). Successions of largely early to middle Paleozoic age constitute the fill of three of the four large intracratonic basins within the continent, the Michigan, Illinois, and Hudson Bay basin (the fourth, the Williston Basin, was markedly affected by Mesozoic-Cenozoic sedimentation during the third phase of continental development, when it received sediment from the uplift and erosion of the Cordilleran orogen). The suturing of northern Laurentia and Baltica formed an orogenic highland at the center of this large combined continent—Laurussia—which has also been called the “Old Red Continent” because it was the source for and the site of the predominantly coarse, red clastics constituting the Old Red Sandstone of western Europe, Svalbard, Greenland, and Atlantic Canada. Detritus was also shed westward across Greenland into the Canadian Arctic, and probably contributed to clastic-wedge formation as far west as Yukon Territory. For much of the Silurian, the basins of Laurentia were partitioned by numerous arches and domes; large areas along the Transcontinental arch lack Silurian deposits. Spectacular reefs dominate the Middle Devonian across Alberta and adjacent British Columbia. Carbonates, mostly dolomite, dominate most basins; evaporates are present in the Hudson Bay, Michigan, and Appalachian basins (Fig. 11). The Great Lakes region is famous for Silurian reefs. Within the Canadian portion of the western Laurentian margin, Cambrian to Triassic sedimentary rocks constitute a structurally relatively conformable succession of continental-margin strata, comprising a classic “miogeocline” (Fig. 18). Areas where section is locally missing (e.g., West Alberta Ridge; Chapter 5) were uplifted as a result of intraplate stresses or possibly as a result of mantle underplating (see Chapter 19). In western Canada and the United States, the strata can readily be subdivided into the Sloss sequences on the basis of regional low-angle unconformities, and they show that strata of Ordovician and Devonian-Mississippian age are the most widespread of the Paleozoic assemblages. Rocks of this age range typically lap onto the Canadian Shield throughout western and Arctic Canada. Distribution of the Absaroka sequence, of Late Mississippian to Early Jurassic age, is somewhat different from that of the older sequences. Within Canada it is much less areally extensive than the preceding sequences (Sloss, 1988). It is absent from the central and northern cratonic interior, but is well represented in the southwestern part of the continent (Chapters 5 and 7). The sequence rests on a profound unconformity that records extensive pre-Late Mississippian denudation, and is also capped by an extensive unconformity. These broad characteristics reflect uplift of the continent in response to orogeny along three of the four continental margins (Alleghanian, Ouachita, and Sonoma orogenies; Figs. 13–15). The final assembly of Pangea during the Late Permian and Triassic (Chapter 6) occurred while several factors likely acted in concert to generate low global sea level. The continental crust of the supercontinent was thickened and thermally elevated; global reduction in rates of sea-floor spreading decreased ridge volume and an increase in global average age of
20 The sedimentary basins of the United States and Canada
FIG. 18 Generalized stratigraphic cross-section through the Western Canada Sedimentary Basin. (Modified from Price et al. (1972).)
oceanic crust depressed sea-floor levels—both increased the volume of the ocean basins (e.g., Worsley et al., 1986). Studies of detrital zircon in the clastic units of the US southwest have suggested that much of the detritus there may have been derived by erosion of the Appalachian and Grenville orogens in the east, and transported west by continental river systems (Chapters 7 and 19).
Sedimentary Evolution of the Eastern Continental Margin Remnants of Iapetus Ocean are now preserved in the Dunnage zone of Newfoundland. These include some pristine fragments of ancient ocean floor, plus some very altered remains that are very difficult to interpret. A complex of small oceans, arcs, and microcontinents formed and were rapidly destroyed during the Taconic orogeny (van Staal et al., 1998). The Iapetan continental margin of North America is spectacularly exposed in Gros Morne Park of Newfoundland, including the thick carbonate debris flows of the Cow Head Breccia (Chapter 3). Remnants of the sedimentary prism are
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 21
present at many places to the south, along the ancient continental margin at the edge of the Appalachian orogen, notably in the Valley and Ridge province of the Appalachian orogen. There, extending from Pennsylvania to Georgia, a miogeoclinal transition can be traced in the Cambrian and Ordovician succession, from a largely carbonate, cratonic, and continentalmargin succession in the west to a deeper water carbonate-clastic succession in the east (Rast, 1989; Chapter 4). Taconic collision and crustal loading commenced first against the St. Lawrence promontory, next against the Alabama promontory, followed by progradation into the Tennessee embayment, then against the New York promontory, followed by migration into the Pennsylvania embayment. Foreland-basin clastic wedges associated with the Taconic, Acadian, and Alleghanian orogenies have long been known in the Appalachian orogen (Fig. 10; King, 1977; Chapter 4). As peri- Gondwanan terranes approached southern Laurentia, leading to the Taconic Orogeny, several thousand meters of orogenic flysch were deposited and then subsequently thrust onto the Laurentian shelf to initiate the Ouachita Orogeny. A major clastic wedge associated with the Acadian orogeny is the coarse-grained assemblage, predominantly red in color, traditionally named the “Catskill delta” (an equivalent of the Old Red Sandstone). The Lackawanna phase of the Alleghanian Orogeny (Early Carboniferous) shed a prominent clastic wedge westward—sandstone reached the Illinois basin. Each clastic wedge appears to radiate from a source area of limited lateral extent (Fig. 10) and is diachronous along strike. Thomas (2006) and Ettensohn et al. (Chapter 4) interpret this as the result of diachronous collision of the approaching terranes with the irregular Iapetan margin. In general, the Devonian is marked by significant unconformities and biologic extinctions. The sub-Devonian unconformity indicates that much of Laurentia was exposed to erosion near the Silurian-Devonian boundary. The extinction near the end of the Frasnian was one of the largest-magnitude extinctions of the Phanerozoic, and a third event near the end of the Famennian was also significant. The unconformities at or near these events are typically overlain by transgressive black shales—currently the targets of widespread fracking to recover rich petroleum reserves. During the Mississippian, except for peninsulas and islands associated with the Transcontinental arch, most of the United States west of the Appalachian Basin and most of Canada west of the Canadian Shield were blanketed in limestone, especially the distinctive crinoidal calcarenite of this period (e.g., the spectacular Rundle Group of Banff National Park). Deepwater facies adjacent to highlands contain turbidites and slump deposits and shallow marine and continental deposits consist of sandstone, conglomerate, and mudstone. Evaporites are common in the Sverdrup and Canadian Maritime basins. Strike-slip and extensional basins of Mississippian to Permian age in Maritime Canada and Newfoundland are characterized mainly by continental and shallow-marine clastics, including significant coal deposits and local thick evaporites (Chapter 6).
