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TRACE FOSSILS AND FLUVIAL ARCHITECTURE OF THE MIOCENE VINCHINA FORMATION, VINCHINA BASIN, NORTHWEST ARGENTINA

A Thesis Submitted to the College of Graduate and Postdoctoral Studies In Partial Fulfillment of the Requirements For the Degree of Master of Science In the Department of Geological Sciences University of Saskatchewan Saskatoon

GUSTAVO LUIS VALENCIA FERMIN

PERMISSION TO USE

In presenting this thesis in partial fulfilment of the requirements for a Postgraduate degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part, for scholarly purposes may be granted by the professor or professors who supervised my thesis work or, in their absence, by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood that any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis. Requests for permission to copy or to make other uses of materials in this thesis/dissertation in whole or part should be addressed to:

Head Department of Geological Sciences University of Saskatchewan 114 Science Place Saskatoon, Saskatchewan, S7N 5E2, Canada

Dean College of Graduate and Postdoctoral Studies University of Saskatchewan 116 Thorvaldson Building, 110 Science Place Saskatoon, Saskatchewan, S7N 5C9, Canada

ABSTRACT

Our understanding of the ichnology of fluvial systems has increased during the last decades due to the integration of trace fossils and sedimentary facies. However, trace-fossil distribution within the framework of fluvial architecture (geometry and three-dimensional assemblage) has been overlooked. The Miocene Vinchina Formation in northwest Argentina is host to massive outcrops of fluvial deposits. In three outcrops along the La Troya creek, deposits of anastomosing and braiding fluvial systems were studied. Eight architectural elements were identified: multistorey sandy channels, amalgamated sandy channels, heterolithic multistorey channels, channels with gravel bars, abandoned channels, muddy floodplain, crevasse splay, and crevasse channels. Three of these elements show bioturbation: crevasse splays, and anastomosing and braided abandoned channels. Vertical simple burrows (Skolithos isp.), freshwater crab burrows (Capayanichnus vinchinensis), and simple horizontal burrows (Palaeophycus tubularis), are the most common trace fossils in the Vinchina Formation. Other elements are Taenidium barretti, Tracheria troyana, Macrauchenichnus isp., and tetradactyl webbed footprints. The ichnofauna identified in the Vinchina Formation illustrates the Scoyenia ichnofacies. In addition, five ichnofabrics are characterized. The position of the water table, substrate consistency, flow energy, and time between depositional events under arid to semi-arid climate conditions were the main parameters controlling bioturbation. Based on detailed observation of the cross-cutting relationship between ichnotaxa, the ichnofabric distribution and the preservation features of the trace-fossils studied, a colonization sequence for each of the subenvironments of the Vinchina Formation is proposed in this study.

ACKNOWLEDGEMENTS

First of all, I would like to thank my advisors Luis and Gabriela for the opportunity of being part of your exceptional research group, and for your patience and outstanding guidance throughout this journey. Both of you are incredible and dedicated professionals and more importantly magnificent humans beings. I will be eternally thankful for all the teachings and time that you dedicated to me during my time at the University of Saskatchewan. I would like to thank the Department of Geological Sciences of the University of Saskatchewan for the scholarship granted in 2019 and for the opportunity if being Teacher Assistant during several terms during my journey in the university. I would like to thank, Veronica Krapovickas, and Martin Farina for their help and guidance during my fieldwork in Argentina and all the subsequence discussions during the project. To my beloved wife Fadhia, my brothers Luis and Fernando and my mom and dad. Without your support, this degree would not be possible.

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TABLE OF CONTENTS PERMISSION TO USE ...... i ABSTRACT ...... ii ACKNOWLEDGEMENTS ...... iii TABLE OF CONTENTS...... iv LIST OF TABLES ...... vii LIST OF FIGURES ...... viii 1. INTRODUCTION ...... 1 1.1 Hypothesis ...... 2 1.2 Research Objectives ...... 2 1.2.1 General Objective ...... 2 1.2.2 Specific Objectives ...... 2 2. METHODOLOGY ...... 3 3. PREVIOUS WORK ...... 4 3.1. Continental Ichnology ...... 4 3.1.1 Ichnology in Fluvial Deposits ...... 4 3.2 Vinchina Formation ...... 6 3.2.1 Regional Tectonics ...... 7 3.2.2 Stratigraphic Framework ...... 8 3.2.3 Fluvial Architecture ...... 9 4. RESULTS ...... 11 4.1 Lithofacies of the Vinchina Formation: ...... 11 4.1.1 Lithofacies Gi ...... 11 4.1.2 Lithofacies SGp ...... 12 4.1.3 Lithofacies SGm ...... 15 4.1.4 Lithofacies Sp ...... 16 4.1.5 Lithofacies St ...... 16 4.1.6 Lithofacies Sh ...... 18 4.1.7 Lithofacies Sm ...... 20 4.1.8 Lithofacies Sr ...... 21

4.1.9 Lithofacies Fm ...... 23 4.1.10 Lithofacies Fl ...... 23 4.2 Fluvial Architectural Elements ...... 24 4.2.1 Multistorey Sandy Channels (CHsm) ...... 24 4.2.2 Channels with Gravel Bars (CHgb) ...... 25 4.2.3 Abandoned Channels (CHa) ...... 26 4.2.4 Heterolithic Multistorey Channels (CHhm)...... 27 4.2.5 Amalgamated sandy channels (CHsa) ...... 27 4.2.6 Muddy Floodplain (Of) ...... 28 4.2.7 Crevasse Splay (Ocs) ...... 28 4.2.8 Crevasse Channels (CHosc) ...... 30 4.3 Trace fossils from the Vinchina Formation...... 30 4.3.1 Invertebrate Trace Fossils ...... 31 4.3.2 Vertebrate Trace Fossils ...... 38 4.4 Ichnofacies and trace-fossil suites in the Vinchina Formation ...... 42 4.4.1 Ichnology of Crevasse Splay Deposits ...... 42 4.4.2 Ichnology of Abandoned Channel Deposits in Anastomosing Systems ...... 44 4.4.3 Ichnology of Abandoned Channel Deposits in Braided Systems ...... 45 4.5 Ichnofabrics from the Vinchina Formation ...... 47 4.5.1 Capayanichnus vinchinensis Ichnofabric 1 ...... 47 4.5.2 Capayanichnus vinchinensis Ichnofabric 2 ...... 48 4.5.3 Skolithos isp.-Palaeophycus tubularis ichnofabric 1 ...... 49 4.5.4 Skolithos isp.-Palaeophycus tubularis ichnofabric 2 ...... 50 4.5.5 Skolithos isp.-Palaeophycus tubularis ichnofabric 3 ...... 52 4.6 Taphonomic Pathways ...... 53 4.6.1 Taphomomic Pathway in Crevasse Splay Deposits ...... 53 4.6.2 Taphomomic Pathway in Abandoned Braided Channel Deposits ...... 54 4.6.3 Taphomomic Pathway in Abandoned Anastomosing Channels Deposits ...... 56 4.7 Fluvial Styles and Depositional Model of the Vinchina Formation...... 57 4.7.1 Anastomosing fluvial style ...... 57 4.7.2 Braided fluvial style ...... 58

5. DISCUSSION ...... 60 6. CONCLUSIONS ...... 62 7. REFERENCES ...... 63

LIST OF TABLES

Table 4.1. Hierarchy model of bounding surfaces used in this work. ………………………………………………..24

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LIST OF FIGURES Figure 3.1. Map of the study area pointing out La Troya Creek and the three outcrops studied in this work. Modified from Limarino et al. (2001)………………………………………………………………………………………………………7

Figure 3.2. Transverse (W-E) cross-section of the Andes at the latitude of the Vinchina basin showing the main morphostructural units by Marenssi et al. (2015)…………………………………………………………………………..8

Figure 4.2. Examples of lithofacies Gi in different locations showing: (A) Planar view of a thin layer of Gi at the base of a channel in LOC-2 outcrop; (B) Cross-section view of sandy channels with lenticular gravel bars of Gi in outcrop LOC-3…………………………………………………………………………………………………………………….12

Figure 4.3 Example of lithofacies SGp showing an erosive contact and planar cross-stratification at the base of a heterolithic sandy channel in outcrop LOC-2………………..……………………………………………………...... 13

Figure 4.4. Stratigraphic section of outcrop LOC-1…………………………………………………………………………………14

Figure 4.5. Examples of the lithofacies SGm showing: (A) different sizes and shapes of mudstone pebbles and cobbles and the erosive contact with the overlying bed in outcrop LOC-2. (B) Common intercalation between SGm lithofacies and mudstone in outcrop LOC-1. (C) Presence of large rounded and tabular intraclasts in one of the beds with SGm lithofacies in outcrop LOC-3……………………………………………………..15

Figure 4.6. Examples of lithofacies St in outcrop LOC-1 (A) and LOC-3 (B)……………………..……………………….16

Figure 4.7. Stratigraphic section of outcrop LOC-2………………………………………………………………………………….17

Figure 4.8. Examples of lithofacies Sh. (A) Panoramic view of the beds in outcrop LOC-3. (B) Thick sandstone beds of lithofacies Sh in outcrop LOC-2…………….…………………………………………………………………..18

Figure 4.9. Stratigraphic section of outcrop LOC-3………………………………………………………………………………….19

Figure 4.10. Cross-section view of a thick amalgamated bed of Sm sandstone (right) underlying an eroded bed of Fl mudstone in outcrop LOC-1…………………………………………………………………………………………………….20

Figure 4.11 Examples of lithofacies Sr showing: (A) View of a rippled sandstone representing a clear example of 2D dunes migration in outcrop LOC-3. (B) 2D dunes on top of a sandstone layer in an amalgamated bed of Sr lithofacies in outcrop LOC-2……………………………………………………………………………..21

Figure 4.12. Stratigraphic location of the sections studied in the Vinchina Formation within the general stratigraphy of this unit showing the paleocurrent general trend measured in Sp and St sandstone in each section (modified from Limarino et al., 2001)…………………………………………………………………………………………22

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Figure 4.13. (A) Example of the lithofacies Fm consisting of thick beds of mudstone in the outcrop LOC-1. (B) Example of the lithofacies Fl in a thin layer of mudstone intercalated with sandstones in the outcrop LOC-2…………………………………………………………………………………………………………………………………………………….23

Figure 4.14. (A) Multistorey sandy channels in the outcrop LOC-1. (B) Channels with gravel bars in the outcrop LOC-1………………………………………………………………………………………………………………………………………..25

Figure 4.15. Photomosaic of a section of outcrop LOC-1. A, B) Original photomosaic and interpretation of the lithofacies and the two architectural elements identified: multistorey sandy channels and crevasse splay deposit. In this photomosaic could be appreciated how the sandstone beds of the crevasse splay are pinching out to the East creating the lenticular shape of the element……………………………………………………26

Figure 4.16. (A) Crevasse splay deposits in outcrop LOC-2. (B) Amalgamated sandy channels in outcrop LOC-3…………………………………………………………………………………………………………………………………………………….27

Figure 4.17. Photomosaic of a section of outcrop LOC-3. A, B) Original photomosaic and interpretation of the lithofacies and the three architectural elements identified: abandoned channels, amalgamated sandy channels and multistorey sandy channels………………………………………………………………………………………………28

Figure 4.18. Photomosaic of a section of outcrop LOC-2. A, B) Original photomosaic and interpretation of the lithofacies and the four architectural elements identified: crevasse channels, abandoned channels, crevasse splay and muddy floodplain……………………………………………………………………………………………………..29

Figure 4.19. Sandy amalgamated channels with an erosive base cross-cutting a muddy floodplain deposit in outcrop LOC-2…………………………………………………………………………………………………………………………………….30

Figure 4.20. (A) Photograph of the top of a Sp sandstone bioturbated at the top by Capayanichnus vinchinensis in outcrop LOC-1. (B) Schematic line drawing of the bioturbated surface where deformation rims surrounding some of the burrows can be distinguished………………………………………………………………….32

Figure 4.22. Examples of Palaeophycus tubularis in (A) LOC-1 and (B) LOC-2………………………………………….34

