INVESTIGATING LINKS BETWEEN CLIMATE PERTURBATIONS, RIVER

DISCHARGE VARIABILITY AND FLUVIAL FANS IN THE PALEOGENE SAN JUAN BASIN, NEW MEXICO, USA

by Kristine L. Zellman © Copyright by Kristine L. Zellman, 2019 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geology).

Golden, Colorado Date

Signed: Kristine L. Zellman

Signed: Dr. Piret Plink-Bj¨orklund Thesis Advisor

Golden, Colorado Date

Signed: Dr. M. Stephen Enders Professor and Head Department of Geology and Geological Engineering

ii ABSTRACT

The San Juan Basin in New Mexico preserves the southernmost sedimentary record of the Paleocene and early-Eocene in North America, and thus represents a critical point for comparison for climatic and biotic change during the Paleogene. This dissertation combines basin-scale 3D outcrop analyses of fluvial architecture and preliminary stable carbon isotope records from the Paleocene upper Nacimiento Formation and the early-Eocene San Jose Formation to investigate the link between Paleogene climate perturbations and depositional trends. Through these investigations, we 1) identify the succession as deposits of variable dis- charge river systems, 2) observe a long-term stratigraphic trend toward increasingly well- drained floodplain deposits, and thus successively more arid conditions from Paleocene into the early-Eocene that we suggest may indicate a long-term shift from a monsoonal climate in the Paleocene to fluctuating humid and arid subtropical and then semi-arid/arid conditions in the early Eocene, 3) identify the Paleocene uppermost Nacimiento Formation and early- Eocene Cuba Mesa and Regina Members of the San Jose Formation as deposits of a large fluvial fan and identify at least two vertical packages of fan progradation, and 4) identify at least two negative carbon isotope excursions that may record the Paleocene-Eocene Thermal Maximum (PETM) and one or more post-PETM hyperthermal events. By combining the stratigraphic analyses with the carbon isotope records, we observe that the packages of fan progradation coincide with the negative carbon isotope excursions, suggesting that changes in water discharge patterns linked to climate were a key control in sedimentary delivery to the basin. These findings have implications for paleoclimate interpretations, variable discharge river and fluvial fan facies models, and improved understanding of San Juan Basin landscape evolution. Furthermore, the recognition of negative carbon isotope excursions suggests that

iii the San Juan Basin may provide a new dataset for studying climatic and biotic changes associated with the Paleocene-Eocene Thermal Maximum and post-PETM hyperthermal climate perturbations.

iv TABLE OF CONTENTS

ABSTRACT ...... iii

LISTOFFIGURES ...... x

LISTOFTABLES ...... xiii

ACKNOWLEDGMENTS ...... xiv

DEDICATION ...... xvii

CHAPTER1 INTRODUCTIONTOTHEDISSERTATION...... 1

1.1 Background ...... 1

1.2 Motivation...... 4

1.3 DissertationOrganization ...... 5

CHAPTER 2 TESTING HYPOTHESES ON PRECIPITATION VARIABILITY SIGNATURES IN THE RIVER AND FLOODPLAIN DEPOSITS OF THE PALEOGENE SAN JUAN BASIN, NEW MEXICO ...... 8

2.1 Abstract...... 8

2.2 Introduction...... 9

2.3 GeologicalSettingandBackground ...... 13

2.3.1 Stratigraphy...... 13

2.3.1.1 PaleoceneNacimientoFormation ...... 14

2.3.1.2 Early-EoceneSanJoseFormation ...... 15

2.4 DatasetandMethods...... 16

2.5 SedimentaryFacies ...... 18

2.5.1 Sandstone facies group 1: sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations or devoid of structure . 18

v 2.5.2 Sandstone facies group 2: sandstones with steeply dipping cross-strataand-lamina ...... 24

2.5.3 Mudrock facies group 1: drab-colored mudrock ...... 26

2.5.4 Mudrock facies group 2: purple-red mudrock ...... 28

2.6 FaciesAssociations ...... 29

2.6.1 Facies association 1 (FA 1) - sandy channel lithosomes ...... 30

2.6.2 Facies association 2 (FA 2) - heterolithic channel lithosomes ...... 31

2.6.3 Facies association 3 (FA 3) - lenticular channel lithosomes ...... 32

2.6.4 Facies association 4 (FA 4) - poorly-drained floodplain lithosomes . . . 33

2.6.5 Facies association 5 (FA 5) - variably-drained floodplain lithosomes . . 35

2.6.6 Facies association 6 (FA 6) - well-drained floodplain lithosomes. . . . . 36

2.7 Discussion...... 37

2.7.1 Signatures of variable discharge in channel deposits and comparison tofaciesmodels...... 37

2.7.2 Signatures of variable discharge in floodplain deposits...... 41

2.7.3 Channel-floodplain associations and their link to hydroclimate . . . . . 43

2.7.4 Evidence of Paleogene climate changes in the San Juan Basin ..... 47

2.8 Conclusions ...... 50

CHAPTER 3 PROGRADATION - RETROGRADATION STYLE OF A PALEOGENE FLUVIAL FAN IN THE SAN JUAN BASIN, NEW MEXICO...... 78

3.1 Abstract...... 78

3.2 Introduction...... 79

3.3 GeologicalSettingandBackground ...... 81

3.3.1 Stratigraphy...... 82

vi 3.3.1.1 PaleoceneNacimientoFormation ...... 82

3.3.1.2 Early-EoceneSanJoseFormation ...... 83

3.3.1.3 Nacimiento-San Jose Formation contact and the Paleocene-Eoceneboundary ...... 85

3.4 DatasetandMethods...... 87

3.5 SedimentaryFacies ...... 88

3.5.1 Sandstonefacies...... 89

3.5.2 Mudrockfacies ...... 91

3.6 FaciesAssociations ...... 93

3.6.1 Channelfaciesassociations...... 93

3.6.2 Floodplainfaciesassociations ...... 95

3.7 PaleocurrentMeasurements ...... 96

3.8 ArchitecturalStyles...... 97

3.9 LateralTrends...... 99

3.10 DepositionalEnvironment ...... 102

3.11 StratigraphicTrends ...... 104

3.12Discussion...... 108

3.12.1 Fluvial fan progradation and retrogradation ...... 108

3.12.2 Controls on fan progradation-retrogradation ...... 110

3.12.3 Link between fluvial fan occurrence, river discharge variability, and climate...... 111

3.12.4 Implications for San Juan Basin landscape evolution and fluvial fan depositionalmodels...... 113

3.13Conclusions ...... 115

vii CHAPTER 4 INVESTIGATING LINKS BETWEEN CLIMATE PERTURBATIONS AND SEDIMENTATION IN THE PALEOGENE SAN JUAN BASIN, NEW MEXICO ...... 137

4.1 Abstract...... 137

4.2 Introduction...... 138

4.3 GeologicalSettingandBackground ...... 142

4.3.1 Stratigraphy...... 142

4.3.1.1 PaleoceneNacimientoFormation ...... 143

4.3.1.2 Early-EoceneSanJoseFormation ...... 144

4.3.1.3 Nacimiento-San Jose Formation contact and the Paleocene-EoceneBoundary ...... 145

4.4 DatasetandMethods...... 147

4.4.1 Measured sections and sample collection ...... 147

4.4.2 Carbonisotopeanalyses ...... 148

4.4.3 Depositionalsystem ...... 149

4.5 FossilWoodSamples ...... 152

4.6 IsotopeAnalyses ...... 152

4.6.1 Fossilwoodorganiccarbonisotopes ...... 152

4.6.2 Bulk-sediment organic carbon isotopes ...... 155

4.6.3 Comparison of trends in fossil wood and bulk sediment carbon isotopecurves...... 157

4.7 Comparison of Carbon Isotope Curves and Stratigraphic Trends ...... 159

4.8 Discussion...... 160

4.8.1 Sedimentary response to long-term warming and hyperthermal events 160

4.8.2 Comparisons with other mid-latitude continental basins...... 162

viii 4.8.3 Concluding remarks on bulk sediment and fossil wood ...... 164

4.9 Conclusions ...... 164

CHAPTER5 CONCLUSION...... 178

5.1 FutureWork...... 179

REFERENCESCITED ...... 181

ix LIST OF FIGURES

Figure 1.1 Distribution of Laramide sedimentary basins formed between late Cretaceous and late Eocene time and the Williston Basin...... 3

Figure 1.2 Previous interpetations of Paleogene stratigraphic relationships in the SJB...... 5

Figure 2.1 Regional maps and Paleogene stratigraphy of the San Juan Basin (SJB). . 52

Figure2.2 Examplesofsandstonefaciesgroup1...... 53

Figure 2.3 Outcrop examples of sandstone facies group 1...... 55

Figure2.4 Examplesofsandstonefaciesgroup2...... 57

Figure 2.5 Example hand samples of mudrock facies group 1...... 58

Figure 2.6 Example hand samples of mudrock facies group 2...... 59

Figure2.7 Examplesofsandstonelithosomes...... 60

Figure 2.8 Representative measured sections and photos from sandstone lithosomes. . 61

Figure2.9 ExamplesofFloodplainLithosomes...... 62

Figure2.10 FA1ChannelCharacteristics...... 63

Figure2.11 FA2ChannelCharacteristics...... 65

Figure2.12 FA3ChannelCharacteristics...... 66

Figure 2.13 Stratigraphic trends in overall percentage of sandstone and mudrock lithofaciesandfaciesgroupcomposition...... 67

Figure 2.14 Vertical trends in channel and floodplain lithosomes in the Arroyo Chijuillastudyarea...... 68

Figure 2.15 Vertical trends in channel and floodplain lithosomes in the Continental Dividestudyarea...... 69

x Figure 2.16 Stratigraphic trends in channel and floodplain lithosomes and associations in the Continental Divide study area...... 70

Figure 3.1 Maps and Paleogene geology of the San Juan Basin (SJB)...... 117

Figure 3.2 Examples of regular cross-stratification and backsetsinoutcrop. . . . . 119

Figure 3.3 Measured Sections from sites selected for detailed investigation...... 120

Figure3.4 DepositionalArchitectureStyles...... 121

Figure3.5 Lateraltrends-flowperpendicular...... 122

Figure 3.6 Channel measurements collected in the central and southernSJB. . . . 123

Figure 3.7 Cuba Mesa Member lateral trends - flow parallel...... 124

Figure 3.8 Examples of outcrops interpreted as proximal, medial, and distal fan faciesintheSanJoseFormationoftheSJB...... 125

Figure 3.9 Stratigraphic trends the Arroyo Chijuilla study area...... 126

Figure 3.10 Different styles of fluvial fan progradation from outcrops in the central andsouthernSJB...... 127

Figure 3.11 Stratigraphic trends in the Continental Divide studyarea...... 128

Figure 3.12 Stratigraphic trends in the Ca˜non Largo study area...... 129

Figure 3.13 Illustration showing stacked prograding fan stratigraphic trends in the SanJuanBasin...... 130

Figure 3.14 Schematic cross-section of hypothesized style of fan progradation and annotatedoutcropphotos...... 131

Figure 4.1 Paleogene geology of the San Juan Basin (SJB) and studyarea. . . . . 168

Figure 4.2 Coalified-charcoalified fossil wood localities...... 170

Figure 4.3 Examples of outcrops interpreted as proximal, medial, and distal fan faciesintheSanJoseFormationoftheSJB...... 171

Figure 4.4 Stratigraphic trends the Arroyo Chijuilla study area...... 172

xi Figure 4.5 Fossil wood and bulk sediment organic carbon isotope records from ArroyoChijuilla...... 173

Figure 4.6 Fossil wood organic carbon isotope record and stratigraphic trends in ArroyoChijuilla...... 174

xii LIST OF TABLES

Table 2.1 Sandstone facies group 1 - sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations, or devoid of structure. . . . . 71

Table 2.2 Sandstone facies group 2 - sandstones with steeply dipping cross-strata and-lamina...... 73

Table 2.3 Mudrock facies group 1 - drab-colored mudrock...... 74

Table 2.4 Mudrock facies group 2 - purple-red mudrock...... 75

Table 2.5 Channel and floodplain facies associations...... 76

Table3.1 Sandstoneandmudrockfacies...... 132

Table 3.2 Sandstone and mudrock facies associations...... 135

Table 4.1 Compilation of coalified-charcoalified fossil wood carbon isotope ratios. . . 175

Table 4.2 Compilation of bulk sediment carbon isotope ratios...... 177

xiii ACKNOWLEDGMENTS

I would first like to thank my advisor, Dr. Piret Plink-Bj¨orklund, for taking me on as a PhD student, funding me, and giving me a chance to do this research in the San Juan Basin. I have learned so much from Piret about sedimentary geology, critical thinking, and scientific writing. I am eternally grateful for the many hours she spent helping me plan fieldwork strategies and improve upon the manuscripts included in this dissertation. Thanks to my committee members: Dr. Alexis Sitchler, Dr. Donna Anderson, and Dr. Jennifer Miskimins. I have so much gratitude for their guidance and support through the thesis proposal defense, comprehensive exams, and defense process. Additional thanks go to Dr. Donna Anderson for agreeing to provide an extra round of peer-review for the papers prior to submittal to journals. A very special thanks goes to Dr. Henry Fricke for his enthusiasm and willingness to collaborate on research in the San Juan Basin, teaching me about sample collection and carbon isotope geochemistry, allowing me access to his lab, introducing me to the PETM community, joining me in the field on multiple occasions, co-authoring papers, and for his constructive criticism of my trenching technique. Fieldwork would not have been productive without the help of my undergraduate field assistants: Leland Spangler, Ariel Rickel, and Eric Miller. To Ariel and Eric, thanks for sticking with me through the extreme heat, steep slopes, vertical cliffs, lack of showers, and so many biting bugs. Leland had the benefit of joining the study after I learned to avoid late-June and early-July fieldwork in the San Juan Basin. He deserves special recognition for his assistance with processing paleochannel measurements which contributed to the paper that is Chapter 3 of this dissertation. I also owe a big thanks to fellow graduate students Mark Hansford, Jianqiao Wang, Dessy Sapardina, and Sonia Campos-Soto who also assisted in the field and provided feedback and enlightening conversation. Thanks to non-geologist

xiv friends, Shelley Falk and Coung Spurlin, who joined in fieldwork to provide safety, support, and good company. The research presented in this dissertation benefitted immensely from conversations and collaborations with professionals from a variety of disciplines and with knowledge of the San Juan Basin. They include: Dr. Scott Wing, Dr. Ellen Currano, Dr. Tom Williamson, Dr. Ross Secord, Dr. Guy Harrington, Dr. Larry Smith, Dr. Spencer Lucas, Don Moore, and Esther May. A special thanks goes to fellow geologist Don Moore and his wife and Cuba, New Mexico historian, Esther May, who provided a safe and comfortable camp, showers, excellent company, and local knowledge during our visits to the San Juan Basin. Many thanks to my colleagues at the U.S. Geological Survey who first encouraged me to pursue a Ph.D and have remained interested in my research. I especially want to acknowledge Dr. Dan Muhs for his mentorship and for agreeing to serve as the internal reviewer for the papers that are being submitted to journal, Dr. Ren Thompson and Scott Minor for supporting my research, and Gary Skipp for giving me access to his lab. Also, a big thanks to my officemates Zach Ancona, Louisa Kramer, and Dr. Paul Henne for your friendship, and for putting up with my loud typing and occasional expletives. Thanks to the many friends who provided encouragement and much-needed outlets for fun and stress-relief over the last several years. If I ever considered quitting, those thoughts were quickly struck down by the knowledge that you would never let me live it down. Last, but certainly not least, I want to thank my family. It truly took a village to get through this PhD, especially since I was pursuing this degree while our family was growing. My parents, Earl and Anita Edwards, joined multiple trips to the field and often camped in less-than-ideal environments (a particular dirt parking lot in Wyoming is coming to mind), all to ensure that I could make progress toward my degree and not be separated from my children during their infancy. In addition to providing childcare while I worked during the day, they outfitted us with a camping trailer so the kids and I would have a comfortable, safe, and climate-controlled place to sleep. I am thankful for the time we were able to spend

xv together and the memories that were made during these fieldwork sessions. My mother-in- law, Nancy Zellman, is not a fan of camping, but she flew from Pennsylvania to Colorado on multiple occasions and for extended visits to us and spend time with the kids. I am eternally grateful to her for everything she has done to enable the completion of this degree, and for the strong relationship that she has built with our children. Finally, none of this would have been possible without the love and support of my husband, best friend, father of my children, and fellow geologist, Mark Zellman. Thanks for remaining unconditionally positive and encouraging through this long process, and for being a loving provider for our children. I am so looking forward to moving on to our next chapter of life together and enjoying many more adventures as a family. This project was supported by the U.S. Geological Survey. Additional support was provided by grants from GSA, AAPG and Timothy Bartshe, and scholarships from Conoco- Phillips, Anadarko, the Hemmesch Fellowship, and the Pustmueller Fellowship.

xvi This dissertation is dedicated to my family: My wonderful husband, Mark Zellman. Our boys, Garrett and Tyler. My parents, Earl and Anita Edwards. My mother-in-law, Nancy Zellman. None of this would have been possible without your ongoing support and many sacrifices.

xvii CHAPTER 1 INTRODUCTION TO THE DISSERTATION

1.1 Background

Earth’s climate has evolved dramatically over the past 60 million years. Complex fluc- tuations in thermal regimes have resulted in extremes ranging from hothouse climates with ice-free poles to icehouse climates with expansive continental ice sheets (e.g. Zachos et al., 2008). These climatic variations can take place gradually over millions of years driven by tectonic processes, they can be cyclic over 10s to 100s of thousands of years driven by orbital forcing, or they can occur as anomalous perturbations (Zachos, 2001). Of particular interest are perturbations that took place during the Paleogene that are characterized by global in- creases in temperature and are thus termed hyperthermal events (Kennett and Stott, 1991). The largest of the hyperthermals, the Paleocene-Eocene Thermal Maximum (PETM) at ∼56 Ma was an abrupt (less than 20 ky onset), transient (∼200 ky duration), and significant (5-8°C) global warming event that is associated with the input of thousands of gigatonnes of 13C depleted carbon into the ocean, atmosphere, and biosphere (Kennett and Stott, 1991; Zachos, 2001; McInerney and Wing, 2011). The source of the 13C depleted carbon release is still debated, and proposed mechanisms include: destabilized methane clathrates from deep sea sediments (Dickens et al., 1995), thermogenic methane released from injection of magma into organic-rich sediments (Frieling et al., 2016), wildfires that burned shallowly buried peat or coal (Kurtz et al., 2003), and rapid thawing of permafrost (DeConto et al., 2012). Regardless of the carbon source, the PETM is the fastest known, massive carbon release throughout the Cenozoic and is estimated at somewhere between current assessments of fossil fuel reserves (1,000-2,000 gigatonnes C) and resources (3,000-13,500 gigatonnes C), and is thus widely considered the best analogue for present and future carbon release (Zeebe et al., 2016).

1 The PETM is known to have caused severe climatic and biotic change in both marine and terrestrial environments (Gingerich, 1989; Thomas, 1989; Kennett and Stott, 1991; Koch et al., 1992; McInerney and Wing, 2011). It is also known that extreme climate fluctuations are likely to affect earth surface processes, but the nature of these effects is often unclear and difficult to study due to non-unique and difficult-to-decode responses (Slyom and Tucker, 2004; Molnar et al., 2006; Tucker, 2009; DiBiase and Whipple, 2011). Thus, it is likely that the best archive of landscape response to past climate is the sedimentary record (Allen, 2008; Armitage et al., 2011; Whittaker, 2012). Globally, there is a growing list of mid-latitude terrestrial PETM datasets that suggest climate change-driven changes to river systems, characterized by alluvial deposits that are unlike those underlying or overlying the PETM interval and suggestive of an increase in the magnitude or variability of river discharge (Schmitz and Pujalte, 2007; Foreman et al., 2012; Foreman, 2014). Three of these datasets are located in North America (Bighorn Basin, Wyoming; Picenance Creek Basin, Colorado; and Uinta Basin, Utah) (Fig. 1.1). More recently, records of post-PETM hyperthermals have also been recognized in some locations (Lourens et al., 2005; Nicolo et al., 2007; Abels et al., 2012), although their environmental and biotic impact is less understood (Sluijs et al., 2009; Abels et al., 2012). The Bighorn Basin of Wyoming is the most extensively researched of the terrestrial PETM records (e.g. Clyde and Gingerich, 1998; Fricke et al., 1998; Bowen et al., 2001; Magioncalda et al., 2004; Wing et al., 2005; Kraus and Riggins, 2007; Secord et al., 2010; Kraus et al., 2013). At present in North America, only the Piceance Creek Basin, Colorado, has been established as an additional well-constrained PETM dataset, however there are multiple suspected PETM datasets in other Laramide basins that present opportunities for ongoing research (Fig. 1.1). The San Juan Basin is the southern-most of these suspected PETM datasets, thus recognition of a PETM interval provides the opportunity to conduct studies of climatic, biologic, and landscape change in response to the climate perturbation.

2 Figure 1.1: Distribution of Laramide sedimentary basins formed between late Cretaceous and late Eocene time and the Williston Basin. Redrawn from Dickinson et al. (1988) and Lawton (2008). Hatch lines denote basins with known Paleocene-Eocene boundary deposits that are included in the proposed study. Sedimentary basins: B, Baca; BH, Bighorn; BM, Bull Mountain; Cab, Cabullona; C-LJ, Carthage-LaJoya; CM, Crazy Mountains; CS, Cutter Sag; D, Denver; EC, El Chanate; EP, Echo Park; ER, El Rito; F, Flagstaff; FC, Fort Crittenden; G, Galisteo; GR, greater Green River; H, Hanna; HP, Huerfano Park; K, Kaiparowits; KL, Klondike; L, Laramie; LHT, Little Hat Top; LR, Love Ranch; McC, Upper McCoy; MP, Middle Park; MV, Monte Vista; NP, North Park; P, Potrillo; PC, Piceance Creek; PR, Powder River; Ra, Raton; R-SR, Ringbone-Skunk Ranch; Ru, Rucker; S, Shirley; SB, Sierra Blanca; SJ, San Juan; SP, South Park; TC, Table Cliffs; U, Uinta; WR, Wind River; W, Williston.

3 1.2 Motivation

Initial motivation for this study came from an interest in making a contribution to PETM and post-PETM hyperthermal event research. The research presented in this dissertation evolved from an initial idea to investigate the river systems spanning the Paleocene-Eocene (P-E) boundary in North American Laramide basins, with the aim to test the hypothesis that signals of increased precipitation peakedness are recorded as variable discharge river deposits throughout the region in conjunction with early-Eocene global warming and the intensification of the hydrological cycle. Over the course of conducting a literature review, it became apparent that the San Juan Basin presented a unique research opportunity. First, the SJB had long been overlooked in the search for additional PETM intervals, due to a long-standing interpretation of a basin-wide depositional unconformity at the P-E boundary (e.g. Baltz, 1967; Cather et al., 2004) (Fig. 1.2a). This interpretation has been propagated through the literature (Dickinson et al., 1988) despite other lines of evidence that suggest that the succession is conformable in the northern SJB (Smith and Lucas, 1991; Williamson and Lucas, 1992) (Fig. 1.2b). Second, the time constraints on the location of the P-E boundary are loose and there is a minimum of 90 m of section separating the last Paleocene fossil locality in the Nacimiento Formation and the first early Eocene fossil locality in the San Jose Formation (Regina Member) (Lucas et al., 1981). Finally, the succession bounded by the existing time constraints shows major shifts in fluvial depositional style with similarities to trends observed in established PETM datasets in other mid-latitude basins (Chapter 3 and 4). After discussions with established researchers from the PETM community (Brady Fore- man, Henry Fricke, and Scott Wing), it became apparent that others have suspected that the San Juan Basin could present an additional PETM and/or post-PETM hyperthermal dataset. As a result, a multi-disciplinary research collaboration has been established that brings together researchers with expertise in sedimentology, geochemistry, and paleontology to test this hypothesis. The research presented in this dissertation is the first stage in this on-

4 Figure 1.2: Previous interpetations of Paleogene stratigraphic relationships in the SJB. A) drawn after interpretations from Cather, 2004. B) modified from Smith and Lucas, 1991.

going investigation of the upper Paleocene Nacimiento Formation and the early-Eocene San Jose Formation (Cuba Mesa - Regina Members) in the SJB. The key aims of this study are to (1) refine the chronostratigraphy of the P-E boundary and determine whether records of early-Eocene hyperthermals, including the Paleocene-Eocene Thermal Maximum (PETM) are preserved, and (2) document Paleocene-Eocene climate changes and their impact on fluvial sedimentation.

1.3 Dissertation Organization

The dissertation is organized into four chapters: Chapter 1 provides an introduction to the dissertation, and is followed by three standalone manuscripts formatted for submittal to peer reviewed journals (Chapters 2, 3 and 4). The first paper (Chapter 2) will be submitted to the Journal of Sedimentary Research. The second paper (Chapter 3) will be submitted to Basin Research. The third paper (Chapter 4) will be submitted to Palaeogeography, Palaeoclimatology, Palaeoecology. A brief summary of the three manuscripts and their key

5 contributions is provided below. Chapter 2 (1st paper) recognizes a signature of variable discharge rivers in the fluvial channel and floodplain deposits of the Paleocene Escavada Member of the Nacimiento For- mation and the early-Eocene Cuba Mesa and Regina Members of the San Jose Formation. We integrate observations and interpretations of channel and floodplain lithosomes with com- parisons to styles of variable precipitation in modern monsoonal and subtropical climates. The results document considerable variability in channel depositional styles and a long-term trend toward increasingly arid conditions in the floodplain deposits. Thus, we suggest that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high intensity precipitation events. Therefore, the existing variable discharge river facies models that consider only channel deposit facies may not capture critical information needed to make accurate interpretations of paleoclimatic conditions. Chapter 3 (2nd paper) identifies the Escavada Member of the Paleocene Nacimiento For- mation and Cuba Mesa Member and Regina Members of the San Jose Formation as deposits of a large fluvial fan. Furthermore, we recognize two stacked packages of fan progradation, separated by a retrogradational interval that may culminate in a previously unrecognized depositional hiatus at the contact between the Cuba Mesa and Regina Members of the San Jose Formation. The recognition that the succession is comprised of variable discharge river deposits adds to a growing body of evidence that formation of large fluvial fans is promoted by fluctuations in discharge linked to hydroclimates with inter- (seasonal) and intra-annual precipitation variability and intense rainfall. In addition, the results of this study have impli- cations for improved understanding of San Juan Basin landscape evolution and recognition criteria for prograding-retrograding fluvial fans in the ancient record. Chapter 4 (3rd paper) introduces preliminary stable carbon isotope records from organic matter in coalified-charcoalified fossil wood fragments and paleosol bulk sediment across the P-E boundary interval. The carbon isotope records are then compared with the stratigraphic

6 trends described in chapters 2 and 3. We observe carbon isotope excursions that we hypoth- esize record the PETM and at least one post-PETM hyperthermal event. Furthermore, the excursions coincide with the two fan progradation packages identified in Chapter 3. Thus, we are able to link fluvial fan progradation and increased discharge variability to potential climate perturbations. These findings suggest that the San Juan Basin could provide a new source of proxy data for climatic and biotic changes associated with the PETM and other early-Eocene hyperthermal events.

7 CHAPTER 2 TESTING HYPOTHESES ON PRECIPITATION VARIABILITY SIGNATURES IN THE RIVER AND FLOODPLAIN DEPOSITS OF THE PALEOGENE SAN JUAN BASIN, NEW MEXICO

2.1 Abstract

This study documents an outcrop dataset of river channel and floodplain lithosomes across the Paleocene-Eocene boundary from the Paleocene upper Nacimiento Formation and the early-Eocene San Jose Formation in the San Juan Basin, New Mexico, USA. We identify the succession as deposits of variable discharge river systems, based on a combination of 1) an abundance of upper flow regime and high deposition rate sedimentary structures indicative of flooding events, 2) preservation of in-channel bioturbation and pedogenic mod- ification indicating periods of prolonged dryness, 3) lack of identifiable braid or point bar strata, 4) alternations of poorly-drained and well-drained floodplain deposits and/or slicken- sides indicating alternating wet-dry cycles, and 5) presence of crevasse splay deposits within floodplain lithosomes that suggest channel avulsions linked to flooding events. We document considerable variability in channel depositional styles and demonstrate that integrating chan- nel lithesome and floodplain datasets may be essential for paleo-climate interpretations. We further observe a long-term stratigraphic trend toward increasingly well-drained floodplain deposits and thus successively more arid conditions from Paleocene into the early-Eocene. Comparison with modern climate types suggest a long-term shift from a monsoonal climate in the Paleocene to fluctuating humid and arid subtropical and then semi-arid/arid conditions in the early Eocene. We show that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high intensity precipitation events. Therefore, the existing variable discharge river facies models that consider only

8 channel deposit facies may not capture critical information needed to make accurate inter- pretations of paleoclimatic conditions.

2.2 Introduction

The surface of the Earth and other planetary bodies is shaped by tectonics and cli- mate, and the geomorphic and sedimentary records are archives of changes to these tectonic and climate regimes (e.g. Wobus et al., 2006; Tucker, 2009). To decipher the geomorphic or sedimentary record effectively, and to distinguish the tectonic and climatic signals, we need a sound understanding of how landscapes respond to changes in tectonic or climatic boundary conditions (e.g. Whittaker, 2012). Such deductions are ambitious because the interaction between climate, tectonics, and landscape is coupled and displays nonlinear dy- namics (e.g. Wobus et al., 2006a; b; Whipple, 2009; Jerolmack and Paola, 2010; DiBiase and Whipple, 2011). Deciphering climatic signals from landscape topography has been proven especially difficult due to non-unique and difficult-to-decode responses (Slyom and Tucker, 2004; Tucker, 2004; Molnar et al., 2006; DiBiase and Whipple, 2011), and it is thus likely that the best archive of landscape response to past climate is the sedimentary record (Allen, 2008; Whittaker et al., 2009; Armitage et al., 2011; Whittaker, 2012). Therefore, there is a particular need for clear recognition criteria for the expression of specific climate changes in the sedimentary record. Rivers are especially sensitive to tectonic and climatic forcing via their channel gradient and discharge (e.g. Macklin et al., 2012; Whittaker, 2012) that also affect the amount and composition of sediment (e.g. Cecil et al., 2003). Tectonic processes create topography and maintain relief through surface and rock uplift (Whipple, 2004; Wobus et al., 2006a; Whittaker et al., 2008). Climate provides the water needed for vegetation growth, soil development, and runoff that facilitates sediment transport, and thus influences the flux of water (discharge) and sediment that ultimately makes it into a river system (Leeder, 2011). Changes in climate can perturb these hydrological and sediment supply variables, thus altering erosion and sedimentation (e.g. Molnar et al., 2006).

9 Rainfall is one of the primary controls on river discharge and sediment yield (Langbein and Schumm, 1958). While traditional approaches (e.g. Langbein and Schumm, 1958) have focused on mean annual precipitation, and thus considered humidity-aridity changes as the key aspect of climate control on river discharge and sediment yields (e.g. Syvitski and Milliman, 2007), the temporal distribution of runoff plays a major role in determining stream channel and floodplain sedimentary processes through its control on the hydrograph and the nature of river morphodynamics, surface sedimentary environments, and subsurface soil development (e.g. Cecil et al., 2003; Goodbred, 2003; Latrubesse et al., 2005; Molnar et al., 2006; Powell, 2009). Rivers that experience high variability in precipitation also transmit variable discharge (Latrubesse et al., 2005; Leier et al., 2005). River discharge variability is a function of seasonal (intra-annual) as well as inter-annual variability (Leier et al., 2005; Fielding et al., 2009; Plink-Bj¨orklund, 2015, but c.f. Fielding et al., 2018). Modern examples of variable discharge rivers have primarily been highlighted in arid subtropics (dryland rivers) (e.g. Graf, 1988; Knighton and Nanson, 1997; Tooth, 2000; Powell, 2009; Reid and Frostick, 2011), but also recognized across sub-humid subtropics (e.g. Fielding et al., 2009) and monsoonal tropics (e.g. Latrubesse et al., 2005). Intense monsoon-driven summer rainfall causes high magnitude flooding in rivers that transmit a relatively low base flow during the sustained dry season (Latrubesse et al., 2005; Fielding et al., 2009; Syvitski et al., 2014; Plink-Bj¨orklund, 2015). As a result, these rivers are shown to deliver most of their water discharge and sediment load during the summer monsoon season (Goodbred, 2003), as only the flood discharge is geomorphically effective and able to transport sediment (Henck et al., 2010; Plink-Bj¨orklund, 2015). Compared to the monsoon domain, most subtropical areas are typically drier, with evapotranspiration exceeding precipitation in many cases. In the monsoon zone and in some subtropics, the intense monsoon driven rain may occur seasonally. More characteristically the subtropical areas have a high inter-annual variability with a precipitation recurrence frequency of multiple years or even decades. This extreme inter-annual precipitation variability is especially distinct in areas that receive monsoon

10 rain only during extreme monsoon trough migration, or during abnormal or strengthened monsoon seasons and associated cyclonic flow. Inter-annual monsoon anomalies have been shown to intensify the precipitation extremes, resulting in catastrophic flooding events and droughts (Seneviratne et al., 2012; IPCC, 2014). In addition, observations and proxy data have shown that monsoon intensity can also vary along longer time scales including inter- decadal, centennial, millennial, orbital, and tectonic time scales (Wang et al., 2014). There has been significant progress towards deciphering the signal of discharge variability in the river records and thus towards developing paleo-climate proxies for precipitation vari- ability. This work has developed sedimentary proxies that highlight an abundance of upper flow regime (UFR) sedimentary structures as a key recognition criterion for river discharge variability (Tunbridge, 1981; Sneh, 1983; Stear, 1985; Deluca and Eriksson, 1989; North and Taylor, 1996; Billi, 2007; Fielding et al., 2009, 2018; Allen et al., 2013; Plink-Bj¨orklund, 2015). This abundance is attributed to high flow velocities, and the characteristically rapid rise, sharp peak and rapid decline of the flood hydrographs. Fielding et al. (2009) further highlighted in-channel vegetation and vegetation-induced sedimentary structures in addition to the abundance of UFR sedimentary structures, and with that developed a facies model and recognition criteria for sub-humid subtropical rivers, through observations of the mod- ern Burdekin River and rock record analogues. Colonization of the channel floor indicates periods of very low flow or lack of base flow, and a seasonally low water table that is more accessible from channel floor, a condition most likely to occur in sub-humid subtropics (Field- ing et al., 2009; Allen et al., 2011, 2013, 2014). Plink-Bj¨orklund (2015) reviewed 17 modern and 35 ancient sedimentary records of variable discharge rivers across the subtropics and the monsoon zone and suggested further recognition criteria, such as high deposition rate sed- imentary structures, in-channel mud layers, thick soft-clast conglomerates, in-channel trace fossils, soft-sediment deformation, and the presence of flood event beds. Lack of barforms or well developed accretion sets, recognizable as point bar or braid bar strata, has been sug- gested as another key characteristic of rivers with variable discharge (e.g. Rhee and Chough,

11 1993; Uba et al., 2005; Billi, 2007; Hampton and Horton, 2007; Fielding et al., 2009; Hulka and Heubeck, 2010; Allen et al., 2011; Plink-Bj¨orklund, 2015; Wang and Plink-Bj¨orklund, in review). Plink-Bj¨orklund (2015) further observed that the sedimentary record of variable discharge rivers exhibits considerable variability in the occurrence of characteristic facies and architectural elements and attempted to link that variability to the presence of a wet season vs erratic storm occurrence, as well as to the presence or absence of the base flow. A different approach was taken by Fielding et al. (2018) with focus on the role of inter-annual peak discharge variability as a sole control on depositional style, based on an observation that modern rivers with similar inter-annual variability displayed similar styles of preserved sedi- mentology, even if the patterns of intra-annual variability were different. In contrast, global river discharge analyses of Hansford et al., (in review) suggests that there is a significant divergence in the style of discharge variability, that is linked to both intra- and inter-annual variability, such as seasonal, erratic and single-storm controlled behavior that all contrast persistent discharge patterns. In summary, despite the progress towards recognition criteria for river discharge variability and thus sedimentary proxies for precipitation variability, there is currently no consensus on how to define discharge variability explicitly, what is the link between different styles of discharge variability and river sedimentary record, or whether variable discharge river records from different climate types can be distinguished. In this paper we document channel and floodplain facies and their depositional styles in the Paleogene San Juan Basin, New Mexico (Fig. 2.1). We aim to 1) identify the Paleocene upper Nacimiento Formation (Escavada Member) and the early-Eocene San Jose Formation (Cuba Mesa and Regina Members) of the San Juan Basin as deposits of variable discharge river systems, 2) explore the differences in the depositional style of variable discharge rivers and their corresponding floodplain deposits, 3) hypothesize about the link between deposi- tional style of river deposits and climate, and 4) test the previous hypotheses for variable discharge river facies models (Fielding et al., 2009, 2018; Plink-Bj¨orklund, 2015) by combin- ing channel and floodplain depositional characteristics. We recognize discharge variability

12 at multiple scales and discuss the potential implications for improving our understanding of Paleocene - Eocene climates in the San Juan Basin.

