Ichnology and sedimentology of a tide-influenced delta, Lower Miocene Chenque Formation, Patagonia, Argentina: Trace-fossil distribution and response to environmental stresses

Ichnology and sedimentology of a tide-influenced delta, Lower Miocene Chenque Formation, Patagonia, Argentina: Trace-fossil distribution and response to environmental stresses

Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 75–86 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Pal...

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Palaeogeography, Palaeoclimatology, Palaeoecology 273 (2009) 75–86

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Ichnology and sedimentology of a tide-influenced delta, Lower Miocene Chenque Formation, Patagonia, Argentina: Trace-fossil distribution and response to environmental stresses Noelia B. Carmona a,⁎, Luis A. Buatois b, Juan José Ponce a, María Gabriela Mángano b a b

Laboratorio de Geología Andina, CONICET — Centro Austral de Investigaciones Científicas (CADIC), B. Houssay 200, C.P. 9410, Ushuaia, Tierra del Fuego, Argentina Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Canada SK S7N 5E2

a r t i c l e

i n f o

Article history: Received 2 July 2008 Received in revised form 2 December 2008 Accepted 2 December 2008 Keywords: Ichnology Tide-influenced delta Miocene Patagonia Chenque Formation

a b s t r a c t Combined sedimentologic and ichnologic analysis of the Lower Miocene Chenque Formation allows recognition of a tide-influenced deltaic succession exposed along the coast of Caleta Olivia city, Patagonia, Argentina. Two main subenvironments were identified, prodelta and delta front, stacked forming a progradational coarsening-upward succession, up to 10 m thick. The prodelta deposits are mainly characterized by heterolithic strata (lenticular and wavy bedding), with low bioturbation intensity and sporadic distribution of trace fossils. The trace-fossil assemblage is dominated by deposit-feeder structures (e.g. Planolites montanus, Protovirgularia isp., and Teichichnus rectus), constituting an impoverished expression of the Cruziana ichnofacies, with respect to their fully marine counterparts. A transition zone between the prodelta and the delta front is discontinuously distributed along this outcrop. This interval consists mainly of flaser-bedded sandstone, almost completely obliterated by equilibrium/adjustment trace fossils of large bivalves (Atrina), and subordinately, by the trace fossils Nereites missouriensis, Phycosiphon incertum, T. rectus, Thalassinoides isp., and Schaubcylindrichnus freyi. The delta-front deposits consist of sigmoidal cross-stratified sandstone with mud drapes. The trace-fossil assemblage is dominated by large Rosselia socialis and Macaronichnus segregatis in the sandier beds, whereas the mud drapes blanketing the sandstone foresets commonly contain N. missouriensis and Protovirgularia isp. Ichnologic characteristics (e.g. shallow-tiered communities, impoverished trace-fossil assemblages, dominance of deposit-feeder structures, and inhibition of suspension-feeder elements) suggest that different paleoenvironmental stresses, such as changes in salinity, water turbidity, and fluctuations in energy and in sedimentation rates, affected the infaunal communities of these tide-influenced delta settings. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Trace fossils constitute valuable tools in the recognition of stresses in marine settings. During the last years there has been an increased interest in the ichnology of deltaic environments (McIlroy, 2004; MacEachern et al., 2005; McIlroy, 2007). These analyses were principally focused on describing the trace-fossil content of river-, wave-, and storm-influenced deltaic deposits. However, the ichnology of tide-influenced deltas remains mostly understudied with the exception of McIlroy (2004, 2007). Shallow-marine environments of the Lower Miocene Chenque Formation of Patagonia, Argentina, are characterized by superbly preserved trace fossils. Recent studies (e.g. Buatois et al., 2003; Carmona, 2005; Carmona et al., 2008a) documented the high abundance and

⁎ Corresponding author. Fax: +54 2901 430644. E-mail addresses: [email protected] (N.B. Carmona), [email protected] (L.A. Buatois), [email protected] (J.J. Ponce), [email protected] (M.G. Mángano). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.12.003

diversity of biogenic structures in these Miocene successions, mainly in shallow-marine deposits developed under normal-marine salinity conditions. In addition, these studies also reflected the occurrence of impoverished trace-fossil suites in estuarine and deltaic deposits. In particular, outcrops along the coast of Caleta Olivia city have been interpreted as formed in a deltaic environment strongly influenced by tides. Therefore, the aims of this study are two-fold: (1) to describe and interpret the sedimentologic and ichnologic characteristics of these deltaic deposits, and (2) to evaluate how these trace-fossil suites reflect paleoenvironmental stresses typical of deltas. 2. Geological setting The Lower Miocene Chenque Formation crops out in the east region of Central Patagonia, Argentina (Fig. 1). This formation comprises a wide variety of shallow-marine and marginal-marine strata that were deposited during two major transgressions (the Leonense and Superpatagoniense transgressions), which occurred during the Early Miocene (Bellosi, 1987, 1995). The Chenque Formation

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Fig. 1. Map showing the distribution of the Chenque Formation, and the studied succession at Caleta Olivia city.

comprises five shallowing-upward sequences. The first two sequences (Sequence I and II) were deposited during the Leonense transgression, and comprise marine successions formed in shelf environments, as well as in shallower-water settings (e.g. estuarine and deltaic environments). The upper three sequences (III–V) were deposited during the Superpatagoniense transgression (late Early Miocene) and comprise mainly very shallow, tide-dominated deposits (Bellosi, 1995). The present study focuses on a succession belonging to the lower sequence (Bellosi, 1987), cropping out along the Atlantic coast of Caleta Olivia city, Santa Cruz province (Fig. 1). 3. Sedimentology, trace-fossil distribution, and depositional environments Two main facies associations have been identified in the studied succession: tide-influenced prodelta, and tide-influenced delta-front (Fig. 2). The tide-influenced prodelta association includes heterolithic facies, encompassing distal- (facies 1) and proximal- (facies 2) prodelta environments, as well as a transition zone between prodelta and delta front (facies 3). The tide-influenced delta-front association includes two sandstone-dominated facies, recording deposition in a distal (facies 4) to proximal (facies 5) delta-front setting.