Figure 4.23. (A) Examples of Skolithos isp. at the base of a sandstone bed, LOC-2. (B) Skolithos isp. at the base of a mud-cracked sandstone bed, LOC-3………………………………………………………………………………………..36

Figure 4.24. (A) Vertical Taenidium barretti showing the backfilling meniscate in a sandstone of lithofacies Sh of outcrop LOC-2. (B) Two horizontal specimens of Taenidium barretti on top of a sandstone in outcrop LOC-3. (C) Specimen of Taenidium barretti on top of a sandstone in outcrop LOC-3………………………………37

Figure 4.25. (A) The footprint Macrauchenichnus isp 1 in outcrop LOC-2, pointing out the three digits and its names. (B) Specimen of Macrauchenichnus isp 1 identified in outcrop LOC-2……………………………………38

Figure 4.26. (A) Top of a fine-grained sandstone (Sr) bioturbated by Large tetradactyl webbed footprints in outcrop LOC-3. (B) Close-up of a large tetradactyl webbed footprint preserved on top of a fine-grained sandstone of lithofacies Sr. (C) Large tetradactyl webbed footprint in outcrop LOC-3……………………………..39

Figure 4.28. (A) Close-up of one of Venatoripes riojanus, outcrop LOC-3. (B) The bioturbated surface with the trackway of footprints, outcrop LOC-3…………………………………………………………………………………………….42

Figure 4.29. (A) Macrauchenichnus isp. reworked by Skolithos isp. in outcrop LOC-2. (B) Tracheria troyana cross-cut by a mud crack in outcrop LOC-2……………………………………………………………………………………………..43

Figure 4.30. Schematic illustration of the trace fossils present in crevasse splay deposits of the Vinchina Formation………………………………………………………………………………………………………………………………………………44

Figure 4.31. Schematic illustration of the trace fossils present in anastomosing abandoned channel deposits of the Vinchina Formation……………..…………………………………………………………………………………………45

Figure 4.32. Schematic illustration of the trace fossils present in braided abandoned channel deposits of the Vinchina Formation………………………………………………………………………………………………………………………….46

Figure 4.33. Schematic illustration of Capayanichnus vinchinensis Ichnofabric 1. Ca: Capayanichnus vinchinensis, Sk: Skolithos isp., Pa: Palaeophycus tubularis……………………………………………………………………..47

Figure 4.35. Block diagram showing ichnofabric distribution in a crevasse splay deposits of the Vinchina Formation………………………………….………………………………………………………………………………………………………….49

Figure 4.36. Schematic illustration of Skolithos isp.-Palaeophycus tubularis ichnofabric 1. Sk: Skolithos isp., Pa: Palaeophycus tubularis…………………………………………………………………………………………………………………….50

Figure 4.37. Schematic illustration of Skolithos isp.-Palaeophycus tubularis ichnofabric 2. Sk: Skolithos isp., Pa: Palaeophycus tubularis, Ta: Taenidum barretti…………………………………………………………………………………50

Figure 4.38. Block diagram showing ichnofabric distribution in anastomosing abandoned channel deposits of the Vinchina Formation………………………………………………………………………………………………………………………51

Figure 4.40. Block diagram showing ichnofabric distribution in braided abandoned channel deposits of the Vinchina Formation……………………………………………………………………………………………………………………………….53

Figure 4.41. Schematic illustration of the taphonomic pathway in crevasse splay deposits of the Vinchina Formation………………………………………………………………………………………………………………………………………………54

Figure 4.42. Schematic illustration of the taphonomic pathway in abandoned braided channel deposits of the Vinchina Formation………………………………………………………………………………………………………………………….55

Figure 4.43. Schematic illustration of the taphonomic pathway in abandoned anastomosing channel deposits of the Vinchina Formation……………………………………………………………………………………………………….56

Figure 4.44. Schematic illustration of the dominant architectural elements in the anastomosing deposits of the Vinchina Formation and their composition. (A) Multistorey sandy channels. (B) Crevasse splays and crevasse channels. (C) Abandoned channel and muddy floodplain………………………………………………………….58

Figure 4.45. Schematic illustration of the dominant architectural elements in the braided deposits of the Vinchina Formation and their composition. (A) Amalgamated sandy channels. (B) Multistorey sandy channels. (C) Heterolithic sandy channels………………………………………………………………………………………………59

CHAPTER 1

1. INTRODUCTION Fluvial deposits represent the preserved record of one of the major continental environments. They accumulate in large and small intermontane valleys, in the broad valleys of trunk rivers, in the wedges of alluvial fans flanking areas of uplift, in the outwash plains fronting melting glaciers, and in coastal plains (Miall, 2006).

Fluvial architecture considers how erosional and depositional processes in channels, overbanks, and valley floors create different kinds of sedimentary bodies and fluvial forms (Allen, 1978; Leeder, 1978; Allen, 1983). Each of these sedimentary bodies is named architectural elements, which are the product of the cumulative effect of sedimentation over periods of tens to thousands of years (Jackson, 1975). They include major channels, bars, flood sheets, and sediment gravity-flow lobes. Miall (1996) defined architectural elements as a component of a depositional system equivalent in size to, or smaller than a channel fill, and larger than an individual lithofacies unit, characterized by a distinctive facies assemblage, internal geometry, external form, and (in some instances) vertical profile.

Architectural element analysis can provide paleoenvironmental information in terms of paleogeomorphology. This information is crucial for facies analysis, fluvial style definition, and paleoenvironmental interpretations. In this regard, the addition of a powerful tool like ichnological analysis could help to develop a more accurate interpretation and explore the interplay between trace-fossil content and distribution with architectural elements, a relationship that has been barely studied in the literature.

During the last decades, our understanding of the ichnology of fluvial systems has increased due to the integration of trace fossils and sedimentary facies (Melchor et al., 2012). However, trace- fossil distribution within the framework of fluvial architecture (geometry and three-dimensional assemblage) has been overlooked. Therefore, some of the main concepts involved in ichnofabric analysis, such as the colonization surface, remain underexplored in the context of fluvial sedimentology and ichnology.

This study aims to characterize selected outcrops of the Miocene Vinchina Formation, a fluvial unit accumulated in a foreland basin related to the Andean orogeny in northwest Argentina. Trace fossils in fluvial deposits formed in braided and/or anastomosing systems are usually scarce and of low ichnodiversity due to the low preservational potential in these environments. As a consequence, bioturbation structures in these deposits are mostly composed of vertical simple dwelling traces, trackways, and simple grazing traces. A detailed sedimentological, architectural, and ichnological study of these fluvial deposits will be performed and, to accomplish that, a series

of research objectives have been proposed. The results of this study will help to define a more accurate depositional model for the sections studied in the Vinchina Formation, and will assess their variability in terms of fluvial styles and colonization by the associated fauna. 1.1 Hypothesis Preliminary data and current literature allow us to generate the following hypotheses: ▪ Fluvial style during deposition of Vinchina Formation changed trough time. ▪ Bioturbation in these fluvial successions was primarily controlled by the duration of colonization window and the position of the water table.

1.2 Research Objectives 1.2.1 General Objective To define the depositional model for the sections studied in the Vinchina Formation through a detailed sedimentologic and ichnologic analysis, and to explore the main concepts of the ichnofacies model and ichnofabric analysis in the fluvial context.

1.2.2 Specific Objectives ▪ To define, describe, and interpret the fluvial facies in each outcrop ▪ To define architectural elements ▪ To identify trace fossils and trace-fossil assemblages ▪ To describe the ichnofacies and ichnofabric distribution ▪ To define the taphonomic pathways of the identified trace-fossil assemblages ▪ To define a depositional model for the sections studied in the Vinchina Formation

CHAPTER 2

2. METHODOLOGY This section includes a brief description of the methods proposed to accomplish the specific objectives of this project:

▪ To define, describe, and interpret the fluvial facies in each outcrop: facies were defined based on lithology, physical sedimentary-structures, contacts, geometry and dimension of the beds, through a detailed analysis of outcrops and following Miall’s code of lithofacies (1978,1983). ▪ To define architectural elements: architectural elements were defined based on fluvial facies analysis and geometrical disposition of bounding surfaces hierarchically classified in the rock bodies; their distribution will be shown in photomosaics for each outcrop. ▪ To identify trace fossils and trace-fossil assemblages: trace-fossil content of sedimentary facies were identified and degree of bioturbation will be semi-quantitatively estimated by using the bioturbation index (BI) proposed by Taylor and Goldring (1993). ▪ To describe ichnofacies and ichnofabric: based on ichnological data, an ichnofacies and ichnofabric model was proposed for the Vinchina Formation. ▪ To define the taphonomic pathways of the identified trace fossil assemblages: based on the distribution and cross-cutting relationships of the trace fossils and their associated colonization surfaces, a taphonomic pathway for each assemblage was proposed. ▪ To define a depositional model of the sections studied in the Vinchina Formation: through the integration of stratigraphy, ichnological information and the paleoenvironmental interpretation, a depositional model was proposed for each section studied in the Vinchina Formation.

CHAPTER 3

3. PREVIOUS WORK

3.1. Continental Ichnology The continental biota is related to sediment through feeding, dwelling, locomotion, reproduction, and searching behaviour, recorded as tracks, trails, burrows, and nests of animals, and plant root traces. Such vestiges are preserved in the geologic record as trace fossils. The lateral and vertical distribution of modern trace-making organisms within an environment is controlled by different parameters, including sediment characteristics, soil moisture, and water- table levels, among other factors. Trace fossils in the geologic record can be used to interpret the palaeoenvironmental, palaeoecologic, palaeohydrologic, and palaeoclimatic settings because a well-defined relationship tends to exist between continental organisms, climate, hydrology, and environment (Hasiotis, 2007; Genise, 2017). The study of continental ichnofaunas has shown an explosive development during the last decade. Integration of ichnologic, sedimentologic, and paleobiologic information is very useful in facies analysis and sequence stratigraphy of continental successions (Buatois & Mángano, 2007). In the specific case of fluvial successions which provide the context of this study, trace fossils typically are not particularly abundant (Buatois & Mángano, 2004, 2007). However, a large number of ichnologic studies in continental successions have been performed in order to expand the ichnofacies model into continental environments. Starting with the recognition of the Scoyenia ichnofacies in continental successions by Seilacher (1967) followed by refinements by several authors (Frey et al., 1984; Buatois & Mángano, 1995, 2007; Genise et al., 2000, 2010,2016; Hunt & Lucas, 2007; Ekdale et al., 2007; Minter & Braddy, 2009: Krapovickas et al.,2016), seven ichnofacies have been recognized (Scoyenia, Mermia, Coprinisphaera, Termitichnus, Celliforma, Octopodichnus- Entradichnus, and Camborygma). Another main concept in trace-fossil analysis, the ichnofabric approach, still remains overlooked in the literature of continental successions. Perhaps, this is due to the commonly low bioturbation index in fluvial deposits. 3.1.1 Ichnology in Fluvial Deposits During the last two decades, several studies have shown the importance of the recognition of trace fossils in fluvial deposits and its implication in paleoenvironmental interpretation as a reflection of different environmental factors. Buatois & Mángano (2011) proposed different ichnoassemblages for each subenvironment from the two main fluvial styles: meandering system and braided system. Four subenvironments with their respective ichnoassemblages were characterized. For active channels, a mono-ichnospecific assemblage of Skolithos was proposed. For abandoned channels, a dominance of shallow-tier trace-fossil assemblage with Taenidium, Beaconites, Diplichnites, Skolithos, Cylindricum and Palaeophycus as the most common ichnotaxa

