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Depositional Environments and Provenance of Early Paleogene Strata in the Huerfano Basin: Implications for Uplift of the Wet Mountains, Colorado, USA

Dirk M. Rasmussen Western Washington University, [email protected]

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Recommended Citation Rasmussen, Dirk M., "Depositional Environments and Provenance of Early Paleogene Strata in the Huerfano Basin: Implications for Uplift of the Wet Mountains, Colorado, USA" (2016). WWU Graduate School Collection. 536. https://cedar.wwu.edu/wwuet/536

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DEPOSITIONAL ENVIRONMENTS AND PROVENANCE OF EARLY PALEOGENE STRATA IN THE HUERFANO BASIN: IMPLICATIONS FOR UPLIFT OF THE WET MOUNTAINS, COLORADO, USA

By Dirk M. Rasmussen

Accepted in Partial Completion of the Requirements for the Degree Master of Science

Kathleen L. Kitto, Dean of the Graduate School

ADVISORY COMMITTEE

Chair, Dr. Brady Foreman

Dr. Bernard Housen

Dr. Katie Snell

MASTER’S THESIS

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Dirk M. Rasmussen

27 May 2016

DEPOSITIONAL ENVIRONMENTS AND PROVENANCE OF EARLY PALEOGENE STRATA IN THE HUERFANO BASIN: IMPLICATIONS FOR UPLIFT OF THE WET MOUNTAINS, COLORADO, USA

A Thesis Presented to The Faculty of Western Washington University

In Partial Fulfillment of the Requirements for the Degree Master of Science

By Dirk M. Rasmussen May 2016

ABSTRACT

Sedimentary basins throughout the Western Interior of North America preserve a record of Late Cretaceous through latest Eocene sedimentation derived from flanking Laramide uplifts. In northern and western basins, the strata contain a well-documented proxy history of Laramide- style exhumation and climatic conditions within the region. However, the tectonic and climatic histories of more southerly basins, such as the Huerfano Basin studied herein, are comparatively underdeveloped despite being key in understanding the spatiotemporal evolution of Laramide tectonism and regional climatic gradients. This study addresses this issue and presents the first detailed lithofacies analysis and provenance analysis of the Poison Canyon, Cuchara, and Huerfano formations in the Huerfano Basin (south-central, Colorado, U.S.A.). We interpret a suite of alluvial sub-environments within these formations whose upsection variability and broad depositional patterns are similar to those observed in other early Paleogene successions in Laramide basins. Specifically, these features include deposition of organic-rich strata during the Paleocene; Eocene strata dominated by red-bed formation; and an anomalously coarse-grained intervening fluvial unit. The major difference in the Huerfano Basin compared to other Laramide basins is the exceptionally coarse-grained nature of all the units, which is likely related to its proximal position to Laramide ranges. Overall, the patterns are consistent with the widespread climatic shift between the wet, cooler Paleocene, and the drier, warmer, potentially more seasonal Eocene climate. As part of our provenance analysis, we characterized petrographic compositions of sand-bodies (N = 31 thin sections) and U–Pb detrital zircon age spectra (N = 848 age determinations) from fluvial sandstones within each of the three formations. The results indicate a new unroofing and source history for the sediment within the basin that contradicts previous hypotheses. Diagnostic zircon peaks at 516-517 Ma, 1423-1430 Ma, and 1678-1687 Ma show that sediment delivered to the Huerfano Basin did not originate in the San Luis Highlands or incipient Sangre de Cristo Mountains, but that the Precambrian crystalline core and associated Cambrian plutons of the Wet Mountains were exposed by the time Laramide deposition initiated in the basin. There are no provenance shifts upsection, indicating a largely stable or lithologically uniform sediment source from the Paleocene through at least ~51 Ma. This suggests the major changes in deposition are more likely related to tectonic and climatic conditions rather than lithologic controls on stratigraphic patterns.

iv ACKNOWLEDGEMENTS

This manuscript benefited from discussions with B. Housen, K. Snell, C. Siddoway, and

H. Fricke. We thank M. Pecha and M. Ibanez-Mejia (MIT) for assistance at the University of

Arizona LaserChron Facility. Thank you to B. Paulson for technical assistance at WWU while conducting lab work. WWU graduate students R. Morris, L. Heywood, and O. Anderson are thanked for their assistance improving petrographic results in this study. We thank WWU undergraduate students C. Curd, S. Holt, L. Gipson, and D. Burns for their assistance in the lab.

The WWU Research-Writing Studio and L. Weiso are recognized for their aid in improving this manuscript. Funding was generously provided from the RMAG Foster Memorial Scholarship,

AAPG Grants-in-Aid, GSA Graduate Research Grant, Colorado Scientific Society Grant, and internal research grants from Western Washington University. Thank you to Wolf Springs Ranch

(Tom Redmond, Paulette Shapland, and ranch staff) for generously granting land access to conduct this research. Thank you B. Donovan for assistance completing fieldwork and continuous encouragement.

v TABLE OF CONTENTS ABSTRACT ...... iv ACKNOWLEDGEMENTS ...... v LIST OF FIGURES AND TABLES ...... vii INTRODUCTION ...... 1 GEOLOGIC SETTING ...... 3 Hinterland Geologic Setting ...... 4 Early Paleogene Deposition ...... 5 METHODS ...... 9 Lithofacies Analysis ...... 9 Cobble Censuses and Sandstone Petrography ...... 10 Detrital Zircon Analysis ...... 11 RESULTS ...... 12 Lithofacies Associations ...... 12 Poison Canyon Formation Coarse-grained Lithofacies ...... 13 Interpretations ...... 14 Poison Canyon Formation Fine-grained Lithofacies ...... 16 Interpretations ...... 17 Cuchara Formation Coarse-grained Lithofacies ...... 18 Interpretations ...... 19 Cuchara Formation Fine-grained Lithofacies ...... 19 Interpretations ...... 20 Huerfano Formation Summary ...... 21 Petrographic Classifications ...... 21 U–Pb Detrital Zircon Age Spectra ...... 23 DISCUSSION ...... 25 Depositional Comparison with other Laramide Basins ...... 25 Provenance Assessment ...... 29 Tectonic Implications ...... 34 CONCLUSIONS ...... 39 REFERENCES CITED ...... 40 FIGURES ...... 51 TABLES ...... 63 APPENDICES ...... 64

vi LIST OF FIGURES AND TABLES

Figure 1. Geologic overview map of Laramide basins within the Western Interior Figure 2. Regional geologic map of the Huerfano Basin study area Figure 3. Poison Canyon stratigraphic section and paleocurrent rose diagrams Figure 4. Images of Poison Canyon Formation lithofacies Figure 5. Images of Cuchara Formation lithofacies Figure 6. Petrofacies classification of early Paleogene sandstones in the Huerfano Basin Figure 7. U–Pb detrital zircon age spectra of early Paleogene strata in the Huerfano Basin

Table 1. Modal compositions of early Paleogene sandstones in the Huerfano Basin Table 2. Summary of cobble lithology for early Paleogene formations in the Huerfano Basin Table 3. Comparison of similarity and overlap statistic for detrital zircon samples

vii INTRODUCTION

The Laramide Orogeny marks a dramatic shift in the tectonic history of western North

America (Tweto 1975; Dickinson et al. 1988; Dickinson 2004; Lawton 2008). During the Latest

Cretaceous through Eocene, the Sevier foreland basin was partitioned into a series of sedimentary basins separated by intervening uplifts (Fig. 1; Dickinson et al. 1988). Areas once inundated by the Cretaceous Interior Seaway were drained and basement-involved reverse thrusts, which commonly reactivated inherited faults, created local highlands that shed sediment into adjacent flexural basins (Dickinson et al. 1988; Jones et al. 2011). Thus, many of the details of the transition from Sevier to Laramide tectonism are recorded in the character of sedimentation within early Paleogene alluvial strata within these basins. For example, previous studies have used various sedimentologic features to (1) constrain the timing of uplifts (DeCelles et al. 1991), (2) identify regional upwarping and tilting of the landscape (Heller et al. 2013), (3) reconstruct shifts in regional catchment structure (Chetel et al. 2011; Dickinson et al. 2012), (4) link basin-scale shifts in paleosol character to grain size provided to the basin (Kraus and

Riggins 2007), and (5) suggest that the onset of lacustrine deposition is linked to exposure of

Precambrian crystalline material and sediment starvation (Carroll et al. 2006). Furthermore, these types of studies allow the testing of tectonic models of the Sevier to Laramide transition and provide the boundary conditions upon which paleoclimatic reconstructions of the time period are based and assessed (Shellito et al. 2003; Sewall and Sloan 2006; Winguth et al. 2010; Heller et al. 2013).

Currently, the most detailed studies are from more northerly and westerly Laramide basins. Specifically, these include the Bighorn, Powder River, Wind River, and Green River basins of Wyoming as well as the Uinta, Kaiparowits, and Piceance basins of Utah and western Colorado (Fig. 1; Keefer 1969; Phillips 1983; Dickinson et al. 1986; Shuster 1987; Dickinson et al. 1988; Roehler 1993; Hoy and Ridgway 1997; Kraus and Gwinn 1997; Clyde and Gingerich

1998; Wing et al. 2005; Lawton 2008; Dickinson et al. 2012; Foreman et al. 2012; Kraus et al.

2015). More southern and eastern Laramide basins are comparatively understudied, despite being equally important for understanding the regional tectonic and climate history. Herein, we focus on the Huerfano Basin of southern Colorado (Figs. 1, 2), and provide the most detailed lithofacies and provenance analysis to date. Studying the Huerfano Basin provides better understanding of tectonic and climatic influences on generation of the stratigraphic record across the Western Interior by enabling systematic comparisons to be made to coeval strata in northern and western basins and constraining sediment provenance.