Sedimentary Evolution of the Southern Margin Along the Ouachita margin, a cratonic carbonate shelf persisted until Mississippian time. Tectonic loading commenced in the east in the Late Mississippian, and extended westward during the Pennsylvanian (Figs. 13 and 14; Arbenz, 1989; Thomas, 2006; Chapter 8), reflecting the last, diachronous, stages of closure of Gondwana against Laurentia. A Cambrian to Lower Mississippian preorogenic, largely clastic succession up to about 3.5 km thick, including turbidites, is present along the Ouachitan margin, with paleocurrent and petrographic data indicating derivation from the craton to the north. This is followed by 12–15 km of largely deep-water turbidite deposits indicating sediment transport along the axis of the remnant ocean basin in a generally west to northwest direction. The somewhat enigmatic Ouachita-Marathon orogen apparently lacked a major continent-continent collision-style mountain range, as it lacks “Appalachian-style” structure (Thomas, 1989). Rather, Royden (1993) interpreted the orogen as the result of a collision between a fast-moving Caribbean- or Carpathian-style arc with the passive North American southern margin.
Sedimentary Evolution of the Arctic Margin At least 10 km of Cambrian to Devonian strata are present in the Franklinian Basin (Trettin, 1989). The origins of the basin are obscure; it probably was initiated by crustal stretching and possibly rift faulting during the breakup of Rodinia toward the end of the Precambrian. A central, deep basin extends from northeast Ellesmere Island southwestwards, at least as far as northwest Melville Island, but this part of the basin is largely covered by younger strata. The deep basin is extensively exposed only in Ellesmere Island as a result of uplift during periods of tectonism during the late Paleozoic and the midCenozoic. There, a transition from deep-water to shallow-shelf environments is well exposed. A similar deep basin and basin-to-shelf transition are preserved to the northeast, in northern Greenland. The shelf is well exposed along a belt extending from northern Ellesmere Island southwestward to Devon and Cornwallis Islands and then westward to Bathurst and Melville Islands. Subsurface data show that the belt continues on southwestward beneath Prince Patrick and Banks Islands.
22 The sedimentary basins of the United States and Canada
Pearya, a small but complex terrane, collided with the Arctic margin in the early Silurian. This area recorded the effects of the Caledonian orogeny, with local uplift and synorogenic sedimentation in the earliest Devonian and again in the Midto Late Devonian, when a major clastic wedge prograded southwestward from sediment sources within the Caledonian orogen of eastern Greenland. Franklinian Basin sedimentation was brought to an end by uplift at the end of the Devonian.
PHASE TWO: DEVELOPMENT OF THE SOUTHERN MIDCONTINENT AND ANCESTRAL ROCKIES Plate-Tectonic Evolution The final closure of Gondwana against Laurentia during the late Paleozoic subjected the southwestern part of Laurentia to a transpressive tectonic regime between the Late Mississippian and Early Permian (Figs. 13–15). Sedimentation and tectonism of the Ouachita-Marathon belt at this time have been referred to previously. The southern portion of the vast, interior cratonic (largely carbonate) platform of the early to mid-Paleozoic, was transformed, beginning in the Late Mississippian, by warping and faulting. Deformation of the craton west of the Mississippi River by transpressive tectonism differentiated this region into a series of basins and uplifts, of which the best known are the Delaware Basin, Central Basin Platform, Midland Basin, plus the Ouachitan foreland basins (Val de Verde, Fort Worth, Anadarko basins: Chapter 8). Tectonism also had a significant effect on Texas and the Four Corners states, where the Ancestral Rocky Mountains developed (Kluth and Coney, 1981; Chapter 7; Figs. 14 and 15). Phase-two tectonism is first in evidence during the Late Mississippian, in the Anadarko basin, along the Texas-Oklahoma border area. This basin underwent particularly rapid subsidence, accompanied by uplift of the adjacent craton, including movement on such trends as the Wichita-Amarillo axis. However, tectonism was most intense during the Middle Pennsylvanian. At this time, the Ancestral Rockies developed across the entire southwestern portion of what is now the United States (from Kansas to Arizona and Idaho to Texas), consisting of broad, block uplifts bounded by narrow fault zones. The final construction of Pangea during the Pennsylvanian and Permian included the Variscan (Hercynian) Orogeny between Europe and Africa (Chapter 6), and led to widespread orogenic uplift of eastern North America and western Europe. Burgess (Chapter 2) attributes this to dynamic topographic uplift over a thermal high caused by supercontinent insulation of the mantle. A reorganization of global plate regimes commenced in the Early Carboniferous, as the final construction of Pangea was underway (Ziegler, 1988). Extensional successor basins developed over some of the areas affected by the Caledonian Orogeny. The Sverdrup Basin, in Canada’s Arctic, is one of these (Chapter 14). Global sea levels were at an all-time low during the Permian-Triassic and remained low until widespread rifting and fragmentation of Pangea commenced in the Jurassic.