2.3 Geological Setting and Background

The San Juan Basin is in the Four Corners region of northwestern New Mexico and southwestern Colorado and is one of several intraforeland basins formed during the Laramide orogeny (Dickinson et al., 1988; Cather, 2004). Laramide tectonics during the late Cretaceous through Eocene caused subsidence of the San Juan Basin in response to uplift of surrounding areas (Cather, 2004). It has been suggested that the San Juan Basin received most of its sediments from highlands to the north and the early San Juan Uplift (Baltz, 1967; Pecha et al., 2018), although source areas in the Brazos-Sangre de Cristo, Nacimiento, Zuni, and Defiance Uplifts are also suspected (Smith, 1992; Cather, 2004; Pecha et al., 2018) (Fig. 2.1b). Long-term evolution of the Cretaceous-Eocene sedimentary succession has been linked to tectonic episodes of subsidence in response to the Laramide orogeny (Fassett, 1985; Cather, 2004). The intraformational angular unconformity and reverse faults in the eastern outcrop area show that the Paleogene units in the San Juan Basin were deposited simultaneous with deformation along the basin-bounding Nacimiento fault (Woodward, 1987; Smith, 1988) (Fig. 2.1b).

2.3.1 Stratigraphy

The Paleogene units preserved in the San Juan Basin are the earliest Paleocene Ojo Alamo Sandstone (Toa), the Paleocene Nacimiento Formation (Tn), and the early Eocene San Jose Formation (Tsc, Tsr, and Tstl) (Fig. 2.1c, d). These units outcrop in a series of badlands and cliff-forming sandstones along the western and southern portions of the San Juan Basin. This study focuses on the upper-most Nacimiento Formation through the Regina member of the San Jose Formation.

13 2.3.1.1 Paleocene Nacimiento Formation

The Paleocene Nacimiento Formation is composed primarily of terrestrial fluvial deposits consisting of a succession of sandstone and varicolored mudrocks that attains a thickness of as much as 525 m (Baltz, 1967; Williamson and Lucas, 1992). The Nacimiento Formation is divided by lithologic and sedimentologic features into three members: The Arroyo Chijuil- lita (oldest), the Ojo Encino, and the Escavada (youngest) (Williamson and Lucas, 1992). The Arroyo Chijuillita Member is characterized by drab mudrocks and sparse heterolithic sandstones. The Ojo Encino Member is also mudrock dominant and is characterized by repetitive black-red-green-white banding that is associated with bentonitic and carbona- ceous mudrocks (black), variegated mudrocks (red and green), and fine-grained sandstones (white) (Williamson and Lucas, 1992). Three persistent black mudrock deposits within the Ojo Encino Member (referred to as the lower black, middle black, and upper black) can be correlated across the basin and serve as useful marker beds (see Leslie et al., 2018). The Escavada Member is characterized by a predominance of sandstones, drab mudrocks, and abundant silcretes (Williamson et al., 1992). Across the southern San Juan Basin, the thick- ness of the Escavada Member varies (19.2-88 m) due to intraformation thinning (Butler and Lindsay, 1985) and channel scouring at the base of the San Jose Formation (Williamson and Lucas, 1992). A north-to-south decrease in grain size and paleocurrent measurements indicate a prevailing north-to-south paleoflow throughout the Nacimiento Formation (Baltz, 1967; Klute, 1986; Sikkink, 1987; Smith, 1992). The early-Paleocene age of the Arroyo Chijuillita and Ojo Encino Members of the Nacimiento Formation are well constrained by biostratigraphy, magnetostratigraphy, and radiometric dating (Williamson and Lucas, 1992; Leslie et al., 2018). The age of the Escavada Member is poorly constrained, with the only age indicator being a zone of normal polarity in the western San Juan Basin that has been correlated to chron 26 (59.24-58.96 Ma), suggesting a late Paleocene age (Butler and Lindsay, 1985).

14 2.3.1.2 Early-Eocene San Jose Formation

The San Jose Formation is the most extensively preserved and exposed Eocene strati- graphic unit in New Mexico (Baltz, 1967; Smith and Lucas, 1991). Similar to the underlying Nacimiento Formation, it is composed of terrestrial fluvial deposits. Previous geological investigations of the southern and southeastern outcrop areas of the San Jose Formation include descriptions of fossils and local physical stratigraphic studies (e.g. Simpson, 1948; Lucas et al., 1981; Smith and Lucas, 1991; Smith, 1992), and regional stratigraphy and mapping (Baltz, 1967; Mytton, 1983; Manley et al., 1987; Smith and Lucas, 1991). The San Jose Formation contains both sandstone-dominated and mudrock-dominated lithofacies. The sandstone vs. mudrock-dominance is the primary characteristic used to distinguish between the Members within the San Jose Formation, listed from oldest to youngest: the Cuba Mesa (sandstone dominant), Regina (mudrock dominant), Ditch Canyon (sandstone dominant), Llaves (sandstone dominant), and Tapicitos (mudrock dominant) (Baltz, 1967; Smith, 1992). The Cuba Mesa Member at the base of the San Jose formation is a 25-240 m thick (based on lithostratigraphy), sandstone-dominated succession that overlies the Paleocene Nacimiento Formation (Baltz, 1967; Smith and Lucas, 1991). It consists of medium- to very coarse-grained, buff-colored sheet sandstones that thicken locally in vertically stacked (amalgamated) channel belts that can attain thicknesses up to 100 m, with channel width- to-thickness ratios ranging from 20 to >1000 (Smith, 1988). The basal sandstone of the Cuba Mesa member is continuous nearly basin-wide over an area of 8000 km2 (Smith, 1988). In some parts of the basin, the contact between the Nacimiento and Cuba Mesa Member of the San Jose Formation can be difficult to distinguish because the Escavada Member of the Nacimiento Formation contains arkosic sandstones that are similar in lithology to those of the Cuba Mesa member (Baltz, 1967; Smith and Lucas, 1991). Like the underlying Escavada Member, no age-diagnostic fossils or dateable materials have been discovered, leaving the deposits of both the upper Nacimiento Formation and the lower San Jose Formation with loose age constraints. It has long been propagated in the literature that there is a large

15 unconformity (≥ 5.6 m.y.; Fassett et al., 2010) that separates the Nacimiento and San Jose Formations in the southern San Juan Basin only (Barnes et al., 1954; Stone, 1983; Smith and Lucas, 1991) or throughout the basin (Cather, 2004). Due to the lack of age constraints within this interval and ongoing re-investigation of the nature of the Nacimiento - San Jose contact (Chapter 4), we have not identified the unconformity on our simplified stratigraphic column and we show that the location of the boundary between Paleocene and Eocene deposits is uncertain (Fig. 2.1c). The Regina Member of the San Jose Formation ranges from 150-460 m in thickness and is composed varicolored mudrock and interbedded sandstone. The sandstone beds are lentic- ular over scales ranging from a few meters to many kilometers (Smith, 1992). Lenticular conglomeratic sandstones in the northernmost outcrops of the member contain cobbles de- rived from Precambrian rocks common in the San Juan uplift north of the San Juan Basin (Smith, 1992). Vertebrate fossils provide the only means by which the early-Eocene age of the Regina Member has been determined (Williamson and Lucas, 1992).

2.4 Dataset and Methods

For this study, we conducted detailed investigations in three locations with excellent upper Nacimiento Formation, Cuba Mesa Member, and Regina Member outcrop exposure: Arroyo Chijuilla (AC) and Continental Divide (CD) in the southern San Juan Basin, and Ca˜non Largo (CL) in the central San Juan Basin (Fig. 2.1d). Approximately 720 m of section were measured between these three sites. Sandstones were described at approximately 10 cm resolution based on bedsets, sedimentary structures, biogenic structures, and grain characteristics (size, shape, sorting, and composition). Mudrock deposits were described based on texture and color of the matrix and mottles at approximately 1-1.5 m resolution. We used a qualitative assessment of texture and color following the methods recommended in Tabor et al. (2017), as time constraints precluded a detailed paleopedological study during this initial investigation. Additional information on soil structure (peds) and total organic carbon (TOC) was documented where mudrock horizons were sampled for carbon isotope

16 analysis (Chapter 4). Rooting structures, burrows, and presence of nodules were noted on the measured section when they were apparent, however since a detailed soil survey was not completed, this record should not be considered comprehensive. Herein, mudrocks are identified as paleosols where visible evidence of pedogensis is observed, such as mottling and moderate- to well- developed ped structures. Mudrocks are identified as protosols where relict laminations, visible plant fragments, or a high content of silt or sand content are observed and indicate an absence of soil formation processes. Photomosaics of outcrops were collected to document macroform characteristics and asso- ciations between sandstone and mudrock depositional styles. Sandstone channel thicknesses and widths were measured from outcrops in the Arroyo Chijuilla study areas using a us- ing a laser rangefinder (TruPulse 360R) mounted to a tripod. Laterally extensive channels were measured by collecting GPS coordinates at channel terminations. Sedimentary facies proportions were quantified using the measured facies thicknesses as compared to the total measured section thickness. In an effort to characterize paleoclimate conditions and their variability across the stud- ied stratigraphic interval, we use comparisons to modern climate conditions by estimating mean annual precipitation and precipitation variability, rather than latitude. We utilize sedimentological signatures of precipitation variability (see Fielding, 2006; Fielding et al., 2009; Plink-Bj¨orklund, 2015) and paleosol characteristics (e.g. Kraus, 1999; Tabor et al., 2017). We do not infer that the past conditions were comparable to every aspect of the mod- ern climate types, but rather use this as a general illustration of paleoclimate conditions. Monsoon climate (Am in the K¨oppen-Geiger modern climate classification, see Peel et al., 2007) is characterized by an annually high and a highly seasonal precipitation pattern, where the precipitation range (wet-dry season) is characteristically high due to the yearly migra- tion of the Inter-Tropical Convergence Zone (ITCZ) (Wang and Ding, 2008). Sub-humid tropical climate types (Aw and As in the K¨oppen-Geiger modern climate classification, see Peel et al., 2007) occur at an intermediate position between the ITCZ and the Subtropical

17 High. Like the monsoon climate, there is a distinct seasonality to precipitation, but with a much shorter and less sustained wet season and a considerably higher inter-annual variability (Wang and Ding, 2008). In arid climate types (Bw and Bs in the K¨oppen-Geiger modern climate classification, see Peel et al., 2007) mean annual precipitation is less than potential evapotranspiration and precipitation events are erratic and may have inter-annual to inter- decadal return frequency. Arid climates thus have an extreme inter-annual precipitation variability, as they receive monsoon rain only during extreme monsoon trough migration or abnormal or strengthened monsoon seasons and associated cylconic flow. Following these definitions we refer to paleoclimate conditions as monsoonal, if there is evidence for pre- cipitation variability and a high soil moisture; as arid if there is evidence for precipitation variability and a low (well-drained) soil moisture; and as sub-humid if there is evidence for precipitation variability and a relatively low soil moisture but with indicators for less well-drained conditions than in the latter.

2.5 Sedimentary Facies

Eleven sandstone sedimentary facies and eight mudrock sedimentary facies were defined. The individual facies are described below and summarized in Tables 2.1-2.4. We have divided the facies into four facies groups. Sandstone facies are grouped based on the characteristics of sedimentary structures: sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations or devoid of structure vs. sandstones with steeply dipping cross-strata and laminae. Mudrock facies are grouped by dominant color: drab vs. purple-red.

2.5.1 Sandstone facies group 1: sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations or devoid of structure

Sandstone facies group 1 is the most abundant, comprising approximately 85% of the sandstone facies present in the three measured sections combined, and includes conglomer- ates (S1.1 - 14%), sandstones with scour and fill structures (S1.2 - 21%), sandstones with planar to low-angle laminations (S1.3 - 19%), sandstones with convex-up long wavelength

18 laminations (S1.4 - 8%), sandstones with soft-sediment deformation (S1.5 - 5%), and struc- tureless sandstones (S1.6 - 18%) (Fig. 2.2, Table 2.1). The percentages indicate proportions of individual facies to the whole sandstone facies dataset, including the sandstone facies group 2. Commonly, these facies transition vertically and/or laterally into one another, forming complex relationships when viewed in outcrop. Facies S1.1: Conglomerates. Facies S1.1 consists of conglomerates overlying flat ero- sion surfaces and channel scours (Fig. 2.2a, b). The conglomerates are composed dominantly of granule- to boulder-sized mudrock rip-up clasts in a sandy matrix, although some also con- tain granule- to pebble-sized clasts of quartzite, chert, and other metamorphic rocks. In the disorganized conglomerates (Fig. 2.2a), the mudrock clasts are angular to subangular, matrix supported, and the conglomerates can reach thicknesses up to 3 m. In most of the stratified and imbricated conglomerates (Fig. 2.2b) the clasts are subrounded, clast-supported, and occur as composite bedsets, ranging from several decimeters to several meters thick and interbedded with tabular or lenticular sandstones. Many of the conglomerates grade into sandstones, while others are bound by erosion surfaces. In some occurrences, the deposits are weakly cemented, and exhibit badland-style erosion that makes them difficult to distin- guish from the surrounding mudrock facies without exposing fresh material. Lignitized plant fragments are common. Facies S1.2: Sandstone with scour-and-fill structures. Facies S1.2 consists of medium- to very coarse-grained sandstone with scour and fill structures (Fig. 2.2c). Grains are poorly to moderately sorted and subangular to subrounded. The primary characteristic of scour and fill structures is a scour filled with upward flattening laminae. The laminae usually fill the scours asymmetrically, although some symmetrical fills occur. Dip angles of laminae range from 20° to <5°, and dip directions of the laminae are multimodal, even within the same bedsets, with dip directions that display minor clustering towards the north- northeast and the southwest. The thickness varies from decimeters to several meters. Widths range from decimeters to 10s of meters. Generally, the size of scour and fill structures

19 increases with bed thickness and grain size. Commonly, scour and fill structures are laterally and vertically amalgamated in outcrop. In some occurrences, the scour and fill structures transition laterally into low-angle laminations (S1.3). Plant fragments occur in places along lamination boundaries. Localized lenses of conglomeratic sandstone with gravel-sized grains occur at the base of some scour and fill structures. Facies S1.3: Sandstone with planar to low-angle laminations. Facies S1.3 consists of fine- to very coarse-grained sandstone with distinct planar to low-angle laminations (<10°) (Fig. 2.2d). Grains are poorly to moderately sorted and subangular to subrounded. Many beds of this facies appear planar, but closer inspection reveals extremely low-angle (in many cases <5°) and long wavelength laminations. In many deposits of this facies, near-planar laminations transition laterally into steeper laminations that are commonly dipping to the north and northwest (Fig. 2.3a). Thickness of lamina sets ranges from a few decimeters to several meters. Plant fragments are common along laminations, especially when the deposit is near the base of a channel. Thin mud layers (<1 cm) are commonly seen between laminae sets. Facies S1.4: Sandstone with convex-up long wavelength laminations. Facies S1.4 consists of medium- to coarse-grained sandstone with convex-up, long-wavelength, and low angle laminations (Fig. 2.2e). Grains are poorly to moderately sorted and subangular to subrounded. Laminae set thicknesses range from decimeters to a few meters. Widths of the convex-up sets range from meters to 10s of meters. There are some examples in outcrop where this facies caps a large scour and fill structure (S1.2) or a thick set of low-angle laminations (S1.3). Similar to S1.3, many deposits of this facies, near-planar laminations transition laterally into steeper laminations that are commonly dipping to the north and northwest (Fig. 2.3a). The convex-up geometries and higher amplitude of laminations distinguish this facies from facies 1.3. Facies S1.5: Sandstone with soft sediment deformation. Facies S1.5 consists of medium- to coarse-grained sandstone with soft-sediment deformation (Fig. 2.2f). Grains

20 are typically poorly-sorted and subangular to subrounded. Occasional lenses of gravel-sized grains are present. Bedding and laminations within this facies are convolute and form anti- clinal to domal structures. Where present, this facies transitions laterally or vertically into other facies within group 1. Thicknesses range from 1 m to 10s of meters. Facies S1.6: Structureless sandstone. Facies S1.6 consists of fine- to coarse-grained sandstone that appears structureless in outcrop (Fig. 2.2g). Grains are poorly to moder- ately sorted and subangular to subrounded. In many occurrences, the deposits are isolated, lenticular or tabular, weakly cemented, vertically bracketed by mudrock deposits, and ex- hibit badland-style erosion that makes them difficult to distinguish from the surrounding mudrock facies (Fig. 2.2h). However, unlike the mudrock facies, these deposits are buff in color, have an erosional base, and commonly exhibit lenticular geometries. This facies is also observed in thick, vertically amalgamated channel deposits where it is well cemented and occurs in association with other facies or group 1. Thicknesses range from decimeters to 10s of meters. Sandstone facies group 1 interpretation: The compositional and textural similarities combined with the close stratigraphic relationship between the mudrock clast conglomerates (S1.1) and the underlying mudrock deposits indicate that the mudrock clasts are locally derived. Disorganized conglomerates with angular to subangular, matrix supported clasts indicate minimal current reworking and are likely to indicate bank failure. Similar deposits have, for example, been documented in the modern Kosi and Brahmaputra rivers, where the banks are undercut during floods, and develop shear cracks, whereby parts of the bank slump during the falling stage of floods (Coleman, 1969; Gohain and Parkash, 1990; Singh et al., 1993a). This bank undercutting and successive collapse is linked to high water power and rapid lowering of water level during the waning stage of floods (e.g. Manley et al., 1987; Gohain and Parkash, 1990; North and Taylor, 1996; Tandon and Gibling, 1997). Flowage of bank material into channels is another failure mechanism, that has been observed in the Brahmaputra River (Coleman, 1969), where flowage develops when the water level falls

21 rapidly, and pore water in the channel banks develops overpressure and results in failure. Such thick conglomerates have also been described from other ancient river deposits (e.g. Deluca and Eriksson, 1989; North and Taylor, 1996; Plink-Bj¨orklund et al., 2014), and suggested to be common in rivers with variable discharge that experience high magnitude floods with high water power and rapid changes in water level (North and Taylor, 1996; Plink- Bj¨orklund, 2015). The stratified and imbricated conglomerates indicate current reworking and transport (Karcz, 1969), as also corroborated by the observed lower degree of clast angularity. The convex- and concave-up sedimentary structures of the sandstone facies 1.2 - 1.4 are similar to experimentally produced sedimentary structures from Froude supercritical flow (e.g. Alexander et al., 2001; Cartigny et al., 2014; Ono et al., subm.), as well as to examples of modern river flood deposits (e.g. McKee et al., 1967; Williams, 1971a; Stear, 1985; Billi, 2007), and interpreted outcrop examples of Froude super- and trans-critical flow or upper flow regime (UFR) deposits of ancient rivers (e.g. Stear, 1985; North and Taylor, 1996; Fielding, 2006; Fielding et al., 2009; Allen et al., 2013; Plink-Bj¨orklund, 2015). Low-angle bedforms can also form by flattening of dunes at the transition to upper stage plane bed (e.g. Naqshband et al., 2017), but the presence of upstream-dipping laminae (backsets) sug- gests that the supercritical-flow origin is more likely, as dunes can only migrate downstream. Experimental flume studies have shown that scour and fill structures (S1.2) are formed by chute-and-pool formation, and/or cyclic steps (Alexander et al., 2001; Cartigny et al., 2014). Both chutes and pools and cyclic steps form at hydraulic jumps where flow abruptly and temporarily transitions from Froude supercritical to subcritical conditions with a large en- ergy loss, resulting in erosion and immediate deposition of the reworked sediments from suspension (Alexander et al., 2001; Cartigny et al., 2014). In these experiments, the struc- tures formed by chutes and pools and cyclic steps display multi-modal variability in laminae dip angles like those observed in the scour and fill structures in this dataset. Experimental studies have shown that planar to low-angle laminations (S1.3) are formed by the migration

22 of antidunes under Froude supercritical flow conditions (Alexander et al., 2001; Cartigny et al., 2014). In these experiments, migration of stable antidunes (with non-breaking surface waves) produces low-angle backset laminae at higher aggradation rates and laminae with approximately planar, sub-horizontal geometry at lower aggradation rates (Cartigny et al., 2014). Unstable antidunes (with breaking surface waves) produce discontinuous lenticular beds with variable internal architecture varying from backsets to low-angle foresets, wherein foresets indicate downstream migration of the antidunes and backsets indicate upstream mi- gration (Cartigny et al., 2014), similar to the observations in facies 1.2 and 1.3 (e.g. Fig. 2.3a). Convex-up low-angle laminae (S1.4) have also been shown in experiments to be formed by antidune migration under Froude supercritical flow conditions (Alexander et al., 2001). However, their formation requires higher deposition rates whereby sediment fallout exceeds bedform migration rates (Alexander et al., 2001). More recent experimental flume studies using a wide range of grain sizes (Ono et al., subm.) have shown that cyclic steps alone can produce complex laminations that include scour and fill structures, planar to low angle laminae with foresets and backsets, and that the nature of the variability is controlled by the size and migration rate of hydraulic jumps. Consistent backsets are only formed when the migration of the hydraulic jumps is stable. When the migration of the hydraulic jumps is unstable, erosionally bound scour and fill structures are generated. Low-angle and planar laminae form downstream of the hydraulic jumps. Regardless of the specific bedform regime responsible for the creation of these structures, facies S1.2, S1.3, and S1.4 are indicative of deposition under Froude supercritical flow conditions. The poor sorting and the common gradational nature of the above described sandstones, together with the experimental results indicate suspension transport of sands (Alexander et al., 2001; Cartigny et al., 2014; Ono et al., subm.), as well as gravels (Ono et al., subm.), under supercritical flow conditions, and high sediment fallout rates at and downstream of hydraulic jumps. Both soft-sediment defor- mation (S1.5) and structureless sandstones (S1.6) have various proposed origins, including movement and deposition in a liquefied state, post-depositional liquefaction processes that

23 modify or obliterate stratification, and high deposition rates and horizontal shear (Postma and Cartigny, 2014). The occurrence of soft-sediment deformation and structureless sand- stones in stratigraphic succession with scour and fill (S1.2) and low-angle laminations (S1.3 and S1.4) have been described in turbidite outcrops (Postma et al., 2009; Postma and Car- tigny, 2014) and produced in supercritical flow flume experiments (Alexander et al., 2001; Cartigny et al., 2014). The experiments show that structureless beds are generated from rapid sediment fallout due to large energy drop immediately following a hydraulic jump, and when there is an abrupt shift in the location of a hydraulic jump. These abrupt shifts can also result in folding of underlying deposits and the creation of flame structures due to rapid porewater expulsion and loading by sediments overlain at high deposition rates (Postma et al., 2009). By comparison with experimental data and their lateral link to other facies in this group, we hypothesize that the deposits of facies S1.5 are likely convolute versions of facies 1.2 - 1.4. Rapid sediment fallout due to an abrupt energy drop after a hydraulic jump may explain the occurrence of some structureless sandstone deposits (S1.6), particularly where the facies occurs in association with other facies in this group (Fig. 2.3b). This interpre- tation is only applied to deposits that are well-cemented. The weakly-cemented deposits may contain structures that are not visible on the surface due to weathering and erosion. Therefore, it is likely that this facies is over-represented in our dataset.

2.5.2 Sandstone facies group 2: sandstones with steeply dipping cross-strata and -lamina

Sandstone facies group 2 (Fig. 2.4; Table 2.2) consists of sandstones with cross-strata and -lamina at or close to the angle of repose and form consistent cross sets. This facies group comprises approximately 15% of the sandstone facies of this dataset and includes cross-stratified sandstone (S2.1 - 11%), sandstone with climbing cross-strata (S2.2 - 0.1%), sandstone with climbing ripple laminations (S2.3 - 2%), and sandstone with overturned cross-strata (S2.4 - 2%).

24 Facies S2.1: Cross-stratified sandstone. Facies S2.1 consists of medium- to lower- coarse grained sandstone with planar or trough cross-strata (Fig. 2.4a). Grains are moder- ately to poorly sorted and subrounded. This facies is recognized as cross-sets with consistent foresets that display no change in dip angle (∼30°). Dip directions are to the south and south- east. Thicknesses of individual cross-sets range from decimeters to ∼1 m. Thicknesses of bedsets range from several decimeters to several meters. Facies S2.2: Sandstone with climbing cross-strata or -laminae. Facies S2.2 con- sists of medium- to lower-coarse grained sandstone with climbing cross-strata or -laminae (Fig. 2.4b). Grains are moderately- to poorly-sorted and subangular to subrounded. De- posits of this facies have the same characteristics as Facies S2.1, except that the set bound- aries are inclined, dipping to the north and opposite to the southerly cross-strata dip di- rection. Thicknesses of individual cross-sets are a few centimeters to several decimeters. Thicknesses of bedsets range from decimeters to 2 m. Facies S2.3: Sandstone with scours and climbing cross-laminae. Facies S2.3 consists of upper fine to medium grained sandstone with scours filled with climbing ripple laminations (Fig. 2.4c). Grains are moderately- to poorly-sorted and subangular to sub- rounded. Deposits of this facies have similar internal characteristics as Facies S2.2, however they are smaller-scale with cross-set thickness <3 cm and are typically composed of finer grain sizes. Thicknesses of bedsets range from a few centimeters to a few meters. Facies S2.4: Sandstone with overturned cross-strata or -laminae. Facies S2.4 consists of medium to lower-coarse grained sandstone with deformed high-angle foresets (Fig. 2.4d). Grains are moderately to poorly sorted and subangular to subrounded. This facies is recognized by clear set boundaries that are deformed and contain oversteepened foreset laminae (>30°) that dip in the southerly direction. Thicknesses of individual cross-sets range from several centimeters to several decimeters and form bedsets that reach thicknesses up to 5 m.

25 Sandstone facies group 2 interpretation: Planar or trough cross-stratification (Fa- cies S2.1) is formed by migration of dunes under subcritical flow conditions (Simons et al., 1965; Allen, 1984). Climbing cross-stratification (Facies S2.2) indicates highly aggradational dune migration under subcritical flow and high deposition rates (Allen, 1984). Climbing ripple laminae (Facies S2.2 and S2.3) are linked to ripple migration under high deposition rates. Overturned cross-strata (Facies S2.4) are interpreted as sets of regular cross-strata that were deformed by the shear stress caused by the overriding current (Mills, 1983). The migration of dunes and ripples in Facies 2.1 - 2.4 occurs in downstream direction as sediment is eroded from the stoss side and deposited on the lee side, thus only the foresets are pre- served (Simons et al., 1965; Allen, 1984; Miall, 2014), and the dip direction of the foresets is a reliable indicator of paleoflow direction. The direction of overturning in Facies S2.4 is consistent with the prevailing southerly paleoflow direction. Collectively, we interpret the facies within this group as indicators of Froude subcritical flow conditions.

2.5.3 Mudrock facies group 1: drab-colored mudrock

Mudrock facies group 1 (Fig. 2.5; Table 2.3) consists of mudrock that is grey, grey-green, or brown (drab) in color. This facies group comprises approximately 64% of the mudrock facies in the three measured sections combined and includes globular grey mudrock (M1.1 - 3%), black mudrock (M1.2 - 3%), brown to dark grey mudrock (M1.3 - 18%), grey-green mudrock (M1.4 - 4%), and grey mudrock (M1.5 - 37%). Facies M1.1: Globular grey mudrock. Facies M1.1 consists of mudrock with large, globular, concave-up soft-sediment deformation (Fig. 2.5a). These deposits appear domi- nantly grey to dark-grey on the surface. Grain size ranges from clay to silt. The thickness of these deposits is >1m. Facies M1.2: Black mudrock.—Facies M1.2 consists of black mu- drock (Fig. 2.5b). Grain size ranges from clay to silt. Deposits of this facies form laterally continuous, dark bands in badland exposures. Deposits of this facies contain subangular blocky peds of moderate structure. Slickensides are common. Where sampled and analyzed for total organic carbon (TOC) content (Chapter 4), these deposits were found to have a

26 slightly higher organic content than surrounding deposits. Facies M1.3: Brown to dark grey mudrock. Facies M1.3 consists of brown to dark grey mudrock (Fig. 2.5c). Grain size ranges from clay to silt. Some deposits of this facies are laminated and contain plant fragments, while other deposits are massive and structureless. Facies M1.4: Grey-green mudrock. Facies M1.4 consists of mudrock with domi- nantly grey-green coloring (Fig. 2.5d). Grain size ranges from clay to silt. Deposits of this facies typically exhibit subangular blocky peds of weak to moderate structure. Slickensides are rare. The dominantly green color is the result of extensive green mottling within an otherwise grey matrix. Facies M1.5: Grey mudrock. Facies M1.5 consists of mudrock with a dominantly grey matrix (Fig. 2.5e). Grain size ranges from clay to silt. Deposits are typically of weak to moderate structure with angular blocky peds and devoid of mottling or slickensides, although some may contain minor amounts of purple-red mottling. Other deposits appear structureless and contain small lignitized plant fragments and visible quartz granules. Mudrock facies group 1 interpretation: The large, globular soft sediment deforma- tion structures (Facies M1.1) likely resulted from loading of wet muds by sand deposited above, similar to the ball-and-pillow structures (Mills, 1983). Black (Facies M1.2) or dark brown colors (Facies M1.3) are commonly attributable to the presence of organic matter or manganese oxide minerals (Vepraskas, 2000; Farnham and Kraus, 2002; Tabor et al., 2017). Grey or green colors (Facies M1.4 and M1.5) are thought to be the result of reduction of ferric minerals or removal of organic matter and oxides, which are processes associated with water saturation and anoxia (Vepraskas, 2000; Farnham and Kraus, 2002; Tabor et al., 2017). De- posits exhibiting weak to moderate structure and blocky peds are interpreted as former soil horizons (paleosols) of varying maturity based on field recognition criteria (Retallack, 1988). Many of the structureless deposits that contain visible plant fragments and quartz granules (Facies M1.5) are likely protosols, indicating that the soils were buried before pedogenesis could take place (Mack et al., 1993; Tabor et al., 2017). In summary, mudrock facies group 1

27 is consistent with floodplain deposits indicating poorly-drained soil conditions and/or lesser degrees of pedogenic development.

2.5.4 Mudrock facies group 2: purple-red mudrock

Mudrock facies group 2 (Fig. 2.6; Table 2.4) consists of mudrock that is dominantly purple, red, or variegated in color. This facies group comprises approximately 36% of the mudrock facies of this dataset and includes purple mudrock (M2.1 - 18%), red mudrock (M2.2 - 16%), and mudrock with variegated color (M2.3 - 2%). Facies M2.1: Purple mudrock. Facies M2.1 consists of mudrock with a purple matrix (Fig. 2.6a). Grain size ranges from clay to silt. Deposits of this facies contain blocky peds of weak to moderate structure. Slickensides are present in some deposits. Yellow-brown mottles are common. Rarely, grey-green mottles are also present. Facies M2.2: Red mudrock. Facies M2.2 consists of mudrock with a red (pink to maroon) matrix (Fig. 2.6b). Grain size ranges from clay to silt. Deposits of this facies have blocky to wedge-shaped peds of moderate to strong structure and nearly always contain slickensides. Grey mottles are common. A few horizons contain large (>mm) carbonate nodules. Facies M2.3: Mudrock with variegated color. Facies M2.3 consists of mudrock with abundant, varicolored mottling of grey-green, yellow-brown, purple, and red (Fig. 2.6c). Grain sizes range from clay to silt. Deposits of this facies typically exhibit wedge-shaped peds of strong structure and slickensides are abundant. Mudrock facies group 2 interpretation: The purple to red color of mudrock facies group 2 is attributed to the presence of hematite, an iron oxide mineral resulting from the soil being moderately- to well-drained and oxidizing (Retallack, 1988). Deposits with a purple matrix are interpreted as having weaker hematite impregnation compared to deposits with a red matrix (Kraus et al., 2013). Grey and yellow-brown mottles are redoximorphic features produced by soil gleying, and indicate that the soils were episodically saturated (Farnham and Kraus, 2002). Grey mottles are also attributed to root traces, and are thought to be

28 a redoximorphic depletion feature formed when stagnating surface water seeps downward along root channels and, in the presence of organic matter, iron is reduced and removed (Kraus and Hasiotis, 2006). The presence of slickensides indicates repeated shrinking and swelling of clays due to fluctuations in the water table (e.g. Bigham et al., 2002; Kraus and Riggins, 2007). Carbonate nodules are formed under well-drained, oxidizing conditions with a net water deficit due to evapotranspiration and their presence indicates periods of dryness (Breecker et al., 2009). Most deposits of these facies are interpreted as paleosols based on the presence of peds, and in some cases, fossil root traces (Retallack, 1988). Deposits with well- developed structure are interpreted as mature paleosols, formed over longer intervals of time, in stable landscapes, or under intense weathering conditions (Marriott and Wright, 1993; Kraus, 1999; Tabor et al., 2017). The wedge-shaped peds that are common in these deposits are thought to form only in soil profiles where seasonal precipitation and abundant fine clay combine to induce shrink-swell processes (Tabor et al., 2017). Deposits with laminations, preserved organic matter, and/or a high silt content are interpreted as protosols. In these cases, the deposits were likely buried before pedogenesis could take place (Tabor et al., 2017). In summary, mudrock facies group 2 is consistent with floodplain deposits indicating moderate to well-drained soil conditions.