The results of this study help to constrain interpretations of Laramide tectonics and provenance within the southern Rocky Mountains. Previously it was hypothesized that exhumation of the Wet Mountains began during the early middle or early late Eocene (Robinson

1966; Scott and Taylor 1975). Additionally, it has been thought that most sediment delivered to the Huerfano Basin during the early Paleogene derived from sources to the west eroding

Paleozoic and Mesozoic sedimentary strata (Burbank and Goddard 1937; Briggs and Goddard

1956; Johnson and Wood 1956; Johnson 1959; Baltz 1965; Briggs 1965; Tweto 1975, 1980;

Flores and Pillmore 1987). Herein, we present detailed descriptions of the depositional environments and their upsection variation in the Huerfano Basin and identify broad patterns in sedimentation similar to those observed in other Laramide basins. Our new provenance analysis examines gravel clast composition, sandstone composition, and U–Pb detrital zircon geochronology. These results reveal a new unroofing and source history for the sediment within the basin that contrasts with previous hypotheses. Specifically, it appears the crystalline

2 basement was exposed in the Wet Mountains by the time Laramide deposition initiated in the basin. The findings presented herein constrain allogenic mechanisms that have been previously invoked to explain lithological changes in early Paleogene stratigraphy within the Huerfano

Basin.

GEOLOGIC SETTING

The Huerfano Basin is a northwestern extension of the Raton Basin encompassing ~800 km2 between the Sangre de Cristo Mountains and Wet Mountains in south-central Colorado

(Figs. 1, 2). The basin is an asymmetrically downwarped depocenter, thickest in the western side of the basin, and was formed during the Laramide Orogeny (Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson 1959; Dickinson et al. 1988). Laramide shortening caused eastward thrusting in the emerging Sangre de Cristo Mountains and faulting within the Wet Mountains, as evidenced by Phanerozoic strata faulted and folded against Precambrian basement rocks along the basin margins (Fig. 2; Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson 1959;

Lindsey et al. 1983). Establishing the context of these basement rocks, as they exist within the broader regional framework of crustal provinces underpinning the region, is a prerequisite for studies of provenance to proceed. Both the Sangre de Cristo Mountains and Wet Mountains are located within the southern Yavapai Province, a transition zone broadly impacted by Yavapai- and Mazatzal-aged orogenesis (~1.6-1.8 Ga) during growth of southern Laurentia (Bickford et al.

1989; Jones and Connelly 2006; Lawton 2008; Dickinson et al. 2012; Laskowski et al. 2013). In order to understand the history of Paleogene deposition it is necessary to examine major aspects of hinterland geology in the adjacent mountain ranges.

3 Hinterland Geologic Setting

The Sangre de Cristo Mountains formed during the Laramide Orogeny, as well as later during the Cenozoic (Fig. 2; Tweto 1975; Lindsey et al. 1986; Lindsey 1998). The modern mountains are thought to have evolved from the San Luis Highlands along reactivated faults related to the Paleozoic Ancestral Rocky Mountains (Tweto 1975, 1980; Weimer 1980; Lindsey et al. 1983; Hoy and Ridgway 2002). Although the exact paleogeographic nature of the San Luis

Highlands within the Ancestral Rocky Mountains is poorly constrained, the highlands are broadly considered to have been a southern extension of the Uncompahgre Uplift (Tweto 1975).

The degree to which the highlands persisted throughout the Mesozoic as a precursor of the

Sangre de Cristo Mountains remains enigmatic. Eastern foothills of the Sangre de Cristo

Mountains define the western margin of the Huerfano Basin forming a complex of imbricate thrust sheets that place upper Paleozoic rocks against basement rocks of the Sangre de Cristo

Mountains. Proterozoic rocks forming the core of the Sangre de Cristo Mountains comprise a compositional suite of felsic to mafic metavolcanic rocks, interlayered metasedimentary rocks, and quartzite (Fig. 2; Jones and Connelly 2006). There is a general southward gradation across the range from felsic gneiss and schist to mafic gneiss and amphibolite (Jones and Connelly

2006). Granitic to mafic Paleoproterozoic plutons and granitic Mesoproterozoic plutons are hosted by Paleoproterozoic metamorphic basement rocks (Fig. 2; Jones and Connelly 2006).

Upper Paleozoic sedimentary rocks extend westward from the edge of the basin to the range crest, and form a partial covering of fine-grained sedimentary rock on the eastern slope of the range (Briggs and Goddard 1956; Johnson 1959; Tweto 1975).

The Wet Mountains are a southern extension of the Front Range and are an inherited feature of the Ancestral Rocky Mountains that became reactivated during the Laramide Orogeny

4 (Boyer 1962; Tweto 1975; Scott and Taylor 1975; Tweto 1980; Weimer 1980; Chapin et al.

2014). A breadth of structural interpretations exists for the Wet Mountains but the exact structural mechanisms by which the range was uplifted remain enigmatic (see discussion for details; Scott and Taylor 1975; Weimer 1980; Jacob 1983; Noblett et al. 1987). This elevated structural block constitutes ~100 km by 30 km of near continuous Proterozoic basement rock

(Thomas et al. 1984; Bickford et al. 1989; Jones et al. 2010). Metamorphic grade generally increases southward across the range indicating differential exhumation of the range (Cullers et al. 1992; Jones et al. 2010). The northern Wet Mountains are predominantly metavolcanic and metasedimentary successions of quartzose and quartzofeldspathic gneisses (Jones et al. 2010).

Assemblages of schist, calc-silicate gneiss, mafic gneiss, and amphibolite are also present (Jones et al. 2010). The central and southern Wet Mountains are predominately interlayered quartzose and quartzofeldspathic gneiss, amphibolite gneiss, and metagabbro (Jones et al. 2010); exposures of schist, marble, and calc-silicate gneiss are rare (Jones et al. 2010). Numerous Paleo- and

Mesoproterozoic granitic to granodioritic plutons intrude metamorphic basement rocks in the

Wet Mountains (Thomas et al. 1984; Bickford et al. 1989). Emplacement depth of the San Isabel batholith, exposed in the southern Wet Mountains, is estimated at 17-23 km (Cullers et al. 1992).

Additionally, three Cambrian-aged alkaline complexes exist in the northern Wet Mountains

(Olson et al. 1977; Armbrustmacher 1984; Bickford et al. 1989; Schoene and Bowring 2006).

Early Paleogene Deposition

Erosion of these mountain ranges delivered sediment to early Paleogene alluvial systems.

Overall, the sedimentary strata exposed in the Huerfano Basin comprise the most complete stratigraphic record of Phanerozoic deposition exposed on the eastern side of the southern Rocky

5 Mountains (Tweto 1980). Similar to other Laramide basins, a widespread unconformity separates

Late Cretaceous marine strata from early Paleogene alluvial strata, and records the onset of

Laramide tectonism (Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson 1959;

Dickinson et al. 1988).

Early Paleogene alluvial strata in the Huerfano Basin are composed of the Poison

Canyon, Cuchara, and Huerfano formations (Burbank and Goddard 1937; Briggs and Goddard

1956; Johnson 1959). The Poison Canyon Formation is characterized by yellowish lenticular beds of cross-bedded, medium-grained arkose and conglomerate. Fine-grained deposits consist of drab organic-rich, micaceous mudrock and siltstone. Outcrops occur along the western margin of the Wet Mountains, wrap around the eastern margin of the Sangre de Cristo Mountains, and extend southward into the Raton Basin. Thickness ranges from a thin layer along the basin margins to ~600 m along the basin axis (Johnson 1959). Deposits of Poison Canyon Formation are thickest in the Huerfano Basin, thin southward, and grade into the Raton Formation (Johnson and Wood 1956; Tyler et al. 1995; Cather 2004). The Poison Canyon Formation is broadly constrained as Paleocene in age based on tropical floras that are similar to those found in the Fort

Union, Denver, and Dawson formations, which are exposed throughout Wyoming and Colorado.

These floras occur ~25 m below the Poison Canyon–Cuchara contact (Burbank and Goddard

1937; Briggs and Goddard 1956).

In the northern Huerfano Basin, the Cuchara Formation is distinctly pink-colored in outcrop and considerably more coarse-grained compared to the Poison Canyon Formation

(Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson 1959). Outcrops of the

Cuchara Formation contain sand-bodies of coarse arkose and conglomerate (Johnson 1959).

Fine-grained deposits in the Cuchara Formation are scarce, and exhibit a wider color range than

6 the fine-grained deposits in the Poison Canyon Formation. Outcrops form a similar geographic distribution to the Poison Canyon Formation but are poorly exposed on the eastern basin margin

(Johnson 1959). Thickness ranges from a thin layer along the basin margins to ~425 m along the basin axis (Johnson 1959). The Cuchara Formation is thickest in the northern Huerfano Basin, and thins southward (Johnson and Wood 1956; Tyler et al. 1995; Cather 2004). This northern coarse-grained facies becomes less gravel rich to the south and more distinctly red in the

Cuchara type section in the Cuchara River Valley near La Veta, CO (Fig. 2; Robinson 1966).

Some authors have considered the Cuchara Formation a time equivalent lateral facies of the

Huerfano Formation (Briggs and Goddard 1956; Robinson 1966). However, at the Poison

Canyon type section, where the most continuous outcrop of early Paleogene section is exposed, the Huerfano Formation distinctly overlies the Cuchara Formation. A single Lambdotherium popoagicum fossil (Wasatchian-7) indicates deposition of the Cuchara Formation at ~53 Ma, assuming a similar age for the biozone as exists in the Bighorn Basin of Wyoming (Robinson

1966; Wing et al. 1991; Clyde et al. 1994). In some areas a thick, heavily bioturbated, and mottled sandstone unit marks the upper boundary of the Cuchara Formation. Elsewhere the boundary appears conformable with the overlying Huerfano Formation. Overall, the Cuchara

Formation represents an unusually coarse-grained and thick fluvial succession of strata within the Paleogene deposits of the Huerfano Basin.