Sedimentary Evolution of the Mid-Continent and Ancestral Rockies Rocks of Pennsylvanian to Permian age (Absaroka sequence) underlie much of the US Mid-Continent region, west of the Mississippi River. Within the craton, from the Dakotas, south to Kansas, and in the intracratonic Illinois basin, they are characterized by the distinctive cyclic repetitions of the classic cyclothems, which were first described from outcrops in Kansas in the 1930s (Chapter 8). Reef carbonates and evaporites were deposited in the classic Permian Basin of west Texas. The margin of the craton at this time lay across central Texas, and the sedimentary succession shows a transition through platform-margin facies into the deep-water deposits of the Ouachita foreland basin to the south. Uplift of Pangea, centering on the Appalachian-Caledonian megasuture, imposed a tilt to the North American continent, downward to the west, upon which was established a major, west-flowing drainage pattern. Late Paleozoic and Mesozoic strata in the basins associated with the Ancestral Rockies of the southwestern United States are consequently rich in Grenville and Appalachian detritus (Fig. 5; provenance and paleogeographic analysis by Dickinson, 1988; Dickinson and Gehrels, 2003). The Ancestral Rockies were characterized by widespread continental sedimentation, including substantial volumes of fluvial and eolian clastics (Chapter 7). One of the more prominent of the active tectonic highs was the Uncompahgre Uplift, which extended from northwestern New Mexico, northwestward across Colorado into Utah. This axis is bounded to the southwest by the Paradox basin, and together they exemplify the pattern of basins and uplifts that characterized the southwest through the Mid-Pennsylvanian to Early Permian. Downthrow on the southwest side of the uplift exceeds 3 km, and banked against the fault is a comparable thickness of coarse, nonmarine clastics of the Cutler Group/Formation, derived by uplift and erosion along the Uncompahgre axis. The adjacent Paradox Basin is filled primarily by a carbonate-evaporite succession recording repeated, cyclic changes in salinity in response to high-frequency changes in sea level. These cycles have been correlated by conodont biostratigraphy to the Mid-Continent cyclothem record, suggesting glacioeustastic
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 23
control (see Chapter 8), but recent studies have suggested that glacioeustatic influences may have been small, the authors favoring an autogenic model for cycle generation (Dyer and Maloof, 2015). Permian eolian deposits in the southwest are interbedded with extensive redbed units and marine carbonates—the cyclicity of the Pennsylvanian continued into the Early Permian as Gondwanan glacial cycles continued.
Sedimentary Evolution of the Western and Northern Continental Margin Sandstone, mudstone, and carbonate were deposited in the Arctic region with the thickest deposits in the Sverdrup basin. In the Cordilleran region, the Havallah basin and Slide Mountain Ocean began to close as the Quesnell, Yukon-Tanana, and Stikine terranes approached the continental margin. The origin and nature of these terranes has been debated, but shown here is the model generally favored by Canadian geoscientists including the oroclinal closing of Stikine against Quesnell trapping the Cache Creek terrane between the two (Fig. 15; Colpron et al., 2007). Permian deposits in the Cordilleran include diverse volcaniclastics and Permian “Tethyan-derived” limestones that were trapped in the oroclinal bend that preserved parts of the Cache Creek Ocean. In Utah and Nevada, thick Permian limestones and sandstones were deposited in the Oquirrh and Bird Spring basins. Equally impressive Permian rocks were preserved in the Pedregoas, Delaware, and Midland (Permian) basins including the global type Middle Permian Guadalupian Series.
PHASE THREE: BREAKUP OF PANGEA AND FORMATION OF THE CORDILLERAN OROGEN Plate-Tectonic Evolution Subduction of Panthalassa probably began in the Mid-Devonian, this being the age of arc-related rocks in Nevada and California, and in the Late Devonian this led to collision of Laurentia with an east-facing arc, giving rise to the Antler Orogeny (Fig. 13; Oldow et al., 1989; Chapter 11). Evidence for convergent-margin tectonism elsewhere along the western margin of the continent is sketchy, although there is some tectonic and stratigraphic evidence for an offshore arc in parts of British Columbia (Chapter 5). Otherwise, the western margin of Canada may have largely remained an extensional margin until the Triassic (Fig. 19). Widespread subduction and arc collision during the Permo-Triassic caused the Sonoma Orogeny of Nevada, which initiated the widespread tectonism of the Cordilleran orogen. The Sonoman Orogeny developed as the Sonoma terrane collided with the extensional margin of North America along an east-facing arc. The Golconda subduction complex developed above (to the east of) this arc (Chapter 11). By late Triassic time the west-facing Nicola arc was well established on the west coast, extending roughly NNW-SSE through the present location of Kamloops, British Columbia (Oldow et al., 1989; Price and Monger, 2003; Chapter 10) and continuing southward into the United States (Chapter 11). At this stage, it was an extensional arc, with a backarc basin now preserved in Canada as the Slide Mountain terrane. The Cache Creek Complex, which extends almost continuously through most of British Columbia and southward into the Sierra Nevada of California, is the well-exposed subduction complex of the Nicola arc, containing rocks and fossils as old as Pennsylvanian. It ranges up to late Triassic in age, and represents one of the major sites where Panthalassa was subducted as North America drifted westward. According to Engebretson et al. (1985), some 13,000 km of Panthalassa oceanic crust were subducted beneath North America between the Early Jurassic and the present day (a width equal to one-third of the Earth’s circumference). As Price and Monger (2003, p. 21) pointed out, this documented 180 m.y. record only represents about half of the probable duration of the period of subduction of Panthalassa, which began at least as far back as the establishment of the Antler arc in the mid-Devonian. The arc may be even older than this. Oldow et al. (1989, p. 159) noted that the arc rocks rest on a Cambrian-Silurian basement in the Klamath Mountains and Sierra Nevada. The Cache Creek subduction complex, which became inactive following the accretion of Stikinia, represents a site of oceanic subduction in addition to that computed by Engebretson et al. (1985). Most of the Jurassic to Recent subduction would have been at locations farther to the west, as terrane accretion took place and subduction shifted outboard. The Triassic was a time of global plate-tectonic transition (although precursors of this transition had appeared during the Carboniferous, as noted previously). An enormous series of rifts developed along what was to become the Gulf and Atlantic borderlands, extending from the Gulf Coast and Florida, to Newfoundland, to northern Greenland, and deep into western Europe, as far east as Poland (Fig. 19; Ziegler 1988; Chapters 4 and 15). Widespread continental sedimentation continued in the southwest, over the eroded roots of the Ancestral Rockies (Chapter 7). On the western margin of North America, thinbedded turbidites exposed near Banff, Alberta (Triassic Spray River Formation) constitute the last major craton-derived sedimentary units before Cordilleran uplift generated western sediment sources and reversed sediment transport directions. They were deposited on the miogeoclinal eastern flank of the Slide Mountain backarc basin, but have been tectonically transported far to the east by Cordilleran thrust faulting.
24 The sedimentary basins of the United States and Canada
FIG. 19 Plate tectonic setting of North America during the Triassic.