2.6 Facies Associations

The sandstone facies occur predominantly as erosionally-based lenticular or laterally and/or vertically amalgamated lithosomes, 2-50 m thick and 30-1500 m wide, that are lat- erally associated with floodplain mudstones of mudrock groups 1 and 2. These sandstone occurrences are interpreted as channel deposits as documented below in facies associations 1-3 (Fig. 2.7 and 2.8; Table 2.5). Some sandstones of Facies S1.6 also occur as tabular sand- stones 0.20-3 m thick interbedded with floodplain mudrock facies as documented below in floodplain facies associations 4-6 (Fig. 2.9; Table 2.5). Some mudrocks also occur in channel deposits as documented in facies associations 2 and 3 (Fig. 2.7 and 2.8; Table 2.5).

29 2.6.1 Facies association 1 (FA 1) - sandy channel lithosomes

Facies association 1 is primarily composed of thick (10-50 m), erosionally-bound sand- stones that have a massive or uniform appearance in outcrop (Fig. 2.7a), except for thick (>1 m) mud-clast conglomerates that occur at the bases of channels and between some sand- stone deposits (Fig. 2.10a, d). Their vertical amalgamation results in sandstone cliffs that can reach thicknesses of >100 m in the northern San Juan Basin. Where measured, individ- ual channels range from 172 - 1564 m in width. The extensive lateral amalgamation of the cross-cutting channel scours gives the channel complexes a tabular appearance. Large, low- angle (<10°) downstream or upstream dipping accretion sets are observed in outcrops that are oriented approximately parallel to the prevailing flow direction (north to south) (Fig. 2.10b). UFR and high deposition rate (HDR) sedimentary structures are dominant (sand- stone facies group 1, 96%) (Fig. 2.8a and 2.9c). Commonly, the UFR structures transition into other UFR structures laterally and vertically (Fig. 2.10c). For example, a commonly encountered facies succession in the Cuba Mesa Member starts with a basal scour overlain by a thick mud-clast conglomerate (Facies S1.1), which is then overlain by vertical alterna- tions of scour and fill (S1.2) and planar to low-angle laminations (S1.3) (Fig. 2.8a). In some examples, the scour and fill structures truncate the underlying planar to low-angle lamina- tions. In other examples, the transitions are the result of upward steepening or flattening of the laminations. It is also common to see thick and laterally extensive deposits with stacked scour and fill structures (Fig. 2.8a and 2.9c). Some deposits are bioturbated from the top down or from accretion set boundaries with vertical burrows that are commonly >1m in length (Fig. 2.10e). Lithosomes of facies association 1 are associated with variably-drained (FA 5) and well-drained (FA 6) floodplain lithosomes. Facies association 1 interpretation: The dominance of UFR and HDR sedimentary structures indicates that most deposition occurred at high flow velocities and high deposition rates (see Stear, 1985; Abdullatif, 1989; Alexander et al., 2001; Fielding, 2006; Cartigny et al., 2014; Plink-Bj¨orklund, 2015). A general lack of transitions from UFR structures to rip-

30 ples or dunes implies extremely rapid decline in flow strength (Jones, 1977). The thick mud- clast conglomerates indicate high floodplain erosion and channel bank collapse and flowage rates (Karcz, 1969; Laury, 1971). In channel bioturbation indicates that the channels only transmitted discharge during floods and were seasonally dry or had low perennial base flow (Hasiotis et al., 2007). These lithosomes appear to consist entirely of high-magnitude flood deposits or flood event beds, such as those produced during flash flood events (Tunbridge, 1981; Stear, 1985). The high degree of channel amalgamation and lack of systematic bar migration (lateral accretion) signifies high avulsion rates (Bryant et al., 1995).

2.6.2 Facies association 2 (FA 2) - heterolithic channel lithosomes

Facies association 2 also consists of abundant sandstones, but these lithosomes contain mudrock layers along accretion set boundaries and display a lesser degree of vertical amal- gamation. As a result, these lithosomes appear less laterally and vertically amalgamated in outcrop than those of FA 1 (Fig. 2.7b and 2.8b). FA 2 lithosome thicknesses range from 5-13 m and widths range from 94-312 m. Accretion sets dip at a low angle (<10°) in the downstream direction in outcrops that are oriented in the flow parallel direction (north to south) (Fig. 2.7b and 2.10a). UFR and HDR sedimentary structures are still dominant (sandstone facies group 1, 89%), but there is a larger proportion of lower flow regime (LFR) structures (sandstone facies group 2, 11%) compared to FA 1 (Fig. 2.8b). The most common LFR structures are cross-stratified sandstone (S2.1 - 10%) and scours filled with climbing ripple lamina (S2.3 - 2%), which are observed overlying UFR sedimentary structures near the top of depositional units (Fig. 2.8b). Lateral variations in facies within individual litho- somes are common. Some deposits are bioturbated from the top down or from accretion set boundaries. Burrows associated with this lithesome are shallower, branching trace fossils (Fig. 2.11b). Mudrock layers of several decimeters in thickness are grey (M1.3 and M1.5) to purple (M2.1) in color and commonly display pedogenic modification in the form of mottling and ped structure (Fig. 2.11a). Some of the dark-grey (M1.3) mudrock layers contain plant fragments. Lithosomes of facies association 2 are associated with poorly-drained (FA 4) and

31 variably-drained (FA 5) floodplain lithosomes. Facies association 2 interpretation: The dominance of UFR and HDR sedimentary structures indicates that most deposition occurred at high flow velocities and high deposition rates (see Stear, 1985; Abdullatif, 1989; Alexander et al., 2001; Fielding, 2006; Postma et al., 2009; Cartigny et al., 2014), similar to FA 1. The vertical transitions from UFR to LFR structures indicate a less rapid decline in flow strength during the waning phase of floods than in FA 1 (Jones, 1977). Deposition of in-channel mud horizons has been linked to the characteristically rapid waning of high-magnitude floods (Williams, 1971; Tunbridge, 1981). Rapidly declining flows leave little trace of lower flow regime conditions, and thus UFR structures may be directly overlain by silt or mud drapes deposited at the final stage of the flood (Williams, 1971; Tunbridge, 1981). Newly-formed mud drapes at the tops of bars, on accretion surfaces or at the channel bottom have been observed in modern variable discharge rivers after major floods (e.g. Stear, 1985; Abdullatif, 1989; Singh et al., 1993; Billi, 2007). For example, after a high magnitude flood of Kosi River in 1984, a 30 cm deep, 20 m wide and 100 m long mud layer was deposited in a channel (Singh et al., 1993a). The presence of bioturbation and pedogenic modification indicate that the channels were seasonally or inter-annually dry or had low perennial base flow (Hasiotis et al., 2007).

2.6.3 Facies association 3 (FA 3) - lenticular channel lithosomes

These lithosomes are isolated and lenticular in shape. The FA 3 thicknesses range from 2-17 m and widths range from 27-282 m. In some examples, lateral accretion sets dipping at 10-30° occur where the outcrop exposure is flow-perpendicular (Fig. 2.12a). These channel deposits range from sandy to heterolithic with mud layers separating sandy accretion sets. Although still abundant, the content of UFR sedimentary structures is considerably lower (sandstone facies group 1, 50%) (Fig. 2.8c). LFR structures (sandstone facies group 2, 50%) include cross-stratified sandstone (S2.1 - 39%), scours filled with climbing ripple lamina (S2.3 - 3%), and overturned cross-strata (S2.4 - 8%). Vertical and lateral transitions between UFR and LFR structures are frequently observed, although some of the smaller lenticular channels

32 are filled entirely with LFR structures (Fig. 2.12b). Lignitized plant and wood fragments are occasionally found in these lithosomes. Channels of facies association 3 are associated with poorly-drained (FA 4) and well-drained (FA 6) floodplain lithosomes. Facies association 3 interpretation: The isolated nature of these channel lithosomes indicates a lower degree of channel amalgamation. The combination of LFR and UFR sed- imentary structures indicates variations between high and low flow velocities, where rapid deposition in supercritical flow conditions alternated with systematic subaqueous dune mi- gration during more sustained subcritical flow conditions. This presence of cross-strata above UFR structures indicates a less rapid decline of flood strength and preservation of the waning phase deposits (Abdullatif, 1989; Plink-Bj¨orklund, 2015). Alternatively, the LFR structures may be the result of post-flood reworking of sediment under subcritical flow conditions (Ab- dullatif, 1989; Plink-Bj¨orklund, 2015). A modern example of such UFR and LFR deposit variability is River Gash in Sudan that experiences high-magnitude floods that last for about a week, but also a wet-season base flow that lasts for the whole monsoon season (Abdullatif, 1989).

2.6.4 Facies association 4 (FA 4) - poorly-drained floodplain lithosomes

Facies association 4 consists of vertically stacked paleosol and protosol deposits of dom- inantly mudrock facies group 1 (drab-colored mudrock, 95%), including globular grey mu- drock (M1.1), black mudrock (M1.2), brown to dark grey mudrock (M1.3), grey-green mu- drock (M1.4), and grey mudrock (M1.5). Deposits of mudrock facies group 2 are rarely observed in this facies association (purple-red mudrock, 3%). Most of the mudrock layers are tabular, however there are also occasional scour surfaces filled with brown to dark grey mudrock (M1.3) (Fig. 2.9a). Vertical successions show alternations of protosols with no vis- ible evidence of pedogenic development and commonly exhibiting preserved organic matter and/or relict laminations (M1.3 and some deposits of M1.5) with more strongly developed paleosols (M1.2, M1.4, and deposits of M1.5 with ped structure). Thin, tabular or lenticular, and poorly-cemented buff-colored sandstones (S1.6, 2%) are commonly interbedded within

33 the mudrock succession (Fig. 2.9a) and range in thickness from 0.20-3 m. These sandy deposits are difficult to distinguish from the surrounding mudrock facies because they all display a similar style of badland erosion, forming steep slopes (>45°) that are commonly capped with well-cemented, cliff-forming channel deposits (Fig. 2.9a) and a thick interval (>1 m) of globular grey mudrock (M1.1) at the base of the channels. Facies association 4 interpretation: The poorly-drained floodplain lithosomes with poorly-cemented sandstones exhibit a range in level of pedogenic development from protosols to more strongly developed paleosols. Similar styles of stacked paleosols are interpreted to record variations in the balance between sediment accumulation and the rate of pedogen- sis (Marriott and Wright, 1993; Kraus, 1999). Pedogenic development can only occur on land surfaces which are relatively stable for a long period of time (Ruhe, 1956; Marriott and Wright, 1993), and vertically stacked, multi-story paleosols are formed in sedimentary systems undergoing net aggradation, where the rate of sedimentation does not overwhelm the rate of pedogensis (Kraus, 1999). The vertically stacked profiles containing protosols and weakly- to moderately-developed paleosols indicate that sedimentation was rapid and unsteady at times, precluding the development of strongly developed soils (sensu compound to composite paleosols of Kraus, 1999). Yet, some scoured surfaces within the floodplain successions indicate that some soil profiles may have been truncated due to erosion of the upper part of a developing soil (Marriott and Wright, 1993; Kraus, 1999). The stacked flood- plain deposits overlying the scour surface indicate that episodes of erosion and truncation were eventually followed by renewed sedimentation (sensu Marriott and Wright, 1993). The combination of compound to composite paleosols and erosion surfaces is thought to result from an unstable floodplain (Marriott and Wright, 1993; Kraus, 1999). The presence of relict planar laminations (some deposits of M1.3) filling some scoured surfaces may indicate the presence of standing water (ponding) on parts of the floodplain. The poorly-cemented buff sandy deposits are likely crevasse splays where thin and tabular, and isolated chan- nel deposits where lenticular. The crevasse splay deposits indicate partial avulsions, which

34 causes channels to shift their positions on the floodplain (Bridge and Leeder, 1979; Jones and Schumm, 1999). This process can terminate sedimentation to part of the floodplain for a period of time, leading to pedogenic development of surface deposits (Kraus, 1999). Meanwhile, the location occupied by the overbank flow of the partial avulsion experiences a high rate of sedimentation that prevents pedogenic development (Kraus, 1999). The overall gleyed (drab) coloring of the facies indicate wet and reduced soils, where anoxic conditions contribute to the preservation of organic matter (Vepraskas, 2000). Taken together, the combination of facies in this association indicates periods of net aggradation through high deposition rates alternating with times of landscape stability or erosion in a frequently wet depositional environment.

2.6.5 Facies association 5 (FA 5) - variably-drained floodplain lithosomes

Facies association 5 consists of vertically stacked paleosol and protosol deposits of both mudrock facies group 1 (drab-colored mudrock, 46%) and mudrock facies group 2 (purple- red mudrock, 22%), with interbedded thin, tabular, and poorly-cemented sandstone deposits (S1.6, 32%) (Fig. 2.9b). Similar to FA 4, these vertical successions show alternations of protosols (M1.3 and some deposits of M1.5) and paleosols. However, the paleosols within this facies association are more strongly developed and display a range of colors including grey-green (M1.4), purple (M2.1), red (M2.2), and highly varicolored (grey-green, yellow- brown, purple, and red mottling, M2.3). Nearly all of the deposits are tabular and scour surfaces truncating paleosol profiles are rarely observed unless they are associated with sandy channel fills. Facies association 5 interpretation: The vertically stacked paleosol profiles indi- cate a sedimentary system undergoing net aggradation. The presence of more strongly developed paleosols, fewer protosols (compared to FA 4), and a lack of truncated paleosol profiles indicates that erosion was insignificant, and sedimentation was unsteady enough to allow for paleosol development (sensu composite paleosols of Kraus, 1999). The combina- tion of drab (reduced) and purple-red (oxidized) paleosols may indicate alternating wet and

35 dry periods on the floodplain during times of low deposition. The presence of interbedded poorly-cemented, buff sandy deposits, interpreted as crevasse splays (see FA 4 interpreta- tion), indicate that avulsion processes were occurring on the floodplain.

2.6.6 Facies association 6 (FA 6) - well-drained floodplain lithosomes

Facies association 6 consists of vertically stacked paleosol and protosol deposits with a high occurrence of purple-red mudrock (mudrock facies group 2, 42%), including purple mudrock (M2.1), red mudrock (M2.2), and mudrock with variegated color (M2.3) (Fig. 2.9c). Vertical successions of this facies association consist of alternations of thick, tabular mudrock layers exhibiting strong pedogenic development with thick deposits of protosols (M1.3 and some deposits of M1.5, 30%) and thin, tabular or lenticular, and poorly-cemented buff- colored sandy deposits (S1.6, 28%) with no visible evidence of pedogenic development. These sandy deposits are difficult to distinguish from the surrounding mudrock facies because they all display a similar style of badland erosion, forming steep slopes (>45°) that are commonly capped with the well-cemented, cliff-forming sandstone channel deposits. Nearly all the deposits are tabular and scour surfaces truncating paleosol profiles are not observed unless they are associated with sandy channel fill. Facies association 6 interpretation: As with FA 4 and FA 5, this facies association indicates a sedimentary system undergoing net aggradation. However, the alternation of thick, strongly developed paleosol profiles (sensu cumulative paleosols of Kraus, 1999) and weakly developed protosols and sandy deposits (sensu compound paleosols of Kraus, 1999) indicate alternations between steady sedimentation and rapid, unsteady sedimentation. The weakly-developed paleosols and protosols are commonly associated with sandy avulsion de- posits. The presence of interbedded poorly-cemented, buff sandy deposits, interpreted as crevasse splays (see FA 4 interpretation), also indicate that avulsion processes were occur- ring on the floodplain. The dominance of purple-red (oxidized) paleosols and a lack of drab (reduced) paleosols indicates that the floodplain was well-drained and frequently dry during times of low deposition.

36 2.7 Discussion 2.7.1 Signatures of variable discharge in channel deposits and comparison to facies models

An abundance of UFR and HDR sedimentary structures, in-channel bioturbation and pe- dogenic modification of in-channel mud layers, thick mud-clast conglomerates, soft-sediment deformation structures, and the lack of preserved braid or point bar strata are the key obser- vations in channel deposits that can be linked to deposition in rivers with variable discharge, characterized by fluctuations between high-magnitude floods and periods of little to no dis- charge (see reviews by Fielding et al., 2009, 2018; Plink-Bj¨orklund, 2015 and references therein). The abundance of UFR sedimentary structures is the most commonly considered indicator of river discharge variability and high peak flows (e.g. Fielding, 2006), and such abundance has been linked to dryland rivers (McKee et al., 1967; Williams, 1971; Frostick and Reid, 1977; Sneh, 1983; Stear, 1985), to more humid subtropical conditions (e.g. Sneh, 1983; Stear, 1985; Abdullatif, 1989; Billi, 2007), as well as to monsoon zone rivers (Singh and Bhardwaj, 1991; Singh et al., 1993a, 2007; Shukla et al., 2001; Jain and Sinha, 2004a; Chakraborty et al., 2010a; Chakraborty and Ghosh, 2010a). Common to all these rivers is extreme variation in discharge with high peak flows and rapid changes in flow stage, such that the flood-stage flow becomes supercritical and there is insufficient time during falling stage for the stream bed to re-equilibrate and rework the bedforms (Fielding, 2006). Most monsoon domain rivers have a perennial base discharge, whereas many rivers in the subtrop- ical zone only transmit discharge in response to the most extreme monsoon rainfall events and are otherwise dry. Exceptions include rivers that have part of their drainage in a differ- ent climate zone (e.g. the Nile). In the subtropics, the monsoon-influenced rainfall events may last for the duration of the monsoon season, similar to the monsoon domain itself, or the whole annual precipitation may fall in a few episodic downpours. The return frequency of extreme rainfall events may be from years to decades, thus some subtropical rivers are ephemeral and may remain dry over these timescales. These rivers characteristically experi-

37 ence high magnitude flash floods, especially in areas with mountainous hinterland and sparse vegetation (e.g. Powell, 2009). High deposition rates are likely to be essential for accumulating large thicknesses of UFR deposits that otherwise have a very low preservation potential (Allen and Leeder, 1980; Cheel, 1986; Bridge and Best, 1988, 1997; Paola et al., 1989; Alexander et al., 2001), and thus high sediment flux and high deposition rates are characteristic for variable discharge river deposits (Plink-Bj¨orklund, 2015). Furthermore, the aggradational nature of the UFR sedimentary structures, as well as the climbing ripple and dune laminations and stratifi- cations indicate deposition from suspension rather than bedload (Allen and Leeder, 1980). Suspension transport of sand and granule size sediment in supercritical flow conditions has also been shown by flume experiments (Alexander et al., 2001; Cartigny et al., 2014; Ono et al., subm.). Together with high flow velocity, high deposition rates and suspension fallout are also responsible for the relatively low abundance of dune cross stratification, as bedform formation is suppressed in high sediment fallout conditions (e.g. Allen and Leeder, 1980). Suspension transport of sand to granule size sediment further signifies that seasonal rivers are likely to have high sediment yields compared to rivers from perennial precipitation zones (see also Cecil et al., 2003). The suspended load in some modern subtropical rivers has been suggested to be about 20 times higher than in persistent discharge rivers (Frostick and Reid, 1977). The thick soft-clast conglomerates further signify high water power (and resultant bank undercutting), and rapid lowering of water level during the waning stage of floods (e.g. Gohain and Parkash, 1990; Singh et al., 1993; North and Taylor, 1996; Tandon and Gibling, 1997). The latter is also the suggested cause for soft-sediment deformation, as a result of pore fluid expulsion generated during late flood stages when current velocities have slowed down and sediment is in the condition of quicksand (McKee et al., 1967), or during rapid loading by overlying sediments (Singh and Bhardwaj, 1991). The overturned cross strata were formed during high flow through partial liquefaction and deformation of water-saturated

38 sediment under the influence of lateral shearing (Stear, 1985; Singh and Bhardwaj, 1991). The latter is especially likely to occur in association with a rapid increase in bottom shear resulting from highly turbulent flow during abrupt transition from lower to UFR (Stear, 1985). The in-channel pedogenic modification and bioturbation indicate that channels were dry for sustained periods of time. We document considerable variability in the percentage of UFR and HDR sedimentary structures (Facies S1.2 - S1.4) (Fig. 2.13). UFR and HDR structures are the most abundant in the Uppermost Nacimiento Formation and Cuba Mesa Member channels which consist of FA 1 and FA 2 channel lithosomes, where they constitute 84% and 7%, respectively, of the sedimentary structures recorded in the measured sections (Fig. 2.13). This high percentage of UFR structures is similar to some of the most extreme modern variable discharge river systems that receive their total annual precipitation in just a few downpours (Sneh, 1983; Stear, 1985; Billi, 2007), and have been shown to consist of 80 - 100% UFR sedimentary structures (Plink-Bj¨orklund, 2015). Similar channel lithosome styles have been hypothesized to consist entirely of high-magnitude flood deposits (Plink-Bj¨orklund, 2015; Wang and Plink- Bj¨orklund, in review) because they share many characteristics with flash flood deposits (Schumm, 1961; McKee et al., 1967; Williams, 1971; Stear, 1985; Billi, 2007). Based on the high percentage of UFR structures, the extremely low percentage of LFR structures (16%), and lack of in-channel mud layers that suggest preservation of waning stage deposits or reworking by perennial baseflow, this seems a valid hypothesis for FA 1. The more heterolithic FA 2 channel lithosomes with a higher vertical and lateral variabil- ity in sedimentary structures, a slightly higher proportion of LFR features (22%), and the presence of mud layers also resemble modern river systems with prevailing deposition from high-magnitude floods, but with some preservation of waning phase deposits (e.g. Stear, 1985; Abdullatif, 1989; Singh et al., 1993; Billi, 2007; Carling and Leclair, 2019). Mud layers have been documented in modern variable discharge rivers that occur in arid and humid subtropics, as well as in the monsoon zone (e.g. Williams, 1971; Tunbridge, 1981; Sneh,

39 1983; Stear, 1985; Abdullatif, 1989; Singh and Bhardwaj, 1991; Singh et al., 1993; Shukla et al., 2001; Billi, 2007; Fisher et al., 2008; Fielding et al., 2009). Similar to FA 1, the FA 2 lithosomes reveal the lack of efficient base flow. FA 1 and FA 2 further lack preserved braid or point bar strata that has been suggested as another characteristic of discharge vari- ability (e.g. Rhee and Chough, 1993; Uba et al., 2005; Billi, 2007; Hampton and Horton, 2007; Fielding et al., 2009; Hulka and Heubeck, 2010; Allen et al., 2011; Plink-Bj¨orklund, 2015), as sustained flow conditions are needed for systematic bar migration. Furthermore, high concentration of sediment carried in suspension during floods inhibits bar formation (Allen and Leeder, 1980). By comparison with the lithosome types described by Fielding et al. (2018), the FA 1 and FA 2 channel lithosomes share characteristics with rivers that display very high peak discharge variance, including the preservation of a variety of bedding structures that include an abundance of UFR structures, transported woody debris, pedo- genically modified mud partings (in FA 2), and little to no evidence of braid or point bar preservation. These features are interpreted to reflect intervals of several successive years with low peak annual flow which is characteristic of rivers with high and very high peak discharge variability (Fielding et al., 2018). The lenticular FA 3 channel lithosomes with an abundance of UFR structures (50%) and a significantly higher proportion of LFR structures (50%) resemble the deposits of variable discharge rivers that transmit effective discharge for the duration of the monsoon season (3 - 4 months) and experience high and low magnitude floods (Abdullatif, 1989; Shukla et al., 2001; Singh et al., 2007 see also discussion in Plink-Bj¨orklund, 2015). In these rivers, peak discharges of short duration that deposit UFR and HDR sedimentary structures alternate with longer periods of lower and more sustained flow conditions that deposit LFR structures (Abdullatif, 1989). Without taking into consideration the characteristics of the surrounding floodplain deposits, we may be inclined to agree with a link between this style of channel lithosome and a more seasonal and sustained discharge during the monsoon season. Fielding et al. (2018), however suggested that the modern rivers in the monsoonal domain with highly

40 seasonal discharge but low inter-annual variability (low peak discharge variance) display architectures typical of classical facies models in showing a dominance of cross-stratification of various scales with lesser UFR sedimentary structures and clear evidence of braid or point bar evolution. This hypothesis however contradicts studies of modern monsoonal river deposits that do report a significant proportion of UFR and HDR sedimentary structures (e.g. Singh and Bhardwaj, 1991; Singh et al., 1993, 2007; Shukla et al., 2001; Jain and Sinha, 2004; Chakraborty et al., 2010; Chakraborty and Ghosh, 2010). The FA 3 lithosomes do not display a dominance of cross-stratification, nor do they show clear evidence of braid or point bar evolution, and instead display features characteristic to the rivers with moderate to high inter-annual variability (intermediate to high discharge variance), including a combination of UFR and LFR sedimentary structures and absent to cryptic macroforms, suggesting that large barforms generated during subcritical flow have been reworked by flood events may (Fielding et al., 2018). Whatever the specific style of discharge variability, the percentage of UFR and HDR sedimentary structures in the Nacimiento, Cuba Mesa, and Regina channels is in contrast with the low occurrence of these structures in persistent discharge river deposits, which consist almost entirely of LFR structures, such as cross-stratification (Miall, 1996). This abundance of UFR and HDR sedimentary structures, combined with the presence of in- channel mud layers, thick soft-clast conglomerates, soft-sediment deformation structures, in-channel bioturbation (vegetation and trace fossils), downstream accreting flood event beds and thus the lack of braid or point bars, and channel mobility by avulsion is consistent with the recently developed sets of recognition criteria for intra- and inter-annually variable discharge river systems (Fielding et al., 2009, 2018; Plink-Bj¨orklund, 2015).

2.7.2 Signatures of variable discharge in floodplain deposits

In addition to evidence that the channels were deposited by variable discharge river systems, the floodplain deposits display indicators of moisture fluctuations and varying de- position rates. The crevasse splay deposits observed in all three floodplain lithosomes may

41 suggest frequent avulsions, which are commonly observed in modern seasonal river sys- tems due to the frequent occurrence of flood events (Abdullatif, 1989; Makaske, 2001; Jain and Sinha, 2003, 2004; Assine, 2005; Billi, 2007; Assine and Silva, 2009; Sinha et al., 2009; Chakraborty and Ghosh, 2010; Donselaar et al., 2013), where highly variable flood discharges and high sediment concentrations result in rapid channel bed aggradation and resultant su- perelevation that is a key threshold for avulsion occurrence (Bryant et al., 1995; Hajek and Edmonds, 2014). The avulsions result in high deposition rates on the floodplain as evidenced by sandy crevasse splay deposits paired vertically with weakly developed compound paleosols and protosols. The type of paleosol that forms in the sedimentary record depends on how rapidly the sediment accumulated, whether that accumulation was steady or discontinuous, and the du- ration of pauses in accumulation (Kraus, 1999). The three floodplain lithosomes (FA 4, FA 5, and FA 6) show alternations of compound paleosols and protosols with more developed com- posite and/or cumulative paleosols, indicating that avulsions were episodic and punctuated by periods of slow deposition allowing for varying levels of pedogenic development. Protosols have also been linked to climatic conditions that inhibit or slow down pedogenesis, such as extreme aridity or cold (Kraus, 1999). The coloration of soil matrix and pedogenic features such as mottles, structure, and nodules in floodplain deposits can provide additional impor- tant information about climate and/or soil drainage conditions (e.g. Vepraskas, 2000; Kraus and Hasiotis, 2006). The most apparent evidence for highly variable soil moisture conditions is seen in the variably-drained floodplain lithesome (FA 5), where the drab (reduced) deposits of mudrock facies group 1 are observed alternating with the purple-red (oxidized) deposits of mudrock facies group 2. However, some indicators of moisture fluctuations are present in all of the floodplain lithosomes observed throughout the study interval, even those identified as poorly-drained floodplain lithosomes. These indicators include features like slickensides and wedge-shaped peds that indicate repeated shrinking and swelling of clays (Tabor et al., 2017). We suggest that these common features indicate that a signature of variable precipitation is

42 present throughout the upper Nacimiento and San Jose Formations, further supporting our conclusion that the deposits throughout our study interval were made by seasonal and vari- able discharge river systems. However, some additional indicators of variable precipitation such as carbonate nodules and more strongly developed ped structures are only observed in association with the variably-drained and well-drained floodplain lithosomes (FA 5 and FA 6). The vertical trend in the occurrence of these lithosomes suggests changes in precipitation seasonality extremes, which are discussed below. It is important to note that our interpreta- tions of the paleosols are based on qualitative assessments of texture and color. Future work should include a detailed paleopedological investigation to provide a quantitative estimate of paleoclimate conditions.

2.7.3 Channel-floodplain associations and their link to hydroclimate

Six styles of channel-floodplain associations occur in the study interval: Amalgamated sandy channels (FA 1) in association with variably-drained floodplain deposits (FA 1 and 5) occur in the Cuba Mesa Member of the San Jose Formation (Fig. 2.14a; Fig. 2.16a) and with well-drained floodplain deposits (FA 1 and 6) in the upper Regina Member of the San Jose Formation (Fig. 2.14b; Fig. 2.16b). Heterolithic channels (FA 2) in association with poorly- drained floodplain deposits (FA 2 and 4) occur in the Escavada Member of the Nacimiento Formation (Fig. 2.14a; Fig. 2.16a) and with variably-drained floodplain deposits (FA 2 and 5) in the Cuba Mesa Member of the San Jose Formation (Fig. 2.14b, c; Fig. 2.15a; Fig. 2.16a). Isolated and lenticular channels (FA 3) in association with poorly-drained floodplain deposits (FA 3 and 4) occur in the Escavada Member of the Nacimiento Formation (Fig. 2.16a) and with well-drained floodplain deposits (FA 3 and 6) in the Regina Member of the San Jose Formation (Fig. 2.14c; Fig. 2.16b). Here, we use our interpretations of the facies associations and the channel-floodplain associations to discuss potential similarities to modern climate types. The channel-floodplain associations are listed in order of stratigraphic occurrence (oldest to youngest).

43 Heterolithic (FA 2) and Isolated Lenticular (FA 3) Channels with Poorly- Drained Floodplain (FA 4). In the Paleocene Escavada Member of the Nacimiento For- mation, the combination of isolated lenticular (FA 3) and amalgamated heterolithic (FA 2) channel lithosomes with drab-colored floodplain lithosomes indicates relatively wet, but sea- sonal conditions based on the interpretations above. By comparison with modern soils, the abundance of grey and grey-green mudrock deposits suggest wet, anoxic conditions (Lottes and Ziegler, 1994) and the formation of black, organic-rich soils requires consistently wet conditions, at least during the summer months when vegetation grows (Lottes and Ziegler, 1994). The associated FA 3 and FA 2 channel lithosomes with an abundance of UFR/HDR sedimentary structures suggest seasonally or inter-annually variable precipitation and vari- able discharge rather than perennial precipitation and persistent discharge. The interval near the top of the Nacimiento Formation that is dominated by the amalgamated FA 2 channel deposits may indicate increased frequency and/or magnitude of precipitation events, over the lower interval that is dominated by FA 3 channel deposits. Based on these interpretations and comparison with modern climate types, the style of precipitation variability suggested by the FA 2 and 4 and the FA 3 and 4 associations may be analogous to modern monsoon domain or the relatively wet subtropics, characterized by a sustained wet-season consisting of variable-magnitude precipitation events during the summer and dry winters, whereby the summer-minus-winter precipitation exceeds 300 mm and the proportion of summer precipi- tation exceeds 55% of the annual total (Wang and Ding, 2008). Amalgamated Sandy (FA 1) and Heterolithic (FA 2) Channels with Variably- drained Floodplains (FA 5). In the Cuba Mesa Member of the San Jose Formation, the combination of amalgamated sandy (FA 1) and amalgamated heterolithic (FA 2) channel lithosomes with variably-drained floodplain lithosomes (FA 5) indicates relatively frequent high-magnitude flood events combined with alternations between dry and wet soil conditions. The drab-colored soils may indicate seasonal precipitation characterized by a sustained wet- season, such as we suggested above for the FA 2 and 4 and the FA 3 and 4 associations.