was put forward. For sandbars, an ichnoassemblage commonly composed by Diplichnites, Helminthopsis, Taenidium and Skolithos with low bioturbation, was proposed. For crevasse splays, an ichnoassemblage of shallow-tier traces with Diplichnites, Taenidium, Rusophycus and Planolites as the common ichnotaxa, showing low bioturbation intensities, was proposed. Finally, the overbank subenvironment was divided into two types, overfilled overbank and desiccate overbank. The overfilled overbank assemblage could be regarded as a depauperate Mermia Ichnofacies, and the desiccate overbank as the archetypal Scoyenia Ichnofacies. Melchor et al. (2012) provided a detailed review of the ichnology of fluvial systems. This study highlighted the utility of the Scoyenia Ichnofacies (typified by meniscate burrows, arthropod trackways, and bilobed burrows/trails) for distinguishing emergent channel-bar tops or the margins of floodbasin ponds. The Mermia Ichnofacies (containing simple horizontal trails and burrows and vertebrate swim traces), and probably also the Characichnos Ichnofacies (tetrapod swim traces) are typical of floodbasin ponds. The Skolithos Ichnofacies (characterized by simple vertical burrows and J, or U-shaped vertical burrows) is related with the top of fluvial bars, although the subaerial or subaqueous nature of the trace fossils must be confirmed in each case. Paleosol ichnofabrics exhibit a greater complexity and variety of trace fossils than in other fluvial subaquatic subenvironments. These authors also reviewed the trace-fossil assemblages of various fluvial subenvironments and different river types. Channel-belt and floodplain facies show a dominance of very simple trace-fossil morphologies, essentially of invertebrate origin. The abundance and diversity of trace fossils are much higher in overbank facies than in channel- belt deposits. Trace-fossil assemblages from anastomosing, braided, meandering, and ephemeral rivers display some differences in composition and abundance, especially when considering floodplain ichnofaunas. Anastomosing rivers appear to preserve trace fossils preferentially in channel facies, where bird tracks and large dinosaur (sauropod) tracks are more common. The trace fossils from ephemeral and braided deposits can be distinguished from those of meandering rivers by the larger proportion of structures preserved on bedding planes. Meandering rivers contain a larger proportion of trace fossils related to pedogenized floodplain settings. The following studies are examples of the application of systematic ichnology and the ichnofacies model in fluvial environments in western Argentina, including some performed in the Vinchina Basin. Buatois & Mángano (2002) studied floodplain deposits in the Carboniferous Tupe Formation of western Argentina. Based on observations in this formation and a review of information from other fluvial units elsewhere, this study identified two recurrent trace-fossil assemblages in floodplain deposits. The first assemblage is characterized by low to rarely moderate diversity of invertebrate trace fossils, moderate to a high diversity of vertebrate structures, and common presence of meniscate backfilled structures, bilobate trace fossils with scratch marks, arthropod trackways, and tetrapod trackways. It is regarded as an example of the Scoyenia Ichnofacies in desiccated floodplains. The second assemblage is characterized by grazing trails, locomotion trails and dwelling burrows, low to rarely moderate ichnodiversity, simple trace

fossils, and superficial to very shallow structures. It is regarded as an example of the Mermia Ichnofacies. Krapovickas et al. (2009) analyzed the ichnology of Miocene fluvial deposits of the Toro Negro Formation, (the formation that is overlaying the Vinchina Formation) in the Vinchina Basin in western Argentina from a paleobiologic and paleoecologic perspective. In this study, several trace fossils were recognized in different architectural elements of this fluvial deposit. In the case of exposed sandbars, tetrapod footprints, Palaeophycus and Helminthopsis were recognized. Crevasse splay deposits preserve abundant meniscate trace fossils and dwelling tubes (e.g. Taenidium, Scoyenia, Palaeophycus). Also, the vertebrate ichnofauna was studied; this group of trace fossils includes avian (Fuscinapeda sirin, incumbent footprint, slender asinodactyl footprint) and mammalian footprints (Macrauchenichnus rector, cf. Venatoripes riojanus, small heteropod footprint, kidney-like footprints, and oval impressions).

Melchor et al. (2010) described large striated burrows from fluvial deposits of the Vinchina Formation, erecting a new ichnogenus and ichnoespecies, Capayanichnus vinchinensis. Also, a paleoenvironmental interpretation was proposed. A setting with a seasonal and semiarid climate, with two main types of rivers: single-channel rivers with frequent channel avulsion and overbank flow, and multichannelized rivers with poorly defined margins, was inferred. A freshwater crab was suggested as the producer of the burrows.

Krapovickas & Nasif (2011) documented large tetradactyl rodent-like footprints from fluvial deposits of the Vinchina Formation, erecting a new ichnoespecies, Tracheria troyana. The morphological analysis of the footprints suggests that they were produced most likely by dinomyid caviomorph rodents or a close relative. Footprints and trackways provide valuable information about behaviour, locomotion, and the spatial and temporal distribution of the producer.

3.2 Vinchina Formation The Vinchina Formation is one of the thickest Cenozoic units related to the Andean orogeny in Argentina totalling more than 5100 m of thickness (Ciccioli et al., 2014). These sediments are part of the filling of a broken Andean foreland trough referred to as the Vinchina Basin that was uplifted during Pliocene times (Ramos, 1999; Limarino et al., 2001; Ciccioli et al., 2008). This unit was mostly deposited in fluvial environments, but short episodes of eolian and lacustrine sedimentation also occurred (Limarino et al., 2001). Along the Sierra de Los Colorados, the Vinchina Formation is a thick red-bed succession composed of sandstone, mudstone, and conglomerate that accumulated under arid to semiarid conditions (Tripaldi et al. 2001; Ciccioli et al. 2013b). Ramos (1970) reported a steady northeastward thinning of the unit and recognized two members based on the composition of volcanic lithic fragments. The contact between the

Lower and Upper members at La Troya creek is marked by progressive unconformities draped by several conglomerate beds (Marenssi et al. 2000). 3.2.1 Regional Tectonics The Andes Mountains in northwestern Argentina include the Precordillera, a part of the Andean thin-skinned fold and thrust belt and the Sierras Pampeanas, which is an area of faulted basement blocks exposed in several ranges directly to the east (Marenssi et al., 2015). The present-day Bermejo Basin is a typical foreland basin that extends along the eastern margin of the Precordillera, whereas in the Cenozoic nonmarine sedimentation also took place in isolated intermontane or broken foreland basins in the Sierras Pampeanas (Jordan and Allmendinger 1986; Jordan 1995; Jordan et al. 2001; Marenssi et al., 2015).

Figure 3.1. Map of the study area pointing out La Troya Creek and the three outcrops studied in this work. Modified from Limarino et al. (2001) The Bermejo Basin is a world-class example of a foreland basin that has evolved by a mixture of thin- and thick-skinned deformation (Beer and Jordan, 1989; Jordan et al., 1993, 2001; Milana et al., 2003; Amidon et al., 2016). The classic model of Jordan et al. (2001) involves a transition from an initially “simple foreland” that is subsequently divided into sub-basins by the rise of mountain ranges along deep-seated basement thrusts to create a “broken foreland”. In this model, the simple foreland stage from (~20-8 Ma) is primarily a flexural response to shortening in the Frontal Cordillera and Precordillera to the west. By ~6.5 Ma, uplift of the Sierras Pampeanas fragments the Bermejo foreland into more localized depocenters (Jordan et al., 2001). One of those sub- basins present within the Bermejo Basin is the Vinchina Basin. This basin represents one of the Cenozoic depocenters of one of the major broken foreland basin related to the Andean orogeny, knows as the northern Bermejo Foreland Basin (Ciccioli et al., 2010). As additional studies have

focused on these localized depocenters, it has become clear that some of the basement uplifts, particularly those in the north, such as the Vinchina Basin, are older and record a complex interplay of thin- and thick-skinned deformation throughout the Miocene (e.g. Dávila and Astini, 2007; Ciccioli et al., 2011, 2013a; Marenssi et al., 2015). The Vinchina Basin is a classic example of a localized depocenter because it is bounded to the north, south, and east by basement-cored uplifts (Amidon et al., 2016). The Toro Negro block to the north and the Umango-Espinal arch to the south are both parts of the Western Sierras Pampeanas, whereas the Sierra de Famatina to the east and Sierra de Narváez to the northeast are part of the Famatina System. To the west, the Vinchina depocenter is bound by the Frontal Cordillera and the northernmost extent of the Precordillera (Amidon et al., 2016).

Figure 3.2. Transverse (W-E) cross-section of the Andes at the latitude of the Vinchina basin showing the main morphostructural units by Marenssi et al. (2015). 3.2.2 Stratigraphic Framework Four major upper Paleogene and Neogene formations are present in the Vinchina depocenter: the Vallecito, Vinchina, Toro Negro, and El Corral formations (Amidon et al. 2016). The Vinchina Formation is 5100 m thick in its type section where the base is covered by alluvium (Tripaldi et al., 2001; Limarino et al., 2001). This formation rests on an erosive surface over eolian sandstone belonging to the late Oligocene-early Miocene Vallecito Formation (Tripaldi and Limarino, 2005; Ciccioli et al., 2010, 2011, 2013b). The Vinchina Formation crops out along the Sierra de Los Colorados (Western Sierras Pampeanas) and the eastern margin of the Precordillera in La Rioja province and records the early distal foreland fill of the northern Bermejo Basin (Ciccioli et al., 2014). The Vinchina Formation was divided into two members by Ramos (1970). The Lower Member is characterized by sandy shale and arkosic sandstone with highly altered volcanic rock fragments. The Upper Member is composed of muddy sandstone with intercalations of tuffaceous sandstone and conglomerate containing both fresh and altered volcanic clasts (Ramos, 1970). The fluvial system of the Vinchina Formation evolved within an eastward- migrating broken foreland basin system. The advance of the fold-and-thrust belt (Precordillera) from the west and simultaneous basement uplift to the north (Sierra del Toro Negro) controlled

subsidence as well as drainage patterns. As a consequence, the Lower Member of the Vinchina Formation records the sedimentation in a more distal position than the Upper Member relative to the fold-and-thrust belt, and at the same time, both of them record a proximal (north) to distal (south) trend with respect to the elevated basement-cored block (Marenssi, 2015).

Figure 3.3. Map showing regional distribution of the Vinchina Formation and its members along the Sierra de Los Colorados, and the seven third-order sequences proposed by Marenssi et al. (2015). 3.2.3 Fluvial Architecture Limarino et al. (2001) studied the architectural elements of the Vinchina Formation following Miall’s code (1983, 1988). In this study, the architectural elements were divided into two major categories: channel-belt (CH) and overbank (O), according to Miall (1978), Allen (1983), and Sánchez-Moya et al., (1996). Three depositional sequences were identified. The older one (S1) comprises anastomosing fluvial deposit with thick floodplain successions and thin intercalation of eolian beds. Sequence S2 comprises a gravelly and sandy braided river deposit. Finally, sequence S3 consists of deposits formed in ephemeral braided alluvial plains and meandering belts. Tripaldi et al. (2001) identified eighteen lithofacies (sensu Miall 1985, 1996) and ten architectural elements, grouped into two categories: channels and floodplain. Also, seven different facies associations in the Vinchina Formation were proposed, based on the distribution and arrangement of the architectural elements previously identified. Those facies associations are the following: I, Anastomosing fluvial system with sand-dominated floodplain; II, Intramountain eolian system; III, Anastomosing fluvial system with mud-dominated floodplain; IV, Braided fluvial system; V, Meandering fluvial system; VI, Ephemeral braided system; and VII, Lacustrine

system. Finally, a map showing the distribution of those facies association in La Troya Creek area was produced. Marenssi et al. (2015) identified the most common architectural elements of the Vinchina Formation, namely composite (multistorey) channel fills (element CHm), isolated (single) channel fills (CHs), gravel bars (element GB), downstream accretion macroforms (DA), lateral- accretion macroforms (LA), sandy macroforms (SB), laminated sand sheets (LS), crevasse channels (CR), crevasse splays (CS), and floodplain deposits (OF). Likewise, five different fluvial styles were proposed, based on facies association and the arrangement of the architectural elements within the deposits. FS1, Anastomosing (either with mixed sand–mud floodplains or with muddy- dominated floodplains) Fluvial Style; FS2, Braided Fluvial System; FS3, Heterolithic High-Sinuosity (meandering) to Low-Sinuosity (wandering) Fluvial System; FS4, Sandy Braided to Low-Sinuosity Fluvial System; and FS5, Ephemeral Sandy Sheet-Like Fluvial System with Fluvial Eolian Interaction.