Although rifting of Pangea commenced in the Triassic (along fractures that, in some cases, began to form in the Permian; Chapter 6), the breakup of the supercontinent and the appearance of the first Atlantic oceanic crust did not take place until the Middle Jurassic (Fig. 20). The oldest part of the Atlantic Ocean is that located off the east coast of the United States (Ziegler, 1988; Sheridan, 1989; Chapter 15). The Gulf of Mexico basin is slightly younger, with oceanic crust there spanning the Middle or early Late Jurassic to Early Cretaceous (Fig. 21; Worrall and Snelson, 1989; Chapter 16). Rifting of the North Atlantic region commenced in the Early Cretaceous, extending northward as far as Svalbard. Separation of Greenland from Canada also began, with rifting initiating the Labrador Sea-Baffin Bay seaway. However, continental breakup and seafloor spreading to generate the North Atlantic did not begin until the mid-Cretaceous, when the Iberian Peninsula separated from the Grand Banks (Fig. 22). In the Late Cretaceous, Atlantic opening extended into the area between Greenland and Britain. Stretching of the Grand Banks switched direction from NW-SE to NE-SW. This succession of rifting episodes with contrasting stretching directions was the key to the structural evolution of what became the Jeanne d’Arc Basin and the Hibernia oil field (Tankard and Welsink, 1987; Chapter 15). In the latest Cretaceous, sea-floor spreading commenced off the margins of Labrador, separating Canada from Greenland for the first time (Fig. 23). A triple-point junction developed off the southern tip of Greenland, and for about 40 million years, from the Late Cretaceous until the Oligocene, Greenland functioned as a separate plate (Srivastava et al., 1981). Greenland rotated away from the Labrador-Baffin Bay margin around a pole that caused it to contract against Ellesmere Island, and this
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 25
FIG. 20 Plate tectonic setting of North America during the Early Jurassic.
had important consequences for the development of the mountains of the northeastern Arctic Islands. The Late Cretaceousmid Tertiary Eurekan orogeny was an episode of transpressive deformation of the northeastern Canadian Arctic Islands. Between the Early Jurassic and the present day, North America drifted westward by some 70 degrees of longitude, relative to the Pacific plate (Fig. 24; Engebretson et al., 1985). Drift was toward the west-northwest, which carried North America some 40 degrees of latitude northward by the Paleocene, before a change in drift trajectories directed the continent as much as 10 degrees back southwestward. For the first time during the Phanerozoic, the northern part of the continent was carried into temperate and then even more northerly latitudes. The Cordilleran orogen was constructed by accretionary tectonics, which, at least locally, began as far back as the Devonian, and within Canada some 500 km of new continental crust has been added to the western margin of the continent (Yorath, 1991; Price, 1994; Price and Monger, 2003; Chapter 10). Figs. 19–23 and 25–27 provide a series of reconstructions that show the plate-tectonic evolution of the western margin of the continent from the Triassic to the Miocene. From Pennsylvanian through Jurassic, the number of arcs and their polarity with respect to North America is extremely controversial; there is not universal agreement about all of the details of these reconstructions (see Price, 1994; Price and Monger, 2003, for a discussion of the controversies). For example, the faunal data of Smith and Tipper (1991) and the paleomagnetic data of Wynne et al. (1995) suggest that Quesnellia, Stikinia, and Wrangellia all lay considerably further south
26 The sedimentary basins of the United States and Canada
FIG. 21 Plate tectonic setting of North America during the Late Jurassic.
until the Jurassic than shown here, implying that the structural relationships shown in these diagrams should be modified by substantial right-lateral displacement of post-Jurassic age. This is complicated by the fact that considerable right-lateral displacement is known to have taken place during the Eocene, as discussed shortly. Partitioning the displacement between the Mesozoic and the Cenozoic is problematic. Collision of the McCloud-Nicola arc against North America took place between the Late Triassic and Early Jurassic, resulting in delamination of the arc and back-arc region, and the beginning of a process of obduction and tectonic wedging of the arc eastward over the Precambrian basement. Subduction stepped out to the outboard margin of what was to become the Intermontane belt in Canada (the amalgamated Stikine, Quesnel, and related terranes). The Bridge River terrane of SW British Columbia, of Permian to mid-Jurassic age, constitutes the subduction complex and related rocks of the marginal arc (Chapter 10). To the south, the Sierra Nevada arc (along the spine of what was to become California) was established in the latest Triassic, and persisted until the early Cenozoic (Chapter 11). Meanwhile, off to the west in Panthalassa, a large terrane now called Wrangellia was approaching the North American margin. Wrangellia consists of two large terranes that are thought to have amalgamated within Panthalassa in the Pennsylvanian (Colpron and Nelson, 2009), creating the Insular superterrane, before suturing to North America (Saleeby, 1983).
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 27
FIG. 22 Plate tectonic setting of North America during the Early Cretaceous.
The Alexander terrane consists of a succession of arc-related rocks resting on a metamorphic basement ranging from Neoproterozoic to early Paleozoic in age (Colpron and Nelson, 2009). The Wrangellia terrane is composed of an arc, overlying plateau basalts, and carbonate sediments of Carboniferous to Triassic age. Both terranes are regarded as far-traveled remnants, Wrangellia of possible Asian origin and Alexander of Baltic affinities (Colpron and Nelson, 2009). Irving et al. (1985, 1996) used paleomagnetic evidence to reconstruct the history of amalgamation and accretion, and coined the term “Baja British Columbia” for the configuration and geographic relationship this superterrane would have had until the midCretaceous. The implication of the paleomagnetic studies of Irving and others is that Wrangellia was originally located far to the south of its present position, with northward translation during the Late Cretaceous and Paleogene. This has long been controversial because of a lack of clear support from the surface geology, for example, no clear evidence for largescale strike-slip displacement. Recent studies of detrital zircon from the Nanaimo Basin, Vancouver Island, by Matthews et al. (2017) have yielded evidence for a major detrital source located in the Mojave-Sonoran region of southwestern North America, lending strong support to the model of major dextral displacement. The orientation of the Insular superterrane was oblique relative to the continental margin, such that the southern end of the terrane collided with North America first, in the area of California (the exact southern margin is highly debated). This took
FIG. 23 Plate tectonic setting of North America during the Late Cretaceous.