44 Whereas the purple and red soils suggest well-drained oxidized conditions and thus relatively arid conditions such as in arid subtropics. Both the grey-green and purple-red floodplain de- posits display moderate to strong pedogenic development, suggesting that varying floodplain deposition resulting in alternations between paleosols and protosols are not alone responsible for the variegated coloring. Instead, the combination of FA 1 and FA 2 channels with FA 5 floodplains seem to suggest a climate that had both considerable seasonal to inter-annual as well as decadal or longer precipitation variability. This longer-term variability, as suggested by the color alternations in the pedogenically-modified floodplain deposits, may indicate mul- tiple fluctuations between humid to sub-humid and arid conditions on a decadal or longer time scales, where the red soils indicate sustained droughts. Possible drivers of inter-annual and longer-term precipitation cyclicity derived from modern and Quaternary observations in- clude El Nino-Southern Oscillation (ENSO, <10 y.), Dansgaard-Oeschger (D/O) oscillations (1500-3500 y.) and precession cycles (20 k.y.). For example, Tuenter et al. (2007) showed that precession cycles do affect the strength of monsoonal systems through their impact on summer insolation, whereby the summer monsoonal system strengthens during precession minima because summer insolation is increased. In addition, Millennial-scale Dansgaard- Oeschger (D/O) oscillations have also been linked to documented monsoon intensification periods over the last glacial period (Rohling et al., 2003; Wang et al., 2008), but clear ex- ternal forcing mechanisms for this scale of cyclicity have not been established (Burns et al., 2019). Isolated Lenticular Channels with Well-drained Floodplain (FA 3 and 6). In the lower Regina Member of the San Jose Formation, more isolated and lenticular channel lithosomes (FA 3) with an increased abundance of LFR sedimentary structures in association with well-drained floodplain lithosomes (FA 6) suggests less frequent flooding events and dry soil conditions. The association of FA 3 channels with well-drained floodplain (FA 6) contradicts suggestions that similar channel lithosomes are linked to monsoonal domain rivers that experience a sustained wet season and transport perennial base-flow, as was

45 hypothesized by Plink-Bj¨orklund (2015). In the Regina Member, the FA 3 channel deposits may be the result of reduced discharge to the portions of the basin included in our study areas. The reduced discharge may be due to lower-magnitude precipitation events and/or losses due to evaporation and infiltration under more arid conditions and increased soil drainage (see Babcock and Cushing, 1942; Ibrahim, 1980; McCarthy and Ellery, 1998). This style of precipitation variability is seen in some modern semi-arid to arid subtropics, or drylands, which are characterized by high, yet variable, aridity reflecting low precipitation totals and high evapotranspiration potential (see Tooth et al., 2013). These areas receive monsoon rain only during extreme, abnormal, or strengthened monsoon seasons and return frequencies of these events may be from years to decades (Wang and Ding, 2008). As a result, the rivers only experience discharge during high magnitude flood events and may remain dry over timescales of years to decades (Powell, 2009). Amalgamated Sandy Channels with Well-drained Floodplain (FA 1 and 6). In the upper Regina Member, the return of sandy amalgamated channel lithosomes (FA 1) in association with well-drained floodplain lithsomes (FA 6) suggests frequent high magnitude flooding events and dry soil conditions. The abundance of well-drained floodplain deposits suggests a dominantly arid environment. The indicators of frequent high magnitude flooding suggest frequent and intense precipitation events. This style of precipitation regime is also within the range of diversity observed within the modern semi-arid to arid subtropics, or drylands (Tooth, 2013). Summary. The diversity and overlap in the channel lithosome styles associated with the floodplain lithosomes within this dataset suggests that it is difficult to differentiate between ancient monsoonal, sub-humid, and semi-arid/arid climates based on channel deposits alone. Analyzing the channel and floodplain lithosomes separately led to conflicting hypotheses about precipitation regimes and their link to climatic conditions. For example, the FA 3 channel lithosome is observed in both the Nacimiento Formation and the Regina Member of the San Jose Formation. We interpreted that the FA 3 lithosome may suggest a system

46 with sustained base flow during an annual wet season with occasional high-magnitude floods, similar to rivers in the sub-humid subtropics (e.g. Abdullatif, 1989) and monsoon domain. However, the associated floodplain deposits indicate vastly different precipitation regimes, with poorly-drained (FA 4) floodplains in the Nacimiento Formation, suggesting a more monsoonal climate, and well-drained (FA 6) floodplains in the Regina Member, possibly suggesting a semi-arid to arid climate. Thus, we were only able to form hypotheses about potential modern climate analogues once we analyzed the channel-floodplain associations and took into consideration the range of precipitation variability within each modern climate type. Furthermore, the observed diversity and overlap in channel-floodplain associations agrees with suggestions that tropical monsoonal, subtropical, and dryland subtropical rivers show many overlapping characteristics when the full-range of river styles is considered within the climate zones (e.g. Knighton and Nanson, 1997; Tooth and Nanson, 2000; Powell, 2009 see also review in Tooth et al., 2013).

2.7.4 Evidence of Paleogene climate changes in the San Juan Basin

As discussed in the previous section, the fluvial deposits of the Escavada Member of the Paleocene Nacimiento Formation through the early Eocene Cuba Mesa and Regina Members of the San Jose Formation show characteristics that suggest a range of variable precipitation trends analogous to those of the modern monsoon and subtropical domains. The FA 3 and 4 and FA 2 and 4 channel-floodplain associations in the Paleocene Escavada Member suggest a precipitation regime that shares characteristics with the modern monsoonal domain, whereas the FA 1 and 5 and FA 2 and 5 channel-floodplain associations seen in the early Eocene Cuba Mesa Member suggests a shift to precipitation regimes that alternate between those charac- teristic of modern humid to sub-humid climates and those characteristic of modern semi-arid and arid subtropics. The overlying FA 3 and 6 and FA 1 and 6 channel-floodplain associations in the Regina Member suggest increasingly arid conditions and decreased return frequency of precipitation events, perhaps analogous to the modern semi-arid to arid subtropics. Here we discuss what these similarities may imply about Paleogene climate changes in the San

47 Juan Basin. The stratigraphic trends in the floodplain deposits suggest an overall trend of increasing aridity from the Paleocene into the early Eocene, but the diversity in the channel lithosome styles found in association with the floodplain lithosomes suggests that considerable variabil- ity in the frequency and magnitude of precipitation and flooding events is superimposed on the long-term drying trend. Furthermore, the stratigraphic clustering of the channel lithe- some styles, as well as the stratigraphic variability in drab vs. purple-red paleosols suggests some long-term trends, and perhaps cyclicity, in the frequency and magnitude of intense precipitation events. Research on modern monsoons and Quaternary paleo-monsoons have shown that monsoon variability occurs across time scales, from inter-annual, inter-decadal, centennial, millennial, and up to orbital and tectonic timescales (see Wang et al., 2014, 2017). The current lack of time-constraints throughout the San Juan Basin study interval precludes further analysis of the frequency and duration of changes in the hydrological cycle, but the deposits within the study interval suggest multiple time scales of precipitation variability. In addition, some of the stratigraphic trends observed among the channel lithosomes may show evidence of threshold climate change beyond inherent monsoon precipitation variabil- ity. For example, the basin-wide FA 1 channel belt at the base of the Cuba Mesa Member that corresponds with a shift from poorly-drained (FA 4) to variably-drained (FA 5) flood- plain deposits may have resulted from sediment flushing as the ambient climate of the region shifted from a monsoonal precipitation regime in the Paleocene to variably sub-humid and arid in the early-Eocene. Reduction in vegetative cover, erosion of soils, and an increase in solid sediment yield are likely to result in a sediment flush under these conditions (Cecil and Dulong, 2003). It is also suggested that once weathering and pedogenic equilibrium are reestablished under the drier conditions, rivers continue to show a relatively high solid sedi- ment yield (Cecil and Dulong, 2003), which may be evidenced in the overlying FA 1 (central San Juan Basin) and FA 2 (southern San Juan Basin) channel lithosomes in the middle and upper Cuba Mesa Member. The return of FA 3 channels in the lower Regina Member could

48 indicate decreased discharge and solid sediment yield as the climate became increasingly arid, as suggested by their association with well-drained FA 6 floodplain deposits. However, the FA 1 channels in the upper Regina Member, which are also found in association with FA 6 floodplain deposits, suggest that rainfall intensity rather than overall aridity is a key control on water discharge and sediment yields. The overall trend towards increasing aridity observed in the floodplain deposits within the study interval coincides with a global long-term warming trend from the Paleocene to the middle Early Eocene upon which transient hyperthermals were superimposed (Zachos et al., 2008; Bijl et al., 2013). It is suggested that the intensity and distribution of pre- cipitation changed as a result of the Paleogene warming trend (Huber and Goldner, 2012), with greater moisture transport to high latitudes (Pagani et al., 2006) and enhanced sea- sonal extremes in some areas (John et al., 2008). In addition, it has been suggested that extremes in the hydrological cycle resulting in increased precipitation peakedness in the sub- tropics and mid-latitudes coincided with the Eocene hyperthermals (McInerney and Wing, 2011; Plink-Bj¨orklund et al., 2014; Carmichael et al., 2016, 2018). The Paleocene and early- Eocene deposits in the study interval suggest an overall long-term decrease in mean annual precipitation and fluctuations in seasonal extremes of precipitation and seem to suggest that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high intensity precipitation events. The overall long-term drying trend suggested in the San Juan Basin paleosols differs from what has been suggested of paleosol records in other mid-latitude sedimentary basins with Paleocene - Eocene boundary deposits. In the Bighorn Basin, Wyoming, a transient decrease in precipitation has been suggested coincident with the Paleocene Eocene Ther- mal Maximum (PETM) hyperthermal event (Kraus and Riggins, 2007; Kraus et al., 2015). These researchers have also observed indicators of wet/dry cycles within the PETM interval paleosols that they suggest may correspond to precessional cycles. The PETM interval is followed by indicators of similar to slightly wetter background soil moisture, using the CAL-

49 MAG mean annual precipitation proxy, linked to four younger hyperthermal events with (Abels et al., 2015). In the Uinta Basin, Utah, paleosol characteristics and trace fossils simi- larly suggest a transient shift to overall drier conditions during the peak PETM, followed by a wetter period with seasonally variable conditions (Golab, 2010). In contrast, conditions in the Piceance Creek Basin, Colorado, are suggested to have been drier in the late-Paleocene and more humid during the PETM due to an observed shift from purple, orange, and red paleosols to dominantly purple paleosols (Foreman et al., 2012). An abundance of red-pink paleosol deposits above the identified PETM interval suggests drier conditions further into the early-Eocene. In the Tremp-Graus Basin, Spain, semi-arid conditions across the Pale- ocene - Eocene boundary are suggested with an increase in intra-annual humidity gradients and associated seasonal flash floods during the PETM, followed by drier conditions imme- diately after the PETM (Schmitz and Pujalte, 2007).

2.8 Conclusions

This study identifies the river and floodplain deposits of the Paleocene upper Nacimiento Formation (Escavada Member) through the Cuba Mesa and Regina Members of the San Jose Formation in the San Juan Basin as formed by variable discharge river systems, resulting from inter- (seasonal) and intra-annual precipitation variability. Evidence for a variable precipitation signature includes:

1. An abundance of UFR sedimentary structures deposited under Froude supercritical flow conditions and high deposition rates within the channel deposits. A high percent- age of UFR and HDR structures is characteristic of modern and ancient monsoonal and subtropical rivers that have highly seasonal discharge and receive most of their annual (or in some cases, inter-annual) precipitation from one to few large downpours resulting in flooding events.

2. Preservation of in-channel bioturbation and mud layers with pedogenic modification indicating that channels underwent periods of prolonged dryness between flooding

50 episodes.

3. Floodplain lithosomes with alternations of poorly-drained and more well-drained de- posits and/or slickensides indicating alternating wet-dry cycles.

4. Floodplain lithosomes with crevasse splay deposits that suggest channel avulsions re- sulting from flooding events.

Floodplain paleosols also evidence an overall long-term decrease in mean annual precipi- tation, whereas channel lithosomes reflect more short-term fluctuations in seasonal extremes of precipitation. Thus, integrating paleo-precipitation proxies from the river channel and floodplain deposits, suggests that floodplain deposits may be a better indicator of ambient climate, whereas channel deposits are records for frequency and magnitude of high inten- sity precipitation events. These data further suggest that the long-term drying trend that coincides with a long-term global warming trend from the Paleocene into the middle early Eocene was superimposed by fluctuations in hydrological cycle extremes. We show that when analyzed together and compared with precipitation variability from modern climate zones, the changes in channel-floodplain associations suggest a shift from a monsoonal precipitation regime in the Escavada Member of the Paleocene Nacimiento Formation to fluctuating sub-humid and arid subtropical in the Cuba Mesa Member and semi-arid to arid in the Regina Member of the early Eocene San Jose Formation. We conclude that the existing variable discharge river facies models that consider only channel deposit facies may not capture critical information needed to make accurate inter- pretations of paleoclimatic conditions.

51 Figure 2.1: Regional maps and Paleogene stratigraphy of the San Juan Basin (SJB). A) Location of the San Juan Basin within the contiguous United States. Base map modified from ESRI, HERE, Garmin, OpenStreetMap, and the GIS user community. B) Regional map of the San Juan Basin (SJB) and adjacent basins and uplifts (SJB = San Juan Basin, G = Galisteo Basin, B = Baca Basin, MV = Monte Vista Basin, RA = Raton Basin, HP = Huerfano Park Basin, CL-J = Carthage-LaJoya Basin. Redrawn from Dickinson et al., 1988; Lawton, 2008; and Cather 2004. C) Simplified Paleogene stratigraphy of the SJB. D) Geological map of the SJB and areas selected for detailed investigations. Arroyo Chijuilla (AC) and Continental Divide (CD) are in the southern SJB. Ca˜non Largo (CL) is in the central SJB. Map modified from Williamson et al., 2008 and Manley et al., 1987.

52 Figure 2.2: Examples of sandstone facies group 1. A) S1.1: Conglomerates - disorganized B) S1.1: Conglomerates - stratified and imbricated. C) S1.2: Sandstone with scour and fill structures. D) S1.3: Sandstone with planar to low-angle laminations. E) S1.4: Sandstone with convex-up long wavelength laminations. F) S1.5: Sandstone with soft-sediment defor- mation. G) S1.6: Structureless sandstone in well-cemented outcrop. H) S1.6: Structureless sandstone as poorly-cemented deposits (marked with white arrows), bracketed by mudrock deposits.

53 Figure 2.2: Continued.

54 Figure 2.3: Outcrop examples of sandstone facies group 1. A) Backsets (annotated in red) with dip directions approximately opposite the prevailing paleocurrent direction (black ar- row). The backsets commonly transition downstream into low-angle (S1.3) or convex up low-angle (S1.4) structures. B) Example of an interpreted hydraulic jump, where abrupt energy drop at the jump resulted in rapid sediment fallout and a structureless deposit (S1.6) that includes mud clasts. Red arrows show inferred flow directions.

55 Figure 2.3: Continued.

56 Figure 2.4: Examples of sandstone facies group 2. A) S2.1: Cross-stratified sandstone. B) S2.2: Sandstone with climbing cross-strata or -laminae. C) S2.3: Sandstone with scours and climbing cross-laminae. D) S2.4: Sandstone with overturned cross-strata or -laminae.

57 Figure 2.5: Example hand samples of mudrock facies group 1. A) Globular grey mudrock. B) Black mudrock. C) Brown to dark grey mudrock with relict laminations and plant fragments. D) Grey-green mudrock. E) Grey mudrock.

58 Figure 2.6: Example hand samples of mudrock facies group 2. A) Purple mudrock with yellow-brown mottles. B) Red mudrock. C) Mudrock with variegated color.

59 Figure 2.7: Examples of sandstone lithosomes. A) Sandy Channel Lithosomes (FA 1). B) Heterolithic Channel Lithosomes (FA 2). C) Lenticular Channel Lithosomes (FA 3). Pale- oflow directions are indicated by the black arrows in lower right corner of annotated images.

60 Figure 2.8: Representative measured sections and photos from sandstone lithosomes. A) Sandy Channel lithsosomes (FA 1). B) Heterolithic channel lithosomes (FA 2). C) Lenticular channel lithosomes (FA 3).

61 Figure 2.9: Examples of Floodplain Lithosomes. Several representative horizons are anno- tated with their corresponding facies abbreviation. A) Poorly-drained floodplain lithsomes (FA 4) are dominated by drab mudrock deposits (mudrock facies group 1) and interbedded thin crevasse splay deposits (S1.6). The arrow points to a scour surface that truncates a succession of floodplain deposits and is subsequently filled with a series of stacked floodplain deposits. B) Variably-drained floodplain lithosomes(FA 5) consist of both drab mudrock (mudrock facies group 1) and purple-red mudrock (mudrock facies group 2), interbedded with occasional crevasse splay deposits (S1.6). C) Well-drained floodplain lithsomes (FA 6) consist of abundant purple-red mudrock (mudrock facies group 2), alternating with some drab mudrock (mudrock facies group 1) and thin crevasse splay deposits (S1.6).

62 Figure 2.10: FA 1 Channel Characteristics. A) Three stacked, thick, and erosionally-bound sandstone deposits (interpreted as flood event beds) separated by mud-clast conglomerates. B) FA 1 Channel with low-angle, upstream-dipping accretion sets. C) FA 1 channel consist- ing almost entirely of scour and fill (S1.2) structures. D) Mud-clast conglomerates (S1.1) overlain by sandstone with low-angle laminations (S1.3). E) In-channel bioturbation (ver- tical burrows) is seen in the lower sandstone deposit, whereas the overlying deposit is not bioturbated and contains visible laminations.

63 Figure 2.10: Continued.

64 Figure 2.11: FA 2 Channel Characteristics. A) Heterolithic channel lithesome with pedogenic modification of in-channel mud layer at a downstream-dipping accretion boundary (marked with red star). B) In-channel bioturbation in FA 2 channel fill.

65 Figure 2.12: FA 3 Channel Characteristics. A) lenticular channel with lateral and vertical accretion sets. B) Lenticular channel filled entirely with LFR structures from sandstone facies group 2 including cross-strata and overturned cross-strata.

66 Figure 2.13: Stratigraphic trends in overall percentage of sandstone and mudrock lithofacies and facies group composition. Simplified measured sections are displayed from Ca˜non Largo (Central SJB), Continental Divide (southern SJB), and Arroyo Chijuilla (southern SJB). The percentage of sandstone vs. mudrock lithofacies and facies group composition are calculated from the three measured sections combined to quantify overall stratigraphic trends that are seen throughout the basin.

67 Figure 2.14: Vertical trends in channel and floodplain lithosomes in the Arroyo Chijuilla study area. A) Heterolithic channel lithosomes (FA 2) in association with poorly-drained floodplain lithosomes (FA 4) in the uppermost Nacimiento Formation, capped by amal- gamated sandy channel lithosomes (FA 1) in association with variably-drained floodplain lithosomes (FA 5) in the lower Cuba Mesa channel complex. B) Heterolithic channel litho- somes (FA 2) in the Middle Cuba Mesa channel complex in association with variably-drained floodplain lithosomes (FA 5). C) Upper Cuba Mesa channel complex (FA 2) in association with variably-drained floodplain lithosomes (FA 5) and overlying lenticular channel litho- somes (FA 3) in association with well-drained floodplain lithosomes (FA 6) in the Regina Member.

68 Figure 2.15: Vertical trends in channel and floodplain lithosomes in the Continental Divide study area. A) middle and upper Cuba Mesa Member tongues consisting of heterolithic channel lithosomes (FA 2) in association with variably-drained floodplain lithosomes (FA 5), capped by the Regina Member, consisting mostly of well-drained floodplain lithosomes (FA 6) in association with rare lenticular channel lithosomes (FA 3) in the lower portion and a return to sandy amalgamated channel lithosomes (FA 1) in the upper portion. B) The uppermost deposits of the Regina Member in the Continental Divide study area consisting of isolated outcrops of well-drained floodplain lithosomes (FA 6) capping the FA 1 and 6 channel-floodplain association.

69 Figure 2.16: Stratigraphic trends in channel and floodplain lithosomes and associations in the Continental Divide study area. A) Isolated and lenticular channel lithosomes (FA 3) and heterolithic channel lithosomes (FA 2) bound by poorly-drained floodplain lithosomes (FA 4) in the uppermost Nacimiento Formation, capped by amalgamated sandy channel lithosomes (FA 1) in association with variably-drained floodplain lithosomes (FA 5) in the Cuba Mesa Member of the San Jose Formation. B) Lenticular channel lithosomes (FA 3) and sandy channel lithosomes (FA 1) in association with well-drained floodplain lithosomes (FA 6) in the overlying Regina Member. Photos taken in the Ca˜non Largo study area, laterally adjacent to the measured section location.

70 Table 2.1: Sandstone facies group 1 - sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations, or devoid of structure.

71 Table 2.1: Continued.

72 Table 2.2: Sandstone facies group 2 - sandstones with steeply dipping cross-strata and - lamina.

73 Table 2.3: Mudrock facies group 1 - drab-colored mudrock.

74 Table 2.4: Mudrock facies group 2 - purple-red mudrock.

75 Table 2.5: Channel and floodplain facies associations.

76 Table 2.5: Continued.

77 CHAPTER 3 PROGRADATION - RETROGRADATION STYLE OF A PALEOGENE FLUVIAL FAN IN THE SAN JUAN BASIN, NEW MEXICO

3.1 Abstract

Basin-scale 3D outcrop analyses of fluvial architecture in the Paleogene San Juan Basin, New Mexico documents lateral and vertical trends in channel, floodplain and paleosol char- acteristics. We identify the Paleocene uppermost Nacimiento Formation and early-Eocene Cuba Mesa and Regina Members of the San Jose Formation as deposits of a large fluvial fan based on lateral trends observed across the basin, including: the large lateral extent, down- stream decrease in degree of channel amalgamation, channel size, grain size, and an increase in the proportion of floodplain deposits, as well as evidence of channel avulsions. Vertical trends suggest at least two packages of fan progradation, separated by a retrogradational interval that may culminate in a depositional hiatus at the contact between the Cuba Mesa and Regina Members of the San Jose Formation. Furthermore, we identify this succession as deposits of variable discharge rivers, based on a combination of 1) an abundance of upper flow regime and high deposition rate sedi- mentary structures indicative of flooding events, 2) preservation of in-channel bioturbation and pedogenic modification indicating periods of prolonged dryness, 3) lack of identifiable braid or point bar strata, and 4) alternations of poorly-drained and well-drained floodplain deposits and/or slickensides indicating alternating wet-dry cycles. This dataset thus adds to a growing body of evidence that formation of large fluvial fans is promoted by fluctuations in discharge, and thus their link to hydroclimates with inter- (seasonal) and intra-annual precipitation variability and intense rainfall. The recognition of a prograding-retrograding fluvial fan system in the San Juan Basin may provide a new explanation for the previously proposed transitional nature of a discon-

78 formity at the Nacimiento-San Jose Formation contact and show that the proposed duration may have been overestimated. Observations of a long-term shift from poorly-drained to increasingly well-drained floodplain deposits across both progradational fan successions in- dicates that the existing predictive model suggesting downstream decreases in soil drainage conditions is not encompassing of all large fan systems, and instead suggests a gradual tran- sition to a more arid climate across the Paleocene-Eocene boundary.

3.2 Introduction

Fluvial fans are extensive fan-shaped fluvial accumulations built by river avulsions (Shukla et al., 2001; Leier et al., 2005; Chakraborty et al., 2010; Latrubesse et al., 2012; Sinha et al., 2012; Weissmann et al., 2015; Ventra and Clarke, 2018). Fluvial fans are aggradational landforms that have the potential to archive important information on the paleoclimate, local tectonic history, and landscape response to various allogenic factors. Perhaps most importantly, fluvial fans are proposed to form the bulk of the continental fluvial records, as they are the principal aggradational fluvial systems able to accumulate significant strati- graphic thicknesses in contrast to non-fan or tributary river systems that are confined to fluvial valleys, and transfer sediment as bypass systems (Weissmann et al., 2010; Ventra and Clarke, 2018 but c.f. Fielding et al., 2012). Recent research efforts have led to the develop- ment of recognition criteria, including conceptual models, for fluvial fan successions in the geologic record (e.g. Singh et al., 1993; Shukla et al., 2001; Weissmann et al., 2010, 2013) that have improved our ability to predict proximal-distal trends in fan deposit architecture, in addition to vertical changes indicative of prograding systems. As a result, ancient fluvial fans are increasingly being identified from stratigraphic investigations (e.g. Kukulski et al., 2013; Weissmann et al., 2013; Gulliford et al., 2014, 2017; Lawton et al., 2014; Owen et al., 2015b; a, 2017; Wang and Plink-Bj¨orklund, 2019). Nevertheless, major gaps still exist in our knowledge of these depositional systems including recognition criteria for progradational- retrogradational systems and controls on progradation-retrogradation processes (Ventra and Clarke, 2018).

79 Typically, tectonism has been invoked as being responsible for upward coarsening and cyclical sedimentation such as associated fan successions (e.g. Blair, 1987; Mack and Leeder, 1999; Jones, 2004; Luzn, 2005; Hirst and Nichols, 2009). More recent research has considered how climatic and tectonic processes influence fluvial fan sedimentation, and how fans respond to these changes (Harvey et al., 2005; Quigley et al., 2007). While it is well-accepted that tectonism provides the opportunity for alluvial fan development through the creation of topography, increasing gradients of river systems supplying sediments, and the creation of accommodation for storage of sediment flux (Frostick and Steel, 1993; Jones, 2002), climate is increasingly being regarded as a significant controlling factor on fan development (Harvey et al., 2005; Leier et al., 2005; Pope and Wilkinson, 2005; Weissmann et al., 2005; Quigley et al., 2007). In particular, variable discharge resulting from precipitation variability is suggested to be essential for fan formation because large fluctuations in discharge contribute to the instability of channels, which results in rapid channel migration and frequent avulsions that ultimately create distinctive fan-shaped sediment lobes (Sinha and Friend, 1994; Horton and DeCelles, 2001; Leier et al., 2005). The excellent outcrop exposure of the Paleogene stratigraphy in the San Juan Basin, New Mexico, provides an opportunity for a 3D basin-scale analysis of fluvial architecture. The ob- jective of this research is to combine channel lithosome and floodplain paleosol characteristics to better understand the depositional processes and forcing mechanisms that may account for evolving fluvial deposition styles. This paper 1) quantifies the lateral and stratigraphic trends in channel lithosome and floodplain deposits and paleosols, including grain size, chan- nel size, and the channel to floodplain ratio; 2) identifies the depositional environment of the uppermost Nacimiento Formation and Cuba Mesa and Regina Members of the San Jose Formation as a fluvial fan; 3) explores the style and potential controls on fan progradation and retrogradation; 4) explores the implications that our findings may have for improved understanding of Paleogene San Juan Basin landscape evolution; and 5) discusses signatures of progradation-retrogradation and climate changes in fluvial fan depositional models.

80 In order to signify that the studied sedimentary succession was built by river avulsions and lacks gravitational deposits, such that are common in alluvial fans (see Blair and McPherson, 1994), we follow the fluvial fan terminology that includes fan shaped bodies built by river avulsions, but excludes alluvial fans (see Moscariello, 2018; Ventra and Clarke, 2018), rather than the distributive fluvial system (DFS) terminology that includes alluvial fans as a part of a continuum of subaerial fan types (Weissmann et al., 2010, 2015).

3.3 Geological Setting and Background

The San Juan Basin (SJB) is in the Four Corners region of northwestern New Mexico and southwestern Colorado (Fig. 3.1a) and is one of several intraforeland basins formed during the Laramide Orogeny (Dickinson et al., 1988; Cather, 2004). Laramide tectonics during the late Campanian (late Cretaceous) through Eocene caused subsidence of the SJB in response to uplift of surrounding areas (e.g. Smith, 1988; Cather, 2004). During the late Paleocene through early Eocene, the SJB was bound to the east by the Nacimiento and Brazos-Sangre de Cristo, the west by the Defiance Uplift, and the north by the San Juan Uplift (Fig. 3.1b; Cather, 2004). Today, the SJB has the form of an asymmetrical syncline with an arcuate axis that dips to the north with steeply dipping northern and eastern limbs and shallowly dipping southern and western limbs. The central, southern, and western SJB is relatively undeformed, with regional dips of <5°. The eastern basin margin is highly deformed along the Nacimiento uplift, with Mesozoic and Cenozoic units steeply dipping to overturned (Woodward, 1987). The northern basin margin is moderately deformed along the San Juan uplift with Paleozoic through Cenozoic units moderately to steeply dipping (Steven et al., 1974). The western and southern basin margins are slightly deformed along the Defiance upwarp and Zuni Mountains uplift, respectively, with Mesozoic and Cenozoic units gently dipping (Baltz, 1967). It has been suggested that the SJB received most of its sediments from highlands to the north and the early San Juan Uplift (Baltz, 1967), although source areas in the Brazos- Sangre de Cristo, Nacimiento, Zuni, and Defiance Uplifts are also suspected (Smith, 1992;

81 Cather, 2004). Past research has linked long-term evolution of the Cretaceous-Eocene sed- imentary succession to three tectonic episodes of subsidence in response to the Laramide orogeny: inception and early-phase (∼78-75 Ma), middle phase (∼74-67 Ma), and late phase occurring during the early Paleogene (Cather, 2004). Intraformational angular unconformi- ties and reverse faults in the eastern outcrop area show that the Paleogene units in the SJB were deposited simultaneous with deformation along the basin-bounding Nacimiento fault (Woodward, 1987; Smith, 1988).

3.3.1 Stratigraphy

The Paleogene units preserved in the SJB are the earliest Paleocene Ojo Alamo Sand- stone, the Paleocene Nacimiento Formation, and the early Eocene San Jose Formation (Fig. 3.1c). These units outcrop in a series of badlands and cliff-forming sandstones along the western and southern portions of the SJB (Fig. 3.1d). This study focuses on the uppermost Nacimiento Formation and the Cuba Mesa and Regina Members of the San Jose Formation.

3.3.1.1 Paleocene Nacimiento Formation

The Paleocene Nacimiento Formation is composed primarily of terrestrial fluvial deposits consisting of sandstone and varicolored mudrocks that attains a thickness of as much as 525 m (Baltz, 1967; Lucas and Williamson, 1992). The Nacimiento Formation is divided by lithologic and sedimentologic features into three members: The Arroyo Chijuillita (oldest), the Ojo Encino, and the Escavada (youngest) (Fig. 3.2b; Williamson and Lucas, 1992). The Arroyo Chijuillita Member is characterized by drab mudrocks and sparse heterolithic sand- stones. The Ojo Encino Member is also mudrock dominant and is characterized by repetitive black-red-green-white banding that is associated with bentonitic and carbonaceous black mudrocks, variegated red and green paleosols, and fine-grained white sandstones (Lucas and Williamson, 1992). Three persistent black paleosols within the Ojo Encino Member (referred to as the lower black, middle black, and upper black) can be correlated across the basin and serve as useful marker beds (see Leslie et al., 2018). The Escavada Member is character-

82 ized by a predominance of sandstones, drab mudrocks, and abundant silcretes (Lucas and Williamson, 1992; Hobbs, 2016). Across the southern SJB, the thickness of the Escavada Member varies 19.2-88 m due to intraformational thinning (Butler and Lindsay, 1985) and channel scouring at the base of the San Jose Formation (Lucas et al., 1981). A north-to-south decrease in grain size and paleocurrent measurements from cross-strata indicate a prevailing north-to-south paleoflow throughout the Nacimiento Formation (Baltz, 1967; Klute, 1986; Sikkink, 1987; Smith, 1992). The early-Paleocene age of the Arroyo Chijuillita and Ojo En- cino Members are well constrained by biostratigraphy, magnetostratigraphy, and radiometric dating (Lucas and Williamson, 1992; Leslie et al., 2018). The age of the Escavada Member, however, is poorly constrained, with the only age indicator being a zone of normal magnetic polarity in the western SJB that has been correlated to chron 26, suggesting a late Paleocene age (Fig. 3.1b; Lindsay et al., 1981; Williamson and Lucas, 1992). If this correlation is cor- rect, the Escavada Member extends to at least the lower part of chron 25 in the western SJB (Lucas and Williamson, 1992), suggesting that that the uppermost Nacimiento deposits may be younger than 58.9 Ma when correlated to the geomagnetic polarity time scale (GPTS - Ogg, 2012) (Fig. 3.1c). A summary of previous work and the results of recent investigations of the Paleocene Nacimiento Formation in the SJB can be found in Hobbs (2016) and Leslie et al. (2018).

3.3.1.2 Early-Eocene San Jose Formation

The San Jose Formation is the most extensively preserved and exposed Eocene strati- graphic unit in New Mexico (Smith and Lucas, 1991; Smith, 1992). Like the underlying Nacimiento Formation, it is composed of terrestrial fluvial deposits with a prevailing north- to-south paleoflow (Fig. 3.1d; Smith, 1988). Previous geological investigations of the south- ern and southeastern outcrop areas of the San Jose Formation include descriptions of fossils and local physical stratigraphic studies (e.g. Simpson, 1948; Lucas et al., 1981; Smith and Lucas, 1991), and regional stratigraphy and mapping (Baltz, 1967; Mytton, 1983; Man- ley et al., 1987; Smith, 1988; Smith and Lucas, 1991). The San Jose Formation contains

83 both sandstone-dominated and mudrock-dominated lithofacies. The sandstone vs. mudrock- dominance is the primary characteristic used to distinguish between the members within the San Jose Formation, listed from oldest to youngest: Cuba Mesa Member (sandstone domi- nant), Regina Member (mudrock dominant), Ditch Canyon Member (sandstone dominant), Llaves Member (sandstone dominant), and Tapicitos Member (mudrock dominant) (Fig. 3.1c; Baltz, 1967; Smith, 1992). The Cuba Mesa and Regina Members are the focus of this study. The Cuba Mesa Member at the base of the San Jose Formation in the SJB is a dis- tinct 40-240 m thick sandstone-dominated succession that overlies the Paleocene Nacimiento Formation (Baltz, 1967; Smith and Lucas, 1991). It consists of medium- to very coarse- grained, buff-colored tabular sandstones that thicken locally in vertically stacked (amalga- mated) channel belts that can attain thicknesses up to 100 m, with width-to-thickness ratios ranging from 20 to >1000 (Smith, 1988). The basal sandstone of the Cuba Mesa Member is continuous nearly basin-wide over an area of 8000 km2 (Smith, 1988). In some parts of the basin, the contact between the Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation can be difficult to distinguish because the Escavada Member of the Nacimiento Formation contains arkosic sandstones that are similar in lithology to those of the Cuba Mesa Member (Baltz, 1967; Smith and Lucas, 1991). Like the underlying Escavada Member, no age-diagnostic fossils or dateable materials have been discovered in the Cuba Mesa Member, leaving the deposits of both the upper Nacimiento Formation and the lower San Jose Formation with loose age constraints (Fig. 3.1c). The Regina Member of the San Jose Formation ranges from 150-460 m in thickness and is composed varicolored grey, purple and red mudrock and interbedded sandstone. The sandstone beds are lenticular over scales ranging from a few meters to many kilometers (Smith, 1992). Lenticular conglomeratic sandstones in the northernmost outcrops of the member contain cobbles derived from Precambrian rocks common in the San Juan uplift north of the SJB (Smith, 1992) (Fig. 3.1b). The early-Eocene age of the Regina Member

84 is determined by Wasatchian (North American Land Mammal Age - NALMA) vertebrate fossils found in the middle to upper Regina Member (Fig. 3.1c; see Lucas and Williamson, 1992 and references within), which loosely constrain the age of the deposits between 55.8 - 50.3 Ma (Alroy et al., 2000).