3.1. Prodelta facies association 3.1.1. Facies 1: Lenticular-bedded mudstone and sandstone This facies consists of regular alternations (millimeter- to centimeter-thick) of very fine-grained sandstone interbedded with mudstone (Figs. 2 and 3). This heterolithic facies is dominated by massive or parallel-laminated mudstone and sandstone showing lenticular bedding. Sandstone lenses contain current-ripple crosslamination, asymmetrical ripples, and mud drapes (Fig. 3A). Although individual beds are commonly lenticular and show lateral thickness changes, bedsets are laterally persistent. Soft-sediment deformation structures and synaeresis cracks are abundant (Fig. 3A–C). Paleocurrent measurements indicate a bipolar pattern with flows directed either to the southeast and northwest (Fig. 3A). Facies 1 shows alternation of unburrowed intervals and moderately bioturbated (BI 0–2) beds. The ichnofauna is mostly composed of deposit-feeder structures, mainly Planolites montanus and Protovirgularia isp. (Fig. 3D), although some isolated specimens of Thalassinoides isp. also occur (Fig. 3A). Interpretation: This facies records deposition in a low-energy setting, with dominance of mud fallout and fluid muds, punctuated by tractive sand deposition. The presence of mud drapes in the cross-

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Fig. 2. Schematic representation of an idealized section at the studied locality, integrating the sedimentologic and ichnologic information.

laminated sandstone lenses reflects mud settling during periods of slack water, and the common bipolar paleocurrent orientation suggest tidal processes. Fluctuations in salinity are inferred by the presence of synaeresis cracks. The impoverished trace-fossil suites also suggest salinity stress, and probably the occurrence of soupgrounds. On the contrary, the more bioturbated intervals may reveal periods of relatively normal-marine salinity conditions, and slower sedimentation rates. These characteristics suggest that facies 1 was deposited as distal bottomsets of a tide-influenced distal prodelta. 3.1.2. Facies 2: Wavy-bedded sandstone and mudstone Facies 2 consists of fine-grained sandstone interbedded with mudstone, displaying wavy and, more rarely, flaser bedding (Figs. 2 and 4A–B). These heterolithic beds show a gradational contact with facies 1. As with facies 1, individual layers display common lateral variations in thickness, but bedsets show tabular geometries. Synaeresis cracks occur in thin mudstone intervals (Fig. 4A–C). The sandstone beds display asymmetrical ripples with mud drapes at the base of this

facies, being generally replaced in some levels by symmetrical ripples at the top of these beds. Soft-sediment deformation structures occur locally (Fig. 4B). Paleocurrent measurements indicate a bidirectional pattern with flows directed to the east and west. This facies records an increase in bioturbation intensity (BI 1–3), and the trace-fossil assemblage is dominated by deposit-feeding burrows (e.g. Asterosoma isp., Planolites montanus, Protovirgularia isp., and Teichichnus rectus) (Fig. 4A, C–D). Subordinate and rare elements include Nereites missouriensis, Phycosiphon incertum, Schaubcylindrichnus freyi, Skolithos isp., and Thalassinoides isp. Interpretation: Dominance of wavy bedding indicates alternation of bedload transport and suspension fallout during slack-water periods. In addition, fluid muds may have also been responsible for accumulation of fine-grained material. Tidal action is also indicated by the bipolar paleocurrent orientation, whereas wave action is only recognized locally by the presence of symmetrical ripples. The presence of synaeresis cracks indicates salinity fluctuations, although these variations are apparently less pronounced than in facies 1.

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Fig. 3. Facies 1. Distal prodelta. A and B. Cross-section view of the thinly interbedded sandy mudstone, with lenticular bedding. This facies displays abundant synaeresis cracks (sy), soft-sediment deformation structures (def), and low bioturbation intensities, with isolated Planolites montanus (Pl) and Thalassinoides isp. (Th). Note also the two opposite current directions indicated by arrows. C. Bedding-plane view showing the mudstone intervals with synaeresis cracks (sy). D. Bedding-plane view with a specimen of Protovirgularia isp. (Pr). Note the poorly defined outline of this trace fossil, suggesting that the substrate was relatively fluid.