FIG. 24 The gradual westward drift of North America, relative to Panthalassa, from the Jurassic to the present (Engebretson et al., 1985).
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 29
FIG. 25 Paleogene.
place in the Mid- to Late Jurassic (Fig. 21), causing the Nevadan orogeny, an event that included the emplacement of ophiolite complexes in northern California (Chapter 11). Collision of the Insular Superterrane with the North American margin was probably not completed until the mid-Cretaceous, in southern British Columbia (Figs. 21 and 22; Trop et al., 2002). During the Late Jurassic and Early Cretaceous, the Intermontane Superterrane became wedged beneath its outboard margin, and the Cordilleran miogeocline was scraped off its basement and accreted to the advancing front of the Intermontane Superterrane, where it formed the oldest part of the Rocky Mountain foreland fold and thrust belt (Price, 1994). During the collision with the Insular Superterrane, the Intermontane Superterrane (Stikinia, Quesnellia, etc.) was pushed northeastward over the margin of the North American continent, scraping off more of the supracrustal rocks of the Western Canada Sedimentary Basin, the convergent motion causing the generation of the rest of the Rocky Mountain foreland fold and thrust belt and the foreland basin (Chapter 9). The accreted terranes were also displaced northwestward, between the mid-Cretaceous and the mid-Eocene, producing a set of major right-lateral strike-slip fault systems (Tintina Trench-Northern Rocky Mountain Trench, Shakwak-Denali, Yalakom-Ross Lake, and Fraser River-Straight Creek faults) that dominate the structural fabric of the Canadian Cordillera. Thus, the Rocky Mountain foreland fold and thrust belt is a
30 The sedimentary basins of the United States and Canada
FIG. 26 Eocene-Oligocene.
transpressional accretionary prism (Price, 1994). The southwestward bend in North America’s path, relative to the Farallon Plate, that took place at about 60 Ma (Fig. 24), explains why transpression became progressively more important from the beginning of the Cenozoic. This culminated in the Eocene with an important phase of NW-SE extension and mafic igneous activity, including dyke emplacement, and extrusion of the Kamloops volcanics in central British Columbia. The Sevier orogeny is the name given to the series of contractional episodes that built a major fold-thrust belt along the eastern front of the Cordilleran orogen, from Arizona to the Yukon between the Early Jurassic and the Eocene (Figs. 4 and 5). Specific episodes of thrust faulting have been identified in the Alberta Rocky Mountains based on radioisotopic dating of fault gouge (Pană and van der Pluijm, 2015). DeCelles and Graham (2015) collated magmatic and tectonic events in the US Cordillera, and documented an eastward shift in the location of thrusting along the Sevier fold-thrust front during the Late Cretaceous. The Western Interior foreland basin was initiated in the mid-Jurassic as a result of the crustal loading caused by arc collision and terrane amalgamation along the length of the Cordilleran orogen (Chapter 9). The Western Interior Seaway, formed during the Jurassic, retreated at the Jurassic-Cretaceous boundary, expanded southward during the Early Cretaceous, and formed a continuous seaway from Alaska to the Gulf of Mexico during most of the Late Cretaceous. The appearance of western sediment sources for this basin seems to have been more or less simultaneous along the length of the basin at about 170 Ma, at least from Alberta to Utah, despite the diachroneity of the terrane collisions that generated contraction and uplift.
The Phanerozoic Tectonic and Sedimentary Evolution of North America Chapter | 1 31
FIG. 27 Miocene.
Sedimentary Evolution of the Western Margin Collisional retroforeland basins are foreland basins developed behind the orogen on the overriding plate at a suture. As Ingersoll (Chapter 11) notes, there do not appear to be any such basins in the Cordillera associated with the terrane collision and amalgamation process. Until the Late Triassic, North America constituted the downgoing plate relative to Panthalassa subduction (Antler and Sonoma orogenies). After arc collapse and collision commenced in the Jurassic, basins are difficult to categorize in terms of simple plate models, and may have undergone evolution through several discrete styles. Arc rocks as old as Middle Permian have been mapped in northwest Mexico and southern California, and by the Late Triassic, prior to the collision with Wrangellia, a major continental arc was established on the North American margin extending from southern Arizona to central California (Fig. 20; Chapter 11). It evolved through several configurations as Wrangellia collided and was displaced northward during the Jurassic-early Cenozoic. Following the late Jurassic Nevadan orogeny a major forearc basin, the Great Valley Basin of California, persisted as a major depository, accumulating a thick succession of sediment-gravity-flow deposits (GVB in Figs. 22, 23, and 25; Chapter 11). Many of the basins within the US and Canadian Cordillera may be described as “successor basins,” that is, basins developed over a deformed orogen. Some are simply residual depressions between elevated blocks, while others were formed by subsidence of a previously deformed region of continental crust. For example, uplift of the Cache Creek terrane during the final amalgamation of Quesnellia and Stikinia provided a sediment source for the Bowser Basin (mid-Jurassic-Lower Cretaceous), which developed over a thermally subsided Stikine basement (Chapter 10).