3.3.1.3 Nacimiento-San Jose Formation contact and the Paleocene-Eocene bound- ary

It has long been propagated in the literature that there is a large unconformity of ≥ 5.6 My (Fassett et al., 2010) that separates the Nacimiento and San Jose Formations in at least the southern SJB (Barnes et al., 1954; Baltz, 1967; Smith and Lucas, 1991). Some researchers have interpreted the contact as an angular unconformity (Baltz, 1967; Smith and Lucas, 1991; Williamson and Lucas, 1992) due to the abrupt change from mudrock- dominated lithofacies to sandstone-dominant lithofacies and localized observations of angular discordance between the Nacimiento mudrock layers and the base of the overlying basal Cuba Mesa channels. During our investigations, we have not observed an angular discordance at the Nacimiento-San Jose contact. However, the erosional base of the laterally continuous basal Cuba Mesa Member sandstone is a composite erosion surface of amalgamated channel bases and can thus create the illusion of a discordance locally. Nevertheless, the top of the basal Cuba Mesa sandbody and the overlying mudrock deposits appear parallel with the underlying Nacimiento mudrock deposits and channel tops. Upon recent review of the aerial photographs used in his interpretation, Smith (pers. com., 2018) confirmed that there is no clear evidence of an angular unconformity at the basin-wide scale. It has also been suggested that the Nacimiento - San Jose Formation contact may be transitional in nature due to interpretations that the contact is conformable in the northern SJB (Barnes et al., 1954; Stone, 1983; Smith and Lucas, 1991) and unconformable in the southern SJB (Barnes et al., 1954; Stone, 1983; Smith and Lucas, 1991) (Fig. 3.1c). Evidence cited for a conformable contact in the northern SJB include intertonguing of the Nacimiento and lowermost San Jose Formation (Cuba Mesa Member) with the late Paleocene upper

85 Animas Formation, where the presence of Tiffanian strata (61.7 - 56.8 Ma; Alroy et al., 2000) indicates that the upper Paleocene strata are preserved below and/or laterally adjacent to the San Jose Formation (Fig. 3.1c; Smith, 1988; Smith and Lucas, 1991). Evidence cited for an unconformable contact in the southern SJB include a fossil gap of at least 6 My between the stratigraphically highest Paleocene mammal find (Torrejonian, 63.3 - 61.7 Ma) and the stratigraphically lowest early Eocene mammal find (Wasatchian, 55.8 - 50.3 Ma) (Fig. 3.1c). However, the fossil localities are separated by a minimum of 90 m of stratigraphy (Lucas et al., 1981). Alternatively, Cather (2004) argues that the Nacimiento - San Jose Formation contact may be disconformable throughout the basin because electric logs for representative wells in two east-west transects show a sharp contact and resemble the electric-log response elsewhere in the southern part of the basin where the contact is suggested to be unconformable. This hypothesis does not appear to account for the biostratigraphic and magnetostratigraphic evidence that late Paleocene deposits are preserved in at least some parts of the San Juan Basin (Fig. 3.1c). Because of the uncertainty over the nature of the contact and a lack of biostratigraphic evidence in the Escavada Member of the Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation, the precise location of the Paleocene-Eocene boundary cannot be determined (Smith and Lucas, 1991). The stratigraphically highest Paleocene mammal find is of early Paleocene age (Torrejonian, 63.3 - 61.7 Ma) and is about 30 m below the Nacimiento - San Jose contact in the southern SJB (Lucas et al., 1981). The stratigraphically lowest fossil mammals in the San Jose Formation are from the Regina Member, at least 60 meters above the Nacimiento - San Jose contact, and are of early Eocene age (Wasatchian, 55.8 - 50.3 Ma) (Lucas et al., 1981) (Fig. 3.1c). Therefore, the Paleocene-Eocene boundary in the south-central SJB is located somewhere between the base of the Escavada Member of the Nacimiento Formation and the top of the Cuba Mesa Member of the San Jose For- mation (Fig. 3.1c; Tsentas and Lucas, 1980; Lucas et al., 1981). Despite this uncertainty, the Paleocene-Eocene boundary is commonly inferred in the literature to reside within the

86 suggested unconformity at the Nacimiento - San Jose contact (e.g. Dickinson et al., 1988). In summary, the nature of the Nacimiento - San Jose contact and the placement of the Paleocene-Eocene boundary in the SJB remains unresolved and requires further investiga- tion.

3.4 Dataset and Methods

For this study, we conducted detailed investigations in three locations with excellent upper Nacimiento, Cuba Mesa, and Regina outcrop exposure: Arroyo Chijuilla (AC) and Continental Divide (CD) in the southern SJB, and Ca˜non Largo (CL) in the central SJB (Fig. 3.1d). Approximately 720 meters of stratigraphic sections were measured between these three sites. Sandstones were described at approximately 10 cm resolution based on bedsets, sedimentary structures, biogenic structures, and grain characteristics (size, shape, sorting, and composition). Mudrock deposits were described based on texture and color of the matrix and mottles at approximately 1 to 1.5 m resolution. A regional investigation was conducted to better understand lateral variability in the deposits across the basin. Data collected in the regional investigation include outcrop pho- tomosaics, channel dimensions (thickness and width), channel fill characteristics, degree of channel amalgamation (qualitative assessment), and paleocurrent measurements (Fig. 3.1d). Paleocurrent directions were measured from dip directions of cross strata and laminae using a Brunton pocket transit. The collection of paleocurrent measurements in fluvial deposits commonly rely on the presence of regular cross-stratification, deposited under lower flow regime (LFR, Froude subcritical flow) conditions. In Chapter 2 we identified that the dom- inant sedimentary structures in the study area are upper flow regime (UFR) sedimentary structures deposited under Froude supercritical flow conditions. These low-angle and scour and fill structures are commonly confused for regular cross strata (Alexander et al., 2001; Plink-Bj¨orklund, 2015) and can result in paleocurrent directions that do not represent true paleoflow when measured using traditional techniques. Due to the low occurrence of lower flow regime (LFR) structures (cross-strata), our measurements are few, therefore we combine

87 our paleoflow measurements with vector means from Smith (1988) (Fig. 3.1d). Smith recog- nized the complexity of collecting paleoflow measurements through the Cuba Mesa Member, and they used vector means of random cross-strata attitudes to indicate average paleoflow directions throughout the basin. Channel measurements were taken from outcrops using a laser rangefinder (TruPulse 360R) mounted to a tripod. Laterally extensive channels were measured by collecting GPS coordinates at channel terminations. Since many outcrop faces in the Ca˜non Largo (CL) study area lie skew to the true channel orientation (paleoflow vector mean = 128°) we later adjusted the channel measurement to account for average paleoflow direction using the following equation:

WT = Wosin(|128° − β|)

Where WT is the adjusted channel width, WO is the outcrop width, 128° is the vector mean paleocurrent direction, and β is the outcrop heading. Channel measurements in the southern SJB were collected from east-to-west oriented outcrops, which is approximately perpendicular to the prevailing north-to-south paleoflow direction, therefore the measurements were not adjusted.

3.5 Sedimentary Facies

We identify two sandstone facies groups, comprised of eleven sedimentary facies, and two mudrock facies groups, comprised of eight mudrock facies (summarized below and in Table 3.1) (see details in Chapter 2). We further identify three channel sandstone facies associations and three floodplain mudrock facies associations (summarized below and in Table 3.2) (see details in Chapter 2). In this study, we use the characteristics, distribution, and abundances of these facies associations to aid in our interpretation of the depositional environments of the Escavada Member of the upper Nacimiento Formation and the Cuba Mesa Member and Regina Member of the San Jose Formation.

88 3.5.1 Sandstone facies

The sandstone facies include conglomerates (S1.1), sandstone with scour-and-fill struc- tures (S1.2), sandstone with planar to low-angle laminations (S1.3), sandstone with convex- up long wavelength laminations (S1.4), sandstone with soft-sediment deformation (S1.5), structureless sandstone (S1.6), cross-stratified sandstone (S2.1), sandstone with climbing cross-strata or -laminae (S2.2), sandstone with scours and climbing cross-laminae (S2.3), sandstone with overturned cross-strata or -laminae (S2.4) (Table 3.1). The dominant facies consist of sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations or devoid of structure (85% - S1.1, S1.2, S1.3, S1.4, S1.5, and S1.6). Conglom- erates (S1.1) consist primarily of disorganized or stratified and imbricated mud rip-up clasts in a sandy matrix. Sandstones with scour-and-fill structure (S1.2) display upward flattening laminae that fill the scours asymmetrically or symmetrically and in places transition laterally to low-angle laminations. Sandstones with planar to low-angle laminations (S1.3) contain long wavelength laminae that dip at <10°. Sandstones with convex-up long wavelength lam- inations (S1.4) also contain laminae that dip at <10° but display laminae with an upward increase in convex structure. Sandstones with soft-sediment deformation (S1.5) contain con- volute laminae that form anticlinal to domal structures. Structureless sandstones (S1.6) occur as thin, tabular, and weakly cemented deposits and as thick well-cemented deposits. Less abundant facies include sandstones with steeply dipping downstream oriented cross- strata and -laminae (15% - S2.1, S2.2, S2.3, S2.4). Cross-stratified sandstones (S2.1) contain consistent foresets that display no change in dip angle (∼30°). Sandstones with climbing cross-strata or -laminae (S2.2) are similar to facies of S2.1, except the set boundaries are inclined, dipping opposite the cross-strata or -laminae dip. Sandstones with scours and climbing cross-laminae (S2.3) contain scours filled with climbing ripple laminations. Sand- stones with overturned cross-strata or -laminae (S2.4) contain deformed and oversteepened high-angle foresets (>30°).

89 Interpretation: The facies displaying scour and fill structures (S1.2), planar to low-angle laminations (S1.3), and convex-up long wavelength laminations (S1.4) resemble experimen- tally produced sedimentary structures from Froude supercritical flow (e.g. Alexander et al., 2001; Cartigny et al., 2014; Ono et al., subm.), as well as to examples of modern river flood deposits (e.g. McKee et al., 1967; Williams, 1971; Stear, 1985; Billi, 2007) and interpreted outcrop examples of Froude super- and trans-critical or UFR flow deposits of ancient rivers (e.g. Stear, 1985; North and Taylor, 1996; Fielding, 2006; Fielding et al., 2009; Allen et al., 2013; Plink-Bj¨orklund, 2015). The preservation of these structures has been attributed to high deposition rates (Cartigny et al., 2014). Both soft-sediment deformation (S1.5) and structureless sandstones (S1.6) have various proposed origins, including movement and depo- sition in a liquefied state, post-depositional liquefaction processes that modify or obliterate stratification, high deposition rates and horizontal shear, and local gravitational collapse at scour or channel margins (Postma and Cartigny, 2014). Based on their stratigraphic proxim- ity to other planar to low-angle sedimentary structures, we hypothesize that the deposits of facies S1.5 are likely convolute versions of facies S1.2 - S1.4. The occurrence of some struc- tureless sandstone deposits (S1.6) may also be the result of rapid sediment fallout under high deposition rates. However, some deposits may contain structures that are not visible on the outcrop surface due to weathering and erosion, especially where poorly cemented, and thus it is likely that this facies is over-represented in our dataset. Thick mudrock clast conglomerates have been linked to bank undercutting and collapse due to high water power and rapid lowering of water level during the waning stage of floods (e.g., Coleman, 1969; Gohain and Parkash, 1990; Singh et al., 1993; North and Taylor, 1996; Tandon and Gibling, 1997). In summary, the sandstones and conglomerates with planar to low-angle, concave- to convex-up laminations or devoid of structure (facies S1.1, S1.2, S1.3, S1.4, S1.5 and S1.6) are likely indicative of deposition under Froude supercritical, upper flow regimes (UFR) and high deposition rates (HDR) (see also Chapter 2).

90 Planar or trough cross-stratification (Facies S2.1) is formed by migration of dunes under subcritical flow conditions (Simons et al., 1965; Allen, 1984). Climbing cross-stratification (Facies S2.2) indicates highly aggradational dune migration under subcritical flow and high deposition rates (Allen, 1984). Climbing cross-strata or -laminae (Facies S2.3) are linked to dune or ripple migration under high deposition rates. Overturned cross-strata or -laminae (Facies S2.4) are interpreted as sets of regular cross-strata or -laminae that were deformed by the shear stress caused by the overlying current (Mills, 1983). The migration of dunes and ripples in Facies 2.1 - 2.4 occurs in the downstream direction as sediment is eroded from the stoss side and deposited on the lee side, thus only the foresets are preserved (Simons et al., 1965; Allen, 1984; Miall, 2014), and the dip direction of the foresets is a reliable indicator of paleoflow direction. The direction of overturning in Facies S2.4 also appears consistent with the prevailing southerly paleoflow direction. Collectively, we interpret the facies with steeply dipping cross-strata and lamina as indicative of Froude subcritical, lower flow regime (LFR) conditions (also see Chapter 2).

3.5.2 Mudrock facies

Mudrock facies consist of drab-colored mudrock or purple-red mudrocks. The drab- colored deposits include globular grey mudrock (M1.1), black mudrock (M1.2), brown to dark grey mudrock (M1.3), grey-green mudrock (M1.4), and grey mudrock (M1.5). The purple-red deposits include purple mudrock (M2.1), red mudrock (M2.2), and mudrock with variegated color (M2.3) (Table 3.1). Globular grey mudrock (M1.1) deposits contain large, concave- up soft-sediment deformation structures. Black mudrock (M1.2) deposits contain subangular blocky peds of moderate structures and occasional slickensides. Brown to dark grey mudrock (M1.3) deposits range from laminated with plant fragments to massive and structureless. Grey-green mudrocks (M1.4) exhibit subangular blocky peds of weak to moderate structure and extensive green mottling. Interpretation: The large, globular soft sediment deformation structures (Facies M1.1) likely resulted from loading of wet muds by overlying deposits. Black (Facies M1.2) or dark

91 brown colors (Facies M1.3) are commonly attributable to the presence of organic matter or manganese oxide minerals (Vepraskas and Craft, 2016; Tabor et al., 2017). Grey or green colors (Facies M1.4 and M1.5) are thought to be the result of reduction of ferric minerals or removal of organic matter and oxides, which are processes associated with water saturation and anoxia (Farnham and Kraus, 2002; Vepraskas and Craft, 2016; Tabor et al., 2017). Deposits exhibiting weak to moderate structure and blocky peds are interpreted as former soil horizons (paleosols) of varying maturity (Retallack, 1988). Many of the structureless deposits that contain visible plant fragments and quartz granules (Facies M1.5) are likely protosols, indicating that the soils were buried before pedogenesis could take place (Mack et al., 1993; Tabor et al., 2017). In summary, the drab-colored mudrock deposits are consistent with poorly-drained soil conditions and/or lesser degrees of pedogenic development in floodplain mudrocks. The coloring of the purple-red deposits is attributed to the presence of hematite, an iron oxide mineral resulting from the soil being moderately- to well-drained and oxidizing (Retallack, 1988). Deposits with a purple matrix are interpreted as having weaker hematite impregnation compared to deposits with a red matrix (Kraus et al., 2013). Grey and yellow- brown mottles are redoximorphic features produced by soil gleying, and they indicate that the soils were episodically saturated (Farnham and Kraus, 2002). Grey mottles are also attributed to root traces, and are thought to be a redoximorphic depletion feature formed when stagnating surface water seeps downward along root channels and, in the presence of organic matter, iron is reduced and removed (Kraus and Hasiotis, 2006). The presence of slickensides indicates repeated shrinking and swelling of clays due to fluctuations in the water table (e.g., Bigham et al., 2002; Kraus and Riggins, 2007). Most deposits of these facies are interpreted as paleosols based on the presence of peds, and in some cases, fossil root traces (Retallack, 1988). Deposits with well-developed structure are interpreted as mature paleosols, formed over longer intervals of time, in stable landscapes, or under intense weathering conditions (Marriott and Wright, 1993; Kraus, 1999; Tabor et al., 2017). The

92 wedge-shaped peds that are common in these deposits are thought to form only in soil profiles where seasonal precipitation and abundant fine clay combine to induce shrink-swell processes (Tabor et al., 2017). Deposits with laminations, preserved organic matter, and/or a high silt content are interpreted as protosols. In these cases, the deposits were likely buried before pedogenesis could take place (Tabor et al., 2017). In summary, the purple-red mudrock deposits indicate moderate to well-drained soil conditions in floodplain mudrocks.

3.6 Facies Associations 3.6.1 Channel facies associations

A majority of sandstones and some of the mudrocks occur in erosionally based lenticular or amalgamated lithosomes. These channel lithosomes are organized into facies associations defined by lithology, channel size, channel architecture, and degree of channel amalgamation (Table 3.2) (Chapter 2). Sandy channel lithosomes (FA 1) are composed of thick, erosionally-bound sandstones that are both vertically and laterally amalgamated. The extensive amalgamation results in sandstone cliffs that have a massive or uniform appearance in outcrop, except for thick mud- clast conglomerates that occur at the bases of channels and between some sandstone deposits. Channel fill is dominated by UFR and HDR sedimentary structures (∼96% - S1.1, S1.2, S1.3, S1.4, S1.5, S1.6). Large, low-angle (<10°) downstream or upstream dipping accretion sets are observed in outcrops that are oriented approximately parallel to the prevailing flow direction (north to south). Some deposits are bioturbated from the top down or from accretion set boundaries with vertical burrows. Heterolithic channel lithosomes (FA 2) also consist of amalgamated sandstone deposits, however the channel fill contains mudrock layers along accretion set boundaries that result in a less amalgamated appearance in outcrop than those of FA 1. UFR and HDR sedimentary structures are dominant (∼89% - S1.1, S1.2, S1.3, S1.4, S1.5, S1.6), but there is a larger proportion of LFR structures (∼11% - S2.1; S2.3) than in FA 1. Accretion sets dip at a low angle (<10°) in the downstream direction in outcrops that are oriented in the flow parallel

93 direction (north to south). The in-channel mudrock facies consist of grey (M1.3 and M1.5) or purple mudrock (M2.1). Lenticular channel lithosomes (FA 3) are isolated and bounded by floodplain mudrock deposits. These channel deposits range from sandy to heterolithic with mud layers separating sandy accretion sets. Although still abundant, the content of UFR sedimentary structures is considerably lower (∼50% - S1.2, S1.3, S1.4, S1.5, S1.6) and LFR structures are more abundant (∼50% - S2.1, S2.3, S2.4). The in-channel mudrock facies consist of grey mudrock (M1.3 and M1.5) or purple mudrock (M2.1). Interpretation: The dominance of UFR and HDR sedimentary structures, the presence of thick mud-clast conglomerates at the base of sandy deposits, and a general lack of tran- sitions from UFR to LFR structures suggest that the FA 1 channels consist entirely of high-magnitude flood deposits or flood event beds. Deposition during flood events generates distinct flood deposits that are bounded by erosion surfaces, or by an erosion surface below and a mud drape above (see flood units of Plink-Bj¨orklund, 2015). The high degree of amal- gamation and lack of systematic bar migration (lateral accretion) signifies high avulsion rates (Bryant et al., 1995). The dominance of UFR and HDR sedimentary structures in the FA 2 channels also suggests that most deposition occurred at high flow velocities and deposition rates. However, vertical transitions from UFR to LFR structures indicate a less rapid decline in flow strength during the waning phase of floods (Jones, 1977). The in-channel mud layers capping flood units may be the result of mud deposited at the final stage of flood events, as has been observed in modern river systems after major floods (e.g. Stear, 1985; Abdullatif, 1989; Singh et al., 1993; Billi, 2007). The isolated nature of the FA 3 channels indicates a lower degree of channel amalgamation. The presence of LFR structures indicates a less rapid decline in flood strength and preservation of waning phase deposits (Abdullatif, 1989; Plink-Bj¨orklund, 2015) or post-flood reworking of sediment under subcritical flow conditions. The presence of in-channel bioturbation in the FA 1 and FA 2 lithosomes and in-channel pedogenic modification in the FA 2 lithosomes indicates that the channels were seasonally

94 dry or had low perennial base flow (Hasiotis et al., 2007). An abundance of UFR and HDR sedimentary structures, in-channel bioturbation and pe- dogenic modification of in-channel mud layers, thick mud-clast conglomerates, soft-sediment deformation structures, and the lack of preserved braid or point bar strata are the key obser- vations in channel deposits that can be linked to deposition in rivers with variable discharge, characterized by fluctuations between high-magnitude floods and periods of little to no dis- charge (see reviews by Fielding et al., 2009, 2018; Plink-Bj¨orklund, 2015 and references therein). Thus, we identify the FA 1, FA 2, and FA 3 channel facies associations as deposits of variable discharge rivers (Chapter 2).

3.6.2 Floodplain facies associations

A majority of the mudrock and some sandstone facies occur in floodplain lithosomes that are organized into facies associations defined by dominant matrix colors, ped structure, and presence of mottling and/or nodules as indicators of soil drainage conditions (Table 3.2) (Chapter 2). Poorly-drained floodplain lithosomes (FA 4) consist of multiple sets of vertically stacked paleosol and protosol mudrock deposits interbedded with thin, tabular or lenticular, and poorly-cemented structureless sandstone (2% - S1.6). The mudrock deposits contain domi- nantly drab-colored facies (95% - M1.1, M1.2, M1.3, M1.4, M1.5). Rare purple deposits are also observed (3% - M2.1). Variably-drained floodplain lithosomes (FA 5) also consist of multiple sets of vertically stacked paleosol and protosol mudrock deposits interbedded with thin, tabular or lenticular, and poorly-cemented structureless sandstone (32% - S1.6). However, the mudrock consist of alternating horizons of drab-colored (46% - M1.1, M1.2, M1.3, M1.4, M1.5) and purple-red facies (22% - M2.1, M2.2, M2.3). Well-drained floodplain lithosomes (FA 6) consist of multiple sets of vertically stacked paleosol and protosol deposits interbedded with thin, tabular or lenticular, and poorly- cemented structureless sandstone (28% - S1.6) and a high occurrence of purple-red mudrock

95 facies (42% - M2.1, M2.2, M2.3) and lesser drab-colored facies, which are almost all classified as protosols (30% - M1.5). Interpretation: Floodplains displaying vertically stacked, multi-story paleosols are linked to sedimentary systems undergoing net aggradation, where the rate of sedimentation does not overwhelm the rate of pedogenesis (Kraus, 1999). Alternations between paleosols and protosols indicate variations in the balance between sediment accumulation and the rate of pedogenesis (Marriott and Wright, 1993; Kraus, 1999). The dominantly drab deposits of FA 4 indicate a frequently wet depositional environment. The interbedding of drab and purple-red paleosols in FA 5 suggests alternating wet and dry periods on the floodplain. The high proportion of purple-red paleosols and a lack of drab paleosols in FA 6 indicates that the floodplain was sustainably dry. The poorly-cemented structureless sandstones (S1.6) are interpreted as crevasse splays where thin and tabular, and isolated channel deposits where lenticular. The crevasse splay deposits indicate partial avulsions, which causes channels to shift their positions on the floodplain (Bridge and Leeder, 1979; Jones and Schumm, 1999). This process can terminate sedimentation to part of the floodplain for a period of time, leading to pedogenic development of surface deposits (Kraus, 1999). Meanwhile, the location occupied by the overbank flow of the partial avulsion experiences a high rate of sedimentation that prevents pedogenic development (Kraus, 1999).

3.7 Paleocurrent Measurements

Paleocurrent directions collected in this study from cross-strata maximum dip directions, combined with vector means from Smith (1988) indicate a dominantly southward (ranging from southwest to southeast) fluvial sediment transport in the Nacimiento and San Jose Formations in the SJB (Fig. 3.1d). In the northern and central SJB, the paleoflow measure- ments show a clear dominance of sediment transport to the southeast. In the southern SJB, paleocurrent measurements are more variable and indicate southwest to southeast transport. Another indicator of a generally north to south paleoflow are trends in grain size and sorting. In general, grain size is larger (Fig. 3.3), and sorting is poorer in the northern Nacimiento

96 and San Jose Formations than in the south. Some of the measurements collected show a bimodal distribution of paleoflow directions (Fig. 3.1e). The deposits where these measurements were collected have the appearance of regular cross-strata (Fig. 3.2a), but when these structures are traced laterally, it is apparent that they grade laterally into near-horizontal, low-angle laminations, which distinguishes them from sets of regular cross-strata (Fig. 3.2b). In the absence of evidence for other de- positional environments commonly associated with bimodal paleoflow directions (e.g. tidal), and by comparison with experimentally produced upper flow regime (UFR) sedimentary structures (Cartigny et al., 2014), these measurements were likely collected from backsets that record upstream migration of antidune and/or cyclic step bedforms under Froude su- percritical flow conditions.

3.8 Architectural Styles

Vertical changes in the proportion of sandstone and mudrock deposits have long been used to distinguish the Escavada, Cuba Mesa, and Regina Members in the San Juan Basin (Baltz, 1967). The principal factors that change between the members are the ratio of sandstone to mudrock lithofacies, channel size, degree of channel amalgamation, and facies composition. Based on these differences, we have identified three major depositional architectural styles: 1) amalgamated channels with minor (<20%) floodplain, 2) channels and (20-60%) floodplain, and 3) isolated channels with dominant (>60%) floodplain (Fig. 3.4). Amalgamated channels with minor floodplain deposits (AS 1). This architectural style consists of vertically and laterally amalgamated sandy channel lithosomes (FA 1) ∼10-50 m thick and 172-1564 m wide, separated by 2-20 m thick successions of floodplain lithosomes (FA 5 or FA 6) (Fig. 3.4a). The lateral and vertical amalgamation of the channel lithosomes results in thick (up to ∼100 m) and laterally extensive packages of this style that can be traced in some outcrops for >1 km. The associated floodplain deposits consist of variably drained floodplain lithosomes (FA 5) in the Cuba Mesa Member and well-drained floodplain lithosomes (FA 6) in the Regina Member. The overall thicknesses of this architectural style

97 ranges from 15-100 m. Heterolithic channels and floodplain deposits (AS 2). This architectural style consists of channel complexes that consist of heterolithic channel lithosomes (FA 2), 5-13 m thick and 94-312 m wide, separated by thick successions (10-25 m) of floodplain lithosomes (FA 4 or FA 5) (Fig. 3.4b). The associated floodplains consist of poorly-drained floodplain lithosomes (FA 4) where this style is present in the Escavada Member of the Nacimiento Formation and variably-drained floodplain lithosomes (FA 5) where present in the Cuba Mesa Member of the San Jose Formation. Crevasse splay deposits make up approximately 17% of the floodplain lithosomes. Isolated channels and dominant floodplain deposits (AS 3). This architectural style consists of isolated and lenticular channels (FA 3), 2-17 m thick and 27-282 m wide, bound laterally and vertically by floodplain deposits 10-34 m thick (FA 4 or FA 6) (Fig. 3.4c). The associated floodplains consist of poorly-drained floodplain lithosomes (FA 4) where this style is present in the Nacimiento Formation and well-drained floodplain lithosomes (FA 6) where present in the Regina Member of the San Jose Formation. Crevasse splay deposits make up on average approximately 30% of the floodplain lithosomes. Interpretation: The high degree of channel amalgamation in architectural styles 1 and 2 indicates high avulsion rates (Jones and Hajek, 2007) in the absence of lateral accretion sets that would signify lateral channel migration. This interpretation is further supported by cross-cutting channel relationships (Jones and Hajek, 2007). The high degree of channel amalgamation is a function high channel return frequency (Hajek and Wolinsky, 2012; Hajek and Edmonds, 2014). Avulsions are most commonly driven by high channel bed aggradation rates (Bryant et al., 1995; Mohrig et al., 2000; Berendsen and Stouthamer, 2001; Makaske, 2001; Jerolmack and Mohrig, 2007; Sinha and Sarkar, 2009; Hajek and Edmonds, 2014), supported by the abundance of HDR structures the channel deposits. This high channel return frequency is likely the reason for low preservation of floodplain facies in architectural style 1. Conversely, the lower degrees of channel amalgamation and higher preservation

98 of floodplain facies in styles 2 and 3 suggests either less frequent avulsions and/or a lower channel return frequency.

3.9 Lateral Trends

Flow perpendicular lateral (east - west) trends are evaluated by comparing the Arroyo Chijuilla and Continental Divide measured sections (Fig. 3.3), field observations, and oblique aerial photos (Fig. 3.5) collected along outcrops in the southern San Juan Basin. The Con- tinental Divide study area is located approximately 9 kilometers west of the Arroyo Chijuilla study area (Fig. 3.1d). The exposure of the outcrops is poor in the area that separates the two study areas, making it challenging to trace channel complexes laterally across the whole distance. Therefore, lithologic similarities in sand-to-mudrock ratio, sandstone and paleosol facies assemblages, facies associations and architectural styles are used correlate the sections, which are hung from a datum placed at the contact between the Cuba Mesa and Regina Members (Fig. 3.3). Due to the somewhat limited outcrop exposure in the Continental Divide study area, comparisons between deposits of the Nacimiento Formation are not possible. The Continental Divide study area contains a greater abundance of mu- drock lithofacies in the Cuba Mesa and Regina Members of the San Jose Formation (46%) than in Arroyo Chijuilla (39%) (Fig. 3.3). The architectural styles 1 and 2 display reduced thicknesses in the Continental Divide study area compared to the Arroyo Chijuilla section (decreases of 53% and 56%, respectively). The architectural style 3 deposits show an in- creased abundance of floodplain and crevasse splay deposits (Fig. 3.5). Due to the increased abundance of mudrock and the decreased thickness of sandstone deposits, the Continental Divide section appears thinner in comparison, with a total of 40.75 meters of Cuba Mesa and 116.67 meters of Regina outcrop measured in Continental Divide and 158.38 meters of Cuba Mesa and 127.49 meters of Regina outcrop measured in Arroyo Chijuilla (Fig. 3.3). Lateral trends in the flow parallel direction (north - south) are evaluated by comparing the detailed measured sections from Ca˜non Largo study area in the central SJB with the Arroyo Chijuilla and Continental Divide study areas, located approximately 80 km to the southeast

99 (Fig. 3.3), and from channel measurements (Fig. 3.6) and photomosaics collected in the northern, central, and southern San Juan Basin (Figs. 1 and 7). Through these comparisons, significant changes from north to south in the sandstone vs. mudrock percentage, grain size (Fig. 3.3), average channel size (Fig. 3.6), and degree of channel amalgamation (Fig. 3.7) can be observed. The percentage of sandstone vs. mudrock is calculated from the measured sections from the Ca˜non Largo, Arroyo Chijuilla, and Continental Divide study areas (Fig. 3.3). Due to the limited amount of the Nacimiento Formation that could be measured from outcrop, we only include data from the Cuba Mesa and Regina Members. These data show a decrease in the percentage of sandstone from north to south, and thus an increased abundance of mu- drock. The Cuba Mesa and Regina deposits in Ca˜non Largo are composed of 72% sandstone lithofacies (22% mudrock, 6% covered), compared to 55% sandstone (39% mudrock, 6% covered) in Arroyo Chijuilla and 50% sandstone (46% mudrock, 4% covered) in Continental Divide. Grain sizes in the sandstone deposits are overall coarser in northern and central SJB compared to the southern study areas, with most common grain sizes in channels changing from an abundance of very coarse-coarse in Ca˜non Largo to a range of grain sizes from very coarse-fine in the southern Continental Divide and Arroyo Chijuilla study areas (Fig. 3.3). In addition, the floodplain deposits show a greater abundance of silt in Ca˜non Largo and a dominance of clay in Continental Divide and Arroyo Chijuilla (Fig. 3.3). There is a decreasing trend in channel size (lithosome thickness and width) from north to south (Fig. 3.6). Upper Nacimiento channels decrease from an average height of 19.8 meters and average width of 358.8 meters in Ca˜non Largo to an average height of 6.0 meters and average width of 138.9 meters in Arroyo Chijuilla (average decrease of 69% in height and 61% in width). The Cuba Mesa channels decrease from an average height of 26.9 meters and average width of 979.2 meters in Ca˜non Largo to an average height of 11.9 meters and average width of 300.7 meters in Arroyo Chijuilla (average of 56% decrease in height and

100 69% decrease in width). The Regina channels decrease from an average height of 8.7 meters and average width of 378.0 meters in Ca˜non Largo to an average height of 7.3 meters and average width of 118.2 meters in Arroyo Chijuilla (average decrease of 16% in height and 69% in width). There is an overall decreasing trend in channel amalgamation from north to south that is best observed in the photopanels of the Cuba Mesa Member (Fig. 3.7). The most laterally and vertically amalgamated channel complexes are in the northern SJB (Fig. 3.7a) and are dominated by the architectural style 1. In Ca˜non Largo (central SJB), the channel complexes are still laterally amalgamated, but there is a visible decrease in the degree of vertical amalgamation (Fig. 3.7b). These channel complexes still meet the recognition criteria for architectural style 2, but they are separated by some intervals of floodplain deposits (∼2-4 m thick). In the southern SJB, channel complexes are typically separated by thicker floodplain deposits (∼10-25 m), although there is still lateral and vertical amalgamation present (Fig. 3.7c). These channel complexes are composed of both the architectural styles 1 and 2. In addition, more crevasse splay deposits are observed in the southern SJB in association with the increased abundance of floodplain deposits (Fig. 3.7c). Interpretation: The most significant basin-scale changes in sandstone vs. mudrock per- centage, grain size (Fig. 3.3), average channel size (Fig. 6), and degree of channel amal- gamation (Fig. 3.7) are observed in the flow-parallel direction from north to south. This spatial trend indicates a proximal - distal relationship between the depositional styles and the location of deposits within the fluvial system. The observed downstream decrease in channel size suggests that distal channels transported lesser discharge than channels in the more proximal parts of the system. Such decreases in discharge may result from high rates of water loss to the floodplain, subsurface, or through evaporation (Fisher et al., 2007; Nichols and Fisher, 2007). The downstream decrease in channel size differs from what is expected of tributive river systems where increasing amounts of water are added to the trunk and the channel capacity increases accordingly. Below, we explore these interpretations further in

101 the context of the depositional environment. In this study, we observe that proximal channel facies show a higher proportion of UFR and HDR structures than the distal channel facies, suggesting that the precipitation signal is clearest at the upper fan and is modified downstream. Downstream declining discharge and flow power with an associated reduction in channel size has been documented in some modern variable discharge river systems, especially dryland rivers where the downstream declines are particularly pronounced on the lower reaches that receive floods and have significant transmission losses to the floodplain, subsurface, and evaporation (Tooth, 1999, 2000).