The Huerfano Formation crops out within most of the central Huerfano Basin. Deposits are predominately pink, red, and maroon silty mudrocks that weather characteristically to badlands-type topography (Johnson 1959; Robinson 1966). Sand-bodies tend to be isolated channel lenses. The prevalence of sandstones generally increases southward into the Raton Basin

(Johnson 1959; Robinson 1966). Thickness of the formation ranges from ~850 m in the

7 Huerfano Basin to ~1500 m in the Spanish Peaks region (Johnson 1959; Robinson 1966). Well- sampled land mammal fossil localities indicate Bridgerian-1a (Gardnerbuttean) deposition of the

Huerfano Formation, which places its age as 51-53 Ma (Robinson 1966; Morlo and Gunnell

2003; Gunnell et al. 2009).

The Poison Canyon, Cuchara, and Huerfano formations were structurally deformed by

Laramide orogenesis and have been previously interpreted, to varying degrees, as synorogenic pulses of sedimentation into the Huerfano Basin (Burbank and Goddard 1937; Briggs and

Goddard 1956; Johnson and Wood 1956; Robinson 1966; Flores and Pillmore 1987; Cather

2004). The diversity of metamorphic and plutonic clasts, stratigraphic distribution of conglomerate facies, unconformities within early Paleogene strata, and paleocurrent directions led early researchers to interpret the major sediment source as the adjacent San Luis Highlands to the north and west (Burbank and Goddard 1937; Briggs and Goddard 1956; Berner and Briggs

1958; Johnson and Wood 1956; Johnson 1959; Briggs 1965). Although several authors have acknowledged the Wet Mountains to the east as a possible secondary sediment source, the default interpretation is that the San Luis Highlands delivered most of the sediment contributing to early Paleogene deposition. Moreover, most interpretations invoke various iterations of unroofing of Phanerozoic strata in order to expose the Precambrian basement (Burbank and

Goddard 1937; Briggs and Goddard 1956; Johnson and Wood 1956; Johnson 1959; Baltz 1965;

Briggs 1965; Tweto 1975, 1980; Flores and Pillmore 1987).

8 METHODS

Lithofacies Analysis

In this study, we present a variety of sedimentologic measurements targeting three areas spanning over ~100 km2 of the Huerfano Basin (Fig. 2). In the field, these measurements included standard lithofacies descriptions of grain size, bedding contacts, sedimentary structures, lithologic unit geometries, paleocurrents, and flow depth estimates from clinoform structures.

Concerted effort focused on the type locality for the Poison Canyon Formation located in

Huerfano County, CO (currently owned by Wolf Springs Ranch). This is the only area with near complete surface outcrop of the Poison Canyon, Cuchara, and Huerfano formations. Based on existing the biostratigraphy, we believe this section has the greatest potential to preserve the

Paleocene–Eocene transition (Robinson, 1966). We trenched and measured a stratigraphic section at this locality from the base of the Poison Canyon Formation, upsection through the entire Poison Canyon and Cuchara formations, ending in the lower Huerfano Formation (Figs. 2,

3; 37.792664° N, 105.275284° W – 37.796772° N, 105.264389° W).

Standard stratigraphic measurements were made using a Jacob’s staff and geologic compass. Both lateral and upsection variability was assessed with lithofacies descriptions and by characterizing grain size variability. Sand-body geometries and stacking patterns were both walked out and measured using a TruPulseTM 360B laser range finder. The laser range finder has uncertainty of ± 0.2 m. To minimize uncertainty, we averaged five iterations of each measurement and corrected for local outcrop orientations.

Paleocurrent measurements were made in the Poison Canyon Formation (n = 35),

Cuchara Formation (n = 35), and Huerfano formation (n = 4). We measured paleocurrent directions primarily from axes of large-scale trough cross-beds determined from the strike and

9 dip of opposing trough limbs (DeCelles et al. 1983). Measurements were calibrated for local declination and stereographically corrected where dip exceeded 20˚ (DeCelles et al. 1983).

Additional paleocurrent directions were measured from small-scale trough cross-bedding, groove casts, and conglomerate imbrication. Finally, we averaged paleocurrent measurements by formation into vector means.

Cobble Censuses and Sandstone Petrography

Assessments of provenance followed several standard methodologies. Several hundred cobbles (100-300 per sample location) were randomly collected from conglomeratic lags at several stratigraphic levels (N = 6) within each formation. We washed, identified, and counted cobbles. Counts were organized into several broad categories (alkali-feldspar granite, granite, total sedimentary, total volcanic, and gneiss) for comparison (Lindsey et al. 2007).

Samples of medium-grained sandstone from fluvial sand-bodies were collected from the

Poison Canyon (n = 18), Cuchara (n = 9), and Huerfano (n = 4) formations spanning ~500 m of stratigraphy over an ~30 km transect in the Huerfano Basin (Fig. 2). From these samples we constructed a suite of covered thin sections for analysis by transmitted light microscopy. Using a petrographic microscope with a rotating point counting stage and a digital lab counter, samples were point-counted using Gazzi-Dickinson methodology (Dickinson 1970; Dickinson and

Suczek 1979; Dickinson et al. 1983; Ingersoll et al. 1984). Orthogonal grid spacing was determined by the maximum grain size of a sample to avoid recounting detrital grains. A minimum of 400 sand-size detrital grains (φ -1-3.5) per sample were identified and counted whenever possible. Points that intersected pores, cement, or matrix were not counted. These

10 counts were tabulated into three categories (Quartz, Feldspar, and Lithics) and plotted as poles on ternary classification diagrams according to Folk (1980) and Dickinson (1985).

Detrital Zircon Analysis

U–Pb detrital zircon ages are derived from samples collected in the Huerfano Basin at the bases of medium-grained sand-bodies in the Poison Canyon stratigraphic section, one sample from each formation (Figs. 2, 3A). We prepared and analyzed the samples in this study according to the protocols at the Arizona Laserchron Center (http://www.laserchron.org; see details in

Gehrels et al. (2008) and Gehrels and Pecha (2014)). We disaggregated samples using a rock crusher and disc mills that were carefully adjusted so as not to fracture individual mineral grains.

Separation proceeded using a standard water table, Frantz magnetic separator, and heavy liquids.

Zircon grains were separated, and then mounted together with samples of the Sri Lanka standard.

Prior to isotopic analysis, sample mounts were sanded to expose the interior of zircon grains, polished, and cleaned. U–Pb ages were determined from randomly selected zircons using a laser ablation multi-collector inductively coupled plasma mass spectrometer (Nu Plasma MC-ICP-

MS) with Photon Machines Analyte G2 laser-ablation system (Gehrels et al. 2008; Gehrels and

Pecha 2014; Pullen et al. 2014). A 30 µm spot size was used and standards were analyzed after every fifth analysis. Ages were determined by randomly selecting ~300 separated zircons from each sandstone sample, and analyzing the selected zircons at random positions within each grain.

Data was reduced using the "AgeCalc" program (details in Gehrels and Pecha (2014)) that converts measured intensities (counts per second) into calculated ages, uncertainties, discordance, and associated information. Analytical error of the MC-ICP-MS is 1-2% (1 sigma) for U–Pb ages. Samples with > 10% discordance were omitted from this dataset. There is

11 ongoing discussion surrounding the utility of probability density functions (PDF) and kernel density estimates (KDE) for detrital zircon data (Vermeesch 2012; Pullen et al. 2014). For completeness in analyzing our detrital zircon datasets, we utilized a traditional histogram, PDF, and KDE, similar to Pullen et al. (2014). The KDE is an adaptive two-stage algorithm that enhances density estimate resolution where sufficient data exists, while smoothing zones with less data (Vermeesch 2012; Vermeesch et al. 2016). The adaptive KDE algorithm involves summing a set of Gaussian distributions and does not explicitly incorporate analytical uncertainties, as does a PDF (Vermeesch 2012; Pullen et al. 2014). This presents issues when dealing with smaller datasets and with younger ages with small age uncertainties, and tends to over-smooth age peaks. However, as the number of total of age determinations increases (i.e., n

>> 100) the two approaches to presenting age distributions converge (Vermeesch 2012; Pullen et al. 2014). In this study, we present larger n-datasets than many previous studies of Laramide strata, and the age distributions do not contain young ages (i.e., Mesozoic or Cenozoic).

RESULTS

Lithofacies Associations

The stratigraphy of the Poison Canyon, Cuchara, and Huerfano formations preserves a suite of alluvial lithofacies indicative of several overbank and fluvial sub-environments (Fig.

3A). Observations in this study are broadly consistent with previous interpretations of depositional environment of these formations as a coarse-grained meander belt or braided plain

(Johnson 1959; Flores and Pillmore 1987). However, detailed bed-by-bed sedimentary logs were not performed prior to this study, and fine-grained deposits were largely overlooked in these previous studies. The following data includes lithofacies descriptions and associations for both

12 coarse- and fine-grained outcrops of the Poison Canyon, Cuchara, and Huerfano formations in the Huerfano Basin (Fig. 2).

Poison Canyon Formation Coarse-grained Lithofacies. ⎯ Sand-bodies in the Poison

Canyon Formation type section represent ~50% of the formation and are composed of arkose and polymict conglomerate (Figs. 3A, 4). Sand-bodies range from dark yellowish orange (10YR 6/6) to very light grey (N8). Sand-body thickness ranges from ~1-10 m (Fig. 3A) and this facies weathers to distinct ridges in the modern topography. Exact sand-body width can be estimated with limited confidence due to restricted lateral exposure. Outcrops can be generally traced laterally tens to hundreds of meters between ridges but the exact geometry (sheet vs. ribbon) can be enigmatic. However, within the sand-bodies themselves there are clear lenticular units marked by concave-up scour surfaces.

Sand-bodies are often partitioned into 1-3 stories, that are each several meters thick, and are recognized by through-going scour surfaces and fining upward sequences. Story breaks are commonly marked by thin, clast-supported conglomerate lags and dense concentrations of mudrock rip-up clasts that form a basal scour surface (Fig. 4C). Well-preserved groove casts are common in these deposits, while cobble imbrication is uncommon. Lower contacts are erosive while upper contacts tend to be more gradational and form crude fining upwards sequences (Fig.