32 The sedimentary basins of the United States and Canada
Some of the more significant sedimentary accumulations in the Cordillera are “overlap assemblages,” that is, sediments derived from terranes that had become amalgamated and uplifted, to become a sediment source. The deposition of detritus from one terrane on the eroded surface of the adjacent terrane provides a minimum age for bracketing the time of collision and amalgamation. The Gravina-Nutzotin, Dezadeash, and Gambier basins in central British Columbia help, in this way, to constrain the timing of the collision of the Wrangellia Superterrane with North America (Chapter 10). Thick marine Triassic and Jurassic rocks of northwest Nevada form an overlap assemblage to the Sonoman orogen (Saleeby and Busby-Spery, 1992). Several basins, including the Queen Charlotte (Lower-Upper Cretaceous), Georgia (Nanaimo: Mid Upper CretaceousNeogene), and Tofino (Paleogene to Recent) basins occupy forearc positions close to the outboard margin of the continent (Chapter 10). Lower and Middle Triassic rocks are restricted to the Arctic, Cordilleran, and Western Interior regions. Thick (>1000 m) carbonates were deposited in a marine basin of uncertain tectonic affinities (foreland basin to Sonoman orogen? remnant Paleozoic miogeocline?) in eastern Nevada, western Utah, and southern Idaho. These rocks thin abruptly to the east and grade into widespread redbeds deposited in coastal plain-sabkha and fluvial environments. West of the Sonoman Mountains, shallow- to deep-water marine rocks were deposited in diverse tectonic settings as the Stikine terrane rotated to trap the Cache Creek terrane and abut the Quesnell terrane. Conglomerates in northern British Columbia and Yukon mark terrane accretion in the Triassic. The Cordilleran region saw the growth of the Cordilleran arc following the Sonoman orogen. Although Permian and Triassic arc rocks are present in Mexico and California, the arc greatly expanded during the Jurassic; the arc was built on cratonic North America south of central Nevada but was a complex of marine arcs farther north. The complexity of Jurassic arcs from California to British Columbia is not well understood—the number, origin, polarity, and overall geometry of Jurassic arcs continue to be debated. Jurassic arc-related rocks include arc plutons and volcanics, forearc, intra-arc, and back-arc sediments, trench deposits, and ophiolites. Today these rocks are strewn across California, Oregon, Washington, and British Columbia. Looming to the west and closing on North America were the Wrangellia and Guerrero terranes. The former forearc region of California (still technically a forearc, albeit 1000 km west of the active arc) developed a series of basins during the Eocene filled with deposits that ranged from fluvial-deltaic to abyssal marine. Sediments were shed from the Nevadaplano and intervening Sierra Nevada region into these basins. Eocene sedimentation is also recorded in the Tyee basin of Oregon and Nanimo basin of Washington-British Columbia.
Sedimentary Evolution of the Western Interior The Western Interior foreland basin is one of the largest and most intensively studied basins in the world (Chapter 9). It has become convenient to regard the Middle to Late Jurassic Nevadan orogeny as the event that initiated the foreland basin, because sediments of this age range in the basin contain the first indication of a westerly derived orogenic source (although arc-derived clasts are present in the Triassic Chinle formation in Arizona). The thickness distribution of the Carmel Formation (Bathonian) in the Colorado Plateau area, and the presence of volcanic detritus, suggests derivation from Cordilleran igneous origins and the beginning of a foreland-basin style subsidence pattern (Chapter 7). Somewhat later in British Columbia, the collision of Quesnellia with the continental margin caused the uplift from which the Kootenay Formation (latest Jurassic and Early Cretaceous) of Alberta and Montana, was derived (Chapter 9). The Morrison Formation (mid-Jurassic-Lower Cretaceous) is the first major, widespread clastic unit in the Rocky Mountain States that could be described as a foreland-basin clastic wedge. It accumulated within the drainage basin of a major, continental-scale fluvial system transporting detritus from the Cordilleran orogen and sources to the south and east, northward to the Boreal Sea (Bhattacharya et al., 2016). To the south, in the area of the Colorado Plateau, vast eolian dune fields were interrupted by several marine transgressions of the Jurassic Western Interior Seaway. Bordering coastal-plain deposits are chiefly redbed sabkha-evaporite deposits. A particularly important tectonic episode occurred later during the Early Cretaceous (Hauterivian-Barremian), when the entire foreland basin was uplifted and eroded for some 9 m.y., forming a widespread unconformity. A thin but widespread sheet of coarse gravels and sands was then spread across the basin from the rising mountains which, by then, extended from northeastern BC to Utah. Across much of Alberta, this gravel, the Cadomin Formation, rests directly on the marine shales of the Fernie Formation (the last preorogenic sedimentary unit) above a major unconformity. Geochemical evidence suggests that uplift of the largely oceanic Quesnel terrane and associated magmatic rocks provided a major sediment source for the foreland basin to the east at this time. Lower Cretaceous clastic wedges (Mannville and Dunvegan formations; Aptian-Cenomanian) reflect renewed contraction and uplift caused by the suturing of the Coastal and Insular belts with the continental margin. Once again, a major
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continental-scale fluvial system drained through the basin toward the Boreal Sea. Detrital zircon studies demonstrate that at various times sediment sources from the west, south, and east fed a major northward-directed axial drainage network (Bhattacharya et al., 2016; Chapter 9). Sources that have been identified in detrital zircon populations include uplifts far west from the interior of the Cordilleran orogen, the magmatic arc to the southwest, and the Appalachian orogen to the east. The Athabasca Oil Sands, part of the Mannville Group of northern Alberta, consist primarily of large point-bar complexes in a tidally influenced river system near the seaward termination of this river system. The world-famous dinosaur country of the Red Deer Valley, Alberta, is located within the fourth of the major pulses of Jurassic-Eocene clastic sedimentation, the Belly River-Edmonton wedge. The last episode of tectonism reflects the period of transpressive orogenic activity that began in the Late Cretaceous and led to the deposition of the widespread Paskapoo Formation, of Paleocene age. Deposition continued at least until the Miocene, but only remnants of the youngest part of this final clastic wedge are now preserved, in locations such as the Cypress Hills of southwestern Saskatchewan. These are the only patches left of sediment that likely was once carried by river systems as far