3.10 Depositional Environment

In addition to the proximal - distal changes in channel size and degree of amalgamation, we have observed other characteristics that diverge from tributary river systems, including the large lateral extent of the system, downstream decrease in grain size. The lateral extent of the Cuba Mesa Member of the San Jose Formation covers at least 8000 km2, based on calculations of the area of the basal Cuba Mesa channel complex (Smith, 1988). Furthermore, the abundant crevasse splay deposits throughout the study interval suggest local avulsions and unconfined flow were the key process of lateral river mobility and sediment distribution within the system (Bryant et al., 1995; Mohrig et al., 2000; Berendsen and Stouthamer, 2001; Makaske, 2001; Jerolmack and Mohrig, 2007; Sinha and Sarkar, 2009; Hajek and Edmonds, 2014). Interpretation: The characteristics of the upper Nacimiento Formation, Cuba Mesa Mem- ber, and Regina Member described above that diverge from tributary river systems are co- incident with those of large fluvial fans (sensu. DeCelles and Cavazza, 1999; Leier et al., 2005; Hartley et al., 2010; Weissmann et al., 2010; Latrubesse, 2015; Ventra and Clarke, 2018). The diagnostic sedimentological attributes that are suggested for the interpretation of ancient fluvial fans include 1) a progressive downstream decrease in grain size, 2) a de- crease in channel depth and width, 3) an increase in overbank preservation, crevasse-splay or sheet flood deposition on a basin scale, and 4) a decrease in lateral and vertical connec-

102 tivity of channel deposits (DeCelles and Cavazza, 1999; Horton and DeCelles, 2001; Shukla et al., 2001; Nichols and Fisher, 2007; Cain and Mountney, 2009; Weissmann et al., 2010). These trends have been documented in modern (Singh et al., 1993; Shukla et al., 2001; Chakraborty et al., 2010) and ancient fluvial fans (Kukulski et al., 2013; Owen et al., 2015b; a, 2017; Chesley and Leier, 2018), and are included in fluvial fan and DFS depositional mod- els (Weissmann et al., 2013, 2015). Below we compare our observations from the San Juan Basin fluvial facies and architecture styles with documented modern and ancient examples of proximal, medial, and distal large fluvial fans. The amalgamated channel architectural style (AS 1) (Fig. 3.8a) is seen in the greatest abundance in the northern and central SJB. The proximal deposits of modern large fluvial fans (Singh et al., 1993; Shukla et al., 2001; Chakraborty et al., 2010) and ancient large fan systems (Kukulski et al., 2013; Owen et al., 2015b; a, 2017) have commonly been documented as consisting of amalgamated channel belts with relatively coarse grains and limited preser- vation of fine-grained floodplain material. The degree of channel amalgamation is highest in the most proximal fan where the fan area is smallest (Fig. 3.8d), and the aggradation rates and channel return frequency are highest (Chakraborty and Ghosh, 2010; Hajek and Wolinsky, 2012; Weissmann et al., 2013, 2015; Ventra and Clarke, 2018). Thus, the intervals that are dominated by amalgamated channels (AS 1) are herein interpreted as proximal fan facies (Fig. 3.8a). This interpretation is further corroborated by the prevailing north-to- south paleoflow direction (Fig. 3.1d) and the increased abundance of FA 1 deposits with a high sand to mudrock ratio as seen in the northern and central SJB. The heterolithic architectural style 2 closely resembles descriptions of medial fan de- posits which characteristically show increased spacing between channel deposits and de- creased channel belt size (Fig. 3.8d; Singh et al., 1993; Shukla et al., 2001; Weissmann et al., 2013; Chesley and Leier, 2018). These proximal to medial changes are attributed to losses from infiltration and the fan covering a wider area. In addition, a higher percentage of fine-grained material is deposited and has the potential to be preserved as a result of lower

103 coarse sediment supply and decreased flow strength (Weissmann et al., 2013), creating more complex, heterolithic internal architecture (Moscariello, 2018) (Fig. 3.8b). The isolated architectural style 3 is consistent with observations of distal fan facies (Fig. 3.8c). Distal fan deposits commonly consist of isolated channel fills within dominantly muddy floodplain deposits (Shukla et al., 2001; Weissmann et al., 2013; Chesley and Leier, 2018; Moscariello, 2018). The more isolated channels are thought to result from a continued widening of the area covered by the fan and additional losses from infiltration (Singh et al., 1993; Shukla et al., 2001; Weissmann et al., 2015). This interpretation is further corroborated by the prevailing north-to-south paleoflow direction and the abundance of FA 3 channels and high proportion of floodplain mudrock seen in the southern SJB. Much of the deposition in distal fans occurs as avulsion deposits (Assine, 2005; Weissmann et al., 2013), which is coincident with the abundance of crevasse splay deposits associated with this architecture style.

3.11 Stratigraphic Trends

The Arroyo Chijuilla, Continental Divide, and Ca˜non Largo study areas each contain a complete sedimentary record of the Cuba Mesa Member (Fig. 3.3). However, the vertical extent of outcrop exposure of the Escavada Member and the Regina Member vary between the study areas. Nevertheless, there are stratigraphic trends shared between the three study areas and throughout the SJB that have long been used to distinguish between the different members of the Nacimiento and San Jose Formations (Baltz, 1967). Here, we explain how the proximal, medial, and distal fan facies described above are distributed through the stratigraphy. The Escavada Member of the uppermost Nacimiento Formation is characterized by an upward shift from distal fan facies to medial fan facies (Fig. 3.9a; Fig. 3.12a). The floodplain deposits within the Escavada member consist of the poorly-drained floodplain lithosome (FA 4). In the northern and central San Juan Basin, this change is observed as a gradual upward- coarsening trend (Fig. 3.12a). In the southern San Juan Basin, the shift appears more abrupt

104 (Fig. 3.9b). The base of the Cuba Mesa Member of the San Jose Formation is characterized as a shift to proximal fan facies (Fig. 3.9a,b,c; Fig. 3.11a; Fig. 3.12a). This easily observed lithologic change characterizes the Nacimiento - San Jose Formation contact in the southern San Juan Basin (Baltz, 1967) (Fig. 3.10a). The transition in the central and northern San Juan Basin is more gradational, making it more difficult to pinpoint the precise location of the contact (Fig. 3.10b). In the southern Arroyo Chijuilla and Continental Divide study areas, the Cuba Mesa Member forms three sandstone intervals (lower Cuba Mesa, Middle Cuba Mesa, and upper Cuba Mesa) that are separated by thick successions of mudrock deposits (Fig. 3.9a,b,c; Fig. 3.11a). In these southern SJB study areas, only the lower Cuba Mesa channel complex consists of proximal fan facies (Fig. 3.9a). The middle and upper channel complexes are more heterolithic (FA 2), and consist of medial fan facies (Fig. 3.9b,c; Fig. 3.11a). In Ca˜non Largo, located in the central SJB, the Cuba Mesa Member is dominated by proximal fan facies and an up-section increase in channel fill heterogeneity and floodplain abundance is not apparent (Fig. 3.12a). In all three study areas, the floodplain deposits within the Cuba Mesa Member consist of the variably-drained floodplain lithosome (FA 5), which is observed between channels and channel complexes (Fig. 3.9b,c; Fig. 3.11a; Fig. 3.12a). The transition from the Cuba Mesa Member to the Regina Member is characterized by an initial return to distal fan facies. However, the bounding floodplain deposits are well- drained (FA 6) (Fig. 3.9c; Fig. 3.11b; Fig. 3.12b), which contrasts with the poorly-drained floodplain deposits (FA 4) observed bounding the isolated channels in the Escavada Member of the Nacimiento Formation. In the Ca˜non Largo and Continental Divide study areas, there is another upward shift to proximal fan facies that occurs higher in the Regina Member (Fig. 3.11b and 3.12b). This shift is abrupt in both study areas. In the Continental Divide study area, this amalgamated sandy channel complex is overlain by another succession of well- drained floodplain deposits (FA 6), although outcrop exposure is very limited (Fig. 3.12b).

105 In summary, there are at least two large-scale upward-coarsening successions that can be observed basin-wide and are characterized by a transition from distal to proximal fan facies (Fig. 3.13). The first extends from the Ojo Alamo and Escavada Members of the Nacimiento Formation and into the Cuba Mesa Member of the San Jose Formation. The second is observed in the Regina Member of the San Jose Formation. In the southern SJB, there is an interval between the coarsening-up succession that extends from the middle to upper Cuba Mesa Member that shows a reversal of the trend, with increased heterogeneity and a return to medial fan facies. This trend is observed up to the Cuba Mesa-Regina contact where the second episode of progradation begins (Fig. 3.13b). In the central SJB, this reversal is not observed (Fig. 3.13a). Outcrop exposure of the upper Regina Member is limited, therefore it is not possible to determine if additional upward-coarsening successions were present. The bounding floodplain deposits transition from poorly-drained (FA 4) in the uppermost Nacimiento Formation to variably-drained (FA 5) in the Cuba Mesa Member of the San Jose Formation. The floodplain deposits become increasingly well-drained through the remainder of the Cuba Mesa Member and into the Regina Member, where purple-red mudrock facies are the most abundant (FA 6) (Fig. 3.13). Interpretation: The general stratigraphic trend where dominantly fine-grained overbank deposits with minor volumes of isolated, coarser channel fills (distal facies) are progressively followed upwards in the stratigraphy by more sand rich, larger, and increasingly amalgamated channel bodies, with lesser volumes of preserved mud-prone overbank strata (medial-proximal facies) resembles the progradational packages that have been documented in modern (Shukla et al., 2001; Chakraborty et al., 2010; Sinha et al., 2014) and ancient (Willis, 1993; DeCelles and Cavazza, 1999; Nakayama and Ulak, 1999; Uba et al., 2005; Weissmann et al., 2013; Wilson et al., 2014; Owen et al., 2015b; a, 2017) fluvial fans. This trend is also a key recognition criterion in the prograding DFS stratigraphic model (Weissmann et al., 2013). Furthermore, the lateral extent of these large-scale stratigraphic trends suggests that they reflect whole fan progradation. Based on this interpretation, we document at least two

106 of these whole fan progradational packages: the first extending from the Ojo Alamo and Escavada Members of the Nacimiento Formation and into the Cuba Mesa Member of the San Jose Formation (Fig. 3.13). The second is observed in the Regina Member of the San Jose Formation. However, proximal - distal changes in the gradational vs. abrupt nature of these packages (Fig. 3.10, Fig. 3.13) suggests additional depositional complexity. In addition, there is an interval in the southern study areas between the progradational packages that extends from the middle to upper Cuba Mesa Member to the Cuba Mesa-Regina contact that appears to show a reversal of the progradational trend, with increased heterogeneity and floodplain deposits (a shift from proximal to medial fan facies) (Fig. 3.13b). This reversal could reflect retrogradation of the fan system, however this trend is not observed in the central SJB (Fig. 3.13a). The improved soil drainage conditions suggested by the floodplain deposits that corre- spond with the first whole fan progradational package (FA 4 in the Nacimiento Formation - FA 5 in the Cuba Mesa Member of the San Jose Formation) is coincident with observations from the stratigraphic records of some ancient prograding fan systems (Trendell et al., 2013; also see examples in Weissmann et al., 2013). Some researchers have linked proximal - distal reduction in soil drainage to shallow water tables in distal fans (e.g. Hartley et al., 2010), leading to the hypothesis that the stratigraphic trends observed in prograding fans are the result of changes in topographic position of paleosols and the proximity of deposits to the water table, rather than changing climatic conditions and (Weissmann et al., 2013). Thus, the existing depositional model used for recognition of prograding fans in the stratigraphic record suggests an up-section change from drab-colored floodplains associated with distal facies overlain by increasingly variegated floodplain associated with medial and proximal facies (Weissmann et al., 2013). However, in the San Juan Basin, the distal deposits in the second fan progradational package (Regina Member) corresponds with entirely well-drained floodplain deposits (FA 6) (Fig. 3.13), suggesting an overall trend toward increased aridity. By comparison with the prograding fan model (Weissmann et al., 2013), we would expect to

107 see a return to drab-colored poorly-drained soils accompanying the retrogradation of the fan system, but instead the floodplains show signs of being increasingly well-drained (Fig. 3.13). This contrast suggests that the existing predictive model which indicates a downstream in- crease in soil moisture variability in fan depositional systems (Weissmann et al., 2013) is not encompassing of all large fan systems. Combined with evidence for changing variable discharge conditions, we conclude that the stratigraphic trend in increasingly well-drained soil conditions was likely the result of changing climatic conditions spanning the Paleocene- Eocene boundary, and that climate was a significant control on the fluvial system in the Paleogene San Juan Basin.

3.12 Discussion 3.12.1 Fluvial fan progradation and retrogradation

Prograding fluvial fan packages are commonly documented in the geological record of ancient fluvial fans (e.g. Willis, 1993; Nakayama and Ulak, 1999; Shukla et al., 2001; Uba et al., 2005; Nichols and Fisher, 2007; Wilson et al., 2014; Owen et al., 2015b; a, 2017). Doc- umentation of stacked progradational packages, such as we propose in the San Juan Basin (Fig. 3.13), are rare (Luzn, 2005; and possibly Schneider et al., 2006 as hypothesized by Weissmann et al., 2013; Wang and Plink-Bj¨orklund, 2019), and recognition criteria for ret- rograding fluvial fans have not been established (Ventra and Clarke, 2018) due to a paucity of retrogradational successions interpreted in the geological record (exceptions include Luzn, 2005; Kukulski et al., 2013). In addition, causal mechanisms for fan progradation - retrogra- dation remain one of the major unanswered questions about fluvial fans. The stacked progradational packages with a sharp change from proximal fan facies to dis- tal fan facies with no depositional evidence of fan backstepping, as observed in the northern and central San Juan Basin (Fig. 3.13a), may suggest a period of fan inactivity followed by reactivation. The stacked progradational packages with an intervening gradual decrease in channel amalgamation observed in the middle and upper Cuba Mesa Member in the south- ern study areas (Fig. 3.13b) may suggest progradation, followed by an initial upstream shift

108 of facies belts as the fan backstepped toward the hinterland. Alternatively, this proximal- medial variability may be the result of lobe avulsions. Modern fans are documented to consist of 3-4 lobes, which are formed due to the clustering of channel avulsions (Assine and Silva, 2009; Chakraborty et al., 2010; Chakraborty and Ghosh, 2010; Assine et al., 2014) and show the presence of active lobe and abandoned older lobes on fan surfaces (Assine, 2005; Chakraborty and Ghosh, 2010; Weissmann et al., 2013). Wang and Plink-Bj¨orklund (Wang and Plink-Bj¨orklund, 2019) suggest that avulsion- and lobe-scale progradation may explain some intermediate-scale upward thickening trends and lateral variations. Regardless of the cause of the decreasing trend in channel amalgamation in the middle and upper Cuba Mesa Member, the overlying relatively sharp transition to isolated channels in the basal Regina Member may suggest that the fan became inactive around the same time that activity ceased in the northern - central San Juan Basin. These lateral differences may indicate that the downstream extent of the fan reached a maximum point of progradation with the deposition of the lower Cuba Mesa Member in the southern San Juan Basin and that sediment delivery decreased prior to shut-off of the fan, whereas the upstream portions of the fan remained active up until shut-off of the fan. Continuing with this hypothesis, the fan was later reactivated, resulting in a second episode of whole fan progradation that deposited the Regina Member of the San Jose Formation. The limited amount of literature on stacked prograding fluvial fan systems provides little basis of comparison with other ancient systems. In the Oligocene - Miocene Ebro Basin, Spain, researchers have interpreted that two coarsening- to fining-upward packages record two cycles of fan progradation - retrogradation (Luzn, 2005). In the central portions of the basin, these trends are characterized by either gradual changes from coarsening- to fining-upward trends or by sudden shifts. In addition, observations of finer-grained deposits overlying coarser-grained deposits have been made in the Kosi megafan in the Himalayan foreland basin, India (Sinha et al., 2014). This trend differs from other Quaternary and Holocene fluvial fans that commonly indicate a period of fan inactivity and incision capping

109 prograding fan packages (Latrubesse et al., 2012; Roy et al., 2012; Assine et al., 2014; Fontana et al., 2014) rather than upstream facies belt shifts as seems to be suggested in the Oligocene - Miocene Ebro Basin (Luzn, 2005) and the Kosi megafan (Sinha et al., 2014).

3.12.2 Controls on fan progradation-retrogradation

While tectonic subsidence is certainly relevant to the long-term preservation of the thick stratigraphic records, there is a lack of evidence for tectonic pulses resulting in episodes of fan progradation. For example, the size of extrabasinal clasts does not change markedly through the stratigraphic column (Fig. 3.3). While grain size does not vary greatly, the size of channels, degree of amalgamation, and the abundance of UFR and HDR sedimentary structures linked to supercritical flow conditions fluctuate considerably. These lines of evi- dence suggest that changes in water discharge patterns linked to climate were a key control in sedimentary delivery to the basin. Examples from Quaternary and Holocene fluvial fans also suggest that climate is a key control on fan progradation and retrogradation. Progradation of fans is commonly linked to conditions of high precipitation variability and associated high discharge variability, such as during the Pleistocene middle to late pleniglacial period (Latrubesse and Kalicki, 2002; Latrubessee, 2003; Latrubesse and Franzinelli, 2005; Latrubesse et al., 2012; Valente and Latrubesse, 2012; Assine et al., 2014) or due to variable discharge in response to glacial out- wash during the last glacial maximum (Fontana et al., 2014). In contrast, fan retrogradation in the form of abrupt shifts towards the hinterland and a period of fan inactivity and incision have been suggested in response to a change from a highly seasonal to more arid climate (Valente and Latrubesse, 2012), a change from semi-arid and flashy precipitation to more sustained rainfall and increased vegetation cover (Assine et al., 2014), an increase in mon- soon intensification (Roy et al., 2012), or a decrease in sediment supply due to glacial retreat (Fontana et al., 2014). Despite the differences, all these interpretations suggest that fan activity is largely controlled by changes in discharge variability ultimately linked to changes in climate.

110 3.12.3 Link between fluvial fan occurrence, river discharge variability, and cli- mate

As discussed above, the progradation of Quaternary and modern fluvial fans is com- monly attributed to conditions that support highly variable river discharge (Latrubesse and Kalicki, 2002; Latrubessee, 2003; Latrubesse and Franzinelli, 2005; Valente and Latrubesse, 2012; Assine et al., 2014; Fontana et al., 2014). Conversely, periods of fan inactivity are com- monly attributed to a decrease in discharge variability resulting from either a shift to more perennial precipitation (Assine et al., 2014) or to overall more arid conditions (Latrubesse et al., 2012), or in some Pleistocene fans, the removal of glacial outwash (Fontana et al., 2014). Nevertheless, whether there is a link between fluvial fan development and specific climate conditions remains a topic of active debate. Some researchers have suggested that the for- mation of large fluvial fans requires seasonal and highly variable precipitation (e.g. Singh et al., 1993; Shukla et al., 2001; Leier et al., 2005), citing evidence that modern and ancient examples of large fluvial fans have been linked to moderate to extreme seasonal fluctuations in discharge that result from highly seasonal precipitation patterns. The tendency for rivers with large fluctuations in discharge to construct large fluvial fans is thought to be related to the instability of channels, which results in rapid channel migration and frequent avul- sions that ultimately create distinctive fan-shaped sediment lobes (Sinha and Friend, 1994; Horton and DeCelles, 2001; Leier et al., 2005). In addition, some researchers have observed that the global distribution of modern megafans is primarily restricted to 15°-35° latitude in the Northern and Southern Hemispheres, corresponding to climatic belts that fringe the tropical climatic zone (Leier et al., 2005). Rivers in this climatic zone receive significant amounts of their surface water supply from monsoon precipitation in the sub-humid to arid subtropics. These regions receive more than 80-90% of their annual precipitation during the summer monsoon season (Wang and Ding, 2008). The intense summer rainfall causes high magnitude flooding in rivers that otherwise transmit a relatively low base flow (Sinha and Friend, 1994; Sinha and Jain, 1998; Latrubesse et al., 2005; Plink-Bj¨orklund, 2015). As a

111 result, these rivers have been observed to deliver most of their water discharge and sedi- ment load during the summer monsoon season as only the flood discharge is geomorphically effective and able to transport sediment (Goodbred, 2003; Plink-Bj¨orklund, 2015). Others challenge the linkage between large fluvial fan development and climate, observing that large DFSs and their catchments are developed in all climatic regimes and are not restricted to any specific latitudinal belt (Hartley et al., 2010, 2013). However, this hypothesis is based on broad climate zones defined by temperature and mean annual precipitation, rather than accounting for precipitation range and variability. Here we show that deposits throughout the study interval show evidence of flood events punctuated by periods of prolonged dryness at various magnitudes (see details in Chapter 2), indicating deposition from variable discharge river systems (see reviews by Fielding et al., 2009, 2018; Plink-Bj¨orklund, 2015 and references therein). We link the occurrence of variable discharge river systems to highly variable inter-annual to seasonal precipitation patterns resembling those of the modern semi-arid to arid subtropics and monsoonal domains (e.g. Latrubesse et al., 2005; Wang and Ding, 2008; Henck et al., 2010). We also suggest that the transition from poorly-drained floodplain deposits to increasingly well-drained floodplain deposits reflects an overall trend toward increased aridity into the early Eocene. These changes become more pronounced in the Regina Member of the San Jose Formation where an increased abundance of purple-red paleosols suggests increased aridity over the underlying deposits of the Cuba Mesa Member. By combining evidence from channel and floodplain facies associations, we hypothesize that the stratigraphic trends in the San Juan Basin suggest a shift from an overall more monsoonal precipitation regime in the Escavada Member of the Paleocene Nacimiento Formation to fluctuating sub-humid and semi-arid in the Cuba Mesa Member of the early Eocene San Jose Formation to semi-arid to arid in Regina Member of the early Eocene San Jose Formation (see also Chapter 2). These findings suggest that the stratigraphic patterns observed in the study interval likely resulted from fluctuations in geomorphically effective discharge attributed to changing magnitudes of precipitation

112 variability and changing climatic conditions. Progradation of the Cuba Mesa may be linked to a system clearing event that we hypothesize may have occurred as the climate shifted toward more arid conditions (Chapter 2). The second episode of fan progradation observed in the Regina Member may have resulted from long-term cycles of monsoon variability at millennial or orbital timescales, but the lack of time constraints throughout the study interval preclude more detailed analysis at this time.

3.12.4 Implications for San Juan Basin landscape evolution and fluvial fan de- positional models

The deposition of a north to south prograding large fluvial fan may provide a new ex- planation for the proposed transitional nature of the Nacimiento - San Jose contact in the SJB (Smith and Lucas, 1991). In the northern and central SJB where the contact is thought by some researchers to be conformable (Barnes et al., 1954; Stone, 1983; Smith and Lu- cas, 1991), the gradual upward transitions from distal to medial to proximal fan facies are coincident with observations from stratigraphic trends from other ancient fans and prograd- ing fluvial fan depositional models (Weissmann et al., 2013; Owen et al., 2015b; a, 2017). However, in the southern SJB the change from distal fan facies to proximal fan facies is more abrupt where previous researchers have called the contact a large unconformity (Baltz, 1967; Lucas et al., 1981; Smith and Lucas, 1991). This seemingly abrupt downstream shift of proximal fan facies is not consistent with other examples of prograding fans (e.g. examples in Weissmann et al., 2013; Owen et al., 2015b) and the depositional model (Weissmann et al., 2013) and raises questions about how prograding fan stratigraphic patterns change from upstream to downstream. A possible explanation is that the parts of the fan closer to the apex experienced more continuous deposition of coarse-grained deposits. As the fan system prograded basinward, younger deposits may have sequentially overlain the older deposits of the distal fan, potentially creating a depositional hiatus and time gap that increased in time with distance from the fan apex, and thus creating a more abrupt stratigraphic change (Fig. 3.14). This wedge-shaped time gap in deposition, combined with the laterally extensive

113 erosional base of the lowermost Cuba Mesa channel complex may have contributed to the interpretation of an angular unconformity by past researchers. Our research has found no evidence of an angular discordance between the floodplain deposits that underlie and overlie the contact in our study areas, therefore we classify the contact between the Nacimiento and San Jose Formations as a possible disconformity or hiatus. In the Oligocene-Miocene Ebro Basin, stacked prograding fluvial fan intervals contain surfaces interpreted as angular uncon- formities at the top of coarsening-upward (prograding) intervals (Luzon, 2005). In addition, major unconformities with soil formation and valley incision are also been observed at the top of Quaternary prograding fan deposits (Fontana et al., 2014). This may suggest that a previously unrecognized significant disconformity is present at the contact between the Cuba Mesa and Regina Members of the San Jose Formation, separating the two packages of whole fan progradation (Fig. 3.13). Because the only time constraints on the depositional age of these deposits comes from lower in the Nacimiento Formation and higher in the Regina Member of the San Jose Formation (Fig. 3.1c), identification of additional disconformities in the succession would suggest that the depositional time gap at the Nacimiento - San Jose Formation (estimated to be ≥ 5.6 My; Fassett et al., 2010) has been overestimated. The lateral change over ∼9 km from relatively thick channel deposits in the Arroyo Chijuilla study area to thinner channel deposits in the Continental Divide study area may indicate that the deposits in Arroyo Chijuilla are closer to the axial lobe than those of Continental Divide. Lateral variability seen across larger distances within the SJB could result from differences in deposition between individual lobes. Thus, in addition to a poten- tially large depositional hiatus between whole fan progradation packages, the shifting areas of deposition may result in multiple lesser-magnitude depositional hiatuses throughout the succession, and the timing and duration of the hiatuses may vary laterally in the flow per- pendicular direction across the basin, adding further to the stratigraphic complexity. The geometries created by these lobes may explain some of the lateral variability in channel thickness and sandstone to mudrock ratio seen across the outcrops in the southern SJB, but

114 further research is needed to test this hypothesis.

3.13 Conclusions

This study identifies the deposits of the upper Nacimiento Formation and Cuba Mesa - Regina Members of the San Jose Formation as deposits of a large fluvial fan system based on the large lateral extent of deposits, progressive downstream decrease in grain size, down- stream decrease in channel depth and width, downstream increase in overbank preservation, downstream decrease in lateral and vertical connectivity of channel deposits, and evidence of channel mobility by avulsion. We recognize two vertically stacked packages of fluvial fan progradation, where dominantly fine-grained overbank deposits with minor volumes of isolated, coarser channel fills (distal facies) are progressively followed upward in the stratig- raphy by more sand rich, larger, and increasingly amalgamated channel bodies, with lesser volumes of preserved mudrock overbank strata (medial-proximal facies). Abrupt shifts from proximal fan facies to distal fan facies suggest a period of fan inactivity occurred between the progradational packages, although the style of retrogradation differs laterally. A sharp change from proximal to distal fan facies in the northern and central San Juan Basin seems to suggest abrupt shut-off of the fan system, whereas, in the southern San Juan Basin, there is an interval that may suggest some initial backstepping of the fan system prior to fan shut- off. The interpretation of stacked prograding fans suggests that a previously unrecognized depositional hiatus separates the fan packages at the contact between the Cuba Mesa and Regina Members of the San Jose Formation. Furthermore, evidence that the deposits were made by variable discharge river systems (Chapter 1), combined with a lack of evidence that the deposits resulted from tectonic pulses and comparisons with Quaternary and modern fans suggests changes in discharge variability linked to climate was a key control on fan activity in the Paleogene San Juan Basin. The hypotheses presented in this study may have implications for improved understand- ing of San Juan Basin landscape evolution and the development of fluvial fan depositional models, including: 1) The San Juan Basin provides a dataset to study multiple packages of

115 fan progradation, and the intermediate surfaces and deposits may improve our understanding of recognition criteria for retrograding fan systems in the ancient record. 2) The identifica- tion of a north to south prograding large fluvial fan that may have deposited progressively younger deposits basinward could provide a new explaination for the previously proposed transitional nature of the Nacimiento-San Jose contact in the San Juan Basin. 3) Lateral changes in the thickness of deposits and sandstone to mudrock ratio may be due to shifting areas of deposition between multiple fan lobes, which may have resulted in multiple lesser- magnitude depositional hiatuses throughout the succession. If these hypotheses are correct, the suggested depositional time gap (estimated to be ≥ 5.6 My) at the contact between the Nacimiento and San Jose Formations is likely overestimated, as the depositional age of the entire study interval is loosely constrained between fossil localities. 4) Distal fan facies asso- ciated with both drab and purple-red floodplain facies suggests that the existing predictive model which suggests downstream decreases in soil drainage conditions is not encompassing of all large fan systems and adds further evidence for a transition to a more arid climate across the Paleocene-Eocene boundary.

116 Figure 3.1: Maps and Paleogene geology of the San Juan Basin (SJB). A) Location of the San Juan Basin within the contiguous United States. Base map modified from ESRI, HERE, Garmin, OpenStreetMap, and the GIS user community. B) Regional map of the San Juan Basin (SJB) and adjacent basins and uplifts (SJB = San Juan Basin, G = Galisteo Basin, B = Baca Basin, MV = Monte Vista Basin, RA = Raton Basin, HP = Huerfano Park Basin, CL-J = Carthage-LaJoya). Redrawn from Dickinson et al., 1988; Lawton, 2008; and Cather, 2004. C) Paleogene stratigraphy and time constraints, modified from Smith and Lucas, 1991 D) Geological map of the SJB, areas selected for detailed investigations, and paleocurrent measurements. Arroyo Chijuilla (AC) and Continental Divide (CD) are in the southern SJB. Ca˜non Largo (CL) is in the central SJB. Solid arrows are paleocurrent measurements collected during this study. Open arrows are paleocurrent vector means from Smith (1988). Map modified from Williamson et al., 2008 and Manley et al., 1987. E) Paleocurrent measurements (this study), not corrected to remove measurements collected from probable backsets.

117 Figure 3.1: Continued.

118 Figure 3.2: Examples of regular cross-stratification and backsets in outcrop. A) sets of reg- ular cross-stratification with dip directions oriented to the prevailing paleocurrent direction (black arrow). B) Backsets (annotated in red) with dip directions approximately opposite the prevailing paleocurrent direction (black arrow).

119 Figure 3.3: Measured Sections from sites selected for detailed investigation. Simplified mea- sured sections are displayed from Ca˜non Largo (central SJB), Continental Divide (southern SJB), and Arroyo Chijuilla (southern SJB). The percentage of sandstone vs. mudrock litho- facies and facies group composition are calculated from the three measured sections combined to quantify overall vertical trends that are seen throughout the basin.

120 Figure 3.4: Depositional Architecture Styles. A) Amalgamated channels and minor flood- plain. B) Channels and floodplain. C) Isolated channels and dominant floodplain.

121 Figure 3.5: Lateral trends - flow perpendicular. The Continental Divide (CD) section appears thinner in comparison with the Arroyo Chijuilla (AC) section due to a greater abundance of floodplain mudrock deposits and reduced thickness of channel complexes. Due to this increased abundance of floodplain mudrock deposits, the CD outcrop may also contain a longer record of Regina Member deposits than the AC section.

122 Figure 3.6: Channel measurements collected in the central and southern SJB. Ca˜non Largo (CL) is located in the central SJB (see Fig. 3.2). Arroyo Chijuilla is located in the southern SJB. Both study areas show that the largest channels (height and width) are located within the Cuba Mesa Member. Both study areas show that the most laterally isolated channels are located within the Regina Member. There is also a downstream decrease in channel size (height and width) between the central and southern SJB (paleoflow is approximately north-to-south).

123 Figure 3.7: Cuba Mesa Member lateral trends - flow parallel. Cuba Mesa Member lateral trends - flow parallel.

124 Figure 3.8: Examples of outcrops interpreted as proximal, medial, and distal fan facies in the San Jose Formation of the SJB. A) Amalgamated channels and minor floodplain, interpreted as proximal fan facies. B) Channels and floodplain, interpreted as medial fan facies. C) Isolated channels and dominant floodplain, interpreted as distal fan facies. D) Satellite image obtained from Google Earth of a 60 km long Megafan in the Taklamakan Desert, Xinjiang, China with approximate locations of proximal, medial and distal fan annotated.

125 Figure 3.9: Stratigraphic trends the Arroyo Chijuilla study area. A) Medial fan facies in the uppermost Nacimiento Formation, capped by proximal fan facies in the lower Cuba Mesa channel complex. B) Medial fan facies in the Middle Cuba Mesa channel complex. C) Medial fan facies of the upper Cuba Mesa channel and overlying abrupt shift to distal fan facies in the Regina Member.

126 Figure 3.10: Different styles of fluvial fan progradation from outcrops in the central and southern SJB. A) Nacimiento - San Jose Formation transition in the central San Jan Basin near the Ca˜non Largo study area (see Fig. 3.2 map). The up-section channel thickening precludes precise placement of the contact boundary in this area. B) The transition from isolated and lenticular channels with an abundance of floodplain deposits in the Nacimiento Formation to the sandy and amalgamated Cuba Mesa Member of the San Jose Formation is more defined in the southern SJB near the Arroyo Chijuilla study area.

127 Figure 3.11: Stratigraphic trends in the Continental Divide study area. A) middle and upper Cuba Mesa Member tongues consisting of medial fan facies, capped by the Regina Member, consisting of distal fan facies overlain by an abrupt shift to proximal fan facies. B) The uppermost deposits of the Regina Member showing the limited amount of outcrop that suggests another possible shift back to distal fan deposits.

128 Figure 3.12: Stratigraphic trends in the Ca˜non Largo study area. A) Distal to medial fan facies in the uppermost Nacimiento Formation, capped by proximal fan facies in the Cuba Mesa Member of the San Jose Formation. B) Overlying abrupt shift to distal fan facies in the Regina Member, overlain by an abrupt shift to proximal fan facies. Photos taken in the Ca˜non Largo study area, laterally adjacent to the measured section location.

129 Figure 3.13: Illustration showing stacked prograding fan stratigraphic trends in the San Juan Basin. A). In the northern - central San Juan Basin we observe two stacked prograding packages (indicated by the white arrows) with no depositional evidence for an upstream shift in facies. B) In the southern San Juan Basin we also observe two stacked prograding packages, but there is an interval that displays upward increasing heterogeneity that could be evidence of gradual fan backstepping further away from the apex. Throughout the basin, we observe a relatively sharp shift to distal fan facies above the contact between the Cuba Mesa and Regina Members of the San Jose Formation that may indicate a depositional hiatus (marked with dashed line) and period of fan inactivity prior to progradation of the second package. We also observe a basin-wide gradual upward increase in soil drainage conditions, characterized by a gradual shift from drab-colored to increased abundance of purple-red floodplain deposits.