4C). The grain size distribution of sand-bodies ranges from lower-fine (φ 2.5) sand to gravel and is dominantly lower-medium (φ 1.5) to upper-coarse (φ 0) sand (Fig. 3A). Matrix supported gravel and heavy minerals lags are commonly concentrated along foresets of large-scale trough cross-beds, which is the dominant sedimentary structure (Fig. 4A). Foreset thickness ranges from

~0.15-0.5 m, and average ~0.3 m. Cross-bedding commonly preserves tangential toes of foresets.

Less common sedimentary structures include small-scale trough cross-bedding, ripple 13 laminations, climbing ripples, planar cross-bedding, and humpback-bedding. Occasionally, elongate sigmoidal structures with ~0.7 m average relief and several meters wide occur within sand-bodies (Fig. 4A). In some cases, cross-beds occur as repetitive sigmoidal structures (i.e., clinoforms) where cross-beds converge tangentially upon a low-angle planar surface. Sets can often be traced laterally, and trend obliquely to the paleocurrent direction. Paleocurrent directions inferred from sedimentary structures indicate southerly flow with a mean vector of

~175˚ ± 7˚ (1 standard error) (Fig. 3B).

Finer-grained sandstone facies represent ~20% of the Poison Canyon Formation and range from silty sandstone to lower-fine sand (φ 2.5) (Fig. 3A). Outcrops tend to be poorly indurated, weather recessively, and form beds that are < 1 m thick which grade into siltstone

(Figs. 4B, D). These deposits have sharp basal contacts and often occur near the top of sand- bodies and/or near story breaks. Deposits are commonly tabular, and preserve fine parallel laminations and ripple laminations. Occasionally, deposits preserve bioturbation similar to that observed in mudrocks.

Interpretations. ⎯ Broadly, the coarse-grained deposits are interpreted as being deposited by fluvial systems. Each story within the sand-bodies represents a channel fill sequence. From bottom to top, this includes conglomeratic facies deposited in the basal portions of the channel or thalweg. Abundant mudrock rip-up clasts and relief of the basal scours of the story breaks suggest that erosion and reworking of both the floodplain and former channel deposits were common. Moreover, story breaks indicate reoccupation by the active channel on the alluvial plain after periods of river migration or avulsion (Kraus and Middleton 1987; Mohrig et al. 2000).

14 Sandy and gravelly channel-fill deposits (Figs. 4A, B, C) represent bed load deposition within active threads of the main channel network. Large-scale trough cross-bedding with matrix-supported gravel along foresets are the diagnostic characteristics for this deposit type, and indicates the presence of sinuous crested dunes migrating through the channel (Fig. 4A). Upper portions of channel-fill deposits are associated with finer-grained fractions and sometimes preserve small-scale trough cross-bedding and ripple laminations. The upward transition to finer material and smaller scale sedimentary structures likely indicates shallowing flow conditions.

Clinoform structures likely represent bar deposition and lateral/oblique bar migration within active channel networks (Fig. 4A; Mohrig et al. 2000). Where clinoforms are present in this field area, they are often partially preserved, and bound by an erosive upper contact. If fully preserved, bar clinoforms represent an approximate measure of flow depth from the thalweg to the free air surface (Mohrig et al. 2000).

Levee and crevasse splay complexes in the Poison Canyon Formation are broadly consistent with well-documented overbank facies models (Miall 2013). In the Poison Canyon

Formation, deposits of this type are thinly interbedded layers of finer-grained sandstone and mudrock. Deposits form tabular beds that commonly fine upwards to silty sandstone and preserve weak parallel laminations and ripple laminations. This style of vertical accretion results from diminishing flow power as discharge expands onto the channel levee and beyond the confined channel, spreading out as a sheet flood (Miall 2013). Preservation of abundant detrital organic matter, root traces, and bioturbation in the Poison Canyon Formation are consistent with this facies model. Levee and crevasse splay complexes have been shown to gently dip and thin away from the channel sand-body (Bown and Kraus 1987). This feature is difficult to evaluate in this case given the recessive weathering of these deposits in the Poison Canyon Formation.

15 Poison Canyon Formation Fine-grained Lithofacies. ⎯ Mudrock represents ~10% of the formation (Fig. 3A). Deposits range in thickness from ~0.1-1.7 m, and average ~0.6 m thick with gradational upper and lower contacts (Figs. 3A, 4B). Hue ranges from 10R-10Y and are dominantly olive grey (5Y 5/2-5Y 3/2) and olive brown (5Y 5/4). Deposits tend to lack primary sedimentary structures, but in some cases are weakly fissile (Fig. 4D). Bioturbation is restricted to silty deposits and is typified by sub-vertical cylindrical burrows up to ~3 mm diameter and ~3 cm in length. Weakly to moderately developed clay cutans and local zones of clay accumulation are pervasive throughout mudrock deposits. Slickensides tend to be more intense in the upper portion of deposits and are otherwise weakly to moderately developed. Mottling is weak to moderate, and tends to be less intense within siltier deposits. Mottles have hues ranging from 5R-

5Y.

Siltstone deposits represents ~10% of the formation. Deposits range in thickness from

~0.1-4.2 m and average ~0.9 m thick (Figs. 3A, 4D). Lower bed contacts are consistently gradational, while upper contacts are both gradational and sharp (Fig. 4D). Hues range from

10R-10Y and are dominantly olive grey (5Y 6/1-5Y 3/2) and olive brown (5Y 5/4. These types of deposits tend to be finely laminated and occasionally massive. Preservation of macroscopic organic fragments is common throughout siltstone deposits. In some instances, local zones that are ~10-20 cm thick yield fossil leaf impressions that occur between thin laminations.

Bioturbation is similar to that described for mudrocks, but is more intense and frequently associated with greenish-grey coloration and more abundant zones of clay accumulation.

Siltstone lithofacies are distinct from mudrock lithofacies as they lack slickenside development and rarely preserve clay cutans. Mottling is uncommon, and occurs only near the top of units.

16 Interpretations. ⎯ Mudrock lithofacies (Figs. 3A, 4B, C) are interpreted as entisols and inceptisols that likely developed on the proximal flood plain and distal alluvial ridge.

Development of cutans, slickensides, and mottles are consistent with Stage 1 pedogenic modification (Bown and Kraus 1987, 1993). Recognition of distinct horizons is rare due to the incipient nature of these soils; however, in some cases there is development of weak B-horizons.

The variety in matrix and mottle coloration suggests differing degrees of soil water saturation related to soil permeability and/or depositional position on the floodplain. In addition to being a primary indicator of climate in some cases, previous studies have argued that grey and drab paleosols represent areas of lower relief and more poorly drained conditions (Kraus 1997; Kraus and Gwinn 1997; Kraus 1999). Yellow-brown and reddish coloration formed in areas with greater relief and/or more permeable alluvium (Kraus 1999). The absence of any clear upsection trends suggests that the distribution of facies is controlled by the alluvial architecture, and generated by depositional processes themselves (i.e., migration of the channel network).

Siltstone lithofacies (Fig. 3A, 4B, D) are interpreted as weakly developed entisols and inceptisols with only minor pedogenic modification of the parent alluvium. Distinguishable soil profiles are absent. Stronger red hues in both matrix and mottle components of these deposits suggest greater permeability and better drainage conditions than the mudrocks. Deposition of siltstone likely represents more proximal deposition to the alluvial ridge where relief is greater compared to more distal parts of the floodplain. Siltstones commonly overlie coarser-grained

(fine-grained sandstone (φ 2-2.5)) levee and crevasse splay deposits. The close association of levee and crevasse splay complexes with siltstone reflects deposition near the alluvial ridge that thins, becoming more mud-rich with more intense pedogenic modification towards the floodplain. Carbonaceous beds with excellent fossil leaf preservation are occasionally part of this 17 lithofacies. These beds possibly represent more distal overbank deposition during closely spaced flooding events and represents a glimpse of vegetation growing on the local floodplain (Davies-

Vollum and Wing 1998).

Cuchara Formation Coarse-grained Lithofacies. ⎯ Coarse-grained deposits in the

Cuchara Formation represent ~85% of the formation and crop out as massive cliff-forming sand- bodies of coarse arkose and polymictic conglomerate (Figs. 3A, 5). The Cuchara Formation ranges from moderate pink (5R 7/4) to very light grey (N8), and is distinctly pink from the

Poison Canyon Formation. Intense dark yellowish orange (10YR 6/6) liesegang banding overprints primary sedimentary structures at some localities. Cuchara sand-bodies are amalgamated and multistoried (Figs. 5A, C). Sand-bodies are both wider and more laterally extensive than the Poison Canyon Formation, and in some cases it is possible to trace Cuchara sand-bodies up to a kilometer. Sand-body thickness ranges from ~1-25 m and average ~3.25 m

(Fig. 3A). Laterally continuous zones with abundant rip-up clasts commonly mark story breaks

(Figs. 5B, C). Story breaks are also associated with lenticular conglomerate lags ~1-3 m wide and ~0.5 m thick. Lower contacts are erosive while upper contacts are diffuse and form fining upwards sequences.

Sand and gravel deposits in the Cuchara Formation are dominantly upper-coarse (φ 0) to gravel (Fig. 3A). Gravel is concentrated near the base of deposits and/or along cross-bed foresets and clinoform surfaces. Rip-up clasts tend to be larger and more densely concentrated than in the

Poison Canyon Formation. Rip-up clasts exceeding 1.5 m in diameter are not uncommon in these zones (Fig. 5B). Large-scale trough cross-bedding is the most abundant sedimentary structure.

Foreset thickness is commonly ~0.5 m. Small-scale trough cross-bedding occurs more commonly than in the Poison Canyon Formation and is associated with fining upwards 18 sequences. Sub-horizontal planar-bedding found higher in units also occurs more commonly than in Poison Canyon deposits. Clinoforms within sand-bodies display ~0.9 m of average relief and are several meters wide. Overall, clinoforms in the Cuchara Formation are more difficult to distinguish given the lesser degree of internal structure compared to the Poison Canyon

Formation. Paleocurrent directions inferred from sedimentary structures indicate southerly flow with a mean vector of ~180˚ ± 6˚ (1 standard error) (Fig. 3B).