Increase in grain size suggests a more proximal position than in facies 1, although the heterolithic nature of these deposits still indicates a position within the prodelta setting (i.e. bottomsets of a proximal prodelta). The increase in both bioturbation intensity and ichnodiveristy may reflect more marine conditions and shorter periods with salinity stress. An increase in ichnodiversity from the distal to the proximal prodelta was also observed by McIlroy (2007) in the Jurassic Lajas Formation, Neuquén Basin, Argentina. He considered that this pattern was possibly an artifact of preservation potential, although he also mentioned that this increment could be linked to a greater range of food resources in the proximal prodelta, and therefore a wider range of behavioral types (McIlroy, 2007).

laminae that form the backfill are cut through centrally by a vertical tube (Stanistreet et al., 1980). However, the general morphology of the Caleta Olivia equilibrium trace fossils have a V-shaped retrusive spreiten, without the central tube and, therefore, are closer to Scalichnus than to Siphonichnus (Hanken et al., 2001). Ichnodiversity is moderate, although bioturbation intensities are relatively high. The reduced mud content and the inferred moderate to high depositional rates, together with the ichnologic characteristics, indicate that this facies was deposited in a more proximal position than facies 1 and 2. More precisely, this facies is interpreted as formed in the transition zone between the prodelta and the delta front. 3.2. Delta-front facies association

3.1.3. Facies 3: Flaser-bedded sandstone Facies 3 consists of muddy sandstone heterolithics displaying mainly flaser bedding, and subordinately wavy bedding (Figs. 2 and 5). Sandstone beds are separated by thin mudstone units. This facies occurs transitionally from facies 2, and is laterally discontinuous. Sedimentary structures are almost completely obliterated by bioturbation, especially by equilibrium/adjustment trace fossils produced by large bivalves (Atrina) (BI 3–5) (Fig. 5B–C). Subordinate trace fossils include Nereites missouriensis (Fig. 5A, C–E), Phycosiphon incertum (Fig. 5A), Teichichnus rectus (Fig. 5D), Thalassinoides isp., and Schaubcylindrichnus freyi. Interpretation: Presence of equilibrium structures reflects moderate to high rates of deposition. In general, the assemblage shows dominance of trace fossils made by deposit-feeding organisms (except for the Atrina structures). Similar bivalve equilibrium trace fossils in other deltaic successions worldwide have been occasionally referred to the ichnogenus Siphonichnus. Siphonichnus comprises vertical backfilled structures, with concave-downward menisci, and the

3.2.1. Facies 4: Sandstone with mud drapes This facies consists mainly of decimeter-thick, fine- to mediumgrained sandstone beds with trough and planar cross-stratification (Figs. 2 and 6). Abundant mud drapes, a few millimeters thick, are preserved along the foresets (Fig. 6A, D–E). Individual beds are mostly tabular, although some small channelized geometries have been recognized. The cross-stratified sandstone shows diffuse and irregular erosive surfaces, and commonly grades upwards into an interval with asymmetrical current ripples. Symmetrical and near-symmetrical ripples are also present on top of the sandstone beds. Reactivation surfaces and mud clasts are also common (Fig. 6A–C). Bioclastic fragments (e.g. bivalves, gastropods) commonly occur at the base of this facies. Intensity of bioturbation is commonly low (BI 1–2), and the trace-fossil assemblage is dominated by Macaronichnus segregatis and large Rosselia socialis in the sandstone intervals (Fig. 6B, E–F), while Nereites missouriensis and Protovirgularia isp. reworked the mud drapes (Fig. 6G–H). Sand dollars in life position also occur in these mudstone levels.

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Fig. 4. Facies 2. Proximal prodelta. A–B. Cross-section view of the wavy-bedded facies. A. Heterolithic facies with Teichichnus rectus (Te) in cross section, and synaeresis cracks (sy). B. Heterolithic interval with synaeresis cracks (sy), and soft-sediment deformation. Isolated specimens of Planolites montanus (Pl) also occur. C. Bedding-plane view of Protovirgularia isp. (Pr), associated to synaeresis cracks (sy). Note the clearly defined outline of this specimen in contrast to the one shown in Fig. 3D. D. Bedding-plane view of Asterosoma isp. (As) with horizontal bulbs.

Interpretation: Dominance of sandstone with cross-stratification and ripples suggests bedload transport. The bimodal ripple crossstratification, as well as the presence of mud drapes, indicates variations in flow velocity and direction (Willis et al., 1999), which is typical of settings with strong tidal-action. Additionally, although not exclusive of tidal settings, the presence of mud clasts also suggests tidal influence (Dalrymple and Choi, 2007). Subordinate wave action is indicated by the presence of symmetrical and near-symmetrical ripples, the latter interpreted as produced by combined flows. This facies represents deposition in a relatively high-energetic setting, with strong tidal influence. Occurrence of biogenic structures adapted to cope with moderate to high energy, such as the ichnogenera Macaronichnus and Rosselia, supports this interpretation. The presence of deposit-feeding structures, such as Nereites missouriensis and Protovirgularia isp. in the mud drapes, indicates opportunistic colonization during intervals of low-energy. This facies is interpreted as deposited in a distal delta-front environment. 3.2.2. Facies 5: Sigmoidal cross-stratified sandstone Facies 5 consists of large-scale (meter-thick) medium-grained, cross-stratified sandstone with slightly erosive base (Figs. 2 and 7). The foresets are sigmoidal and locally covered by mud drapes. Paleocurrent measurements from cross-beds indicate a predominantly westward direction, suggesting dominance of flood-currents. Thickness of individual cross-strata is variable. Bioturbation intensities are low (0–1), and the only recognized biogenic structures are Macaronichnus segregatis and isolated Rosselia socialis specimens. Interpretation: Lower abundance of mud drapes in facies 5 most likely reflects stronger currents than those acting during deposition of facies 4. The low-diversity trace-fossil suite, with predominance of deposit-feeding structures and absence of suspension-feeding trace