130 Figure 3.14: Schematic cross-section of hypothesized style of fan progradation and annotated outcrop photos. A) Northern San Juan Basin. B) Central San Juan Basin. C) Southern San Juan Basin

131 Table 3.1: Sandstone and mudrock facies.

132 Table 3.1: Continued.

133 Table 3.1: Continued.

134 Table 3.2: Sandstone and mudrock facies associations.

135 Table 3.2: Continued.

136 CHAPTER 4 INVESTIGATING LINKS BETWEEN CLIMATE PERTURBATIONS AND SEDIMENTATION IN THE PALEOGENE SAN JUAN BASIN, NEW MEXICO

4.1 Abstract

The San Juan Basin of New Mexico and Colorado is the southern-most Laramide basin where early-Eocene age deposits overlie deposits of Paleocene-age. Nevertheless, the basin has been so far overlooked, despite that interest in climate change across the Paleocene- Eocene boundary has increased in recent years due to the recognition of the Paleocene- Eocene Thermal Maximum as potentially the best paleoanalogue for current greenhouse gas warming. In this study, we present preliminary stable organic carbon isotope records derived from paleosol bulk sediment and fragments of coalified-charcoalified fossil wood across the Paleocene-Eocene boundary in the San Juan Basin. We then compare the isotope records with a detailed stratigraphic analysis of the Paleocene upper Nacimiento formation (Escavada Member) and the early-Eocene San Jose Formation (Cuba Mesa - Regina Members) to test whether the Paleogene climate changes affected landscape evolution. Preliminary results show fluctuations in the carbon isotope ratios of fossil wood fragments that are indicative of a negative carbon isotope excursion (CIE) spanning the stratigraphic extent of the Cuba Mesa Member and additional smaller negative excursions are indicated in the Regina Member. The carbon isotope ratios display high variability in the record obtained from paleosol bulk sediment and are not indicative of clear negative excursions as a stan- dalone dataset. However, when compared with the fossil wood record, intervals containing fluctuations between excursion and non-excursion values are observed in coincidence with two CIEs identified in the fossil wood record. We hypothesize that the Cuba Mesa Member CIE could record the Paleocene-Eocene Thermal Maximum (PETM), and the stratigraphically higher CIEs could record one or more post-PETM hyperthermal events. Furthermore, the

137 hypothesized PETM CIE coincides with a pronounced stratigraphic transition from isolated fluvial channel deposits bounded by drab-colored mudstones in the Nacimiento Formation to laterally-continuous, amalgamated fluvial channel complexes in the Cuba Mesa Member, interpreted as a prograding large fluvial fan. Changes in channel deposits are consistent with an increase in deposition and discharge rates into the Eocene, while associated changes in mudstone floodplain deposits from drab to increasingly variegated (grey-purple-red) mud- stones are consistent with improved soil drainage. These changes in fluvial deposits share similarities with those associated with identified PETM intervals in Laramide basins further to the north that have also been interpreted as having been deposited under highly seasonal precipitation regimes. The hypothesized post-PETM hyperthermal CIE(s) may coincide with a second episode of fluvial fan progradation in the Regina Member. Overall, these initial results suggest that the San Juan Basin may present a new PETM and early-Eocene hyperthermal dataset that can provide new insights on climatic and biotic changes during the Paleogene.

4.2 Introduction

Earths climate has evolved dramatically over the past 60 million years. Complex fluc- tuations in thermal regimes have resulted in extremes ranging from hothouse climates with ice-free poles to icehouse climates with expansive continental ice sheets (e.g. Zachos et al., 2008). These climatic variations can take place gradually over millions of years driven by tectonic processes, they can be cyclic over 10s to 100s of thousands of years driven by orbital forcing, or they can occur as anomalous perturbations (Zachos, 2001). Of particular interest are perturbations that took place during the Paleogene that are characterized by global in- creases in temperature and are thus termed hyperthermal events (Kennett and Stott, 1991). Hyperthermals are associated with rapid changes to the near-surface carbon cycle associated with the addition of large amounts of carbon to atmospheric ocean reservoirs (e.g. Bowen et al., 2006; Zachos et al., 2008; Abels et al., 2012), and thus may represent the best pa- leoanalogues for current greenhouse gas warming (Zachos et al., 2008). The study of these

138 events from the geologic past can advance our understanding of how climate might change in the future, and what the impacts of such change may be in both marine and terrestrial environments. The largest of the hyperthermals, the Paleocene-Eocene Thermal Maximum (PETM) at ∼56 Ma is known to have caused severe climatic and biotic change in both marine and terrestrial environments (Gingerich, 1989; Thomas, 1989; Kennett and Stott, 1991; Koch et al., 1992; McInerney and Wing, 2011). More recently, records of post-PETM hyperthermals have also been recognized in some locations (Lourens et al., 2005; Nicolo et al., 2007; Abels et al., 2012), although their environmental and biotic impact is less understood (Sluijs et al., 2009; Abels et al., 2012). Changes to the global carbon cycle associated with hyperthermal events are recorded in the stratigraphic record in both marine and terrestrial deposits as negative carbon isotope excursions (CIEs). These globally correlated, transitory, negative excursions in carbon iso- tope (δ13C) values indicate episodic additions of massive amounts of isotopically depleted carbon to the ocean-atmosphere system (Koch et al., 1992; Hesselbo et al., 2003; Ruhl et al., 2011; Abels et al., 2012). The PETM is likely the most well-studied hyperthermal event (e.g. Anagnostou et al., 2016), yet the source of the 13C depleted carbon release is a topic of debate. Proposed mechanisms include: destabilized methane clathrates from deep sea sediments (Dickens et al., 1995), Thermogenic methane released from injection of magma into organic-rich sediments (Frieling et al., 2016), Wildfires that burned shallowly buried peat or coal (Kurtz et al., 2003), and rapid thawing of permafrost (DeConto et al., 2012). The PETM CIE is generally recognized as having 3 phases: a rapid onset during the period of carbon release (4-20 kyr), a relatively long body (80-160 kyr) of sustained negative carbon isotope composition, and a shorter recovery (70-80 kyr) during which isotopic composition returned to pre-PETM values (Bowen et al., 2006; McInerney and Wing, 2011). Environmen- tal changes coinciding with the PETM CIE include a global temperature increase of 5-8°C (Fricke et al., 1998; Bowen et al., 2006; Sluijs et al., 2006, 2010; Zachos et al., 2007; Handley

139 et al., 2008), large shifts in geographic ranges and rapid evolution of terrestrial and marine organisms (for overview, see McInerney and Wing, 2011), extinction of benthic foraminifera (Thomas, 1989), and enhanced terrestrial runoff indicating increased precipitation intensity on land (Thiry and Dupuis, 2000; Carmichael et al., 2017). The PETM event has been studied in global marine environments (e.g. Kennett and Stott, 1991) and global terrestrial deposits (e.g. Koch et al., 1992; McInerney and Wing, 2011 and references therein; Foreman et al., 2012; Plink-Bj¨orklund et al., 2014). The Bighorn Basin of Wyoming is the most extensively researched of the terrestrial PETM records (e.g. Clyde and Gingerich, 1998; Fricke et al., 1998; Bowen et al., 2001; Magioncalda et al., 2004; Wing et al., 2005; Kraus and Riggins, 2007; Secord et al., 2010; Kraus et al., 2013). As recent interest in the PETM has expanded, fueled by suggestions that the event is analogous to anthropogenic global warming, additional suspected terrestrial PETM deposits have been identified (McInerney and Wing, 2011 and references therein; Foreman et al., 2012; Plink- Bj¨orklund et al., 2014). Consequently, the PETM is likely the most studied and best known past global warming event (e.g. Anagnostou et al., 2016), despite the large uncertainties

in CO2 concentrations (Zeebe and Zachos, 2013), and differences with the modern carbon release rate (Zeebe et al., 2016). Soil carbonate nodules, plant lipids, herbivore tooth enamel, and bulk sediment organic matter are the materials most commonly used for δ13C analysis in terrestrial depositional environments (McInerney and Wing, 2011 and references therein). Carbon-isotope ratios of fossil wood have been confirmed as a proxy for changes in the isotopic composition of paleoatmosphere (e.g. Hasegawa, 1997; Gr¨ocke et al., 1999, 2005; Arens et al., 2000; Hesselbo et al., 2002, 2007), but only limited studies have been carried out using coalified-charcoalified fossil wood material for carbon-isotope chemostratigraphy in terrestrial settings (e.g. Yans et al., 2010), and this method has not been used on early-Eocene datasets. Carbon isotope ratios in higher-plant organic matter have been shown to be closely related to the carbon isotope composition of the ocean-atmosphere carbon reservoir (Gr¨ocke, 2002), thus fossil

140 wood is hypothesized as being a suitable material for δ13C. Another emerging paleoclimate proxy is the sedimentary record itself. For example, some of the most compelling evidence for dramatic hydrological changes at the PETM are geomorphological and sedimentological changes, documented in both terrestrial and marine archives (Carmichael et al., 2017). Rivers are especially sensitive to tectonic and climatic forcing via their channel gradient and discharge (e.g. Macklin et al., 2012; Whittaker, 2012). Thus, rivers are capable of responding to climatic and tectonic perturbations on short timescales (e.g. 1-10 ka, Goodbred, 2003; Simpson and Castelltort, 2012). Such deductions are however ambitious because the interaction between climate, tectonics, and landscape is coupled and displays nonlinear dynamics (e.g. Wobus et al., 2006a; b; Whipple, 2009; Jerolmack and Paola, 2010; DiBiase and Whipple, 2011; Romans et al., 2016). For example, rivers may respond on short time scales, but may continue to re-equilibrate over longer time scales (e.g. 1-10 My; Allen, 2008; Armitage et al., 2011), and the terrestrial response to hydrological changes may be complex and, in some cases, prolonged (Foreman et al., 2012; Romans et al., 2016). Despite these uncertainties, global trends are emerging, with sedimentological and geomorphological changes indicating that precipitation became more episodic and intense during the PETM, regardless of changes in total amount (Carmichael et al., 2017). The San Juan Basin in northwestern New Mexico is the southernmost Laramide basin that preserves Paleocene and early-Eocene deposits (Baltz, 1967), which could make it the southernmost record of the PETM and post-PETM hyperthermal events in North America. Herein, we present a preliminary investigation of organic carbon isotope records derived

13 13 from bulk sediment (δ Cbulk sed) and coalified-charcoalified fossil wood (δ Cwood) as part of ongoing research to improve our understanding of the links between climate perturbations and sedimentological response, as well as depositional controls on the Paleocene-Eocene boundary interval in the San Juan Basin, New Mexico. To this end, we 1) analyze the trends in δ13C ratios to determine if there is evidence of negative CIEs within the study

141 13 13 interval, 2) compare the δ Cbulk sed and δ Cwood records to determine if the records suggest similar trends, 3) compare the carbon isotope stratigraphy with sedimentary record, and 4) explore the link between potential climate perturbations and observed changes in channel and floodplain depositional trends and discuss uncertainties surrounding the interpretation of the δ13C records generated from the two sources of organic carbon.

4.3 Geological Setting and Background

The San Juan Basin (SJB) is in the Four Corners region of northwestern New Mexico and southwestern Colorado (Fig. 4.1a) and is one of several intraforeland basins formed during the Laramide Orogeny (Dickinson et al., 1988; Cather, 2004). Laramide tectonics during the late Campanian (late Cretaceous) through Eocene caused subsidence of the SJB in response to uplift of surrounding areas (e.g. Smith, 1988; Cather, 2004). It has been suggested that the SJB received most of its sediments from highlands to the north and the early San Juan Uplift (Baltz, 1967; Donahue, 2016), although source areas in the Brazos-Sangre de Cristo, Nacimiento, Zuni, and Defiance Uplifts are also suspected (Fig. 4.1b; Smith, 1992; Cather, 2004; Donahue, 2016). Past research has linked long- term evolution of the Cretaceous-Eocene sedimentary succession to three tectonic episodes of subsidence in response to the Laramide orogeny: inception and early-phase (∼78-75 Ma), middle phase (∼74-67 Ma), and late phase occurring during the early Paleogene (Cather, 2004). Intraformational angular unconformities and reverse faults in the eastern outcrop area show that the Paleogene units in the SJB were deposited simultaneous with deformation along the basin-bounding Nacimiento fault (Woodward, 1987; Smith, 1988).

4.3.1 Stratigraphy

The Paleogene units preserved in the SJB are the earliest Paleocene Ojo Alamo Sand- stone, the Paleocene Nacimiento Formation, and the early Eocene San Jose Formation. These units outcrop in a series of badlands and cliff-forming sandstones along the west- ern and southern portions of the SJB (Fig. 4.1c). This study focuses on the uppermost

142 Nacimiento Formation and the Cuba Mesa and Regina Members of the San Jose Formation.

4.3.1.1 Paleocene Nacimiento Formation

The Paleocene Nacimiento Formation is composed primarily of terrestrial fluvial deposits consisting of sandstone and varicolored mudrocks that attains a thickness of as much as 525 m (Baltz, 1967; Williamson and Lucas, 1992). The Nacimiento Formation is divided by lithologic and sedimentologic features into three members: The Arroyo Chijuillita (oldest), the Ojo Encino, and the Escavada (youngest) (Fig. 4.1d; Williamson and Lucas, 1992). The Arroyo Chijuillita Member is characterized by drab mudrocks and sparse heterolithic sand- stones. The Ojo Encino Member is also mudrock dominant and is characterized by repetitive black-red-green-white banding that is associated with bentonitic and carbonaceous black mu- drocks, variegated red and green paleosols, and fine-grained white sandstones (Williamson and Lucas, 1992). Three persistent black paleosols within the Ojo Encino Member (referred to as the lower black, middle black, and upper black) can be correlated across the basin and serve as useful marker beds (see Leslie et al., 2018). The Escavada Member is characterized by a predominance of sandstones, drab mudrocks, and abundant silcretes (Williamson et al., 1992; Hobbs, 2016). Across the southern SJB, the thickness of the Escavada Member varies 19.2-88 m due to intraformational thinning (Butler and Lindsay, 1985) and channel scouring at the base of the San Jose Formation (Lucas et al., 1981). A north-to-south de- crease in grain size and paleocurrent measurements from cross-strata indicate a prevailing north-to-south paleoflow throughout the Nacimiento Formation (Baltz, 1967; Klute, 1986; Sikkink, 1987; Smith, 1992). The early-Paleocene age of the Arroyo Chijuillita and Ojo En- cino Members are well constrained by biostratigraphy, magnetostratigraphy, and radiometric dating (Williamson and Lucas, 1992; Leslie et al., 2018). The age of the Escavada Member, however, is poorly constrained, with the only age indicator being a zone of normal magnetic polarity in the western SJB that has been correlated to chron 26, suggesting a late Paleocene age (Fig. 4.1d; Lindsay et al., 1981; Williamson and Lucas, 1992). If this correlation is cor- rect, the Escavada Member extends to at least the lower part of chron 25 in the western SJB

143 (Williamson and Lucas, 1992), suggesting that that the uppermost Nacimiento deposits may be younger than 58.9 Ma when correlated to the geomagnetic polarity time scale (GPTS - Ogg, 2012) (Fig. 4.1d). A summary of previous work and the results of recent investigations of the Paleocene Nacimiento Formation in the SJB can be found in Hobbs (2016) and Leslie et al. (2018).

4.3.1.2 Early-Eocene San Jose Formation

The San Jose Formation is the most extensively preserved and exposed Eocene strati- graphic unit in New Mexico (Smith and Lucas, 1991; Smith, 1992). Like the underly- ing Nacimiento Formation, it is composed of terrestrial fluvial deposits with a prevailing north-to-south paleoflow. Previous geological investigations of the southern and southeast- ern outcrop areas of the San Jose Formation include descriptions of fossils and local physical stratigraphic studies (e.g. Simpson, 1948; Lucas et al., 1981; Smith and Lucas, 1991), and regional stratigraphy and mapping (Baltz, 1967; Mytton, 1983; Manley et al., 1987; Smith, 1988; Smith and Lucas, 1991). The San Jose Formation contains both sandstone-dominated and mudrock-dominated lithofacies. The sandstone vs. mudrock-dominance is the primary characteristic used to distinguish between the members within the San Jose Formation, listed from oldest to youngest: Cuba Mesa Member (sandstone dominant), Regina Member (mu- drock dominant), Ditch Canyon Member (sandstone dominant), Llaves Member (sandstone dominant), and Tapicitos Member (mudrock dominant) (Fig. 4.1c,d; Baltz, 1967; Smith, 1992). The Cuba Mesa and Regina Members are the focus of this study. The Cuba Mesa Member at the base of the San Jose Formation in the SJB is a dis- tinct 40-240 m thick sandstone-dominated succession that overlies the Paleocene Nacimiento Formation (Baltz, 1967; Smith and Lucas, 1991). It consists of medium- to very coarse- grained, buff-colored tabular sandstones that thicken locally in vertically stacked (amalga- mated) channel belts that can attain thicknesses up to 100 m, with width-to-thickness ratios ranging from 20 to >1000 (Smith, 1988). The basal sandstone of the Cuba Mesa Member is continuous nearly basin-wide over an area of 8000 km2 (Smith, 1988). In some parts of

144 the basin, the contact between the Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation can be difficult to distinguish because the Escavada Member of the Nacimiento Formation contains arkosic sandstones that are similar in lithology to those of the Cuba Mesa Member (Baltz, 1967; Smith and Lucas, 1991). Like the underlying Escavada Member, no age-diagnostic fossils or dateable materials have been discovered in the Cuba Mesa Member, leaving the deposits of both the upper Nacimiento Formation and the lower San Jose Formation with loose age constraints (Fig. 4.1d). The Regina Member of the San Jose Formation ranges from 150-460 m in thickness and is composed varicolored grey, purple and red mudrock and interbedded sandstone. The sandstone beds are lenticular over scales ranging from a few meters to many kilometers (Smith, 1992). Lenticular conglomeratic sandstones in the northernmost outcrops of the member contain cobbles derived from Precambrian rocks common in the San Juan uplift north of the SJB (Smith, 1992) (Fig. 4.1b). The early-Eocene age of the Regina Member is determined by Wasatchian (North American Land Mammal Age - NALMA) vertebrate fossils found in the middle to upper Regina Member (Fig. 4.1d; see Lucas and Williamson, 1992 and references therein), which loosely constrain the age of the deposits between 55.8 - 50.3 Ma (Alroy et al., 2000).

4.3.1.3 Nacimiento-San Jose Formation contact and the Paleocene-Eocene Bound- ary

It has long been propagated in the literature that there is a large unconformity of ≥ 5.6 My (Fassett et al., 2010) that separates the Nacimiento and San Jose Formations in at least the southern SJB (Barnes et al., 1954; Baltz, 1967; Smith and Lucas, 1991). Some researchers have interpreted the contact as an angular unconformity (Baltz, 1967; Smith and Lucas, 1991; Williamson and Lucas, 1992) due to the abrupt change from mudrock- dominated lithofacies to sandstone-dominant lithofacies and localized observations of angular discordance between the Nacimiento mudrock layers and the base of the overlying basal Cuba Mesa channels. During our investigations, we have not observed an angular discordance at

145 the Nacimiento-San Jose contact. However, the erosional base of the laterally continuous basal Cuba Mesa Member sandstone is a composite erosion surface of amalgamated channel bases and can thus create the illusion of a discordance locally. Nevertheless, the top of the basal Cuba Mesa sandbody and the overlying mudrock deposits appear parallel with the underlying Nacimiento mudrock deposits and channel tops. Upon recent review of the aerial photographs used in his interpretation, Smith (pers. com., 2018) confirmed that there is no clear evidence of an angular unconformity at the basin-wide scale. It has also been suggested that the Nacimiento - San Jose Formation contact may be transitional in nature due to interpretations that the contact is conformable in the northern SJB (Barnes et al., 1954; Stone, 1983; Smith and Lucas, 1991) and unconformable in the southern SJB (Baltz, 1967; Lucas et al., 1981; Smith and Lucas, 1991) (Fig. 4.1d). Evidence cited for a conformable contact in the northern SJB include intertonguing of the Nacimiento and lowermost San Jose Formation (Cuba Mesa Member) with the late Paleocene upper Animas Formation, where the presence of Tiffanian strata (61.7 - 56.8 Ma; Alroy et al., 2000) indicates that the upper Paleocene strata are preserved below and/or laterally adjacent to the San Jose Formation (Fig. 4.1d; Smith, 1988; Smith and Lucas, 1991). Evidence cited for an unconformable contact in the southern SJB include a fossil gap of at least 6 My between the stratigraphically highest Paleocene mammal find (Torrejonian, 63.3 - 61.7 Ma) and the stratigraphically lowest early Eocene mammal find (Wasatchian, 55.8 - 50.3 Ma) (Fig. 4.1d). However, the fossil localities are separated by a minimum of 90 m of stratigraphy (Lucas et al., 1981). Alternatively, Cather (2004) argues that the Nacimiento - San Jose Formation contact may be disconformable throughout the basin because electric logs for representative wells in two east-west transects show a sharp contact and resemble the electric-log response elsewhere in the southern part of the basin where the contact is known to be unconformable. This hypothesis does not appear to account for the biostratigraphic and magnetostratigraphic evidence that late Paleocene deposits are preserved in at least some parts of the San Juan Basin (Fig. 4.1d).

146 Because of the uncertainty over the nature of the contact and a lack of biostratigraphic evidence in the Escavada Member of the Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation, the precise location of the Paleocene-Eocene boundary cannot be determined (Smith and Lucas, 1991). The stratigraphically highest Paleocene mammal find is of early Paleocene age (Torrejonian, 63.3 - 61.7 Ma) and is ∼30 m below the Nacimiento - San Jose contact in the southern SJB (Lucas et al., 1981). The stratigraphically lowest fossil mammals in the San Jose Formation are from the Regina Member, at least 60 meters above the Nacimiento - San Jose contact, and are of early Eocene age (Wasatchian, 55.8 - 50.3 Ma) (Lucas et al., 1981) (Fig. 4.1d). Therefore, the Paleocene-Eocene boundary in the south-central SJB is located somewhere between the base of the Escavada Member of the Nacimiento Formation and the top of the Cuba Mesa Member of the San Jose Forma- tion (Fig. 4.1c; Tsentas and Lucas, 1980; Lucas et al., 1981). Despite this uncertainty, the Paleocene-Eocene boundary is commonly inferred in the literature to reside within the suggested unconformity at the Nacimiento - San Jose contact (e.g. Dickinson et al., 1988).

4.4 Dataset and Methods

The study section is located in the southern San Juan Basin, approximately 11 km west of the village of Cuba, NM in outcrops adjacent to Chuilla Road 1101. We have named the study area Arroyo Chijuilla due to its geographic proximity to the drainage with the same name (Fig. 4.1e). Samples for geochemical analysis were collected from the Arroyo Chijuilla study area. Sedimentological data were also collected from Arroyo Chijuilla and a second study area, Continental Divide, located approximately 9 km to the west (Fig. 4.1c).

4.4.1 Measured sections and sample collection

We measured 349 m of stratigraphic section in Arroyo Chijuilla and 162 m of stratigraphic section at Continental Divide spanning the uppermost Nacimiento Formation and Cuba Mesa and Regina Members of the San Jose Formation using a 1.5 m Jacobs Staff and verifying accuracy with a TruPulse 360r laser rangefinder. Sandstones were described at approximately

147 10 cm resolution based on bedsets, sedimentary structures, biogenic structures, and grain characteristics (size, shape, sorting, and composition). Mudrock deposits were described based on texture and color of the matrix and mottles at approximately 1-1.5 m resolution. Two sources of organic carbon were collected along the measured section for stable or- ganic carbon isotope analysis: coalified-charcoalified fossil wood and bulk sediment. Eighty hand samples of sandstones containing coalified-charcoalified fragments of fossil wood were collected from sandstone channel lithofacies at 33 levels of the stratigraphic succession. Some of the fossil wood localities contained a single, recognizable root or tree branch (Fig. 4.2a). Other localities contained multiple small fragments embedded in sandstone and the num- ber of species present could not be determined (Fig. 4.2b). Still other localities presented as a thin layer or lag of small plant fragments (hash) within channel lithofacies that likely contain material from many species (Fig. 4.2c). Where possible, multiple samples were col- lected from each stratigraphic level, especially when the fossil localities appeared to contain material from a single species. Bulk sediment samples were collected from mudrock paleosols within floodplain litho- somes at 39 levels of the stratigraphic succession. Due to the degree of lateral variability in channel and floodplain lithosomes in the study area, samples were often collected in flood- plain intervals lateral to the measured section. Samples were collected from outcrops by removing weathered surface material to expose fresh, well-consolidated rock.

4.4.2 Carbon isotope analyses

Coalified-charcoalified fossil wood samples were picked from disaggregated sandstone hand samples and carefully cleaned of additional debris with a needle in the laboratory under a binocular microscope. Black and lustrous fossil wood fragments were selected over brown fragments for analysis whenever possible to provide inter-sample consistency. Many of the samples had the appearance of preserved wood structure. Bulk sediment samples ground to a fine powder using a handheld Dremel tool. The powdered sample was placed in a centrifuge tube and 0.1 N hydrocholoric acid was added

148 and mixed with the sample and left to sit for 20 minutes to remove carbonate. Samples were then washed with deionized water, spun in a centrifuge, and decanted. This process was repeated until a neutral solution was achieved when measured on pH paper. The samples were then dried overnight in a 70° C oven and powdered again prior to analysis. Carbon isotope ratios and weight percent total organic carbon (TOC) were measured in duplicate using a continuous-flow gas-ratio mass spectrometer (Finnigan Delta PlusXL) at the University of Arizona Environmental Isotope Laboratory. Samples were combusted using an elemental analyzer (Costech) coupled to the mass spectrometer. Standardization is based on acetanilide for elemental concentration and IAEA-CH-7 and USGS-24 for δ13C. Preci- sion is better than ± 0.08 for δ13C, based on repeated internal standards. Carbon isotope values are reported in delta notation (δ13C, where δ = (Rsample/Rstandard - 1)*1000‰) normalized to the international Vienna Peedee Belemnite (VPDB) standard (Table 4.1 and 4.2).

4.4.3 Depositional system

The river and floodplain deposits of the Paleocene upper Nacimiento Formation (Es- cavada Member) through the Cuba Mesa and Regina Members of the early Eocene San Jose Formation have been recognized as a large fluvial fan (Chapter 3), formed by vari- able discharge river systems (Chapter 2), resulting from inter- (seasonal) and intra-annual precipitation variability. Observations that provide evidence for discharge variability and thus also a variable precipitation signature include: 1) An abundance of UFR sedimentary structures deposited under Froude supercritical flow conditions and high deposition rates within the channel deposits. A high percentage of UFR and HDR structures is charac- teristic of modern and ancient monsoonal and subtropical rivers that have highly variable discharge and receive most of their annual (or in some cases, inter-annual) precipitation from one to few large downpours or during a distinct wet season resulting in high magnitude floods (McKee et al., 1967; Williams, 1971; Frostick and Reid, 1977; Stear, 1985; Abdullatif, 1989; Singh and Bhardwaj, 1991; Singh et al., 1993, 2007; Shukla et al., 2001; Billi, 2007;

149 Fielding et al., 2009; Chakraborty et al., 2010; Chakraborty and Ghosh, 2010). 2) Preser- vation of in-channel bioturbation and mud layers with pedogenic modification indicating that channels underwent periods of prolonged dryness between flooding episodes (Hasiotis et al., 2007). 3) Lack of recognizable braid or point bar strata, characteristic for variable discharge rivers (Wang and Plink-Bj¨orklund, in review; Rhee and Chough, 1993; Uba et al., 2005; Billi, 2007; Hampton and Horton, 2007; Fielding et al., 2009; Hulka and Heubeck, 2010; Allen et al., 2011; Plink-Bj¨orklund, 2015). 4) Floodplain lithosomes with alternations of poorly-drained and more well-drained deposits and/or slickensides indicating alternating wet-dry cycles (Kraus, 1999; Kraus and Hasiotis, 2006; Vepraskas and Craft, 2016; Tabor et al., 2017). 5) Floodplain lithosomes with crevasse splay deposits that suggest channel avulsions resulting from flooding events (Abdullatif, 1989; Makaske, 2001; Jain and Sinha, 2003, 2004; Assine, 2005; Billi, 2007; Assine and Silva, 2009; Sinha et al., 2009; Chakraborty et al., 2010; Chakraborty and Ghosh, 2010; Donselaar et al., 2013). Stratigraphic trends in the floodplain paleosols from drab-colored in the uppermost Nacimiento Formation to increasing abundances of purple-red deposits in the San Jose Formation suggests an overall long-term decrease in mean annual precipitation (Fig. 4.4). When stratigraphic trends in channel-floodplain facies associations are analyzed together and compared with precipitation variability from modern climate zones, the changes may suggest a shift from a monsoonal precipitation regime in the Escavada Member of the Paleocene Nacimiento Formation to fluctuating sub-humid and arid subtropical in the Cuba Mesa Member and semi-arid to arid in the Regina Member of the early Eocene San Jose Formation (Chapter 2). Evidence for deposition in a large fluvial fan includes: 1) the large spatial extent of deposits (at least 8000 km2) 2) progressive downstream (north to south) decrease in grain size, 3) downstream decrease in channel depth and width, 4) downstream increase in over- bank preservation, 5) downstream decrease in lateral and vertical connectivity of channel deposits, and 6) evidence of channel mobility by avulsion. At least two stacked packages of whole fluvial fan progradation are recorded in the study interval and on a basin-wide

150 scale (Chapter 3), characterized by dominantly fine-grained overbank deposits with minor volumes of isolated, coarser channel fills (distal facies) are progressively followed upwards in the stratigraphy by more sand rich, larger, and increasingly amalgamated channel bodies, with lesser volumes of preserved mud-prone overbank strata (medial-proximal facies) (Fig. 4.3). In the Arroyo Chijuilla study area, the first of these progradational packages extends from the uppermost Nacimiento Formation to the lower Cuba Mesa Member channel complex (Fig. 4.4a). The high degree of lateral and vertical amalgamation and dominance of UFR and HDR sedimentary structures characterize this channel complex as proximal fan facies and suggests that this channel complex may record the maximum point of fan progradation to the southern San Juan Basin. This upward-coarsening prograding trend is overlain by an interval of decreased amalgamation in the middle and upper Cuba Mesa Member channel complexes, resembling medial fan facies, that may suggest some initial backstepping of the fan system in the southern part of the basin (Fig. 4.4b). The contact between the Cuba Mesa and Regina Members of the San Jose Formation is characterized by an abrupt shift from medial fan facies to isolated and lenticular channels and dominant floodplain deposits, interpreted as distal fan facies (Fig. 4.4c). The second progradational package extends from the contact between the Cuba Mesa and Regina Members to the top of the Arroyo Chijuilla stratigraphic section (Fig. 4.4c). In the Arroyo Chijuilla study area, only the distal fan facies are exposed in Regina Member outcrop, but lateral correlations with a study area located ∼9 km to the west suggest that the Regina Member records a second episode of fan progradation with proximal fan deposits overlying the distal deposits. We hypothesize that the abrupt shift from proximal fan facies to distal fan facies at the contact between the Cuba Mesa and Regina Members of the San Jose Formation suggest a period of fan inactivity between the progradational packages. The interpretation of stacked prograding fans suggests that a previously unrecognized depositional hiatus of unknown duration separates the fan packages at the contact between the Cuba Mesa and Regina Members of the San Jose Formation.

151 If this hypothesis is correct, the suggested depositional time gap (estimated to be ≥ 5.6 My; Fassett et al., 2010) at the contact between the Nacimiento and San Jose Formations is likely overestimated, as the depositional age of the entire study interval is loosely constrained between fossil localities (Fig. 4.1d). Evidence that all the deposits in the study interval are from variable discharge river systems (Chapter 2), combined with observations that the genesis and progradation of Qua- ternary and modern large fluvial fans are linked to highly variable discharge suggest that climate was a key control on the depositional system in the Paleogene San Juan Basin.

4.5 Fossil Wood Samples

The color of the fossilized wood specimens throughout the study interval range from light brown to black, perhaps indicating different levels of coalification and/or charcoalification (Scott and Glasspool, 2007; Yans et al., 2010). Precise classification of the inertinite group macerals to which the samples belong is beyond the scope of this initial investigation, how- ever, the preservation of apparent woody structure as observed under binocular microscope may suggest that most of the samples have undergone charcoalification (Scott, 2000, 2001). Fossil charcoal typically shows excellent preservation of cellular structures and is the result of incomplete combustion from wildfire or volcanogenic activity thought to provide direct evidence of paleowildfires (Cope, 1980; Scott, 2000, 2001). Coalified samples do not show good cellular preservation and their formation is less understood but may be linked to sul- phur bacteria in wet and anoxic conditions providing sulphuric acid (Gerards et al., 2008), although the origin of the sulphur in terrestrial environments, and formation of sulphides within the cells of higher plants are still debated (Grimes et al., 2001; Rickard et al., 2007).

4.6 Isotope Analyses 4.6.1 Fossil wood organic carbon isotopes

Stable isotopic analyses performed on the fossil wood organic carbon samples generated

13 δ Cwood values ranging from -28.2 to -20.1‰ and a baseline value of ∼22.0‰ (Fig. 4.5a; Ta-

152 ble 4.1). Superimposed upon this baseline are 4 potential negative CIEs. The first and largest

13 of the negative CIEs (W1; Fig. 4.5a) begins as gradual decrease in δ Cwood values that may begin as low as the 37 m stratigraphic level in the Nacimiento Formation, although sample

13 density is poor below the Nacimiento-San Jose contact and the range in δ Cwood values is high (-20.2 to -24.3‰) over the short interval spanning the 37-40 m stratigraphic levels. The

13 W1 CIE reaches a δ Cwood peak negative value of -28.2‰ at the 108 m stratigraphic level in the middle Cuba Mesa Member channel complex, which measures as a magnitude 6.2‰ negative shift (calculated as peak negative value minus non-excursion baseline value) over as

13 much as 123 m. Thereafter, a gradual trend toward increased δ Cwood values over 68 m is observed, resulting in a return to near-baseline values (-22.1‰). The return to near-baseline values occurs at the 231.5 m stratigraphic level, corresponding with the contact between the Cuba Mesa and Regina Members of the San Jose Formation. The second negative excursion (W2; Fig. 4.5a) occurs immediately above the contact between the Cuba Mesa and Regina Members at the 231.5 m stratigraphic level. This excursion is of slightly lesser magnitude

13 (-5.7‰), reaching a δ Cwood peak negative value of -27.7‰, and occurs over a considerably shorter interval of 9 m. The excursion is followed by a gradual return to near-baseline values (-22.9‰ at the 248 m stratigraphic level) over 11 m. The third negative excursion (W3; Fig. 4.5a) is of still lesser magnitude (-4.5‰), reaching a peak negative value of -26.5‰ at the 260 m stratigraphic level, occurring over an interval of 12 m, and is followed by a return to near-baseline values (21.4‰) at the 284.5 m stratigraphic level which occurs over an interval of 24.5 m. The fourth and final excursion in the study interval (W4; Fig. 4.5a) occurs in

13 the uppermost outcrops of the Regina Member and culminates in a δ Cwood peak negative value of -28.0‰ at the 305 m stratigraphic level. The 6.0‰ magnitude negative excursion occurs over an interval of 20.5 m, and the peak negative value is followed by a gradual return to near- to slightly above- baseline values (-21.1‰) over an interval of 44 m. Analyses of multiple samples from the same stratigraphic level show a small amount of isotopic offset (0.1-0.3‰) between samples that are likely to contain fragments from multiple species, such

153 as those collected from plant fragment drapes within channel sandstones, and a moderate to large amount of isotopic offset (0.8-3.1‰) when samples consist of individual fossil wood fragments indicative of one species (Table 4.1). The TOC content of the samples included within the dataset range from 20.1-65.3%, with the exception of 10 samples that displayed

13 anomalously low TOC values (0.4-3.18%). These δ Cwood values were not included when

13 13 calculating the average δ Cwood curve, which displays the averaged δ Cwood values for each stratigraphic interval (Fig. 4.5a).