Interpretations. ⎯ Coarse-grained facies of the Cuchara Formation resulted from bed load deposition within the active channel belt. The characteristic fining upward sequences, cross- bedding, and bar clinoforms indicate fluvial deposition. However, the thicker, more coarse- grained sand-bodies of the Cuchara Formation, as well as the greater lateral extensiveness and more frequent erosional surfaces compared to the Poison Canyon Formation, suggest a transition to braided conditions or mobile channel system resulting in sheet deposition (Gibling 2006). The coarser-grained fraction and distribution of sedimentary structures is consistent with this interpretation. The lack of thinly interbedded fine-grained sandstone, siltstone, and mudrock, suggest that levee and crevasse splay complexes are apparently absent from the Cuchara

Formation or, that channel movement removed this facies. Transport of coarser sediment, wider range of story thicknesses, and more laterally extensive sand-bodies potentially implies a steepened river gradient and/or greater peak discharge in the system compared to the Poison

Canyon Formation (Paola and Borgman 1991; Paola and Mohrig 1996). The Cuchara sand- bodies are distinct not only in color, but also in grain size, size, geometry, and internal structure.

Cuchara Formation Fine-grained Lithofacies. ⎯ Mudrock lithofacies in the Cuchara

Formation (Figs. 3A, 5D) are scarce, and represent ~5% of the formation (Fig. 3A). Recessively weathered and covered intervals may represent this type of fine-grained deposition, but even so, 19 fine-grained deposition is minor in the Cuchara Formation. Where present, mudrocks have red hues (10R) and are reddish brown (10R 5/4-10R 3/4). Beds are generally laminated, less than

~0.5 m thick, and comprise the uppermost component of silty deposits. Mudrock deposits lack pedogenic modification.

Siltstone lithofacies represent ~10% of the formation. Deposits range in thickness from

~0.1 to 2.85 m and average ~0.8 m thick. Lower bed contacts are generally diffuse, while upper contacts are both diffuse and sharp (Fig. 5D). Hues range from 5R-10Y and are dominantly olive grey (10Y 6/2-10Y 4/2) and reddish brown (10R 5/4-10R 3/4). These deposits tend to lack primary sedimentary structures. Macroscopic organic fragments are less common than in the

Poison Canyon deposits of this type. Clay-filled cylindrical burrows, similar to those described for the Poison Canyon Formation, are common. Siltstone deposits lack development of slickensides and cutans but exhibit moderate to intense mottling with strong red hues (10R).

Interpretations. ⎯ Fine-grained deposits in the Cuchara Formation are very minor compared to the Poison Canyon Formation. Mudrock and siltstone facies are interpreted as incipient entisol development on the alluvial ridge. Intense mottling with strong red hues indicates that these zones were probably well-drained. The lack of organic matter compared to

Poison Canyon deposits could indicate reduced floodplain productivity and vegetative cover, or oxidation within the coarser-grained sediment. Alternatively, rapid sedimentation rates may have prevented accumulation of organic-rich layers. The reduction of fine-grained facies combined with both the lack of pedogenesis and less observed organic matter broadly indicate more braided sedimentation or mobile fluvial channels during deposition of the Cuchara Formation.

Alternatively, this lithological shift could reflect a period of reduced subsidence and slower net sedimentation rates leading to greater channel-stacking (Heller and Paola 1996).

20 Huerfano Formation Summary. ⎯ Robinson (1966) made detailed descriptions of the

Huerfano Formation during his extensive paleontological work in the Huerfano Basin. For this reason, our study provides only a summary of key lithological observations of the Huerfano

Formation. The Huerfano Formation is composed of arkose sandstone, siltstone, and micaceous mudrock. Beds, regardless of grain size, have strong red hues (10R) that are characteristically dark reddish brown (10R 3/4) with relatively uniform values and chromas. Sand-bodies are highly lenticular in geometry and represent single, isolated stories usually < 1.5 m thick. These deposits typically have a thin gravel or conglomerate basal scour that grade into very fine- grained sandstone (φ 3-3.5) with small-scale trough cross-bedding. Mudrock and siltstone lithofacies tend to have gradational contacts between beds and are weakly laminated. Carbonate nodules are present in mudrock deposits, however, these may be a secondary precipitate rather than primary soil feature. Previous studies of paleodrainage have suggested south-directed drainage networks during the Eocene in the Huerfano Basin (Dickinson et al. 1988; Cather

2004). Although paleocurrent directions in the Huerfano Formation are limited to only a few measurements in this study (n = 4), these measurements are compatible with previous interpretations of south-directed drainage networks.

Petrographic Classifications

Petrographic results, including point counts (N = 31) of sandstone composition and cobble censuses (N = 6) for Poison Canyon, Cuchara, and Huerfano samples, are summarized in

Tables 1 and 2. Point counting results from this study are plotted as poles on ternary classification diagrams (Figs. 6A, B). Previous petrographic results from Boggs (1978) and

21 Cullers (1995) are included for reference (Figs. 6A, B). Cobble census results are plotted as pie diagrams (Fig. 6C).

Sandstone compositions plot within the arkose domain according to Folk (1980) (Fig.

6A). The average modal percent quartzose grains (Qm + Qp) for the Poison Canyon, Cuchara, and Huerfano formations are 55.8%, 56.7%, and 56.7%, respectively. The average modal percent feldspar (K + P) are 37.7%, 36.3%, and 38.3%, respectively. The average modal percent lithic fragments (Lm + Lv + Ls + L-unclassified) are 6.5%, 7.0%, and 5.0%, respectively. Results of this study are consistent with previous petrographic results by Boggs (1978) and Cullers (1995).

Within a provenance plot, ~70% of the data occur firmly within the continental block domain and transitional continental subdomain (Fig. 6B; Dickinson 1985). A few data points, also part of the main data cluster, plot within the basement uplift and recycled orogenic subdomains.

Cobbles (n = 1414 clasts) from six sand-bodies in Poison Canyon stratigraphic section were classified and counted (Table 3; Fig. 6C). Alkali-feldspar granite is the dominant cobble lithology (~44-62%) for all three formations. However, the Poison Canyon Formation has ~15-

20% less alkali-feldspar granite than the Cuchara and Huerfano formations. Granite is the second most abundant lithology (~5-37%) and consistently decreases upsection by formation.

Sedimentary cobbles include sandstone, chert, and metasedimentary lithologies (~13-30%). The

Poison Canyon and Cuchara formations have similar percentages of sedimentary cobbles but there is an abrupt increase (~15%) in the Huerfano formation. Volcanic cobbles (~3-6%) range from felsic to mafic and are only slightly more abundant in the Cuchara Formation. Gneissic cobbles are minor (1-5%) and are most abundant in the Huerfano Formation.

22 U–Pb Detrital Zircon Age Spectra

U–Pb detrital zircon age spectra for the Poison Canyon, Cuchara, and Huerfano formations (N = 848) are plotted as normalized KDE, PDP, and histogram functions (Figs. 7A,

B, C). Average discordance of zircon ages is < 2% (Figs. 7D, E, F). Zircon ages range from

465.3 Ma ± 36.2 Ma to 1.85 Ga ± 12 Ma. No maximum depositional age is determined here because the youngest zircon ages considerably predate other Phanerozoic chronostratigraphic constraints. The most striking feature in the age spectra is the complete absence of Mesozoic- and Cenozoic-aged zircons.

Taken as a whole, zircon ages are organized into three major unimodal age populations that comprise > 85% of the data (Fig. 7). These populations are recognizable from previously determined regional zircon studies (Bickford et al. 1989; Dickinson et al. 2012; Laskowski et al.

2013). The oldest zircon ages are within the late Paleoproterozoic (1.6-1.8 Ga) Yavapai-

Mazatzal provenance population (32%). The second oldest age population is consistent with

Mesoproterozoic (~1.31-1.58 Ga) anorogenic plutonism (15%). The youngest zircon ages are

Cambrian-aged and are consistent with ages of local alkaline magmatic intrusions (39%) (see discussion; Bickford et al. 1989; Schoene and Bowring 2006). The remainder of zircon ages are non-overlapping and do not occur within any major age population group (~15%).

The Poison Canyon sample (n = 282 age determinations) has three major spectral peaks at 1687 Ma, 1430 Ma, and 517 Ma, respectively (Fig. 7C). The oldest Yavapai-Mazatzal population accounts for ~43% of the Poison Canyon ages. In addition, this population has a minor peak that occurs at 1607 Ma (~4%), which is well within the Yavapai-Mazatzal range. The next most abundant peak is the Cambrian alkali population, which includes ~39% of the ages.

Lastly, the anorogenic plutonic population represents ~8% of the ages for this sample.

23 The Cuchara sample (n = 281) has three major spectral peaks at 1687 Ma, 1430 Ma, and

516 Ma, respectively (Fig. 7B). ~54% of the zircon ages are within the Cambrian alkali population, which forms a peak at 516 Ma. This peak is unique in that the amplitude is ~15-30% greater than equivalent peaks in the Poison Canyon and Huerfano samples. The Yavapai-

Mazatzal population accounts for ~20% of the Cuchara sample. Zircon ages within the anorogenic plutonic population comprise ~11% of the Cuchara sample. A minor peak (~1%) occurs at 1523 Ma within the early limit of the anorogenic plutonic population. Finally, a minor transitional peak (~1%) occurs at 1578 Ma and represents either latest anorogenic plutonism or earliest Mazatzal ages.

The Huerfano sample (n = 285) has three major spectral peaks at 1678 Ma, 1423 Ma, and

517 Ma, respectively (Fig. 7A). The Yavapai-Mazatzal population accounts for ~33% of the

Huerfano sample. ~25% of the zircon ages for this sample are within the Cambrian alkali population. Zircon ages within the anorogenic plutonic population accounts for ~24% of the

Huerfano sample age determinations. Similar to the Cuchara age spectra, there is a minor transitional peak (~3%) that occurs at 1536 Ma.