fossils, reflects strong heightened water turbidity conditions (Buatois and López-Angriman, 1992; Gingras et al., 1998). Additionally, relatively strong currents and salinity stress may have also affected the infaunal suites. This facies is interpreted as formed in a proximal delta-front setting. 3.3. Depositional environment 3.3.1. Evidence of tidal influence Several lines of evidence suggest that the studied succession has been deposited under strong tidal influence. (1) Oppositely dipping ripple cross-lamination and dune cross-stratification are common throughout the whole succession, indicating flood and ebb flows (Klein, 1977; Willis, 2005). (2) There is an abundance of mud drapes mantling ripple and dune foresets, reflecting fluctuations in current velocities and mud fallout during slack water (Klein, 1977). (3) The lower interval is strongly heterolithic and displays flaser, wavy and lenticular bedding, indicating alternating traction sedimentation and suspension fallout (Klein, 1977; Willis, 2005). (4) Sigmoidal contacts in cross-bedded strata, which are typical of tidal bodies (Mutti et al.,1985; Willis, 2005), occur in the delta-front deposits. (5) Abundant mud intraclasts occur at several sandstone beds, and are regarded as common in tide-influenced settings (Dalrymple and Choi, 2007). 3.3.2. Evidence of other subordinate processes The Caleta Olivia succession is overwhelmingly dominated by tidegenerated structures. However, evidence of other subordinate processes is present, albeit locally. Synaeresis cracks are thought to be formed due to freshwater input, and reveal the participation of river processes. Similar cracks were experimentally produced by varying the concentration of salt in the fluid (Foster et al., 1955; Weiss,

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Fig. 5. Facies 3. Transition zone. A. Cross-bedding view of the interbedded muddy sandstone, with Phycosiphon incertum (Ph) and Nereites missouriensis (Ne) specimens. B–C. Heterolithic interval containing abundant equilibrium structures (eq) of Atrina. Planolites (Pl) and Nereites missouriensis (Ne) also occurs in this facies. D. Cross-section view of the heterolithic interval, specimens of Teichichnus rectus (Te) and Nereites missouriensis (Ne). E. Bedding-plane view of facies 3, with Nereites missouriensis (Ne) specimens.

1958, Donovan and Foster, 1972). Soft-sediment deformation structures suggest high rates of sedimentation in relatively steep, unstable slopes. Although large-scale deformation features are more typical of river-dominated deltas (e.g. Bhattacharya and Davies, 2004), smallerscale structures, such as those in the Caleta Olivia succession, may also occur in strongly tide-influenced deltaic settings. Evidence of wave influence is restricted to wave and combined-flow ripples. Hummocky cross-stratification is notably absent. 3.3.3. Evidence of deltaic deposition Although tidal influence can be established on the basis of physical sedimentary structures and associated facies, identifying if tidal facies were formed in a delta, estuary or open-marine setting is not straightforward because these facies may occur in more than one depositional environment. Careful documentation of stratal stacking pattern are required also (Willis and Gabel, 2001; Dalrymple et al., 2003; Dalrymple and Choi, 2007). In the studied progradational succession, a tide-influenced delta is preferred over other tidal scenarios, such as an estuary or an open seaway. The documented ichnofauna clearly points to stressed conditions typical of marginal-marine environments rather than fully marine settings. Recognition of brackish-water ichnofaunas requires knowledge of the open-marine expression of the basin to act as a template for comparison (Buatois et al., 2005). In this case, coeval deposits of the Chenque Formation formed under normal-marine conditions contain intensely bioturbated deposits having highly diverse ichnofaunas (Buatois et al., 2003; Carmona et al., 2008a). Accordingly, the stressed nature of the Caleta Olivia trace-fossil suites can be firmly established. However, stressed conditions due to dilution of normal-marine waters can be produced in various depositional settings influenced by freshwater discharge. The Caleta Olivia ichnofauna, although depauperate with respect to its fully marine counterparts in the basin, shows

some departures with respect to classic brackish-water ichnofaunas, such as those present in estuaries or interdistributary bays (e.g. MacEachern and Pemberton, 1994; MacEachern and Gingras, 2007). Some common ichnotaxa in the Caleta Olivia succession (e.g. Phycosiphon) are unusual in more stressed brackish-water settings, but may occur in prodelta environments, in which freshwater dilution alternates with periods of normal-marine salinity conditions (e.g. Buatois et al., 2008). Also, comparison with coeval estuarine deposits in the Chenque Formation shows that the Caleta Olivia ichnofauna is more diverse. In addition, the stratal stacking pattern indicates that the succession reflects deltaic progradation rather than the backstepping pattern typically displayed by transgressive estuarine deposits (Dalrymple and Choi, 2007). A progradational stacking pattern occurs also in estuarine valleys due to progradation of the bay-head delta during the subsequent highstand (Zaitlin et al., 1994). However, ichnodiversity levels in the Caleta Olivia succession are higher than those of a bay-head delta, which usually experiences strong salinity dilution (MacEachern and Pemberton, 1994; MacEachern and Gingras, 2007). 3.3.4. Paleoenvironmental reconstruction Although the studied succession is clearly progradational, it reflects a short-term regressive event within an overall transgressive trend within the basin (Leonense and Superpatagoniense Atlantic transgressions; Bellosi, 1986, 1995). Tidal dominance occurs in areas with high tidal range and low influence of waves, and is commonly associated to transgressive intervals (Willis et al., 1999; Nummedal et al., 2003; Willis, 2005; Ta et al., 2005; Porębski and Steel, 2006). Amplification of tidal range may also occur in structurally controlled basins independent of the sea-level cycle (Martinius et al., 2001; McIlroy, 2004; McIlroy et al., 2005), but tectonically generated confinement does not seem to be present in the case studied. Low