13 Interpretation: The range δ Cwood values (-28.2 to -20.1‰) is representative of C3 veg- etation (values range between -23 and -33‰; Saltzman and Thomas, 2012) affected by coal- ification and/or charcoalification, which have been demonstrated to cause a 13C-enrichment in wood that results in a positive ∼0.5‰ to 3‰ shift in δ13C values (Gr¨ocke et al., 1999 and references therein).

13 The moderate to high degree of isotopic offset observed in the δ Cwood values obtained from individual fossil wood fragments collected from the same stratigraphic level is likely the result of samples coming from different species of plants, or even different parts of the same species of plant. It is well established that there is strong δ13C variability between different species of plants and the δ13C values of the different compounds and tissues (leaves, seeds, cuticles, etc.) within a single plant can also vary (Gr¨ocke et al., 1999 and references

13 therein). The low degree of isotopic offset observed in the δ Cwood values of samples collected in plant fragment drapes within sandstone channels may indicate that multiple species and

13 plant tissues are present, and the mixing may have resulted in an averaged δ Cwood value. The -6.2% negative W1 CIE may be of significant enough magnitude to overwhelm the isotopic offsets, and the stratigraphic thickness of the negative shift suggests that the interval records a hyperthermal event. Existing terrestrial datasets show a mean -4.7±1.5 shift in δ13C associated with the PETM CIE (McInerney and Wing, 2011), and thus the W1 CIE in the San Juan Basin may record the PETM hyperthermal event (Fig. 4.5a). However,

13 accurate placement of the onset of the hypothesized PETM CIE given the current δ Cwood

154 13 dataset is precluded by low sample density. The average δ Cwood curve seems to suggest that the excursion may start in the Nacimiento Formation, but higher sampling density is required before conclusive interpretations are made, especially considering that the Nacimiento-San Jose contact is thought to be a major disconformity. In addition, short-duration, lower mag- nitude hyperthermal events have been recorded in Paleocene datasets, therefore the negative

13 shift in the δ Cwood in the Nacimiento formation could also record an earlier hyperthermal event (Saltzman and Thomas, 2012).

13 The hypothesized PETM CIE in the δ Cwood record ends abruptly at the contact be- tween the Cuba Mesa and Regina Members of the San Jose Formation. The length of the stratigraphic section above the hypothesized PETM CIE (121 m) and significant magnitude of the W2, W3, and W4 excursions may suggest that post-PETM hyperthermal events are preserved in the San Juan Basin. In other early-Eocene δ13C datasets, the CIEs associated with post-PETM hyperthermal events have been shown to be of shorter duration, and lesser magnitude than the PETM CIE (e.g. Abels et al., 2016). However, the low sample density included in this preliminary investigation, especially for the W3 and W4 excursions preclude further interpretation of potential post-PETM hyperthermal events in the San Juan Basin at this time.

4.6.2 Bulk-sediment organic carbon isotopes

Stable isotopic analyses performed on the bulk-sediment organic carbon samples gener-

13 ated δ Cbulk sed values ranging from -29.2 to -23.50‰ (Table 4.2). Samples that underwent

13 duplicate analysis were averaged to create a sample averaged δ Cbulk sed curve with values ranging from -27.8 to -23.8‰. The pre-excursion baseline value ∼24.5‰. Superimposed upon

13 this baseline is at least one interval that contains multiple negative shifts in δ Cbulk sed values that could be suggestive of a CIE (B1: Fig. 4.5b), however the curve shows high frequency and high magnitude fluctuations between excursion and pre-excursion values through this interval. Furthermore, duplicate analyses of some samples display considerable intra-sample isotopic offset ranging from 0.2-2.8‰, and in one sample the difference between duplicate

155 runs was 4.0‰ (Fig. 4.5b; Table 4.2). The TOC content of the samples included within the dataset ranges from 0.02-0.5% with an average of 0.1%. The B1 interval containing possible excursion values begins at the first sample collected above the Nacimiento-San Jose Formation contact, within the lower Cuba Mesa Member at the 85 m stratigraphic level, however precise placement of the onset is not possible at this time due to poor sample density below this stratigraphic level (B1; Fig. 4.5b). A

13 peak negative δ Cbulk sed value of -29.2‰ is reached at the 108 m stratigraphic level, which measures as a magnitude ∼4.7‰ negative shift (calculated as peak negative value minus pre- excursion baseline value) over a minimum of 23 m. When averaged with repeat sample runs from the 108 m stratigraphic level, the peak excursion value is -27.80 (magnitude ∼3.3‰

13 negative CIE). Due to the variability in the δ Cbulk sed values and low sample density of this initial dataset, it is not possible to determine the upper stratigraphic extent of the B1 potential negative CIE. A second interval of possible lesser-magnitude negative CIEs occurs in the Regina Member

13 from stratigraphic levels 259-341 m (B2; Fig. 4.5b) A peak negative δ Cbulk sed value of -27.1 is observed at the 286 m stratigraphic level (magnitude ∼2.6 negative shift) and suggests that additional CIEs could be present, or at least continued high frequency variability between pre-excursion and excursion values. Interpretation: The magnitude 2.6-4.7‰ negative excursions from baseline are suggestive of carbon cycle response to hyperthermal events. The magnitude of the average CIE derived from bulk sediment organic matter in other terrestrial datasets associated with PETM in- tervals is -3.5‰ (see McInerney and Wing, 2011) and those associated with post-PETM hyperthermals are of lesser magnitude (Abels et al., 2016). However, the high frequency and high magnitude fluctuations displayed in the curve within the lower possible excursion in- terval where sample density is highest (B1: Fig. 4.5b), combined with intra-sample isotopic

13 offset is concerning when assessing the validity of the δ Cbulk sed results. Furthermore, the low sample density above and below the B1 interval may not be sufficient to record poten-

156 13 tially significant fluctuations in δ Cbulk sed values, and thus the appearance of the potential negative CIE intervals and returns to near-baseline values that are suggested by the average

13 13 δ Cbulk sed curve may be deceiving. Therefore, as a standalone dataset, the δ Cbulk sed val- ues derived from this initial investigation is inconclusive for evidence of hyperthermal events in the Paleogene San Juan Basin. While it is possible that increased sample resolution may

13 aid in further interpretations of the δ Cbulk sed dataset, the intra-sample variability suggests that the floodplain paleosols within the study interval may not be a reliable material for sta- ble carbon isotope analysis, perhaps due to the low TOC content of the samples (0.02-0.5%).

13 The low TOC content could indicate that δ Cbulk sed values are particularly susceptible to distortion from input of allochthonous organic carbon from the erosion and reworking of older sediments. It also cannot be ruled out that the low TOC content may have led to skewed measurements resulting from slight variations in sample preparation between batches.

4.6.3 Comparison of trends in fossil wood and bulk sediment carbon isotope curves

13 Both the δ Cwood hypothesized PETM CIE and occurrence of excursion-magnitude

13 δ Cbulk sed values in the B1 interval occur within the Cuba Mesa Member of the San Jose

13 Formation (Fig. 4.5). The limited δ Cbulk sed measurements suggest that the onset of the excursion is above the Nacimiento-San Jose contact, and the negative shift observed in the

13 13 δ Cwood record at the 40 m stratigraphic level is not recorded in the δ Cbulk sed sample

13 from the same level. Peak negative δ Cbulk sed values in the B1 interval are observed ∼50

13 m below peak negative δ Cwood values in the hypothesized PETM CIE. The peak negative

13 δ Cwood values occur above an interval that appears to show an overall upward trend toward

13 higher δ Cbulk sed values, approaching pre-excursion baseline values. Given the poor bulk sediment sampling density, it is not possible to accurately determine the upper extent of the B1 interval, but the stratigraphically lower peak negative values seem to suggest that the

13 record may be truncated compared to the δ Cwood hypothesized PETM CIE. Higher in the stratigraphic section (Regina Member), substantial variations in the trends of the curves are

157 observed. For example, the W2 excursion at the 237 m stratigraphic interval is not recorded

13 13 in the δ Cbulk sed curve. Peak negative δ Cbulk sed values for the Regina Member occur at

13 the 286 m level, which is 19 m below the level that peak negative δ Cwood are observed (305 m). Despite these differences, the W3 and W4 CIEs are coincident in stratigraphic placement with the B2 interval (Fig. 4.5).

13 Interpretation: In the Cuba Mesa Member, the negative W1 CIE in δ Cwood values,

13 combined with the onset of a high concentration of δ Cbulk sed values of CIE-magnitude may indicate that there is evidence across both δ13C datsets of the PETM hyperthermal event

13 in the San Juan Basin. However, if this hypothesis is correct, the δ Cbulk sed curve may be stratigraphically truncated or the magnitude of the CIE may be reduced compared to the

13 13 δ Cwood curve. In the Bighorn Basin, Wyoming, researchers compared δ Cbulk sed values from paleosols with δ13C taken from other sources (carbonaceous shales, pedogenic carbon- ate, leaf wax n-alkanes, and tooth enamel) and observed similar styles of high-frequency variability, reduced CIE magnitude, and truncated thickness of the PETM CIE in the in the paleosol dataset (Baczynski et al., 2016). They suggest that the differences seen in the

13 13 paleosol δ Cbulk sed values are the result of degradation and C enrichment of organic mat- ter during pedogenic processes and/or the addition of transported 13C-enriched allochtonous carbon from the erosion of older rocks. However, this hypothesis does not explain the intra-

13 sample variability in δ Cbulk sed values observed in the San Juan Basin dataset, which may have resulted from the previously discussed factors that may be associated with the low TOC content of the samples. Based on these lines of evidence combined with insufficient sample

13 density in this initial dataset, we conclude that the δ Cbulk sed curve may not provide an accurate representation of carbon cycle dynamics within the study interval.

13 Terrestrial plants take up CO2 directly from the atmosphere and therefore the δ Cwood dataset may provide a more reliable measure of CIEs within the study interval, despite the potential variability introduced by different plant types and/or constituents. Hereafter, we

13 focus primarily on the results from the δ Cwood in the investigation of potential climate

158 perturbations and their influence on sedimentation in the San Juan Basin.

4.7 Comparison of Carbon Isotope Curves and Stratigraphic Trends

13 13 The onset of the hypothesized PETM CIE in both the δ Cwood and δ Cbulk sed datasets coincides with the deposition of the lower Cuba Mesa Member channel complex, which we have interpreted as the maximum extent of fan progradation in the southern San Juan Basin

13 (Fig. 4.4a; Fig. 4.6; Chapter 3). Based on the δ Cwood curve, the hypothesized PETM CIE appears to span the Cuba Mesa Member, with peak negative CIE values occurring in coincidence with deposition of the middle Cuba Mesa Member channel complex, which we suggest are medial fan deposits that may record some initial retrogradation of the fluvial fan system, and just above the first occurrences of purple-red paleosols (Fig. 4.4b; Fig. 4.6). The

13 gradual increase in δ Cwood values through the remainder of the middle and upper Cuba Mesa Member channel complexes may record the continuation of the main body PETM CIE or the PETM recovery interval. The top of the hypothesized PETM CIE terminates abruptly

13 with a return to near-baseline δ Cwood values immediately above the contact between the Cuba Mesa and Regina Members, which is also characterized by an abrupt shift from medial fan facies to distal fan facies (Fig. 4.4c; Fig. 4.6). We have suggested that the contact may record a depositional hiatus between two packages of prograding fluvial fans, and the

13 abrupt 2.8‰ positive shift in δ Cwood seems to corroborate this hypothesis (Fig. 4.6). If this hypothesis is correct, the PETM recovery interval could be truncated or missing from the record in the southern San Juan Basin. Alternatively, the recovery interval could be recorded laterally in the basin if the shift is due to fan lobe avulsion. The distal fan facies in the lower Regina Member contain both the W2 and W3 CIEs

13 (Fig. 4.4c; Fig. 4.6). Fluctuations between peak negative δ Cwood values and near-baseline

13 δ Cwood values in isolated sandstone channels occur within this interval. The W4 CIE, which may record a post-PETM hyperthermal event coincides with a gradual upward increase in channel size through the Regina Member, that we suggest provides evidence of the second package of fan progradation (Chapter 3).

159 4.8 Discussion 4.8.1 Sedimentary response to long-term warming and hyperthermal events

The overall trend towards increasing aridity observed in the floodplain deposits within the study interval coincides with a global long-term warming trend from the Paleocene to the middle Early Eocene upon which transient hyperthermals were superimposed (Zachos et al., 2008; Bijl et al., 2013). Therefore, the floodplain paleosols may record a long-term shift from climates that experienced sustained precipitation for the duration of the monsoon season to increasingly arid climate states (Chapter 2). It is suggested that the intensity and distribution of precipitation changed as a result of the Paleogene warming trend (Huber and Goldner, 2012), with greater moisture transport to high latitudes (Pagani et al., 2006) and enhanced seasonal extremes in some areas (John et al., 2008). It has also been suggested that enhanced extremes in the hydrological cycle resulting in increased precipitation peakedness in the subtropics and mid-latitudes occurred in response to the Eocene hyperthermal events (McInerney and Wing, 2011; Plink-Bj¨orklund et al., 2014; Carmichael et al., 2017, 2018). The progradation of a large fluvial fan in the lower Cuba Mesa Member as suggested by the stratigraphic analysis (Chapter 3), in coincidence with the onset of the hypothesized PETM CIE indicates that the frequency and magnitude of high intensity precipitation events may have accompanied the hyperthermal event in the San Juan Basin region. The high degree of amalgamation and abundance of UFR and HDR sedimentary structures in the lower Cuba Mesa Member channel complex may record sediment flushing occurring as a response to the onset of the PETM and/or a response to the shift in the ambient climate of the region from a monsoonal precipitation regime in the Paleocene to variably sub-humid and arid in the early-Eocene. Reduction in vegetative cover, erosion of soils, and an increase in solid sediment yield are likely to result in a sediment flush under these conditions (Cecil and Dulong, 2003), and could lead to fan progradation. The reduced amalgamation, interpreted as medial fluvial fan facies, seen in the middle and upper Cuba Mesa Member and coinciding with peak negative hypothesized PETM

160 CIE values may suggest that the fluvial system continued to experience high discharge, but solid sediment yield may have decreased once weathering and pedogenic equilibrium were reestablished under drier conditions (Cecil and Dulong, 2003) brought on the by PETM and/or the long-term drying trend.

13 The abrupt shift to distal fan facies coinciding with an abrupt positive shift in δ Cwood values that we interpret as the upper stratigraphic extent of the hypothesized PETM CIE may indicate that fluvial fan activity ceased for a period of time during or after the PETM recovery interval. As a result, there may be a depositional hiatus at the contact between the Cuba Mesa and Regina Members of the San Jose Formation that truncates the CIE record

13 in the southern San Juan Basin. The abrupt positive shift in the δ Cwood values strengthens the hypothesis that there is likely a depositional hiatus at the contact (Fig. 4.6). Interpretations of sedimentary response to potential post-PETM hyperthermal events is less straightforward through the Regina Member due to decreased sample density. The shift to distal fan facies consisting of isolated channels and dominant floodplain with increasing abundance of purple-red paleosols in the lower Regina Member may suggest decreased dis- charge and solid sediment yield post-PETM and as the climate became increasingly arid. However, if the Cuba Mesa-Regina Member hiatus hypothesis is correct, the reestablishment of deposition to the southern San Juan Basin suggests that the fan system was reactivated. The hypothesis that the fan system ceased activity and was reactivated is supported by another package of upward increasing channel amalgamation in the Regina Member, which we have interpreted as possible second episode of fan progradation. These deposits are coin- cident with two negative CIEs of shorter stratigraphic thickness (W2 and W3). Indications of an additional negative CIE (W4) near the top of the Regina Member may suggest that a post-PETM hyperthermal event may be linked to the abrupt basinward shift of proximal fan facies observed in the uppermost Regina Member (Chapter 3), but not exposed in outcrop within the Arroyo Chijuilla study area.

161 Overall, these findings strengthen our hypothesis that the Paleocene and early-Eocene deposits in the study interval show evidence of an overall long-term decrease in mean an- nual precipitation and fluctuations in seasonal extremes of precipitation. We show that fluctuations in seasonal extremes of precipitation appear to be linked to hyperthermal cli- mate perturbations characterized by negative CIEs, including the Paleocene-Eocene Thermal Maximum.

4.8.2 Comparisons with other mid-latitude continental basins

Further evidence that the San Juan Basin may contain deposits of PETM-age comes from similar changes observed in depositional architectural trends associated with other PETM datasets. In particular, there is a growing list of datasets displaying anomalous alluvial deposits associated with PETM intervals in mid-latitude terrestrial settings (e.g. Schmitz and Pujalte, 2007; Foreman et al., 2012; Foreman, 2014). In these examples, the deposi- tional units are unlike those underlying or overlying the PETM CIE. In the Bighorn Basin, Wyoming, a channel sandstone accumulated during the PETM that is significantly thicker and wider than any other pre- and post-PETM channel complex. The deposition of the large channel sandstone is suggested to have initiated after the PETM onset and termi- nated before the end of the PETM recovery phase (Foreman, 2014). In the Piceance Creek Basin, Colorado, channel deposits become more laterally and vertically amalgamated during the PETM, although there is some degree of offset between the depositional trends and the negative CIE, interpreted as indicative of lag times in response and recovery to the hyperther- mal event (Foreman et al., 2012). In the Uinta Basin, Utah, channel deposits of suspected PETM-age are thicker, wider, and more amalgamated than suspected pre- and post-PETM channel complexes (Plink-Bj¨orklund et al., 2014). In the Tremp-Graus Basin, Spain, channel deposits associated with the PETM are also thicker and wider on average than pre-PETM channel complexes, however degrees of channel amalgamation during the PETM interval vary, with one study area showing an increased proportion of channels and amalgamation and another study area showing a decrease in the proportion of channel deposits (Schmitz

162 and Pujalte, 2007). Changes in floodplain paleosols associated with the pre-PETM, PETM, and post-PETM intervals are more variable between the terrestrial datasets and differ from our interpreta- tion of a long-term drying trend in the San Juan Basin. In the Bighorn Basin a transient decrease in precipitation has been suggested coincident with the Paleocene Eocene Ther- mal Maximum (PETM) hyperthermal event (Kraus and Riggins, 2007; Kraus et al., 2015). These researchers have also observed indicators of wet/dry cycles within the PETM interval paleosols that they suggest may correspond to precessional cycles. The PETM interval is followed by indicators of similar to slightly wetter background soil moisture, using the CAL- MAG mean annual precipitation proxy, linked to four younger hyperthermal events with (Abels et al., 2015). In the Uinta Basin, Utah, paleosol characteristics and trace fossils simi- larly suggest a transient shift to overall drier conditions during the peak PETM, followed by a wetter period with seasonally variable conditions (Golab, 2010). In contrast, conditions in the Piceance Creek Basin, Colorado, are suggested to have been drier in the late-Paleocene and more humid during the PETM due to an observed shift from purple, orange, and red paleosols to dominantly purple paleosols (Foreman et al., 2012). An abundance of red-pink paleosol deposits above the identified PETM interval suggests drier conditions further into the early-Eocene. In the Tremp-Graus Basin, Spain, semi-arid conditions across the Pale- ocene - Eocene boundary are suggested with an increase in intra-annual humidity gradients and associated seasonal flash floods during the PETM, followed by drier conditions imme- diately after the PETM (Schmitz and Pujalte, 2007). The anomalous sedimentation events that appear to characterize the PETM interval in these terrestrial basins suggest that an increase in the magnitude or variability of river discharge is associated with the PETM, at least in the middle-latitudes. This increase in variability is likely related to stronger seasonality and increase in sediment supply due to intensification of physical weathering, and a decrease in the density of vegetation cover in response to the extreme warming event. Differences in the floodplain deposits that suggest

163 varying levels of soil drainage and thus wet vs. dry conditions, particularly outside the PETM interval, may suggest that ambient MAP was more variable between locations. At present, the most detailed studies of PETM terrestrial basins in North America have focused on basins located at or above 40 degrees latitude (e.g. Bighorn Basin, Piceance Creek Basin, and Uinta Basin). The San Juan Basin is located at ∼36 degrees latitude, thus recognition of a more southerly dataset recording the PETM and post-PETM hyperthermal events provides an opportunity to conduct proxy studies along a ≥ 4 degree latitudinal transect. Such studies could provide additional insight into climatic and biotic changes in response to hyperthermal events in the San Juan Basin and across the Laramide basins.

4.8.3 Concluding remarks on bulk sediment and fossil wood

The observations of intra-sample variability between repeated measurements and poten-

13 tially reduced magnitude and/or truncation of the CIE in the δ Cbulk sed record from the San Juan Basin add to an increasing body of evidence that caution should be exercised when interpreting δ13C records to address global carbon cycle dynamics (e.g. Smith et al., 2007; Wynn, 2007; Diefendorf et al., 2010; Baczynski et al., 2016). In addition, the use of coalified-charcoalified fossil wood fragments for δ13C analysis is very limited, thus there

13 is little basis for determining the expected magnitude of δ Cwood CIE associated with the PETM and subsequent hyperthermal events. Higher sample density and collection of addi- tional material for δ13C analysis should help resolve and validate these records, and aid in more definitive identification of hyperthermal events in the San Juan Basin.

4.9 Conclusions

We conducted preliminary stable organic carbon isotope analysis on bulk sediment from

13 13 paleosols (δ Cbulk sed) and coalified-charcoalified fossil wood (δ Cwood) as part of an on- going stratigraphic study to improve our understanding of the links between climate per- turbations and sedimentological response, as well as depositional controls on the Paleocene- Eocene boundary interval in the San Juan Basin. Stable isotopic analyses performed on

164 13 the fossil wood organic carbon samples generated δ Cwood values indicative of 4 negative carbon isotope excursions (CIEs). Analyses performed on the bulk-sediment organic carbon

13 samples generated a δ Cbulk sed record with two intervals containing multiple negative shifts

13 in δ Cbulk sed values that could be suggestive of a CIE, however, there are high frequency and high magnitude fluctuations between excursion and pre-excursion values through the in-

13 tervals. Furthermore, duplicate runs revealed high intra-sample δ Cbulk sed variability. This variability may be attributed to low total organic carbon (TOC) content combined with input of allochthonous organic carbon from the erosion and reworking of older sediments.

13 Despite the variability in the δ Cbulk sed values, an interval with a high concentration

13 of negative excursion values corresponds with a pronounced negative CIE in the δ Cwood

13 13 record. By analyzing the δ Cwood and δ Cbulk sed together, we also found evidence for at least one additional potential CIE of lesser magnitude and duration higher in the section. The more pronounced CIE is interpreted as a potential record of the Paleocene-Eocene Thermal Maximum (PETM), and the subsequent CIE may record a post-PETM hyperthermal event.

13 13 Comparisons of the δ Cwood and δ Cbulk sed records reveal that in addition to intra-sample

13 variability, the δ Cbulk sed record is potentially of reduced magnitude and the potential

13 PETM interval may be truncated compared to the δ Cwood record, adding to an increasing body of evidence that caution should be exercised when interpreting δ13C records to address global carbon cycle dynamics. Comparisons with our analyses of the stratigraphic record reveal that the potential PETM CIE corresponds with the progradation of a large fluvial fan in the lower Cuba Mesa Member of the San Jose Formation that is characterized as an anomalous, large, and laterally and vertically amalgamated sandstone channel. The prograding fan depositional environment and abundance of upper flow regime (UFR) and high deposition rate (HDR) sedimentary structures suggests that increased frequency and magnitude of high intensity precipitation events may have accompanied the PETM in the San Juan Basin region. This corresponds with suggestions that enhanced extremes in the hydrological cycle resulting in increased pre-

165 cipitation peakedness in the subtropics and mid-latitudes occurred in response to the Eocene hyperthermal events. The lower Cuba Mesa Member channel complex could also record sed- iment flushing as a response to the onset of the PETM and/or a response to the shift in the ambient climate of the region from a monsoonal precipitation regime in the Paleocene to variably sub-humid and arid in the early-Eocene. The reduced amalgamation, interpreted as medial fluvial fan facies, observed in the middle and upper Cuba Mesa Member and co- inciding with peak negative hypothesized PETM CIE values may suggest that the fluvial system continued to experience high discharge, but solid sediment yield may have decreased once weathering and pedogenic equilibrium were reestablished under drier conditions. The

13 PETM CIE culminates in an abrupt positive shift in δ Cwood values that coincides with an abrupt shift to distal fan facies that may indicate that fluvial fan activity ceased for a period of time during of after the PETM recovery interval, and suggests that there is a depositional hiatus at the contact between the Cuba Mesa and Regina Members of the San Jose Forma- tion. The reestablishment of deposition to the southern San Juan Basin, recorded by the Regina Member, suggests that the fan system was later reactivated. The hypothesis that the fan system ceased activity and was reactivated is supported by another package of upward increasing channel amalgamation in the Regina Member, which we have interpreted as a pos- sible second episode of fan progradation. Indications of an additional negative CIE near the top of the Regina Member outcrop suggests that a post-PETM hyperthermal event may be linked to the abrupt basinward shift of proximal fan facies near the top of this second episode of fan progradation. In addition, an overall trend towards increasing aridity observed in the floodplain deposits across the study interval coincides with a global long-term warming trend from the Paleocene to the middle Early Eocene upon which transient hyperthermals were superimposed. Overall, these findings suggest that the Paleocene and early-Eocene deposits in the study interval show evidence of an overall long-term decrease in mean annual precip- itation and fluctuations in seasonal extremes of precipitation. We show that fluctuations in seasonal extremes of precipitation appear to be linked to hyperthermal climate perturbations

166 characterized by negative CIEs, including the Paleocene-Eocene Thermal Maximum. Anomalous sedimentation events appear to characterize the PETM interval in other ter- restrial basins, suggesting that an increase in the magnitude or variability of river discharge is associated with the PETM, at least in the middle-latitudes. This increase in variabil- ity is thought to be related to stronger seasonality and increase in sediment supply due to intensification of physical weathering, and a decrease in the density of vegetation cover in response to the extreme warming event. Meanwhile, inter-basin differences between flood- plain deposits that suggest varying levels of soil drainage and thus wet vs. dry conditions, particularly outside the PETM interval, may suggest that ambient MAP was more variable between locations.

167 Figure 4.1: Paleogene geology of the San Juan Basin (SJB) and study area. A) Location of the San Juan Basin within the contiguous United States. Base map modified from ESRI, HERE, Garmin, OpenStreetMap, and the GIS user community. B) Regional map of the San Juan Basin (SJB) and adjacent basins and uplifts (SJB = San Juan Basin, G = Galisteo Basin, B = Baca Basin, MV = Monte Vista Basin, RA = Raton Basin, HP = Huerfano Park Basin, CL-J = Carthage-LaJoya). Redrawn from Dickinson et al., 1988; Lawton, 2008; and Cather, 2004. C) Geological map of the SJB. Map modified from Williamson et al., 2008 and Manley et al., 1987. D) Satellite imagery of the Arroyo Chijuilla study area and outcrops. Red letters correspond with outcrop photos in Figure 4. Base imagery from Google Earth. E) Paleogene stratigraphy and time constraints, modified from Smith and Lucas, 1991.

168 Figure 4.1: Continued.

169 Figure 4.2: Coalified-charcoalified fossil wood localities. A) Partially coalified-charcoalified fossil branch or root embedded in a sandstone channel. B) Fragment of coalified-charcoalified wood embedded in channel fill. C) Drapes of coalified-charcoalified plant fragments imbedded in channel fill.

170 Figure 4.3: Examples of outcrops interpreted as proximal, medial, and distal fan facies in the San Jose Formation of the SJB. A) Amalgamated channels and minor floodplain, interpreted as proximal fan facies. B) Channels and floodplain, interpreted as medial fan facies. C) Isolated channels and dominant floodplain, interpreted as distal fan facies. D) Satellite image obtained from Google Earth of a 60 km long Megafan in the Taklamakan Desert, Xinjiang, China with approximate locations of proximal, medial and distal fan annotated.

171 Figure 4.4: Stratigraphic trends the Arroyo Chijuilla study area. A) Medial fan facies in the uppermost Nacimiento Formation, capped by proximal fan facies in the lower Cuba Mesa channel complex. B) Medial fan facies in the Middle Cuba Mesa channel complex. C) Medial fan facies of the upper Cuba Mesa channel and overlying abrupt shift to distal fan facies in the Regina Member. Location of outcrops is shown in red letters on Figure 1e and match the subfigure labels.

172 Figure 4.5: Fossil wood and bulk sediment organic carbon isotope records from Arroyo 13 Chijuilla. A) δ Cwood values and interpreted potential carbon isotope excursions (CIEs). Samples that contained anomalously low total organic carbon (TOC) are denoted by triangle 13 markers and are not included in the calculation of the average δ Cwood curve (blue line) B) 13 δ Cbulk sed values and interpreted potential carbon isotope excursions (CIEs). Approximate baseline value is marked with the black vertical lines.

173 Figure 4.6: Fossil wood organic carbon isotope record and stratigraphic trends in Arroyo Chijuilla. Red arrows show packages of progradation-retrogradation of fan system. Outcrop photos corresponding to intervals of fan progradation-retrogradation shown in Figure 4 and indicated by the red letters.

174 Table 4.1: Compilation of coalified-charcoalified fossil wood carbon isotope ratios.

175 Table 4.1: Continued.

176 Table 4.2: Compilation of bulk sediment carbon isotope ratios.

177 CHAPTER 5 CONCLUSION

In this study, we combined basin-scale 3D outcrop analyses of fluvial architecture and preliminary stable carbon isotope records from the Paleocene upper Nacimiento Formation and the early-Eocene San Jose Formation to investigate the link between Paleogene climate perturbations and depositional trends. Through these investigations, we 1) identified the succession as deposits of variable discharge river systems, 2) observed a long-term strati- graphic trend toward increasingly well-drained floodplain deposits, and thus successively more arid conditions from Paleocene into the early-Eocene that we suggest may indicate a long-term shift from a monsoonal climate in the Paleocene to fluctuating humid and arid subtropical and then semi-arid/arid conditions in the early Eocene, 3) identified the Pale- ocene uppermost Nacimiento Formation and early-Eocene Cuba Mesa and Regina Members of the San Jose Formation as deposits of a large fluvial fan and identified at least two ver- tical packages of fan progradation, and 4) identified at least two negative carbon isotope excursions that may record the Paleocene-Eocene Thermal Maximum (PETM) and one or more post-PETM hyperthermal events. By combining the stratigraphic analyses with the carbon isotope records, we observed that the packages of fan progradation coincide with the negative carbon isotope excursions, suggesting that changes in water discharge patterns linked to climate were a key control in sedimentary delivery to the basin. These findings have implications for paleoclimate interpretations, variable discharge river and fluvial fan facies models, and improved understanding of San Juan Basin landscape evolution. Fur- thermore, the recognition of negative carbon isotope excursions suggests that the San Juan Basin may provide a new dataset for studying climatic and biotic changes associated with the Paleocene-Eocene Thermal Maximum and post-PETM hyperthermal climate perturbations.

178 5.1 Future Work

A detailed paleopedological study of floodplain deposits within the study interval is nec- essary to validate the hypothesis that there was an overall trend toward increased aridity into the early-Eocene. In particular, a more detailed stratigraphic study that records sed- imentary structures and icnology, in addition to soil color and mottling could provide a high-resolution, qualitative record of changes in soil moisture. In addition, quantitative es- timates of precipitation can be derived using the CALMAG method, which uses a paleosol weathering index for estimating mean annual precipitation. The coalified-charcoalified fossil wood sample density should be increased to improve the

13 resolution of the δ Cwood record. These efforts should focus on the key intervals immedi- ately above and below the Nacimiento-San Jose contact and in the Regina Member where additional CIEs indicative of post-PETM hyperthermal events may be recorded. Increased resolution in the uppermost Nacimiento and immediately above the contact with the Cuba Mesa Member could resolve the stratigraphic position of the PETM onset. Due to the uncertainty surrounding the reliability of the δ13Cbulk dataset and the lim- ited use of coalified-charcoalified fossil wood, attempts should be made to locate additional sources of organic carbon for δ13C analysis. Leaf wax n-alkanes are thought to have fewer diagenetic or source effects on their isotopic composition than δ13Cbulk and could provide an

13 opportunity to validate δ Cwood record in the San Juan Basin. It is also possible that a de- tailed paleopedological study could result in the discovery of additional horizons of pedogenic carbonate nodules. Carbon isotope ratios should also be measured from samples collected in additional sections across the basin to determine if the results that indicate CIEs can be replicated. Paleontological surveys to locate plant and vertebrate samples and palynological analysis of bulk sediment samples in the hypothesized PETM intervals may provide proxy data for biotic change across the P-E boundary and in response to hyperthermal events in the San Juan Basin and across a longer latitudinal transect than has been previously available. In

179 addition, such studies may provide additional age-constraints on the Cuba Mesa Member and the location of the P-E boundary. Finally, a key goal of ongoing research should be developing a better understanding of the stratigraphic location and nature of the Nacimiento-San Jose Formation contact, which has implications for improving our understanding of San Juan Basin landscape evolution and fluvial fan progradation styles.

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