CHAPTER 4

4. RESULTS Sedimentary facies, facies associations, architectural elements, trace fossils, stratigraphic sections, taphonomic pathway models, and photomosaics were defined and/or constructed from field-work observations in three (3) outcrops (LOC-1, LOC-2 and LOC-3) along La Troya Creek, close to the town San Jose de Vinchina in the northwest area of La Rioja Province in northwest Argentina.

Figure 4.1. Satellite image of the study area pointing out the outcrops (LOC-1, LOC-2, LOC-3) along La Troya creek in La Rioja province, northwest Argentina. Modified from Google Earth 2018. 4.1 Lithofacies of the Vinchina Formation: The lithofacies of the Vinchina Formation were defined following the fluvial lithofacies code proposed by Miall (1985, 1996), and adding specific details about features that the rocks show in the three outcrops studied (LOC-1, LOC-2, LOC-3). The following ten lithofacies were identified: 4.1.1 Lithofacies Gi Description: This lithofacies consists of erosively based massive matrix-supported conglomerate containing tabular and sub-rounded mudstone pebbles (0.5-6 cm wide) and cobbles (6-20 cm wide). Individual beds are present in two main shapes, 0.3-1.5 m thick lenticular bodies and thin (0.1-0.5 m) layers at the base of sandy channels. The Gi lithofacies is present in the three outcrops studied. Interpretation: Lithofacies Gi is interpreted as debris flow deposits. Generated by curled-up and broken mud cracks incorporated to wadi deposits (Glennie, 1970) or slumped banks sediments

incorporated to hyper-concentrated flow (Coleman, 1969). This lithofacies was recognized at the base of sandy amalgamated channel-fills, multistorey sandy channel-fills and in sandy channel- fills with gravel bars.

4.1.2 Lithofacies SGp Description: This lithofacies consists of erosively based, medium to coarse-grained sandstone with planar cross-stratification containing tabular and subrounded mudstone pebbles (0.5-6 cm wide), and in lower quantity cobbles (6.1-10 cm wide) consisting of mudstone intraclasts. Individual tabular beds are 0.2-0.8 m thick, and form units up to 2 m thick. This lithofacies is present in the outcrops LOC-2 and LOC-3. Interpretation: Lithofacies Sgp is interpreted as representing transverse and linguoid bedforms (Miall, 1996). This lithofacies records the reactivation of 2D dunes after a period of low energy when mudstone layers were deposited and then washed out by the high energy flow. The mudstone intraclasts are the remanent of those mudstone layers. This lithofacies was identified

at the base of heterolithic multistorey channel-fills and in a lower occurrence in multistorey sandy channel-fills.

Figure 4.3. Example of lithofacies SGp showing an erosive contact and planar cross-stratification at the base of a heterolithic sandy channel in outcrop LOC-2.

Figure 4.4. Stratigraphic section of outcrop LOC-1.

4.1.3 Lithofacies SGm Description: This lithofacies consists of massive coarse- to medium-grained sandstone containing tabular and subrounded pebbles (0.5-6.4 cm wide) and cobble-size (6.5-25 cm wide) mudstone intraclasts at the base (Fig.8). Diffuse planar lamination is present locally. Individual tabular beds are 0.3-1 m thick, and form units up to 2 m thick, which are commonly intercalated with mudstone intervals. This lithofacies is present in all three outcrops. Interpretation: This lithofacies suggests rapid deposition from heavy sediment-laden flows during waning floods (Todd, 1989; Maizels, 1993). Incipient stratification resulted from high depositional rates during deposition (Baker et al., 1983; Fisher et al., 2007). Lithofacies SGm is interpreted as the product of deconfinement or avulsion of hyper-concentrated flows, as the flow is laterally or distally unconfined and spread-out in sheet-floods. The high grain fallout rate associated with flow deceleration inhibited the formation of tractive structures on channels (Horn et al., 2018). The mudstone intraclasts are the product of the erosion of these high energy flows over the mudstone beds deposited during intervals of low energy. This lithofacies was recognized in amalgamated sandy channel-fills, multistorey sandy channel-fills, heterolithic sandy channel-fills, and sandy channel-fills with gravel bars.

4.1.4 Lithofacies Sp Description: This lithofacies consists of erosively based, planar cross-stratified, medium- to coarse-grained sandstone. Individual beds are present in two main shapes, tabular beds and lenticular bodies. Individual tabular beds are 1-3 m thick, and form units up to 40 m thick. Lenticular bodies are 0.2-1 m thick at the top of channel-fill deposits. This lithofacies is present in the three outcrops studied. Interpretation: Lithofacies Sp is interpreted as representing transverse and linguoid bedforms (2- D dunes) (Miall, 1996). This lithofacies was recognized in sandy multistorey channel-fills, channel- fills with gravel bars, heterolithic multistorey channel-fills, abandoned channel-fills and amalgamated sandy channel-fills, representing 2-D dune migration along the riverbed. 4.1.5 Lithofacies St Description: This lithofacies consists of erosively based, trough cross-stratified, medium- to coarse-grained sandstone. Individual beds are 0.1-0.5 m thick, and form units up to 1 m thick. This lithofacies is present in all three outcrops studied, but with low occurrence. Interpretation: Interpreted as deposits of unidirectional flow with the development of 3-D dunes, lower flow regime (Miall, 1978; Harms et al., 1982; Collinson et al., 2006). This lithofacies was identified in multistorey sandy channel-fills and in channel-fills with gravel bar deposits.

Figure 4.6. Examples of lithofacies St in outcrop LOC-1 (A) and LOC-3 (B)

Figure 4.7. Stratigraphic section of outcrop LOC-2.

4.1.6 Lithofacies Sh Description: This lithofacies consist of sharp-based, parallel-stratified, fine- to coarse-grained sandstone. Individual tabular beds are 0.1-1.5 m thick, and form units up to 50 m thick. This lithofacies is present in all three outcrops studied, but is more common in outcrops LOC-2 and LOC-3 (fig 11). Interpretation: This lithofacies was formed under plane-bed flow (critical flow) (Miall, 1996). Planar-bedded deposits originated via upper flow regime under high deposition rates, resulting in thick stratification (North and Taylor, 1996; Billi, 2007). This lithofacies was identified in multistorey sandy channel-fills and crevasse splay deposits.

Figure 4.8. Examples of lithofacies Sh. (A) Panoramic view of the beds in outcrop LOC-3. (B) Thick sandstone beds of lithofacies Sh in outcrop LOC-2.

Figure 4.9. Stratigraphic section of outcrop LOC-3.

4.1.7 Lithofacies Sm Description: This lithofacies consists of sharp-based, massive, very coarse- to fine-grained sandstone. Individual tabular beds are 0.5-1 m thick, and form units up to 4 m thick. At large scale, these beds have a lenticular shape with planar top and concave base (Fig. 13). This lithofacies is present in all three outcrops studied. Interpretation: This lithofacies resulted from rapid deposition from heavy sediment-laden flows during waning floods (Todd, 1989; Maizels, 1993); incipient stratification suggests high depositional rates during sedimentation (Baker et al., 1983; Fisher et al., 2007). Massive sandstone resulted from deconfinement or avulsion of hyper-concentrated flows, as the flow is laterally or distally unconfined and spread-out in sheet-floods. The high grain fallout rate associated with flow deceleration inhibits the formation of tractive structures on channels (Horn et al., 2018). This lithofacies was identified in amalgamated sandy channel-fills, heterolytic multistory channel-fills and channel-fills with gravel bars deposits.

Figure 4.10. Cross-section view of a thick amalgamated bed of Sm sandstone (right) underlying an eroded bed of Fl mudstone in outcrop LOC-1.

4.1.8 Lithofacies Sr Description: This lithofacies consists of current-ripple cross-laminated, very fine- to medium- grained sandstone beds. Individual beds are grouped into two types, thin beds (0.1-0.3 m thick) and thick beds (1-2 m thick). Thin beds form units up to 1 m thick. Thick beds form units up to 4 m thick. This lithofacies is present in outcrops LOC-2 and LOC-3. Interpretation: Lithofacies Sr records subaqueous current ripple migration under unidirectional, lower flow regime (Allen, 1963; Miall, 1978, 1996); critical ripples formed with the decline of flow velocity (Mckee et al., 1967; Fisher et al., 2008). This lithofacies was identified in channel-fills with gravel bars, amalgamated sandy channel-fills, multistorey sandy channel-fills, abandoned channel-fills, and crevasse splays and floodplain deposits.

Figure 4.11. Examples of lithofacies Sr showing: (A) View of a rippled sandstone representing a clear example of 2D dunes migration in outcrop LOC-3. (B) 2D dunes on top of a sandstone layer in an amalgamated bed of Sr lithofacies in outcrop LOC-2.

4.1.9 Lithofacies Fm Description: This lithofacies consists of sharp-based, massive dark brown mudstone. Individual tabular beds are 0.1-1 m of thick, and form units up to 5 m thick. This lithofacies is present in all three outcrops studied. Interpretation: This lithofacies represents suspension settling of grains of eolian or fluvial origin on overbank areas; post-depositional reddening was produced under oxidizing conditions (Miall, 1978,1996; Foix et al., 2013). This lithofacies was identified in crevasse splay and floodplain deposits. 4.1.10 Lithofacies Fl Description: This lithofacies consists of sharp-based, parallel-laminated mudstone. Individual laminae are 1-10 mm thick, beds are 0.1-0.5 m thick. The latter forms units up to 4 m thick. This lithofacies is present in all three outcrops studied. Interpretation: Lithofacie Fl records suspension settling from standing water (Rogers and Astin, 1991; Mángano et al., 1994). This lithofacies was identified in abandoned channel-fills, and crevasse splay and floodplain deposits.

4.2 Fluvial Architectural Elements Eight architectural elements were identified in the three outcrops studied along La Troya creek. The recognition of the elements was performed using the hierarchy model of bounding surfaces (Table 1) proposed for this area by Tripaldi et al. (2001). Five of them are channelized elements, and the other three are elements of the overbank area. The channelized elements are the following: 4.2.1 Multistorey Sandy Channels (CHsm) The CHsm element is characterized by 5-30 m thick, bed-sets of multistorey sandy channels (Fig.16-A) with up to 150 m of lateral continuity. Each channel complex is limited by a 5th order surface and composed mostly by 0.5-2.5 m thick Sp sandstone with thin layers of SGm (residual channel deposit) at the base of each channel unit. The multistorey sandy channel is the most recurrent element in the study area, and it is present in all three outcrops.

Order Description Surface Geometry

Limits channel complexes or big 5 Flat or concave upwards multistorey channels 4a. Upper surface of preserved a. Flat or convex upward macroforms

b. Concave upwards 4 4b. Basal surface of minor channels

c. Flat. (in places convex at 4c. Surface that limits big lithosomes large scale) within overbank deposits Reactivation surface that cuts cross- 3 stratified bed-sets within the Flat or irregular

macroforms

Limits bed-sets or lamina-sets of 2 Flat or slightly irregular cross-stratified bodies

Limits beds or lamina or cross- 1 Flat lamination

Table 4.1. Hierarchy model of bounding surfaces used in this work. Modified from Tripaldi et al. (2001).

Figure 4.14. (A) Multistorey sandy channels in the outcrop LOC-1. (B) Channels with gravel bars in the outcrop LOC-1.

4.2.2 Channels with Gravel Bars (CHgb) The CHgb element is characterized by mantiform bodies of coarse to medium-grained Sp sandstone channels with lenticular bodies of Gi conglomerates (Fig. 16-B). The erosive base of the element represents a 5th order bounding surface. The sandy channel-fills show internally planar cross-stratification and several reactivation surfaces of 3rd order that represent episodes of erosion of gravel and sandy bars. Thickness is 0.5-1.5 m. This element is present in all three outcrops.

4.2.3 Abandoned Channels (CHa) This element comprises simple sandy channels 1.5-4 m thick, that are interbedded with floodplain deposits (Fig. 19). It is characterized by simple Sp sandstone channels that commonly have Sr sandstone thin layers on top, and are mantled by Fl mudstone. Some of them have SGm sandstone in the lower interval of the element. The thickness of the mudstone beds on top of the channels varies from a few centimeters to around 1 m. The erosive base of these channels represents a 4th order bounding surface.