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Fig. 6. Facies 4. Distal delta front. A. Cross-stratified sandstone with mud drapes, dipping in opposite directions. Note also the presence of mud clasts and reactivation surfaces marked by white dash lines. B. Interval with mud drapes, flaser and abundant mud clasts (mc). Macaronichnus segregatis (Ma) commonly reworks sandstone beds. C. Bedding-plane view of facies 4, showing a mud-clast lag (mc) along an erosive surface. D. Dune-scale strata with mud drapes in rippled beds. E. Cross-section view of facies 4, showing an inclined specimen of Rosselia socialis (Ro) occurring in the rippled and cross-stratified beds. F. Bedding-plane view of facies 4, with abundant sand dollars (sd) and Rosselia socialis (Ro). Lens cap is 5.5 cm. G–H. Ichnofauna associated with mud drapes. G. Nereites missouriensis (Ne). Coin is 2.4 cm. H. Epichnial preservation of Protovirgularia isp. (Pr). Coin is 1.8 cm.

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Fig. 7. Facies 5. Proximal delta front. A. General view of facies 5. B. Line drawings of bedding in A. Paleocurrent measurements from cross-stratified strata indicate dominance of flood currents (black arrow).

wave influence and high tidal range were most likely present in the low-gradient platform San Jorge Basin during deposition of the Chenque Formation (Bellosi, 1986, 1995). In modern tide-influenced deltas, tidal channels and adjacent tidal flats are common (Coleman and Wright, 1975). Interestingly, in the studied succession we only recognized delta-front and prodelta deposits, with no representation of delta-top tidal facies or evidence of subaerial exposures (Fig. 8). The delta-top facies generally have lowpreservation potential and are normally eroded during transgressive ravinement (Bhattacharya and Willis, 2001), and accordingly, very few

ancient examples of these more proximal deposits have been recorded (McIlroy et al., 2005; McIlroy, 2007). Alternatively, Willis et al. (1999) explained that absence of tidal-flat facies and tidal channels may reflect the fact that the tide-influenced delta-front deposits can be generated several kilometers basinward of the subaerial deposits, particularly in low-accommodation settings (see McIlroy, 2007). In the present example, absence of lag deposits produced by ravinement or other evidence of erosion at the top of the delta front, most likely suggests that the studied deposits were formed seaward from the delta-top tidal flats and channels.

Fig. 8. A. Correlation scheme of the sections measured along the Atlantic coast in Caleta Olivia city, with facies distribution. Sections are oriented mostly parallel to the inferred paleocoast. Facies stacking pattern displays a coarsening-upward trend. B. Paleocurrent diagrams from facies 1, 2 and 4, showing bipolar currents both in the prodelta and in the delta-front facies. C. Correlation panel with less vertical exaggeration, showing the dominantly tabular geometries that characterize these deposits.

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The Caleta Olivia deposits are different from those of the Fly River delta recently described by Dalrymple et al. (2003). The Fly River delta deposits are mud-dominated and, while having a heterolithic delta front, the prodelta is essentially muddy. In contrast, the Caleta Olivia succession contains a larger proportion of sand. The Caleta Olivia succession is similar in scale and sedimentologic characteristics to that of Eocene tidal bodies from the Baronia Formation of Spain documented by Mutti et al. (1985). The Baronia tidal bodies form coarsening-upward successions that were attributed to progradation from bottomsets to bar-slope and bar-crest facies. The Baronia tidal deposits have resulted in a tidal-bar model that became popular during the nineties (e.g. Dalrymple, 1992; Johnson and Baldwin, 1996). Although Mutti et al. (1985) regarded these deposits as formed in the distal portion of an estuary or a tidedominated delta, this interpretation has been disputed by other authors. Dalrymple et al. (2003), in their study of the tide-dominated Fly River delta, noted that deltaic tidal bars generate lateral-accreted deposits instead of forward-accretion deposits. These authors suggested that the Baronia deposits are most likely compound dunes rather than tidal bars. Willis (2005) suggested that the Baronia deposits may represent tidal bars of a prograding delta or dunes in waters 20–25 m deep of a shallow shelf. Dalrymple and Choi (2007) revised the diagnostic sedimentologic features of tide-dominated environments, and concluded that the Baronia sandbodies are more likely the deposits of compound dunes or deltas. Preliminary results of a study conducted by Olariu et al. (2008) suggest that the Baronia sandbodies were formed by subtidal compound dunes within a strait or a seaway. The Baronia ichnofauna seems to be diverse and illustrates an archetypal Cruziana ichnofacies, which is consistent with an open-marine setting. In contrast, the Caleta Olivia deposits show evidence of stress conditions, as indicated by their more restricted and depauperate ichnofauna, suggesting that this tidal sandbody was not emplaced under fully marine conditions (i.e. open sea), but within a deltaic system. 4. Organism responses to environmental stresses Recently, MacEachern et al. (2005) analyzed in detail the most important controls on the distribution of trace fossils in deltaic systems. Because tide-influenced deltas are still understudied, the main paleoenvironmental stresses acting on their infaunas are poorly known (McIlroy, 2004; Brandsæter et al., 2005; MacEachern et al., 2005; McIlroy et al., 2005; McIlroy, 2007). Consequently, in the following paragraphs we discuss the role played by each environmental parameter on the development of the infaunal communities in this Miocene tide-influenced delta. 4.1. Salinity Reduced salinity can be inferred from the general low diversity of the ichnofauna, particularly in more proximal settings. The presence of echinoid sand dollars in these proximal positions, which are usually considered stenohaline organisms, seems to contradict this view. However, modern clypeasteroids (e.g. Mellita) are known from brackish habitats, with salinities as low as 20‰ (Smith, 1984). Furthermore, sand dollars have also been documented by Kuehl et al. (2005) in the foreset beds of the tide-dominated Ganges-Brahmaputra delta. Interestingly, McIlroy (2007) found biogenic structures attributed to echinoderms (e.g. Asteriacites, Scolicia) in tide-dominated deltaic deposits of the Jurassic Lajas Formation, suggesting tolerance of these organisms to lowered salinities. West and Ward (1990), and Mángano et al. (1999) also mentioned the occurrence of ophiuroid trace fossils in lowered salinity environments. In prodelta deposits, fluctuations in salinity are evidenced from the alternation between moderately-burrowed and unburrowed intervals. The presence of synaeresis cracks in these deposits also suggests fluctuations in salinity (MacEachern and