U–Pb detrital zircon age spectra are highly consistent between the three formations.

Although the relative proportion between each age population varies somewhat between formations, the relative peak heights amongst sample populations are consistent (Fig. 7A).

Overlap and similarity statistics are given in Table 3 (Gehrels 2000). Overlap values indicate the degree to which two age probabilities share peaks (1.0 is perfect overlap, 0.0 is no overlap;

Gehrels 2000). Overlap values between Poison Canyon, Cuchara, and Huerfano samples range from ~0.8-0.95. Similarity is a measure of whether proportions of overlapping ages are alike in the relative abundance of the ages (1.0 is perfect similarity, 0.0 is no similarity; Gehrels 2000).

24 Similarity between the samples here are all ~0.9. Overall, these statistics yield values that are >

0.8, and this strongly suggests that samples of the Poison Canyon, Cuchara, and Huerfano formations are derived from the same hinterland sources.

DISCUSSION

Depositional Comparison with other Laramide Basins

The Huerfano Basin displays a similar progression and pattern of deposition during the early Paleogene to other Laramide basins. Nearly all Laramide basins contain a widespread unconformity that was generated by the Laramide Orogeny that separates Late Cretaceous fluviodeltaic and marine strata from alluvial strata of the early to middle Paleocene (Dickinson et al. 1988; Lawton 2008). This unconformity marks the initiation of Laramide tectonism, regression of the Cretaceous Interior Seaway, and transition to alluvial deposition (Dickinson et al. 1988; Lawton 2008). Recently, the unconformity and subsequent sheet-like fluvial conglomerate that exists in southern Laramide basins has been linked to upwarping and progressive, regional surface tilting due to subduction of an oceanic plateau (Heller et al. 2013;

Heller and Liu 2016). For example, in the Piceance and Uinta basins, a widespread unconformity separates the Mesaverde Formation from the Ohio Creek Conglomerate (Johnson and Finn 1986;

Johnson and Flores 2003; Dickinson et al. 2012; Heller et al. 2013). This unconformity likely represents ~10 Myr of missing time, and is marked by widespread bleaching, soil development, extensive bioturbation, and progradation of a coarse-grained, sheet-like fluvial unit (Heller et al.

2013). In contrast, within the Huerfano Basin the onset of Laramide tectonism is marked with an angular unconformity that is less distinct compared to the western basins. It occurs between fine- grained marine rocks of the Pierre Shale and fine-grained alluvial rocks of the Poison Canyon

25 Formation. There are no distinct sedimentologic markers indicating a hiatus or erosion, and the contact is commonly covered and vegetated (Johnson 1959). Existing biostratigraphic constraints, although sparse, suggest the unconformity is longer in duration as well, potentially by several million years (Robinson 1966).

Throughout Laramide basins, the Paleocene is predominantly represented by the Fort

Union Formation and its equivalents. The Fort Union Formation contains lithofacies of mudrock, siltstone, and fine-grained sand-bodies that represent low gradient, ponded, and poorly drained alluvial environments (Ethridge et al. 1981; Wing and Bown 1985; Bown and Kraus 1987). In addition, coal layers are abundant in the northern exposures in the Powder River and Williston basins of Wyoming, North Dakota, and Montana (Ethridge et al. 1981; Flores 1981; Gerhard et al. 1982; Tyler et al. 1995). However, in central and western Laramide basins such as the Green

River, Piceance, and San Juan basins, the Fort Union Formation and equivalents (e.g., lower

Wasatch Formation, Ojo Alamo, and Nacimiento formations) tend to lack coal deposits or they are significantly less abundant (Donnell 1969; Sikkink 1987; Williamson and Lucas 1992; Wilf

2000; Johnson and Flores 2003). Similarly, the Poison Canyon Formation lacks true coal deposits, but does contain layers with high organic contents. In contrast, further south in the

Raton Basin, prominent coal beds occur in the Raton Formation. The Raton Formation is considered a more distal lateral facies equivalent to more proximal lower beds of the Poison

Canyon Formation (Johnson and Wood 1956).

Organic preservation and coals are commonly taken as an indicator of wetter, cooler climates. Similarity of tropical floras in the Poison Canyon Formation to floras in the Denver

Basin suggest equable and wet conditions within the region during the early Paleogene (Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson and Ellis 2002). Wet early Paleogene

26 conditions in the Huerfano and Raton basins are also predicted by regional climate modeling

(Sewall and Sloan 2006). Thus we hypothesize the lack of coal may be related to coarser-grain size and higher sedimentation rate, rather than a primary climatic indicator; this is a phenomenon documented in the Late Cretaceous Kaiparowits Formation (Roberts 2007). This hypothesis is supported by the observation that the lower Poison Canyon Formation is thicker than the

Paleocene portion of the Raton Formation (Johnson and Wood 1956). Overall, the Poison

Canyon Formation appears to be one of, if not the most, coarse-grained Paleocene formation within Laramide basins. Despite this, it still displays broad climatic indicators and sedimentologic features consistent with a cooler, wetter Paleocene for the Western Interior.

Eocene deposition throughout Laramide basins is represented by the Willwood and

Wasatch formations and their equivalents. Lithofacies include fine-grained sand-bodies, as well as highly variegated, often intensely red, mudrocks that lack coal. These facies represent widespread development of well-drained soils on a relatively open alluvial plain (Bown and

Kraus 1987; Kraus 1987; Bown and Kraus 1993; Kraus and Bown 1993; Kraus 1996, 1997). In the Bighorn and Powder River basins, exquisite soil horizonation, distinct avulsion deposits, and multi-storied fluvial sand-bodies exist within the Willwood Formation (Bown and Kraus 1987;

Kraus 1987; Bown and Kraus 1993; Kraus and Bown 1993; Kraus 1996, 1997). In the Wasatch

Formation of the Piceance, Green River, and Wind River basins, similar lithofacies associations exist (Donnell 1969; Keefer 1969; Roehler 1993; Tyler et al. 1995). Broadly, this difference between the Paleocene and Eocene strata has been ascribed to higher temperatures, higher atmospheric CO2 levels, and regional drying during the early Eocene relative to the Paleocene

(Zachos et al. 2001; Kraus and Riggins 2007; Smith et al. 2008). The Huerfano Basin also preserves this common shift to red bed deposition within the Eocene-aged strata. The Cuchara

27 and Huerfano formations are consistent with interpretations of warming and regional drying trends. However, the soil horizonation is far less distinct, and development of carbonate nodules is less apparent in the Huerfano Formation compared to the Willwood Formation. These observations are broadly consistent with the coarser nature of sediment deposited in the basin and sedimentation rates that outpace soil development.

Lastly, the existence of an anomalously coarse-grained fluvial unit between Paleocene and Eocene strata in the Huerfano Basin is similar to other Laramide basins. In the Bighorn and

Piceance basins, either a solitary or coarse-grained package of fluvial units are coeval with the

Paleocene–Eocene Thermal Maximum (PETM), a transient global warming event (McInerney and Wing 2011; Foreman et al. 2012; Foreman 2014; Kraus et al. 2015). In other basins, such as the San Juan and Denver basins, similar coarse-grained units occur between definitively

Paleocene- and Eocene-aged rocks, but the genesis of these units is more enigmatic (Sikkink

1987; Williamson and Lucas 1992; Raynolds 1997, 2002; Fricke et al. 2011). In these basins, the existing chronostratigraphic framework and climate proxy resolution is insufficient to differentiate between climatic or tectonic forcings at the Paleocene–Eocene boundary. In the

Huerfano Basin, we suggest the Cuchara Formation represents this coeval coarse-grained package that is broadly associated with the Paleocene–Eocene boundary and separates strata of

Paleocene age from strata of Eocene age. The origin of the Cuchara Formation is beyond the scope of this study, and requires a more detailed age model and chemostratigraphic framework to assess its generation and timing. However, given its uniqueness within the thick stratigraphic succession and its regional prevalence, it likely represents a substantial change in depositional processes, regardless of whether the driver of that change was climatic or tectonic in nature.

28 Provenance Assessment

Previous authors have ascribed coarse-grained facies in the Poison Canyon Formation and the abrupt sedimentologic shift defined in the Cuchara Formation to episodic uplifts that involved hinterland unroofing, erosion, and a flux of sediment during the Laramide Orogeny

(Burbank and Goddard 1937; Briggs and Goddard 1956; Johnson 1959). Authors argue that uplift episodes increased the coarse-grained sediment fraction due to steepened alluvial gradients and rapid basin subsidence. Based on initial provenance analyses, previous workers proposed that sediment was derived from recycled Paleozoic and Mesozoic strata, as well as exhumation of plutonic and metamorphic rocks (Burbank and Goddard 1937; Briggs and Goddard 1956;

Johnson 1959). The San Luis Highlands, west of the Huerfano Basin, is ubiquitously cited as the major source region for Poison Canyon, Cuchara, and Huerfano sediment (Burbank and Goddard

1937; Briggs and Goddard 1956; Johnson 1959; Briggs 1965; Robinson 1966; Tweto 1980;

Flores and Pillmore 1987; Lindsey 1998; Cullers 1995; Cather 2004; Chapin et al. 2014).

Although some authors recognize the Wet Mountains to the north and east of the Huerfano Basin as a possible secondary hinterland, the de facto interpretation has been that the San Luis

Highlands supplied the majority of sediment to the Huerfano Basin throughout the Late

Cretaceous and early Paleogene. Our study provides several lines of evidence to the contrary of these studies, and thus we argue that the Wet Mountains were a significant sediment source to the basin.