Figure 4.16. (A) Crevasse splay deposits in outcrop LOC-2. (B) Amalgamated sandy channels in outcrop LOC-3.

4.2.4 Heterolithic Multistorey Channels (CHhm) This architectural element is characterized by up to 2 m thick lenticular bodies of SGm sandstone. The erosive base of these channels represents a bounding surface of 5th order. Internally this element usually shows cross-stratification at the base and is massive or shows parallel stratification in the upper part of the body. This element is present in all three outcrops but is more common in the outcrop LOC-3. 4.2.5 Amalgamated sandy channels (CHsa) The CHsa element is characterized by tabular thick bodies (up to 30 m) of amalgamated sandy channels (Figs. 19 and 21). Most of the individual channels do not show clear boundaries as a result of amalgamation, but in some cases, thin layers of SGm sandstone, characterized by mudstone intraclasts allow us to visualize the erosive bases of the channels. Internally, the channels exhibit planar cross-stratification in Sp sandstone, parallel stratification in Sh sandstone, trough cross-stratification in St sandstone, and current-ripple cross-lamination in Sr sandstone.

This element is an important constituent of the Vinchina Formation deposits and is present in all three outcrops.

4.2.6 Muddy Floodplain (Of) This element is characterized by 0.2-4.0 m tabular deposits of Fl and Fm mudstone that commonly show desiccation cracks and rare intercalation of thin layers of Sm sandstone (Fig. 20). This element is present in outcrops LOC-1 and LOC-2. 4.2.7 Crevasse Splay (Ocs) This element is characterized by thick units (1–20 m) of Sh and Sr sandstone beds interbedded with Fl and Fm mudstone beds (Fig. 20). At a large scale, the lenticular shape of these deposits is

apparent and sandstone bodies can be traced laterally by tens of meters. This element is present in outcrops LOC-1 and LOC-2.

4.2.8 Crevasse Channels (CHosc) This element comprises simple channels that cross-cut the underlying crevasse splay deposits (Fig. 20). These channels-fills are characterized by bodies of Sp sandstone with SGm sandstone at the base. Some of these channels show ripple cross-lamination (Sr) on top. This element is present in outcrops LOC-1 and LOC-2.

Figure 4.19. Sandy amalgamated channels with an erosive base cross-cutting a muddy floodplain deposit in outcrop LOC-2.

4.3 Trace fossils from the Vinchina Formation. In this section, the ichnotaxa identified in the three studied outcrops of the Vinchina Formation are described. The ichnotaxa are grouped into two main categories: invertebrate and vertebrate trace fossils.

4.3.1 Invertebrate Trace Fossils

Capayanichnus vinchinensis Melchor et al., 2010 Description: Unlined or thinly lined, “L” shaped or simple vertical burrow with oval to subcircular cross-section showing a distinct surface texture composed of sets of three to five slightly curved and parallel ridges arranged obliquely to burrow axis on burrow wall (Fig. 23). Most of the specimens are vertical but some of them are predominantly horizontal, especially in interbedded sandstone and mudstone. Length of sets of ridges is comparable to burrow diameter. Width of individual sets of ridges is about 20 to 60% of the long axis of burrow cross-section. In straight sections of burrows, sets of ridges converge with an acute angle along a line parallel to burrow axis describing a herringbone or chevron pattern. On the opposite side of the burrow filling, the same pattern appears. Sets of ridges transverse to burrow axis are absent. Blunt to rounded or slightly tapered burrow terminations, lacking enlargements. The ratio between the major and minor axis of burrow cross-section ranges between 1.0 and 1.5. Structureless burrow fill is typically composed of sandstone with abundant mudstone intraclasts. Preserved as full-relief. Ethology and trophic type: Interpreted as dwelling trace or Domichnia (Melchor et al., 2010). In terms of trophic type, active predation is inferred based on the producer, which is an omnivore crab (Melchor et al., 2010). Producer: Interpreted as produced by freshwater crabs (Melchor et al., 2010) Remarks: This ichnotaxon was identified in outcrops LOC-1 and LOC-2, where is relatively abundant. Colonization may have occurred under low energy conditions due to the presence of laminae or thin layers of mudstone on top of the beds containing Capayanichnus vinchinensis. This ichnotaxon was identified in Sp, St, Sm and Sr sandstone of crevasse splay and deposits. This ichnogenus is distinguished from other related ichnotaxa, such as Spongeliomorpha, Glyphichnus, Camborygma, Macanopsis, Skolithos and Katbergia, by a combination of a predominantly vertical orientation, overall “L” shape (when fully developed), absence of lining and burrow bifurcation, distinctive surface texture, and lack of burrow enlargements (Melchor et al., 2010).

Figure 4.21. (A) Bedding-plain view of Capayanichnus vinchinensis with sub-horizontal displacement in a Sh fine-grained sandstone in outcrop LOC-2. (B) Cross-section view of a vertical C. vinchinensis in which the “L” shape of the burrow and the presence of mudstone intraclasts in the burrow fill can be appreciated. Outcrop LOC-1. Palaeophycus tubularis Hall, 1847 Description: Simple horizontal to sub-horizontal, straight to slightly curved burrows with smooth wall and thin lining. Diameter is 3-7 mm. Observe burrow length is up to 12 cm. Burrow infill is similar to the surrounding rock. Preserved as full relief or as positive epirelief. Ethology and trophic type: Interpreted as dwelling trace or domichnia (Pemberton and Frey, 1982). In marine environments, Palaeophycus is typically regarded as produced by suspension feeders or predators (Pemberton and Frey, 1982). The former interpretation may apply in the case of the Vinchina Formation only for those deposits that were colonized under subaqueous conditions. Predation may be inferred in the case of burrows produced subaerially. The passive

infill and absence of grain size contrast between burrow fill and host sediment argue against a deposit- or detritus-feeding type. Producer: Palaeophycus tubularis is a facies-crossing trace fossil that has been recorded in a wide variety of environments (e.g., Buatois and Mángano, 2011). This simple burrow is difficult to assign to a definite producer (Melchor et al., 2015). In continental settings, Chamberlain, (1975) suggested rove beetles, and caddisflies, mayflies and dragonflies larvae as possible producers. Krapovickas et al., (2010) proposed semiaquatic insects (orthopterans and hemipterans) or semiaquatic and non-aquatic beetles as potential producers. Remarks: This ichnotaxon was identified in several spots of outcrops LOC-1 and LOC-2. It is associated with thin laminae of mudstone (Fl) deposited on top of thick sandstone beds (Sm, Sh, and Sp), which are rhythmically intercalated with mudstone (Fl). Palaeophycus differs from Planolites by the presence of a wall lining and infill similar to that of the host rock (Pemberton and Frey, 1982). Palaeophycus tubularis differs from P. striatus, P. heberti, P. sulcatus, and P. alternatus, by the presence of thin-walled burrows and the lack of continuous parallel striae, irregularly anastomosing striae and alternating striations and annulations (Pemberton and Frey, 1982).

Figure 4.22. Examples of Palaeophycus tubularis in (A) LOC-1 and (B) LOC-2.

Skolithos isp. Haldeman, 1840 Description: Straight vertical to slightly inclined cylindrical burrows. Burrows do not branch, cross, nor interpenetrate. Burrow boundaries are smooth with structureless fill. Diameter is 4-7 mm. Preserved as full-relief. Ethology and trophic type: Interpreted as dwelling trace or domichnia (Alpert, 1974). In continental settings, Skolithos isp. is produced by terrestrial deposit feeders, herbivores or predators (Netto, 2007).

Producer: Given its morphological simplicity and long stratigraphic range throughout the Phanerozoic, Skolithos has been attributed to the activity of many different organisms from various phyla (Knaust et al., 2018). In continental environments, there is a broad consensus that Skolithos is made by arthropods, chiefly insects and arachnids (Netto, 2007). Remarks: This ichnotaxon is one of the most abundant in all three outcrops studied. All the specimens identified appears on tops and in bases of channelized amalgamated medium- to very fine-grained sandstone units (Sp, St, and Sm) in some cases cross-cutting interbedded mudstone (Fl) in crevasse splay and abandoned channel deposits. All the specimens were observed in plain view, so the vertical expression of the burrows is not known. Skolithos differs from Monocraterion by the lack of a prominent funnel-shaped aperture or structure at the top of the burrow. Vertical burrows, such as Diplocraterion and Arenicolites, are also similar to Skolithos, but are U-shaped and may have funnel-shaped apertures. (Alpert, 1974). Polykladichnus differs from Skolithos by the presence of branching (Schlirf and Uchman, 2005).

Figure 4.23. (A) Examples of Skolithos isp. at the base of a sandstone bed, LOC-2. (B) Skolithos isp. at the base of a mud-cracked sandstone bed, LOC-3. Taenidium barretti Bradshaw, 1981 Description: Straight to sinuous, mostly horizontal and rarely vertical, simple burrows characterized by a well-defined meniscate backfilling. No wall is observed. Diameter is 2-4 mm. Menisci are of alternating grain size, strongly arcuate, and their thickness is variable in different specimens (1-3 mm thick), although within each individual it remains uniform. Preserved as full relief. Ethology and trophic type: Taenidium barretti is classically regarded as a locomotion trace (repichnia) and/or a feeding structure (fodinichnia) (Díez-Canseco et al., 2016). Meniscate fill is an active backfill that results from mechanic manipulation or ingestion by the animal (e.g., Bromley, 1990; Buatois and Mángano, 2011). Vertically upward oriented, non- compartmentalized, meniscate structures may be regarded as repichnia, whereas oblique, lateral or vertically downward-oriented structures may represent adjustment or escape from lower water tables, desiccation, erosion or predation (Keighley and Pickerill, 1994). In terms of trophic type, Taenidium is produced by deposit or detritus feeders (D’Alessandro and Bromley, 1987). Producer: Savrda et al. (2000) suggested that the available data on Taenidium in continental deposits is insufficient to identify the specific producer(s) with any confidence. Buatois & Mángano (2007) suggested that a vagile organism, moving through the substrate in search of

food and utilizing a combination of bypassing and ingestion, is a potential candidate for the producer. Taenidium barretti may involve ingestion and excretion of an animal that transports the sediment through the body as it has been associated with deposit or detritus feeding organisms, most likely worm-like animals (Bown and Kraus, 1983; Squires and Advocate, 1984; D’Alessandro and Bromley, 1987; Sarkar and Chaudhuri, 1992; Schlirf et al., 2001). On the other hand, if the meniscate fill is produced by an animal that passes material along the sides of its body and compacts it behind them by forward motion, but with poor evidence of ingestion (Bradshaw, 1981), then locomotion by insects is an alternative interpretation (Frey et al., 1984; O’Geen and Busacca, 2001; Gregory et al., 2004). Remarks: Taenidium barretti was found in outcrops LOC-2 and LOC-3. This ichnotaxon occurs on top of mudstone laminae (Fl) that are overlying thin fine-grained sandstone beds (Sr) in outcrop LOC-3 and, in fine-grained sandstone (Sp) in outcrop LOC-2. Taxonomy of simple, unbranched meniscate burrows, such as Ancorichnus, Beaconites and Taenidium, has been reviewed by D’Aessandro and Bromley (1987) and Keighley and Pickerill (1994). Ancorichnus is a burrow containing a central meniscate fill and a structured mantle (Heinberg, 1974), Beaconites is a walled meniscate burrow (Keighley and Pickerill, 1994), and Taenidium is an unwalled meniscate backfilled burrow (Keighley and Pickerill 1994). Taenidium barretti is distinguished from other ichnospecies of Taenidium in hemispherical or deeply arcuate, tightly packed meniscate backfill (Keighley and Pickerill, 1994).