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Pemberton, 1994). However, significant size reductions in the ichnotaxa were not observed, except for a few intervals of the distal-prodelta facies (e.g. Planolites). 4.2. Sedimentation rates Sedimentation was moderate to high, and certainly played a major role controlling the distribution of trace fossils. This seems to have been particularly important in the more proximal settings, where biogenic structures show equilibrium strategies (e.g. trace fossils of the bivalve Atrina, Fig. 5B–C). High sedimentation rates were also observed by Ta et al. (2005) for the tide-dominated interval of the Holocene Mekong River Delta. These authors suggested that during the evolution of the Mekong River delta, the tide-dominated interval recorded higher accumulation rates (40 m/kyr) than in the subsequent mixed wave- and tide-dominated period (4.7–7.1 m/kyr). Additionally, the maximum accumulation rate (39 mm/yr) during the tidedominated period was recorded in the delta-front slope (Ta et al., 2005). This agrees with our observations that the highest sedimentation rates occurred in the transition zone between the delta-front and the prodelta. 4.3. Hydrodynamic energy and water turbidity Moderate energy most likely affected the more proximal settings, especially the proximal and distal delta fronts, with large Rosselia socialis and Macaronichnus segregatis as dominant elements of these suites (Fig. 6B, E–F). Interestingly, there are no elements typical of the Skolithos ichnofacies in the proximal settings, probably indicating that water turbidity precluded suspension-feeding behavior. Particularly in the delta-front settings the water turbidity is high, and this commonly reduces ichnodiversity and bioturbation intensities, as well as produces restriction of suspension- and filter-feeder organisms (Buatois and López-Angriman, 1992; Gingras et al., 1998). These characteristics (e.g. impoverishment of Skolithos ichnofacies structures, dominance of elements of the Cruziana ichnofacies) are considered typical of deltaic conditions (e.g. Moslow and Pemberton, 1988; Gingras et al., 1998; MacEachern et al., 2005). 4.4. Hyperpycnal flows These flows are produced by direct fluvial discharges, and are generally related to river floods (Mulder and Syvitski, 1995). Although hyperpycnal conditions are inferred to be common in deltaic deposits (MacEachern et al., 2005), there is no evidence of these flows in the studied sections. The seaward distance with freshwater influence directly depends on the intensity of tidalmixing and the amount of river discharge (Willis, 2005). In particular, tides can modify depositional patterns imposed by the river, by modulating flow of channel-mouth plumes, changing the rates of river- and basin-water mixing, and grain sorting within and away from distributaries, and can also rework sediments between river floods (Willis, 2005). Therefore, if there were hyperpycnal discharges in the analyzed succession, their deposits would have been probably completely reworked by the tidal action. Additionally, there is no evidence of phytodetrital content in the analyzed interval. Absence of phytodetritus seems to be reasonable with our interpretation that these deposits formed in the seaward end of the tideinfluenced delta, far from any direct influence of the river, as it is suggested by the absence of tidal channels and mud flats in the upper part of the succession. 4.5. Fluid muds McIlroy (2004), and MacEachern et al. (2005) observed that in tide-influenced environments, clay flocculation and fluid muds