Sandstone classifications and cobble censuses are inconsistent with a hinterland source composed of primarily sedimentary strata. Sedimentary clasts both in thin sections and cobble censuses are only a minor compositional component. Although sedimentary clasts would have weathered relatively quickly to their framework components once mobilized, fluvial transport is

29 insufficient to change overall ratios between framework minerals, even over hundreds of kilometers in some cases (Dickinson 1985). Petrographic classifications in this study plot within the continental block domain with low Qm-F ratios and high Q-L ratios (Fig. 6B); both results are consistent with erosion of exposed basement rock. Sands within the recycled orogen domain

(not observed here) that derive from fold-thrust systems of stratified sedimentary and low-grade metamorphic strata have characteristically low components of feldspar and volcanic clasts

(Dickinson and Suczek 1979; Dickinson 1985). Moreover, the abundance of first-generation alkali-feldspar indicates a primarily plutonic source rock rather than sedimentary recycling.

Given both the close proximity of deposits to possible source areas and the significant thickness of Phanerozoic strata to be eroded in order to expose the Precambrian core, previously proposed unroofing hypotheses to expose basement rocks seem unlikely.

U–Pb age spectra of early Paleogene samples in the Huerfano Basin allows for comparison to age spectra of proposed sources for these formations. Three major zircon populations that are observed in these samples generally indicate provenance within the southern

Yavapai and Yavapai-Mazatzal transition provinces (Lawton 2008; Dickinson et al. 2012;

Laskowski et al. 2013). Age spectra of the Sangre de Cristo Formation, a previously proposed source, exhibit a single broad age population at ~1.6-1.8 Ga (Bush and Horton 2015; Bush et al.

2015) that cannot fully reconcile all three major age populations observed in early Paleogene formations in the Huerfano Basin.

Age populations at ~1.4 Ga are broadly related to regional anorogenic plutonism in both the Sangre de Cristo and Wet mountains (Thomas et al. 1984; Bickford et al. 1989; Jones and

Connelly 2006). Although this population is not explicitly diagnostic between the Sangre de

Cristo and Wet Mountains, it remains a useful indicator for provenance. Ages in our data are

30 more consistent with expansive exposures of plutonic rocks of similar age in the Wet Mountains than basement rocks in the Sangre de Cristo Mountains (Thomas et al. 1984; Bickford et al.

1989; Jones et al. 2010). In contrast, plutonic exposures in the Sangre de Cristo Mountains are far less abundant and basement rocks tend to be metamorphic (Jones and Connelly 2006). This inference is supported by the abundance of granitic cobbles and feldspathic gravel, as well as the lack of gneissic material.

Arc-derived, Mesozoic-aged zircon populations from the Western Cordilleran (~60-280

Ma) are well-documented in the Jurassic and Cretaceous strata (e.g., Entrada, Morrison, Dakota,

Benton, Niobrara, Pierre formations) that are present in areas adjacent to the Huerfano Basin and in the basin subsurface (Dickinson et al. 2012; Dickinson and Gehrels 2003, 2008). These formations also contain Paleozoic-aged zircons, originally derived from Appalachian and northern Laurentian sources (Dickinson et al. 2012; Dickinson and Gehrels 2003, 2008).

Ostensibly, these formations also covered the Precambrian core of the San Luis Highlands prior to Laramide deformation and would have needed to be at least partially eroded prior to exposure of the Precambrian crystalline core. The complete absence of Mesozoic-aged zircons and a single, tight Cambrian-aged peak is wholly inconsistent with a recycled sedimentary strata provenance for the Poison Canyon, Cuchara, and Huerfano formations. Thus, either the San Luis

Highlands were not a significant contributor of sediment or the sedimentary carapace had been removed prior to deposition of the Poison Canyon Formation.

In contrast, the Wet Mountains must have been a significant sediment source. Apart from the absence of Mesozoic-aged zircons, the most conspicuous feature of the age spectra is the

~516-517 Ma age peak. Within the northern Wet Mountains, several Cambrian-aged alkaline magmatic complexes are exposed, which include McClure Mountain, Democrat Creek, and Gem

31 Peak complexes. Preliminary ages of these rocks were determined to be ~520 Ma using fission- track, K–Ar, and Rb–Sr isotope systems (Fenton and Faure 1970; Olson et al. 1977). Subsequent isotopic determinations have yielded crystallization ages of 536 ± 4 Ma (U–Pb), 535 ± 5 Ma

(Rb–Sr), and 523.98 ± 0.12 Ma (ID-TIMS U–Pb) for the McClure Mountain Complex, and 511

± 8 Ma (Rb–Sr) for the Democrat Creek Complex (Armbrustmacher and Hedge 1982;

Armbrustmacher 1984; Bickford et al. 1989; Schoene and Bowring 2006). Regionally, the next closest Cambrian-aged igneous rocks known to exist are isolated magmatic intrusions of the Iron

Hill complex ~150 km west (McMillan and McLemore 2004). Although the Iron Hill complex is likely genetically related to the alkali complexes in the Wet Mountains, it dates to ~570 Ma, which is significantly older than the observed Cambrian zircon population in the data presented here (Olson et al. 1977).

Together these observations suggest that early Paleogene sediment of Huerfano Basin was derived primarily from the Wet Mountains, not the San Luis Highlands or Sangre de Cristo

Mountains as previous authors have suggested. Petrographic classifications indicate erosion of continental block basement rocks, not recycling of Paleozoic and Mesozoic sedimentary strata.

Likewise, cobble censuses indicate erosion primarily of plutonic rocks, and the clastic sedimentary component on average is < 10%. A primarily plutonic source area is difficult to reconcile with the previous interpretations of the San Luis Highlands as the primary sediment source, because as Permian–Pennsylvanian sedimentary rocks of the Sangre de Cristo Formation mantle the eastern flank of the Sangre de Cristo Mountains (Tweto 1975; Dickinson 1985).

Consistently south-directed paleocurrents from the Paleocene to early late early Eocene independently support the correlation of the Cambrian zircon population to alkaline magmatic complexes at McClure Mountain and Democrat Creek, which are directly north of the study area.

32 Moreover, the modest surface exposure of the alkaline magmatic complexes, only ~60 km2, compared to the abundance of Cambrian-aged zircons relative to older populations suggests that the Wet Mountains are the primary provenance for sediment in the Huerfano Basin.

Given south-directed paleocurrents and extensive exposure of the Pikes Peak batholith

(~3100 km2; Tweto 1987) only ~50 km north, the lack of Mesoproterozoic-aged (1.08 Ga) Pikes

Peak granite zircons in our data is striking (Smith et al. 1999). This phenomenon could result from a paleodrainage divide as early as the Paleocene, which separated the northern Wet

Mountains from the southern limit of the Pikes Peak batholith. If this hypothesis is correct, the modern catchment divide could be a vestige of the Laramide Orogeny. Alternatively, the lack of

Pikes Peak-aged zircons could indicate that exhumation of the Pikes Peak batholith post-dates deposition of the Huerfano Formation. However, this seems highly unlikely given the close correlation between exhumation of the Pikes Peak batholith and deposition of the Dawson and

Denver formations further north along the Front Range (Raynolds 1997, 2002).

The absence of Mesoproterozoic-aged Pikes Peak zircon populations raises the question of whether sediment delivered to the Huerfano Basin could have derived from farther afield.

McMillan and McLemore (2004) illustrate the distribution of Cambrian–Ordovician magmatism along the eastern margin of Colorado Plateau throughout Colorado and New Mexico. These are areas that can be entertained as possible alternative source areas to the Wet Mountains. However, all of these sources are inconsistent with south-directed paleocurrents. Furthermore, Jurassic and

Cretaceous strata with arc-derived zircon populations from the Western Cordilleran are extensively exposed in surrounding areas south and west of the Huerfano Basin. Erosion of these strata together with southern Cambrian sources is inconsistent with the absence of Mesozoic ages in our data as discussed previously. A more distal sediment source would require that additional

33 Cambrian-aged sources existed elsewhere within exposures of the crystalline basement prior to the Paleocene, have been fully eroded, and exist only as detritus deposited in the Poison Canyon,

Cuchara, and Huerfano formations. Olson et al. (1977) noted that the existence, past or present, of additional Cambrian intrusions in southern Colorado is possible, but entirely unknown. We argue that the evident correlation to exposed Cambrian sources in the Wet Mountains, together with the remarkable degree of similarity between age spectra, petrographic classifications, and south-directed paleocurrents suggests a stable sediment source in the Wet Mountains during the

Late Paleocene and early Eocene. If the majority of the early Paleogene sediment is derived from the Wet Mountains, the implication is that basement rocks in the Wet Mountains were fully exposed by the late Paleocene.

Tectonic Implications

Results of this study constrain the relative unroofing histories of the Wet Mountains and

Sangre de Cristo Mountains. There is general consensus that the angular unconformity separating

Late Cretaceous and early Paleogene rocks in the Huerfano Basin, and other Laramide basins, signifies the onset of the Laramide Orogeny (Johnson 1959; Cather 2004). Likewise, shortening across the Huerfano Basin is evidenced by folding and faulting of strata as young as the

Huerfano and Farasita formations (Burbank and Goddard 1937; Johnson 1959; Lindsey 1998) and implies active Laramide deformation until the late early Eocene. However, there is disagreement as to the exact spatiotemporal nature of subsequent uplifts in relationship to basin sediment supply (Cather 2004).

Interpretations of Laramide faulting and their spatiotemporal interactions range widely.

Faults within the buried San Luis Highlands are north striking and steep to moderately dipping

34 (Cather 2004). Important dextral components of these fault complexes are controversial (Cather

2004). In contrast, faults in the Sangre de Cristo Mountains are gently to moderately eastward dipping (Baltz and Myers 1999). Imbricate thrust sheet complexes along the eastern front of the

Sangre de Cristo Mountains are related to tight fault-propagation folds that frequently have overturned limbs (Johnson 1959; Lindsey 1998). In the Wet Mountains, northeast-oriented

Laramide compression was transmitted across the Wet Mountain block (Tweto 1975). The Ilse fault zone forms a northwest-oriented division of the Wet Mountains that separates the range into two distinctive segments with distinctive metamorphic protoliths and unique structural trends

(Noblett et al. 1987). During the early Laramide Orogeny, ~16 km of right-lateral strike-slip offset is estimated to have occurred along this zone (Noblett et al. 1987). Given that strike-slip offset is restricted to motion between basement blocks of the Wet Mountains located northeast of the Huerfano Basin, the possibility for biased paleocurrents due to block rotation is likely minimal, if rotation occurred at all within the basin. Interpretations of the Wet Mountain thrust that bounds the eastern margin of the range vary from a low-angle thrust near the surface, with several kilometers of overhang (Jacob 1983), to a high-angle reverse fault (Weimer 1980).