4.3.2 Vertebrate Trace Fossils

Macrauchenichnus isp. Description: Trydactyl footprints with an average length of 124 mm and an average width of 129 mm. The posterior margin of the footprint shows a slightly concave curvature. Digits are short, very broad and with blunt tips. Divarication angle among digits is approximate of 50°. Digit III is somewhat longer than the others (II and IV, sub-equal in length), as well as twice as wide. Preserved as positive hyporelief. Ethology and trophic type: Macrauchenichnus isp. represents repichnia or locomotion traces (Krapovickas et al., 2009). Based on the inferred producer, this ichnotaxon is thought to have been made by a herbivore (Fariña, 1996). Producer: Macraucheniids are regarded as producers (Angulo and Casamiquela, 1982; Krapovickas et al., 2009). Remarks: Several of these footprints were identified in outcrops LOC-2. In all cases, the footprints are preserved as positive hyporelief at the base of fine-grained sandstone, which are intercalated with laminae or thin beds of mudstone (Fl) in crevasse splay deposits. Macrauchenichnus isp. exhibits the same pattern of digits and general outline of Eumacrauchenichnus patachonichus (Aramayo and Manera de Bianco, 1987), differing mainly on the larger size of the latter (Krapovickas et al., 2009).

Figure 4.25. (A) The footprint Macrauchenichnus isp 1 in outcrop LOC-2, pointing out the three digits and its names. (B) Specimen of Macrauchenichnus isp 1 identified in outcrop LOC-2.

Large tetradactyl webbed footprints Description: Four thin digits footprint, with an average length of 20 cm and an average width of 18 cm. The digit III is the longest, while the digit I is the shortest. Digits II and IV are of approximately the same size. These tracks display a random orientation and typically occurs as clusters of high density. Preserved as negative epirelief, usually on top of rippled fine-grained sandstone in abandoned channel deposits. Ethology and trophic type: These are interpreted as repichnia or locomotion traces. Based on an interpretation of their avian producer, the trophic type is inferred to be predation. Producer: It is regarded as possibly produced by a large bird. Remarks: This trace fossil was identified in two spots of outcrop LOC-3, in both cases in lithofacies Sr, usually on top of rippled fine-grained sandstone in active channel deposits. This trace is currently under revision and, accordingly, it is left in open nomenclature.

Tracheria troyana Krapovickas and Nasif, 2011 Description: Tetradactyl manus and pes impressions of sub-equal size, with long and robust digits with strong elongated claw marks. The manus footprints are asymmetrical. The digit III impressions are the longest, while digits II and IV impressions are sub-equal and following in size, and finally, digit V is the shortest and at times do not print. In the pes imprints, the lateral digits (II–V) are shorter than the central digits (III-IV), resulting in a symmetrical arrangement. Elongated phalangeal and metapodial pad impressions are present, occasionally printing two heel pads. Trackways are narrow with footprint impressions near to the midline, and pes impressions are placed in front of, and more lateral to the midline, than the manus impressions. Preserved as positive hyporelief. Ethology and trophic type: These trackways are regarded as repichnia or locomotion traces (Krapovickas and Nasif, 2011). Based on the inferred producer, this ichnotaxon is inferred to have been made by a herbivore (Vucetich et al., 2015). Producer: These trace fossils are attributed to large dinomyid caviomorph rodent (Krapovickas and Nasif, 2011). Remarks: Tracheria troyana was identified in outcrop LOC-2. The footprints are preserved at the base of fine- to very fine-grained sandstone of lithofacies Sh interbedded with mudstone laminae (Fl) in crevasse splay deposits. In comparison with all known South American taxa with tetradactyl manus and pes, traces made by the extinct rodent-like notoungulates (i.e., hegetotheriids and mesotheriids) can be ruled out. Hegetotheriids, such as Hegetotherium, Pachyrukhos and Hemihegetotherium, have digit III longer than digits II and IV, and digit V is greatly reduced (Sinclair 1909). Besides, Pachyrukhos has arrow-like hoofed ungual phalanges that do not correspond with the long, heavy, and pointed claws observed in the Vinchina specimens. Mesotheriids, such as Trachytherus and Mesotherium, have a pentadactyl manus with a reduced digit V and the remaining four digits subequal in size (Shockey et al. 2007).

Figure 4.27. (A) Tracheria troyana in outcrop LOC-2. (B) Tracheria troyana in a surface with mud cracks and the footprint Macrauchenichnus isp in outcrop LOC-2. (C) Tracheria troyana, outcrop LOC-2. Venatoripes riojanus Frenguelli 1950 Description: Hindfoot imprint of an oval shape, about 500 mm long and 260 mm wide, neither digits nor plantar pad are evident. Footprints are slightly concave medially and, the anterior and posterior margin of the imprints are of similar width. Pace length is very short, of about the same length of the hindfoot. No forefoot impressions could be clearly identified. Preserved as negative epirelief. Ethology and trophic type: These trace fossils are regarded as repichnia or locomotion traces (Krapovickas et al., 2009). Based on the inferred producer, this ichnotaxon is attributed to a herbivore (Frenguelli, 1950). Producer: These footprints are interpreted as produced by medium-large ground sloth (Krapovickas et al., 2009). Remarks: The footprints were identified in one bed of fine-grained sandstone of the lithofacies Sr of an abandoned channel deposit in outcrop LOC-3. There are various related large oval- shaped pes footprints preserved in Neogene deposits of Argentina. Neomegatherichnum pehuencoensis (Aramayo and Manera de Bianco 1987), from the Pleistocene of Pehuen-Có in Buenos Aires province, has a similar general outline, although it is larger. In well-preserved specimens, the impression of the third digit claw is clear as a triangular mark on the anteromedial margin of the footprint. Other ichnotaxa from the Neogene of Argentina, such as Megatherichnum oportoi (Casamiquela, 1974), Acuñaichnus dorregoensis Casamiquela, 1983

and Mylodontichnus rosalensis (Aramayo and Manera de Bianco, 1987), are comparatively narrower on the posterior margin of the footprint (Krapovickas et al., 2009).

Figure 4.28. (A) Close-up of one of Venatoripes riojanus, outcrop LOC-3. (B) The bioturbated surface with the trackway of footprints, outcrop LOC-3. 4.4 Ichnofacies and trace-fossil suites in the Vinchina Formation Overall, the trace-fossil identified illustrate the Scoyenia Ichnofacies, as recorded in three main fluvial subenvironments and/or architectural elements: crevasse splays, and anastomosing and braided abandoned channels. For each of these subenvironments, trace-fossil suites were identified. In this study, a trace-fossil suite is understood as a group of trace fossils that reflects contemporaneous time of emplacement (Buatois and Mángano, 2011). 4.4.1 Ichnology of Crevasse Splay Deposits Ichnofaunas from the crevasse splay deposits of the Vinchina Formation are characterized by low diversity and abundance, and additionally by the dominance of simple vertical dwelling burrows like Capayanichnus vinchinensis and Skolithos isp. However, if the bioturbation of crevasse splays is compared with the other architectural elements identified in this study, it shows relatively high density and diversity. The following trace-fossil suites have been identified in crevasse splay deposits: 4.4.1.1 Palaeophycus tubularis suite: This suite consists of the predominantly horizontal unbranched cylindrical burrow Palaeophycus tubularis. It is common in the crevasse splay deposits, typically occurring in low to moderate densities. This suite is present at the top of fine- to very fine-grained Sh sandstone, which is mantled by thin layers of Fl or Fm mudstone, preserved as positive epirelief. Also, it is present in the base of fine to very fine-grained Sh sandstone preserved as positive hyporelief. This suite is present in the three outcrops studied.

4.4.1.2 Macrauchenichnus isp. and Tracheria troyana suite: This suite comprises vertebrate footprints, namely Macrauchenichnus isp. and Tracheria troyana. This suite is present in outcrop LOC-2. The footprints are present at the bottom of Sh sandstone, preserved as positive hyporelief and typically showing low density. Some of them are cross-cut by desiccation cracks or reworked by Skolithos isp. (Fig. 31).

Figure 4.29. (A) Macrauchenichnus isp. reworked by Skolithos isp. in outcrop LOC-2. (B) Tracheria troyana cross-cut by a mud crack in outcrop LOC-2. 4.4.1.3 Capayanichnus vinchinensis and Skolithos isp. suite: This suite comprises vertical dwelling burrows, namely Capayanichnus vinchinensis and Skolithos isp. This suite is present in most of the crevasse splay deposits exposed in outcrops LOC-1 and LOC-2. These trace-fossils are present in medium- to fine-grained Sh sandstone interbedded with Fl mudstone and preserved as full-relief, showing low to moderate density. Many of them, specially Capayanichnus vinchinensis, cross-cut desiccation cracks.

Figure 4.30. Schematic illustration of the trace fossils present in crevasse splay deposits of the Vinchina Formation

4.4.2 Ichnology of Abandoned Channel Deposits in Anastomosing Systems Sandy channels are the most common component of the Vinchina Formation, but trace fossils are present in just a few of them. Some of these bioturbated channelized elements represent abandoned channels deposited of anastomosing systems. These channels, identified in outcrops LOC-1 and LOC-2, are characterized by the presence of thin layers of mudstone, which were deposited over the channel-fill sediments after abandonment. The ichnofauna of these channels is dominated by simple grazing trails and dwelling burrows, and displays low diversity and density. The following trace-fossil suites were identified: 4.4.2.1: Palaeophycus tubularis suite: This suite consists of the predominantly horizontal unbranched cylindrical burrow Palaeophycus tubularis. This suite is present in outcrop LOC-2. This suite is preserved mostly as positive hyporelief. This suite is present at the top of fine-grained Sp or Sr sandstone, which are mantled by thin layers of Fl or Fm mudstone. 4.4.2.2: Taenidium barretti suite: This suite comprises meniscate predominantly horizontal sinuous burrow Taenidium barretti. However, some vertical specimens were identified in outcrop LOC-2. Most of the specimens are preserved as positive epirelief. Some of them are

reworked by Skolithos isp. This suite is present at the top of fine-grained Sp or Sr sandstone, which are mantled by thin layers of Fl or Fm mudstone. 4.4.2.3: Skolithos isp. suite: This suite comprises vertical dwelling burrow, namely Skolithos isp. This suite is present in outcrops LOC-1 and LOC-2. These trace-fossils are preserved as full- relief in thin mudstone layers that mantled fine-grained Sp, Sr and St sandstone. Some specimens cross-cut desiccation cracks, Palaeophycus tubularis, and Taenidium barretti.

Figure 4.31. Schematic illustration of the trace fossils present in anastomosing abandoned channel deposits of the Vinchina Formation. 4.4.3 Ichnology of Abandoned Channel Deposits in Braided Systems These bioturbated channelized elements represent abandoned channels deposited of a braided fluvial system (identified in the outcrop LOC-3). The ichnofauna of these channels displays low diversity and density (Fig. 33). Additionally, these deposits are characterized by the dominance of Skolithos isp. and the scarcity of fine-grained sediment (Fl, Fm). The following trace-fossil suites were identified: 6.4.3.1: Vertebrate Footprint suite: This suite consists of the large tetradactyl webbed footprint and the trackway Venatoripes riojanus identified in one bed of outcrop LOC-3. These

footprints are preserved as negative epirelief at the top of thin layers of fine-grained Sr sandstone overlying Sp and St sandstone. Some of them are cross-cut by desiccation cracks or reworked by Palaeophycus tubularis. 6.4.3.2: Palaeophycus tubularis and Taenidium barretti suite: This suite consists of the predominantly horizontal unbranched cylindrical burrow Palaeophycus tubularis and the meniscate horizontal sinuous burrow Taenidium barretti. It has been identified in outcrops LOC- 2 and LOC-3. This suite is preserved mostly as positive epirelief on top of fine-grained Sr sandstone and very fine- to fine-grained Sh sandstone. Some specimens are cross-cut by desiccation cracks and others cross-cutt large tetradactyl webbed footprints. 6.4.3.3: Skolithos isp. suite: This suite comprises vertical dwelling burrow, namely Skolithos isp. It has been identified in all three outcrops studied. These trace-fossils are preserved in very fine- to fine-grained Sh, Sr and St sandstone as full-relief. Some specimens cross-cut desiccation cracks and vertebrate footprints.