as the eastern continental margin of Canada. Further south, from Colorado to Texas, parts of the central seaway were clear enough at least twice to deposit widespread chalks during the Late Cretaceous. Within a region extending from Montana to northernmost Mexico, the Western Interior was disrupted, beginning in the Late Cretaceous (about 75 Ma), and continuing to the Late Eocene, by basement-involved tectonism, that broke up the foreland basin into a series of relatively small basins and uplifts (the largest, the Powder River Basin, is about 400 km long). This phase of tectonism is quite different from the classic “thin-skinned” tectonism of the Sevier and Rocky Mountain (Alberta) fold thrust belts, and gave rise many years ago to the term Laramide Orogeny. The term Laramide has subsequently been used in diverse ways, for example, to refer to all Cordilleran orogeny of this age range, a tendency that should be avoided. Various explanations have been advanced for the unusual style of tectonism. Lawton (Chapter 13) discusses the model of “flat-slab” subduction, first advanced by Dickinson and Snyder (1978). The basis of this model is that an area of near-horizontal subduction of a buoyant Pacific (Farallon) plate beneath western North America generated disruption and shortening through the overlying continental crust (Saleeby, 2003). Laramide basins are mostly bordered by fault-bounded, basement-cored uplifts (Chapter 13). Faults are typically steeply dipping reverse faults showing displacements that may exceed 10 km. Basin fills are almost entirely nonmarine, commonly consisting of lacustrine and coarse fluvial facies. Many of the basins in the center of the Cordillera, from southern Montana to SW Utah, are classified as “ponded basins” and contain fills that are largely lacustrine in origin. The Rocky Mountain uplifts also sent a large wave of clastic sediment into the Gulf of Mexico via the Mississippi-Missouri river system (Chapter 16). The middle Cenozoic (Late Eocene and Oligocene: Fig. 26) was a time of widespread erosion across much of North America. Other than a few thin deposits on the High Plains, Rocky Mountains, and Colorado Plateau, sedimentary rocks of this age are restricted to the coastal regions. Except for carbonates in Florida, Yucatan, and the Bahamas, sediments are clastic. In the Cordilleran region, flat subduction ended and the locus of arc magmatism migrated back toward the subduction zone and left a trail of Oligocene volcanic fields behind it. In the forearc region, Oligocene deposits are commonly fluvial and overlie deep marine Middle Eocene rocks. But significant change was coming as the East Pacific Rise approached the West Coast. The late Cenozoic marks a time of dramatic change in Western North American tectonics (Fig. 27). The subduction of the East Pacific Rise placed the relatively slow-moving North American Plate adjacent to the rapidly moving Pacific Plate. The Pacific Plate had pulled parts of the American Southwest from Central Mexico to Northern California away from the North American Plate. The results were two major events: extension in the Basin and Range, and the development of a transform margin between the Pacific Plate with its North American cargo and the remainder of North America. Meanwhile on the East Coast, Cenozoic patterns of sedimentation continued. The Western Interior has relatively minor Miocene and Pliocene deposits as erosion and possibly renewed uplift affected both regions. Widespread, thick clastic sediments were deposited in basins across the Basin and Range with a broad spectrum of lithologies represented—conglomerates and breccias are common in most basins. As the San Andreas transform system developed (the San Andreas Fault is Late Miocene to recent, but older transform faults in the system date to the Oligocene), a dynamic series of basins formed along both margins of the fault system (Chapter 12). Deep water deposits dominate and include turbidites, siliceous shale, marl, sandstone and conglomerate. Sedimentation was locally extremely thick—the Ventura Basin has some of the thickest Pliocene deposits on Earth. The Pasadenan orogen is defined by uplift, folding, and thrusting of Miocene and Pliocene deposits—an indication of very recent tectonic activity. It is not unusual to see Late Miocene and Pliocene abyssal sedimentary rocks sharply folded and truncated and unconformably overlain by Pleistocene continental and marine rocks. Processes typical of Miocene and Pliocene deposits are active today in the basins of the California Continental Borderlands, the broad area of offshore California on the Pacific Plate.
34 The sedimentary basins of the United States and Canada
Sedimentary Evolution of the Arctic Margin As noted previously, a phase of extension in the Carboniferous triggered the development of the Sverdrup Basin in the Canadian Arctic, on the northern margin of the continent. Carbonate and evaporite sedimentation dominated in this basin, together with a belt of marginal clastics, until the Permian, after which sedimentation became predominantly clastic (Chapter 14). Late Paleozoic uplift of the newly formed Pangea (Chapter 2), and a northward drift of the continent into higher latitudes, are the probable cause of this major facies change (compare Figs. 14, 15, 19–23). Sedimentation was terminated by the Eurekan orogeny, in the mid-Cenozoic, during Greenland’s brief life as a separate plate. Arctic paleogeography was profoundly altered by a minor phase of sea-floor spreading in the Early Jurassic. Until then, Canada’s northern Yukon and Arctic Islands region had continued to the north into a region called “Crockerland” (Chapter 14). This land area (the precursor to Chukotka-Arctic Alaska: Fig. 22) formed a northern margin and sometime sediment source for Sverdrup basin and basins in northern Alaska. Evidence for rifting of Crockerland away from North America is found in fault-bounded basins of Late Jurassic age in the western Canadian Arctic. Sea-floor spreading eventually rotated Crockerland counterclockwise and it collided with and underthrust northern Alaska, generating the Brooks Range in the process (compare Figs. 22 and 23; Embry, 1990). This series of events generated a new extensional continental margin for Canada, where the northwest margin of the Arctic Islands is now located (Chapter 14). To the southwest, this passes into the Beaufort Mackenzie Basin (Chapter 17). The east side of this basin is comparable in structure and stratigraphy to the extensional Arctic Islands margin, whereas the western part of the basin was affected by thin-skinned tectonism between the latest Cretaceous and the late Miocene. The basin there has the character of a foreland basin adjacent to the Brooks Range fold-thrust belt. Active sedimentation continues in the Beaufort-Mackenzie Basin. It is currently the depository for the Mackenzie River, one of the major rivers draining the North American interior. During the Paleogene, the Arctic Ocean was topographically isolated several times from the global ocean and essentially became a huge freshwater lake.