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constitute two important factors that affect infaunal communities. In particular, these mud-prone settings are characterized by high depositional rates and important accumulations of flocculated fluid mud, generating soupground conditions (MacEachern et al., 2005). These soupgrouds are commonly only colonized by mobile depositfeeders, excluding large endobenthic deposit-feeder and suspension-feeder organisms. Interestingly, in the prodelta facies (facies 1 and 2) and in the mud drapes observed in the delta-front facies (facies 4), structures interpreted as the locomotion of depositfeeder bivalves (Protovirgularia isp.) are common (Figs. 3D, 4C and 6H). Furthermore, the morphologic analysis of Protovirgularia isp. reveals important differences in substrate consistency in each of these facies (Carmona et al., 2008b). For instance, those specimens of Protovirgularia isp. in muddier beds (e.g. in facies 1) show irregular and poorly defined chevrons. This type of sediment allows bivalves to penetrate easily, but offers poor anchorage for the foot (Trueman et al., 1966; Mángano et al., 1998), resulting in a highly deformed trace-fossil morphology where the substrate is too fluid. Conversely, the specimens of Protovirgularia isp. which occur in relatively coherent, sandier beds (e.g. facies 4) show sharp, closely-spaced chevrons, revealing that the tracemaker experienced some friction while advancing through this more resistant sediment. 4.6. Oxygen Oxygen conditions do not seem to have been very restricted in the studied succession, mainly in the proximal facies. Trace fossils typical of low-oxygenated settings (e.g. Chondrites) are not common. Additionally, most of the biogenic structures display normal sizes. However, the ichnofauna is impoverished and it does not show important penetration depths, particularly in the distal facies. Low diversity and shallow tiers are probably more related to deposition of fluid muds and generation of soupgrounds than to a decline in oxygen content, although generation of fluid muds may also lead to temporal dysoxic or anoxic conditions, so both factors may be interdependent (Wignall and Pickering, 1993; MacEachern et al., 2005). In any case, the redox boundary layer most likely was relatively close to the sediment surface. 5. Comparisons with other deltaic ichnofaunas There are few ichnologic studies of tide-dominated deltas that serve for comparison. We restrict this discussion to prodelta and delta-front deposits, which are the ones represented by the Caleta Olivia succession. McIlroy (2004) documented the ichnology of the Jurassic Ile Formation of Norway. This formation is interpreted as a tide-dominated delta with strong riverine influence, deposited in a microtidal setting. He noted overall lower ichnodiversities than in wave-dominated deltas and poor development of the Skolithos ichnofacies. The prodelta deposits contain a moderately diverse trace-fossil assemblage, including Palaeophycus, Phoebichnus, Planolites, Rhizocorallium, Chondrites, Phycosiphon, Teichichnus, Taenidium, Thalassinoides, and Schaubylindrichnus. The delta-front deposits show the highest ichnodiversity and intensities of bioturbation in the Ile Formation. Skolithos, Ophiomorpha, Diplocraterion, Arenicolites, Rosselia, Asterosoma, Lockeia, Teichichnus, Chondrites, Trichichnus, Planolites, Palaeophycus, Gyrochorte, and Taenidium are present in delta-front mouth bars. Collectively both ichnofaunas illustrate an archetypal Cruziana ichnofacies. Ichnodiversity is higher than in the Caleta Olivia succession. This may be due to the fact that the succession is microtidal and higher-energy large-scale bedforms, such as those that characterize settings with high tidal range, may not have been formed. Colonization of large tidal bedforms is commonly inhibited or only possible during short-term windows (Pollard et al., 1993).

McIlroy (2007) documented the ichnology of a tide-dominated succession in the Jurassic Lajas Formation of Argentina. The prodelta deposits are intensely bioturbated, and contain Teichichnus, Schaubcylindrichnus, Chondrites, Rhizocorallium, and Parahaentzchelinia, illustrating an impoverished Cruziana ichnofacies. The deltafront mouth-bar deposits show the highest ichnodiversity, including deposit-feeding/infaunal predator trace fossils (e.g. Asterosoma, Helminthorhaphe, Macaronichnus, Palaeophycus, Planolites, Rosselia, Schaubcylindrichnus, Teichichnus), suspension-feeding trace fossils (e.g. Diplocraterion, Parahaentzschelinia, Siphonichnus), and biogenic structures assigned to facultative suspension or deposit feeders (e.g. Ophiomorpha, and Thalassinoides). Delta-front ichnofaunas illustrate an archetypal Cruziana ichnofacies. McIlroy (2007) considered that the observed increase in ichnodiversity from the prodelta to the delta front may be an artifact of preservational potential, although he also noted that the combination of episodically strong currents with periods of slack-water may have provided optimal conditions for both suspension- and deposit-feeder organisms in the delta-front setting. Although the Lajas and Caleta Olivia successions seem to have been deposited in a similar setting, overall ichnodiversity is higher in the former. This may be linked to the fact that the salinity stress affected mainly the ebb-tidal channel deposits (McIlroy, 2007), and not the whole succession, as revealed by the analysis of the Caleta Olivia deposits. Also, a number of sedimentologic studies have documented the facies and stratal stacking pattern of tide-influenced deltas (e.g. Willis et al., 1999; McIlroy et al., 1999; Bhattacharya and Willis, 2001; Martinius et al., 2001; McIlroy 2004; Brandsæter et al., 2005; McIlroy et al., 2005), and their ichnologic content has been subsequently addressed by McIlroy (2004), MacEachern et al. (2005). Willis et al. (1999), and Bhattacharya and Willis (2001) discussed tide-dominated deltaic deposits of the Cretaceous Frontier Formation of Wyoming (see also review in MacEachern et al., 2005). Prodelta deposits of the Frontier Formation are unburrowed to weakly bioturbated and trace fossils are sporadically distributed. The prodelta ichnofauna consists of Planolites, Piscichnus, Teichichnus, Thalassinoides, Chondrites, “Terebellina”, escape trace fossils, and bivalve escape/adjustement structures, illustrating an extremely impoverished Cruziana ichnofacies. Delta-front deposits are unburrowed or very weakly bioturbated and ichnofossils are extremely sporadically distributed. The delta-front ichnofauna consists of Arenicolites, Ophiomorpha, “Terebellina”, Thalassinoides, Planolites, Cylindrichnus, Diplocraterion, Lockeia, Zoophycos, Macaronichnus, Palaeophycus, Piscichnus, Skolithos, escape trace fossils, and bivalve escape/adjustement structures, illustrating the Cruziana ichnofacies. Size reduction was noted and most of the bioturbated zones are associated with pauses in deposition (MacEachern et al., 2005). Martinius et al. (2001) analyzed tide-dominated deltaic deposits of the Jurassic Tilje Formation of Norway (see also review in MacEachern et al., 2005). Prodelta deposits of this unit are unburrowed or show low degrees of bioturbation. They contain Planolites, Teichichnus, Diplocraterion, Thalassinoides, Bergaueria, Siphonichnus, Palaeophycus, Skolithos, Rhizocorallium, Cylindrichnus, Chondrites, Taenidium, Phycosiphon, and escape trace fossils illustrating an archetypal Cruziana ichnofacies. Delta-front deposits are unburrowed to sparsely bioturbated and contain similar ichnogenera, with the addition of Gyrochorte. The delta-front ichnofauna represents a proximal expression of the Cruziana ichnofacies. Although overall ichnodiversity is moderate, trace fossils are sporadically distributed and commonly concentrated at pause planes. Fluid muds and freshets seem to have been the most important stress factor in the Tilje Formation (Martinius et al., 2001; MacEachern et al., 2005). Although it is premature to propose some general ichnologic model for tide-influenced deltas, several common features are emerging. First, tide-influenced delta fronts and prodeltas seem to