Figure 4.32. Schematic illustration of the trace fossils present in braided abandoned channel deposits of the Vinchina Formation.

4.5 Ichnofabrics from the Vinchina Formation Five ichnofabrics have been recognized. Use of the infaunal ecospace can be grouped in three main tiers: the deepest tier characterized by vertical and sub-horizontal Capayanichnus vinchinensis; a mid-tier occupied by Skolithos isp; and the shallow-tier characterized by Palaeophycus tubularis, Taenidium barretti, Tracheria troyana, Macrauchenichnus isp., and tetradactyl webbed footprints. The ichnofabrics identified are the following: 4.5.1 Capayanichnus vinchinensis Ichnofabric 1 Description: This ichnofabric is dominated by deep-tier vertical Capayanichnus vinchinensis in thick Sh medium- to fine-grained sandstone beds intercalated with thin layers of Fl mudstone. Mid-tier Skolithos isp. and shallow-tier Palaeophycus tubularis are less abundant. The intensity of bioturbation in these deposits is low (BI 1-2). This ichnofabric is present in outcrops LOC-1 and LOC-2.

Figure 4.33. Schematic illustration of Capayanichnus vinchinensis Ichnofabric 1. Ca: Capayanichnus vinchinensis, Sk: Skolithos isp., Pa: Palaeophycus tubularis.

Interpretation: The cross-cutting relationships among the different trace fossils are interpreted as recording separate episodes of bioturbation at the colonization surface. The dominance of Capayanichnus vinchinensis is due to the time between sedimentation events that allowed the producer (freshwater crab) to burrow. Based on neoichnological observations in Río Pilcomayo National Park (Argentina), Melchor et al., (2010) suggested that this type of burrows probably responds to the reproductive cycle of these crabs by which the male crab isolates its couple in the burrows from possible contact with other males. Also, the isolation would be a protection for juveniles if they remain with the female until released in water bodies, as the ditch is far from main river courses where single males are concentrated. The presence of Palaeophycus tubularis, a shallow-tier dwelling burrow cross-cut by desiccation cracks, is interpreted as the result of the dewatering of the original colonization surface (ponded area in the floodplain). Finally, Skolithos

isp., a mid-tier vertical burrow, is interpreted as controlled by the lowering of the water table due to the dewatering generated by surface exposure to subaerial conditions. These characteristics are consistent with conditions present in the medial part of crevasse splay subenvironments, where the deposits are dominated by medium- to fine-grained sandstone which is differentiated from proximal splay facies by their finer grain size and the preservation of Fl mudstone beds and lamina. In this area of the crevasse splay, the surface is ponded during flooding events and subsequently exposed to subaerial conditions when the water table deepens due to dewatering. The semi-arid climate during the deposition of the Vinchina Formation (Tripaldi et al., 2001) is likely to generate seasonal floodings followed by extensive droughts evidenced by the extensive presence of desiccation cracks in the formation.

4.5.2 Capayanichnus vinchinensis Ichnofabric 2 Description: This ichnofabric is dominated by the deep-tier vertical and sub-horizontal burrow Capayanichnus vinchinensis in intercalations of Sh sandstone and Fl mudstone with desiccation cracks on top. Shallow-tier Palaeophycus tubularis is locally cross-cut by shallow-tier Macrauchenichnus isp. and Tracheria troyana, which in some cases are reworked by mid-tier Skolithos isp. The intensity of bioturbation in the host deposit is low (BI 1-2). This ichnofabric is present in LOC-2.

Figure 4.34. Schematic illustration of Capayanichnus vinchinensis Ichnofabric 2. Ca: Capayanichnus vinchinensis, Sk: Skolithos isp., Pa: Palaeophycus tubularis, Ma: Macrauchenichnus isp., Tt: Tracheria troyana. Interpretation: This ichnofabric differs from the Capayanichnus vinchinensis ichnofabric 1 in two main aspects. First, some of the Capayanichnus vinchinensis specimens are sub-horizontal. This could be due to the higher presence of mudstone layers in between the sandstone that probably holds humidity during more time, so the producer did not have to burrow vertically to reach the water table. These sub-horizontal crab burrows probably have the same function during the reproductive cycle of the crabs than those observed by Melchor et al. (2010) in the neoichnological examples of Río Pilcomayo National Park. Second, the presence of vertebrate

footprints represented by Macrauchenichnus isp. and Tracheria troyana that marks the early stage of the substrate dewatering evidenced by the desiccation cracks that cross-cut many of these tracks. Integration of ichnofabric analysis and sedimentologic data suggests a distal crevasse splay deposit. This area of the splay is characterized by a higher presence of Fl and Fm mudstone in comparison with proximal facies, and is dominated by Sh fine to very fine-grained sandstone due to lower energy levels. Scarce thin layers of Sr sandstone also appear interbedded with mudstone.

Figure 4.35. Block diagram showing ichnofabric distribution in a crevasse splay deposits of the Vinchina Formation.

4.5.3 Skolithos isp.-Palaeophycus tubularis ichnofabric 1 Description: This ichnofabric is characterized by mid-tier Skolithos isp. and shallow-tier Palaeophycus tubularis. It is present at the upper part of Sp sandstone beds mantled by thin layers of Fl mudstone. Degree of bioturbation in these deposits in low (BI 1-2) This ichnofabric is present in LOC-1 and LOC-2.

Figure 4.36. Schematic illustration of Skolithos isp.-Palaeophycus tubularis ichnofabric 1. Sk: Skolithos isp., Pa: Palaeophycus tubularis. Interpretation: Deposition of a thin layer of mudstone represents a shift to lower energy levels with respect to the environmental conditions inferred for the underlying Sp and Sr sandstone. The top of this mudstone layer represents the colonization surface in this ichnofabric. Palaeophycus tubularis was emplaced first, followed by Skolithos isp. in response to a lowering water table. These features are consistent with conditions generated in abandoned anastomosing channels deposits, where sediments deposited by an active channel are followed by accumulation of thin layers of mudstone due to by suspension fall-out in a remaining stationary body of water product of the channel abandonment. Also, it could be inferred that the conditions before the shift of energy prevented bioturbation, probably due to high energy and bedform mobility. 4.5.4 Skolithos isp.-Palaeophycus tubularis ichnofabric 2 Description: This ichnofabric is characterized by the horizontal and sub-horizontal shallow-tier Palaeophycus tubularis and Taenidium barretti and the mid-tier Skolithos isp. in Sp and Sr sandstone beds mantled by thin layers of Fl mudstone. Degree of bioturbation in these deposits is low (BI 1-2). This ichnofabric is rare, and is just present in a few beds in outcrop LOC-2.

Figure 4.37. Schematic illustration of Skolithos isp.-Palaeophycus tubularis ichnofabric 2. Sk: Skolithos isp., Pa: Palaeophycus tubularis, Ta: Taenidum barretti.

Interpretation: This ichnofabric shares most of the features of the Skolithos isp.-Palaeophycus tubularis ichnofabric 1. The only difference is the presence of Taenidium barretti in the shallow- tier. This ichnotaxon could be evidence of a locally higher level of organic particles available in the substrate, allowing colonization by the deposit-feeders or detritus feeding producers, most likely worm-like organisms (Bown and Kraus, 1983; Squires and Advocate, 1984; D’Alessandro et al., 1987; Sarkar and Chaudhuri, 1992; Schlirf et al., 2001). These ichnologic and sedimentologic characteristics are consistent with an abandoned anastomosing channel deposit.

Figure 4.38. Block diagram showing ichnofabric distribution in anastomosing abandoned channel deposits of the Vinchina Formation.

4.5.5 Skolithos isp.-Palaeophycus tubularis ichnofabric 3 Description: This ichnofabric is characterized by shallow-tier large tetradactyl webbed footprints, Palaeophycus tubularis, Taenidium barretti, and mid-tier Skolithos isp. preserved in thin beds of Sr sandstone, which are typically overlaying Sp sandstone. The intensity of bioturbation in these deposits is low (BI 1-2). This ichnofabric occurs in outcrop LOC-3.

Figure 4.39. Schematic illustration of Skolithos isp.-Palaeophycus tubularis ichnofabric 2. Sk: Skolithos isp., Pa: Palaeophycus tubularis, Ta: Taenidum barretti, Wf: tetradactyl webbed footprint. Interpretation: The Sr sandstone on top of these deposits is indicative of low energy during lower flow regime. Therefore, in a braided fluvial context, it could be inferred that this ichnofabric was developed in a channel that was progressively abandoned. During this process, the channel experimented significant drops of flow and energy levels until final abandonment. The first bioturbators (those producers of tetradactyl webbed footprint), arrived right before or during the abandonment of the channel. Subsequently, once the channel became disconnected from the main flow and dried out. In contrast to similar ichnofabrics in anastomosing abandoned channels, the colonization surface is on sandstone rather than mudstone. Subsequent organisms colonizing the surface were the producers of Palaeophycus tubularis, Taenidium barretti. Finally, dewatering process resulted in deepening of the water table and some organisms started to burrow vertically generating Skolithos isp. More prolonged drying was conducive to the formation of desiccation cracks.

Figure 4.40. Block diagram showing ichnofabric distribution in braided abandoned channel deposits of the Vinchina Formation. 4.6 Taphonomic Pathways Different factors are involved in the preservation of trace-fossils. In the Vinchina Formation, four main controlling parameters were identified: water table fluctuation, current flow energy, substrate consistency, and time between depositional events. Based on detailed observation of the cross-cutting relationship between ichnotaxa, the ichnofabric distribution and the preservation features of the trace-fossils studied, a colonization sequence for each of the subenvironments is proposed.

4.6.1 Taphomomic Pathway in Crevasse Splay Deposits First stage: This stage started after a flooding event that resulted in the formation of crevasse splays in the overbank area of the river. At this time, the medial and distal areas of the crevasse splay were ponded due to the high level of the water table. In ponded areas, thin layers of mud were deposited on top of the crevasse-splay sand due to suspension fall-out. These soft layers of mud were colonized by the organisms that produced Palaeophycus tubularis. Second stage: This stage was characterized by a drop in the water table. This process generated dewatering in the sediment producing a firmer substrate and some desiccation cracks. During

this period, the substrate was bioturbated by large vertebrates that trampled the crevasse splay area, generating the trackways Macrauchenichnus isp. and Tracheria troyana. The formation of mud cracks continued all through this stage. Third Stage: This stage was characterized by increased dewatering of the sediment concomitant with a continuous drop of the water table, resulting in the formation of large desiccation cracks all over the colonization surface. Under these conditions, the producer of Skolithos isp. and Capayanichnus vinchinensis burrowed down the sediment possibly looking for more humid conditions, resulting in the establishment of vertical burrows.

Figure 4.41. Schematic illustration of the taphonomic pathway in crevasse splay deposits of the Vinchina Formation.

4.6.2 Taphomomic Pathway in Abandoned Braided Channel Deposits First stage: This stage started after a drop in the levels of flow and energy in an active channel as a consequence of the dry season that typified the arid to semi-arid climate conditions

dominant in the area. During this time, thin layers of ripple-cross laminated sandstone were deposited on top of the channel fill deposits. The top of these thin layers of Sr sandstone represents the colonization surface. The surface of these layers was colonized by an unidentified large bird, which trampled the surface, generating the large webbed tetradactyl footprints trace fossils. Second stage: During this stage, the channel was abandoned by the main water flow and dried out (possibly in a short time due to the sandy nature of the substrate and a lower position of the water table). These conditions allowed colonization by various organisms, probably semiaquatic insects (orthopterans and hemipterans) or semiaquatic and non-aquatic beetles, which are possible producers of Palaeophycus tubularis and Taenidium barretti. Third stage: This stage is characterized by the advanced stage of dewatering of the substrate, as evidenced by desiccation cracks in the sediment. In response to these conditions, some organisms, probably insects or arachnids, started to burrowing vertically, generating the trace fossil Skolithos isp.

Figure 4.42. Schematic illustration of the taphonomic pathway in abandoned braided channel deposits of the Vinchina Formation.