Sedimentary Evolution of the Atlantic and Gulf Margins The Atlantic margin was initiated as a series of rift basins, including the well-known Newark rifts, of Triassic and Early Jurassic age (Figs. 19 and 20). Sedimentation of coarse clastics along basin margins passed, in many cases, into evaporites in basin centers (Chapter 15). Evaporite sedimentation persisted into the Jurassic throughout the central Atlantic, from Newfoundland and Nova Scotia (Argo Salt) to the Gulf of Mexico (Louann Salt). At this time, the incipient Atlantic-Gulf ocean lay between about 15–30°N latitude, and had minimal connections to the global oceanic circulation system (Fig. 20). Rifting extended into the northern Atlantic margins and the Baffin Bay-Davis Strait area in the Early Cretaceous (Fig. 22). Sedimentation there was predominantly clastic. Evaporite sedimentation did not occur in these more northerly basins, which were located in cooler climates. During most of the Jurassic and into the earliest Cretaceous, carbonate sedimentation extended along much of the Atlantic margin (Chapter 15) and into the Gulf Coast basin (Chapter 16), where such classic carbonate units as the Smackover Formation were deposited. However, as North America drifted slowly northwestward (Fig. 24), this style of sedimentation came to an end. It ended by Neocomian time on the Scotian Shelf and Georges Bank, and by Cenomanian time on the Blake Plateau, to be replaced mainly by shallow-marine clastics (Jansa, 1981). On the Bahamas platform and part of the Florida margin, carbonate sedimentation continues to the present day. Loading and displacement of the basal salt layers had profound consequences for the evolution of the Gulf Coast basin, as discussed in detail by Ewing and Galloway (Chapter 16). This basin was where modern North American petroleum geology found its feet, in the early part of the 20th century. Prospecting for evaporite diapirs using primitive gravity and seismic devices was one of the first applications of scientific methods, anywhere, to the exploration for petroleum. The Gulf Coast is the depository for several of the major river systems draining the interior of North America, including the Mississippi-Missouri and the Rio Grande. These have remained in essentially the same positions since the early Cenozoic (Fisher and McGowen, 1967; Blum and Pecha, 2014). Sedimentation has extended the continental margin of the Gulf by >300 km since the Early Cretaceous, much of it in the Paleocene-Eocene when Laramide uplifts fed sediment to southeast-flowing rivers. Sedimentary pulses may be correlated to major tectonic episodes within the continental interior, especially episodes of deformation and uplift of the Cordilleran orogen (Chapter 16). Laramide deformation commenced in the Campanian (Chapter 13), but the main phase of deformation, as indicated by coarse clastic supply into Laramide basins, extended from Maastrichtian to late Eocene, roughly contemporaneous with the Wilcox depositional episode in the Gulf Coast. Uplift and erosional truncation of Laramide structures indicates the end of this tectonic episode by late Eocene time, with subsequent deposition of the early Oligocene Bishop Conglomerate in the Uinta Basin contemporaneous with the beginning of the Frio-Vicksburg depositional episode in the Gulf Coast Basin.
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Mid-Late Cenozoic Tectonism of the Western Margin At about 40 Ma, subduction of the Panthalassa Ocean brought the sea-floor spreading ridge between the Pacific and Farallon plates against the California continental margin. Relative motions between the North American and Farallon plates were oblique, and subduction of the spreading ridge led to the development of the right-lateral San Andreas transform fault (Atwater and Molnar, 1973; Chapter 12). Changes in intraplate stress across the western part of the continent that were triggered by this event are now thought by most workers to be the main cause of the change from a contractional to an extensional regime within the US Cordillera. A major phase of crustal extension took place between the Oligocene and the Miocene, leading in places to a doubling of the width of some regions of continental crust. The resulting tectonic province, the Basin and Range, extends from southern British Columbia to northern Mexico (Oldow et al., 1989). Sedimentary basins in this province are characteristically of graben or half-graben style, bounded by steep listric faults that typically flatten out at midcrustal levels (Hamilton, 1987). Basin fills are entirely nonmarine, except those bordering the Salton Trough and the Gulf of California. Many small basins bounded by strike-slip faults are located within the San Andreas transform system (Nilsen and Sylvester, 1995; Chapter 12), including nonmarine basins onland (e.g., Crowell and Link, 1982), and offshore basins that are largely filled with turbidites and other deep-marine clastics (e.g., Howell et al., 1980). The mostly submarine California Continental Borderlands are locally highly extended continental crust marked by sharp uplifts (e.g., Catalina Island) and major deep-marine troughs (Fisher et al., 2009; Chapter 12).
LATE CENOZOIC MODIFICATIONS Continental glaciation during the late Cenozoic spread a thick blanket of moraine and outwash deposits across the northern interior, as far south as Illinois. Glaciation disrupted continental drainage patterns. A major east-flowing river system is thought to have crossed the continental interior until the late Cenozoic, transporting detritus from the northern Cordillera to the continental margin of Baffin Bay (McMillan, 1973; Duk-Rodkin and Hughes, 1994). This river system was dammed and diverted by advancing continental ice. Outflow found a course to the northwest, and this is probably the origin of the present-day Mackenzie River system. Postglacial reworking of the glacial blanket has triggered major slumping and gully erosion of the Baffin Bay margins (Chough and Hesse, 1976). Meanwhile, tidewater glaciers continue to shed vast quantities of clastic debris into the Gulf of Alaska (Eyles et al., 1991). Melting of the continental ice caps generated enormous proglacial lakes (including Bonneville, Missoula, Agassiz, Iroquois, and other super-Great Lakes), the draining of which generated major meltwater channels and carried significant volumes of sediment out to the Pacific, Arctic, and Atlantic margins. During part of the melt-back phase, the Great Lakes drained southward through the Mississippi system (Rittenour et al., 2007), and was responsible for depositing a giant submarine fan system on the floor of the Gulf of Mexico (Feeley et al., 1990; Weimer, 1990). Large areas of North America may be at the beginning of another long period of peneplanation, comparable to that which occurred toward the end of the Precambrian. The Appalachian orogen, in the east, is already exposed to its roots. In the west, some 3 km of the sediment-fill have been removed from the Alberta Basin, following postorogenic uplift (Beaumont, 1981). The mid-Cenozoic extension of the southwest Cordillera to form the Basin and Range region initiated a significant reduction in relief in that area. By contrast, although there is an absence of obvious targets for terrane collision in the Pacific Ocean, continued subduction of the Gorda and Pacific plates beneath the western margin, from northern California to Alaska, is likely to maintain active tectonic uplift in the northwest. Uplift of the Colorado Plateau is an exception to the general trend, and possible future activity of the Yellowstone hot spot may complicate this simple picture, but in the very long term, the rest of the continent seems likely to have entered another hundreds-of-millions-of-years-long phase of erosional reduction.
Acknowledgments The first edition of this chapter was reviewed by all the contributing chapter authors or senior coauthors of this book, who pointed out errors and suggested a number of useful points of clarification. The Canadian time chart (Fig. 4) was reviewed by Reg Wilson, Jim Mortenson, and Les Fyffe. Ron Blakey and Ray Ingersoll provided useful comments on the time chart for the United States (Fig. 5).
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