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be more stressed than wave-dominated ones but less stressed than river-dominated ones. Accordingly, tide-influenced delta-front and prodelta deposits tend to be more sparsely bioturbated, and ichnofaunas are less diverse than in wave-dominated deltas. Intensity of bioturbation and ichnodiversity is typically higher than in river-dominated prodeltas and delta fronts. However, high ichnodiversity and intense bioturbation have been noted in some tide-influenced deposits (e.g. McIlroy, 2004, 2007). Also, ichnodiversity levels of tide-influenced deltas seem to be higher than in tide-dominated estuaries (McIlroy, 2007). Second, bioturbation is commonly associated with bedform mantling, suggesting colonization along pause planes (e.g. MacEachern et al., 2005). Third, bivalve equilibrium trace fossils seem to be common, reflecting vertical accretion of the sea floor. Fourth, suppression of suspension-feeding elements is apparently quite common as a response to high content clay in the water column. An expanding dataset is needed in order to further evaluate how tide-influenced deltaic ichnofaunas are shaped by the relative influence of the different stress factors. 6. Conclusions The lower Miocene succession outcropping along the Atlantic coast of Caleta Olivia city was deposited in a tide-influenced delta environment, and consists of two main facies associations: deltafront and prodelta facies. The ichnologic analysis reveals that the trace-fossil suites developed in these environments show low diversities and low to moderate abundance of trace fossils, poor development of tiering structure, and sporadic distribution. In particular, the suites developed in the delta-front settings show reduced ichnodiversity, dominance of deposit- and detritus-feeding structures (mainly those considered to be tolerant to energetic conditions), and suppression of suspension-feeding elements. Dominance of structures typical of the Cruziana ichnofacies, and impoverishment of elements attributable to the Skolithos ichnofacies are considered typical of deltaic environments, and probably reflect major stressful conditions, generated not only by freshwater input, but also by tidal action (e.g. fluctuation in sedimentation rates and in current velocities, flocculated muds, and water turbidity). On the other hand, the trace-fossil suites developed in the prodelta settings display low to moderate diversity, dominance of depositfeeding structures (mainly facies-crossing forms), and sporadic distribution. These assemblages reflect an impoverished expression of the Cruziana ichnofacies. Most likely, occurrence of fluid muds (with the subsequent reduction in oxygenation), as well as salinity fluctuations, commonly precludes development of diverse communities in this tide-influenced prodelta environment. Although there is still a poor understanding of the organism responses to tidal action in deltaic settings, this study shows that tidal dynamics generate distinctive effects on the infauna. These effects relate not only to fluctuations in energy and water salinity, but also (and probably more significant), to changes in substrate consistency, water turbidity, and sedimentation rates. Acknowledgments We thank Eduardo Olivero, Jeremías González, and M. Isabel López-Cabrera for discussion in the field. Alvar Sobral is thanked for preparation of polished samples. Duncan McIlroy and an anonymous reviewer provided useful criticism. Financial support for this study was provided by a Post-Doctoral Grant from the Argentinean Research Council (CONICET), an International Association of Sedimentologists Postgraduate Grant Scheme, and PICT 840 awarded to Carmona. Additional funds were provided by the Canadian Natural Sciences and Engineering Research Council (NSERC) Discovery Grants 311727-05 and 08 and 311726-05 and 08 awarded to Mángano and Buatois, respectively.

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