4.6.3 Taphomomic Pathway in Abandoned Anastomosing Channels Deposits First stage: This stage started after the abandonment of the active channel by the main water flow. A stationary water body remained over the abandoned channel, generating deposition of mud by suspension fall-out on top of the channel-fills sand. The top of this mudstone layer represents the colonization surface. This surface was colonized by a variety of invertebrates, most likely semiaquatic insects (orthopterans and hemipterans) or semiaquatic and non-aquatic beetles, which are inferred as the producers of Palaeophycus tubularis and Taenidium barretti. Second Stage: This stage was characterized by a lower level of the water table. This condition generated and incipient dewatering in the substrate and as a consequence, the substrate became firmer than in the previous stage generating some thin desiccation cracks on top of the surface. At this point, in response to the lower level of the water table, some organisms, probably midge larvae, burrowed vertically in the sediment producing Skolithos isp.

Figure 4.43. Schematic illustration of the taphonomic pathway in abandoned anastomosing channel deposits of the Vinchina Formation.

4.7 Fluvial Styles and Depositional Model of the Vinchina Formation. Based on the assemblage of lithofacies, architectural elements, ichnofabrics and ichnofacies distribution, two different fluvial styles were recognized within the three outcrops studied in the Vinchina Formation. In general, these fluvial styles are consistent with those proposed for this area by Limarino et al. (2001), Tripaldi et al. (2001) and then refined by Marenssi et al. (2015) and Schencman et al. (2018).

4.7.1 Anastomosing fluvial style This fluvial style is recorded in the deposits studied in outcrops LOC-1 and LOC-2, which correspond in the Lower Member of the Vinchina Formation. This fluvial style is characterized by the dominance of multistorey sandy channel, abandoned channels, crevasse channel, crevasse splay, and muddy floodplain architectural elements (Fig. 46). The thick units of multistorey sandy channels are made of Sh, Sp, Sr and St sandstone, which are internally separated by fourth or third-order surfaces and do not show any evidence of bioturbation. Abandoned channels are characterized by simple channels, up to 4 m deep, represented by Sp, Sr and St sandstone which in some cases are bioturbated on top, showing the Skolithos isp.-Palaeophycus tubularis ichnofabric 1 and Skolithos isp.-Palaeophycus tubularis ichnofabric 2. Crevasse splays deposits consist of tabular beds of Sh, Sr and Sm sandstone interbedded with Fl and Fm mudstone beds. These deposits are characterized by the Capayanichnus vinchinensis Ichnofabric 1 and the Capayanichnus vinchinensis Ichnofabric 2. Crevasse channels consist of simple or two-storey channels usually less than 1.5 m deep that appear associated with crevasse splay deposits and do not show any evidence of bioturbation. Finally, muddy floodplain deposits, which are composed of thick beds of Fm or Fl mudstone, do not show any evidence of bioturbation either. This fluvial style is interpreted as anastomosing due to the presence of thick multistorey channel belts encased within flood-plain deposits made of stacked coarsening-upward crevasse splay and muddy flood-basin deposits (Limarino et al., 2001; Tripaldi et al., 2001; Marenssi et al., 2015), and the lack of lateral accretion sandy bars that would suggest a meandering style. Rust (1981) documented a similar case in a modern example of an anastomosing fluvial system in an arid- zone in Cooper’s Creek, central Australia, suggesting that arid-zone anastomosing fluvial systems deposits are characterized by the presence of thick muddy successions with deep desiccation cracks and scarce evaporites layers. The mud encloses isolated channel sands, which show little evidence of lateral migration, but may accrete vertically until they terminate, due to avulsion. Bank cohesion in these deposits is attributed to the dominance of fine sediments. This characteristic would also explain the bank stability in the Vinchina Formation anastomosing deposits, where there is no evidence of plant roots.

4.7.2 Braided fluvial style This fluvial style was identified in outcrop LOC-3 corresponding to the Upper Member of the Vinchina Formation. These deposits are characterized by the dominance of coarse- to very coarse-grained sandstone and conglomerate elements, such as amalgamated sandy channels up to 15 m thick, multistorey sandy channels, abandoned channels, heterolithic multistorey channels, and channels with transversal gravel bars (Fig. 47). Maximum channel depth has been estimated as 4.5 m. The amalgamated sandy channels are made up essentially of coarse- to medium-grained Sp and Sh sandstone; some of these have SGm sandstone in their bases, and do

not show any trace fossil. The multistorey sandy channels are characterized by thick units up to 10 m of stacked sandy channels with tabular shape made up essentially of Sp medium-grained sandstone mantled by Sr medium- to fine-grained sandstone and scarce thin layers of Fl mudstone in between some channel-fills. This element does show bioturbation structures, but just in a few cases. These bioturbated channels are simple channels of Sp and Sr sandstone on top that are separated by thin beds of Fl mudstone. The bioturbation structures comprise the Skolithos isp.-Palaeophycus tubularis ichnofabric 3. Heterolithic sandy channels are common. These channel-fills are characterized by SGm, Sm and Sp sandstone, and scarce lenses of Gi conglomerate; these elements are up to 5 m thick. The dominance of multistorey channels composed of gravelly and sandy bar deposits without evidence of lateral migration, combined with the paucity of floodplain fines, suggest deposition in braided fluvial systems (Limarino et al., 2001; Tripaldi et al., 2001).

CHAPTER 5

5. DISCUSSION Buatois and Mángano (2004, 2007) noted that some overbank ichnofaunas are emplaced in water bodies that have been progressively desiccated (desiccated overbank), whereas others record subaqueous colonization in water bodies filled by the vertical accretion of overbank deposits without experiencing desiccation (overfilled overbank). These two ichnofaunas commonly display contrasting characteristics. The overbank deposits in the Vinchina Formation show clear evidence of desiccation, therefore, can be defined as desiccated overbank deposits. In general, the ichnofauna in desiccated overbank deposits are characterized by the predominance of arthropod trackways (e.g. Diplichnites, Protichnites, Hexapodichnus, Trachomatichnus), meniscate trace fossils (e.g. Scoyenia, Taenidium, Beaconites), ornamented burrows (e.g. Spongeliomorpha, Tambia), bilobate trace fossils with scratches (e.g. Cruziana, Rusophycus), and vertical burrows (e.g. Skolithos, Cylindricum) and insect and arachnid nesting structures (e.g. Celliforma, Coprinisphaera ) (e.g. Bromley and Asgaard, 1979; Bracken and Picard, 1984; Squires and Advocate, 1984; Kim and Paik, 1997; Gand et al., 1997; Eberth et al., 2000; Savrda et al., 2000; Gierliński et al., 2004; Buatois et al., 2007; Lucas et al., 2010, Melchor et al., 2010). In the Vinchina Formation, crevasse splays deposits are characterized by the dominance of ornamented burrows (Capayanichnus vinchinensis), vertical burrows (Skolithos isp.), horizontal simple burrows (Palaeophycus tubularis), meniscate trace fossils (Taenidium barretti), and vertebrate footprints (Tracheria troyana and Macrauchenichnus isp.). The lack of arthropod trackways and insect and arachnid nesting structures in the desiccate overbank deposits in the Vinchina Formation is due to different reasons. First, the lack of arthropod trackways is probably due to taphonomic factors. In continental settings, trace fossils, such as Skolithos, Palaeophycus and Taenidium, are considered to be produced by arthropods. Accordingly, the absence of trackways may reflect a preservational bias rather than the absence of arthropods. Second, insect and arachnid nesting structures are typical of paleosols (Genise, 2017). Their absence in the Vinchina Formation is consistent with the lack of soil development. This is probably related to the arid to semi-arid conditions and the high sedimentation rates that were dominant during the deposition of the Vinchina Formation. Climate ranks as one of the most significant controls on paleosol ichnofaunas (Genise et al., 2000, 2004, 2010, 2017). The importance of climate as a limiting factor on paleosol ichnofaunas is due to the overwhelming dominance by insect nests, which contain larvae provisioned with organic matter. Larvae and provisions are strongly sensitive to microclimatic conditions (e.g. moisture and soil temperature)

because an excess of moisture inside cells leads to decay of provisions and insufficient moisture is conducive to larval dehydration (Genise et al., 2000, 2017; Buatois and Mángano, 2011). In the case of abandoned channels (anastomosing and braided), their ichnofauna is typically characterized by meniscate trace fossils (e.g. Beaconites, Taenidium), vertical to inclined burrows (e.g. Skolithos, Cylindricum), arthropod trackways (e.g. Diplichnites), simple horizontal burrows (e.g. Palaeophycus), and locally vertebrate footprints (e.g. Allen and Williams, 1981; Graham and Pollard, 1982; Bamford et al., 1986; Sarkar and Chaudhuri 1992; Miller and Collinson, 1994; Miller, 2000; Keighley and Pickerill, 2003; Lockley et al., 2003; Morrissey and Braddy, 2004; Lucas et al., 2006; Buatois et al., 2007; Falcon-Lang et al., 2007). In the case of the Vinchina Formation, the ichnofauna of these deposits is characterized by meniscate trace fossils (Taenidium barretti), vertical burrows (Skolithos isp.), simple horizontal burrows (Palaeophycus tubularis) and vertebrate footprints (large tetradactyl webbed footprints and Venatoripes riojanus). The bioturbation structures in channelized elements the Vinchina Formation is typical of these deposits elsewhere. Ichnotaxa in the Vinchina Formation are dominantly preserved in medium- to very fine-grained sandstone and in thin laminae of mudstone deposited over sandy deposits (abandoned channels and crevasse splays) with no identified occurrence in finer-grained (muddy floodplains) and coarser-grained elements (heterolithic sandy channels, amalgamated sandy channels, channels with gravel bars). MacNaughton and Pickerill (1995), documented a similar case in the Triassic Lepreau Formation (New Brunswick, Canada), where this profound difference in degree of bioturbation between channel and overbank facies was attributed to an environmental control associated with arid to semi-arid conditions. Gierlowski-Kordesch (1991) emphasized the impact of water availability in determining the habitats of nonmarine burrowing organisms. Water supply is mostly available within stream channel sediments and may have allowed preferential colonization of channel sediment (MacNaughton and Pickerill, 1995). In the case of the anastomosing deposits of the Vinchina Formation, water supply was also available in crevasse splays deposits, probably due to the capacity of this sediments to maintain ponded water bodies, as a consequence of the intercalation of “reservoir” sandstone (Sh, Sr) with “seals” of mudstone (Fl, Fm) (Figs. 17, 20 and 46). This information suggests that under arid to semi-arid conditions, channel sands are more likely to be colonized and bioturbated than overbank elements. In contrast, in more humid regimes, overbank deposits of similar river systems tend to be thoroughly bioturbated (e.g. Buatois et al., 2007).

CHAPTER 6

6. CONCLUSIONS

1. Two different fluvial styles were recognized in the studied sections of the Vinchina Formation (anastomosing and braided). Integration of sedimentology and ichnology can provide a detailed model of the different subenvironments and their distribution in the fluvial context. Also, ichnology can be used to help identify architectural elements by the fact that trace-fossil associations are excellent indicators of environmental conditions that change across the different subenvironments. 2. The Vinchina Formation ichnofauna displays low diversity and abundance. In general, it is dominated by simple horizontal burrows, vertical dwelling burrows, and vertebrate trackways. 3. The ichnofacies model and the ichnofabric approach can be used in conjunction to characterize in detail the trace-fossil distribution within the different subenvironments of fluvial systems. The trace fossils of the Vinchina Formation collectively illustrate the Scoyenia Ichnofacies. In addition, five different ichnofabrics have been defined. These ichnofabrics are distributed in the deposits of three subenvironments: crevasse splays, and anastomosing and braided abandoned channels. 4. The infaunal ecospace in the Vinchina Formation deposits could be divided into three main tiers. The deep-tier dominated by Capayanichnus vinchinensis, the mid-tier dominated by Skolithos isp. and the shallow-tier dominated by Palaeophycus tubularis, Taenidium barretti and vertebrate footprints. 5. The main controlling parameters on bioturbation process in the Vinchina Formation are water table fluctuation, energy of the current flow, substrate consistency, and time between depositional events. All these parameters reflect arid to semi-arid climatic conditions during deposition.

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