Noblett et al. (1987) suggests east-vergent reverse motion with lateral offset along a high-angle fault. In contrast, Taylor (1975) interpreted the Wet Mountain fault as a normal fault.

Interpretations of the Westcliffe fault separating the Wet Mountains and Huerfano Basin range from high-angle to near vertical reverse and normal faulting (Johnson 1959). Perhaps the clearest conclusion from these structural studies is that character, timing, even direction of tectonic activity associated with the Huerfano Basin is anything but clear. While provenance cannot resolve the exact timing and style of deformation, our results help to broadly indicate which mountain ranges had enough relief to supply significant sediment to the basin.

35 Our provenance results encourage a reappraisal of the Wet Mountains tectonic history.

The provenance results presented herein reveal that the Precambrian crystalline core of the Wet

Mountains was at least partially uplifted and fully uncovered by no later than the Paleocene (i.e., deposition of the Poison Canyon Formation). This is in contrast to apatite fission-track cooling ages in the Wet Mountains that indicate preservation of the late Mesozoic partial annealing zone and lack Laramide cooling ages (Kelley and Chapin 2004). This led to the interpretation that the

Wet Mountains were thrust laterally during Laramide deformation and that Laramide arching in the Wet Mountains was minimal (Kelley and Chapin 2004). Apparently, the sedimentary carapace was removed rapidly during initial Laramide stages as hypothesized by Tweto (1975).

Our analysis indicates that substantial erosion of the Wet Mountains crystalline rocks occurred during the early Paleogene, at least enough for > 550 m of basin-fill. We propose that westward draining tributaries delivered sediment from the crest of the Wet Mountains to a southward dipping alluvial slope and trunk river network in the Huerfano Basin. We argue the San Luis

Highlands and emerging Sangre de Cristo Mountains existed as a low relief structural platform adjoined to the western margin of the Huerfano and Raton basins during the early and middle

Laramide, as suggested by Cather (2004). This arrangement would imply that the evolving thrust faults of the Sangre de Cristo Mountains were probably blind underneath synorogenic sediment during much of the Laramide Orogeny (Cather 2004). In this scenario, significant growth of the

Sangre de Cristo Mountains likely did not occur until late stages of the Laramide Orogeny (i.e.,

~51-53 Ma) or potentially even post-Laramide, which post-dates deposition of the Huerfano

Formation (Cather 2004). The lack of exhumation rectifies the absence of Mesozoic-aged zircons and the rarity of lithic grains and sedimentary cobbles. Furthermore, the lack of exhumation implies that hypotheses ascribing coarse-grained sand-bodies in the Poison Canyon and Cuchara

36 formations to pulses of tectonism in the San Luis Highlands cannot be valid due to the fact that sediment was not coming from there. Thermochronologic studies are compatible with this hypothesis, and suggest that negligible exhumation of the incipient Sangre de Cristo Mountains occurred during the early and middle Laramide (Kelley and Chapin 1995).

However, if the majority of sediment in the Huerfano Basin was transported south from the uplifted Wet Mountains, it becomes difficult to reconcile the fact that the thickest deposits both in the Huerfano and Raton basins are along the western basin axis where subsidence, putatively from flexure of a uplifted load, appears to have been greatest (Johnson and Wood

1956). Yet Cather (2004) pointed out that it remains unknown if the present Laramide deformation front represents initial thrusting, or if deformation originated further west along older faults. If the Laramide deformation front was evolving further west during deposition of the Poison Canyon, Cuchara, and Huerfano formations, it could help to explain flexural loading in the western part of the Huerfano and Raton basins with a primary sediment source from the

Wet Mountains to the east and north. Systematic northward migration of the Raton Basin depocenter during the Late Cretaceous to late Eocene implies that deformation occurred, at lease in part, to the west of the Huerfano and Raton basins during early to middle phases of Laramide orogenesis (Lindsey 1998; Cather 2004). Depocenter migration is ascribed to diachronous thrust sheet emplacement in the Culebra Range of the incipient Sangre de Cristo Mountains flexing the lithosphere and creating the basin (Lindsey 1998). However, apatite fission-track cooling ages in the Culebra Range and elsewhere in the Sangre de Cristo Mountains indicate late Oligocene to early Miocene cooling due to epeirogenic denudation (Kelley and Chapin 1995). The few existing Laramide ages that occur are mostly late Eocene ages (Kelley and Chapin 1995). If

37 uplift in the Culebra Range and Sangre de Cristo Mountains post-dated deposition of the

Huerfano Formation, then these uplifts cannot account for basin subsidence patterns.

Although detrital zircon age spectra and south-directed paleocurrents in the Huerfano

Basin documented in this study are inconsistent with a sediment source in the Culebra Range, our interpretations are not mutually exclusive with this hypothesis. If significant uplift in the

Culebra Range and southern Sangre de Cristo Mountains did occur during early Laramide stages as proposed by Lindsey (1998), this uplift would need to be coeval with uplift of the Wet

Mountains to create a separate fluvial system in the Raton Basin with similar depositional environments to deposits in the Huerfano Basin during the same time interval. This would imply a later uplift history for the northern Sangre de Cristo Mountains, because sediment deposited in the Huerfano Basin was not derived from western sources. If drainage systems in the Huerfano and Raton basins interacted, then Cambrian-aged detrital zircon populations should be present in samples of early Paleogene formations further south. Overall, structural–sedimentation interpretations require further testing and additional spatial and temporal controls are necessary.

If the majority of sediment in the Huerfano Basin was delivered from the relatively stable

Wet Mountains during the early Paleogene, then the abrupt lithologic shift from the Poison

Canyon Formation to Cuchara Formation cannot be wholly explained by a tectonic forcing. In our preliminary examination, the repetitive depositional patterns between overbank and fluvial channel deposits in the Poison Canyon Formation may be more consistent with autogenic channel organization, similar to observations by Hajek et al. (2010) in the Hana Basin of

Wyoming, rather than tectonic pulses. The Cuchara Formation, however, is distinctive compared to autogenic fluvial deposition of the Poison Canyon Formation. Reassessments of thick fluvial sand-bodies at the Paleocene–Eocene boundary in other basins have led to shifts in

38 interpretations from tectonic drivers to climatic drivers (Kraus and Middleton 1987; Lorenz and

Nadon 2002; Foreman 2014; Foreman et al. 2012). Based on existing stratigraphic constraints in the Huerfano Basin, a climatic forcing cannot be rejected at this time.

CONCLUSIONS

Alluvial strata in the Huerfano Basin are comparatively understudied relative to coeval strata in other Laramide basins, but contain an important record of early Paleogene climate and tectonism. Detailed stratigraphic measurements, sandstone petrography, and detrital zircon analysis from this study yielded the following conclusions: (1) Paleocene and Eocene deposition followed a similar progression to other Laramide basins, (2) detrital zircon age spectra and petrographic results from early Paleogene formations in the Huerfano Basin suggest that the primary provenance were plutonic rocks of the Wet Mountains, not the San Luis Highlands and incipient Sangre de Cristo Mountains, (3) the remarkable similarity of the detrital zircon age spectra and petrographic results amongst the formations indicates a largely stable or lithologically uniform sediment source from the Paleocene through at least earliest Bridgerian time, and (4) that the crystalline basement of the Wet Mountains was fully exposed by the

Paleocene. Finally, this study establishes the stratigraphic and provenance framework within which to assess autogenic and allogenic forcings on basin-scale stratigraphy as well as future paleoclimatic and paleobiologic studies of the region.

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50 FIGURES Figure 1. (Caption on p. 52)

51 Figure 1. Geologic map of the Western Interior adapted from Dickinson et al. (1988) and Lawton (2008) of Laramide sedimentary basins and major Laramide faults of the central Rocky Mountains. Relationships are compiled from local and regional geologic maps. The Huerfano Basin study area is outlined (red) and detailed in Figure 2.

52 Figure 2. (Caption on p. 54)

53

Figure 2. Regional geologic map study area showing major tectonic features, exposed hinterland lithologies in the Sangre de Cristo and Wet Mountains, and basin fill lithologies in the Huerfano and greater Raton basins (adapted from Bickford et al. 1989; Green 1992; Jones and Connelly 2006). The inset map outlined in green shows extent and location Poison Canyon, Black Mountain, and Gardner Klippe, the three primary areas where stratigraphic measurements were obtained in this study. Locations of detrital zircon samples are marked with red stars. Note the exposures of Cambrian-aged alkaline magmatic intrusions in the northern Wet Mountains mapped in green.

54

Figure 3. (Caption on p. 56)

55 Figure 3. A) Stratigraphic section through the Poison Canyon, Cuchara, and lower Huerfano formations at the Poison Canyon type locality in Huerfano County, Colorado (coordinates for the base and top of section are given). Stratigraphic levels of detrital zircon samples and fossil leaf quarries are marked with red and green stars, respectively. B) Rose diagrams indicating consistently south-directed paleocurrent directions for the Poison Canyon and Cuchara formations. Southerly paleocurrent directions for the Huerfano Formation are documented by Dickinson et al. (1988) and Cather (2004). Red circles represent vector means and n is the number of measurements.

56 Figure 4. Photographs illustrating described lithofacies in the Poison Canyon Formation. A) Upsection, east-facing vantage of the lower Poison Canyon Formation showing a representative channel sand-body with clinoforms structure (~1.5 m Jacob’s staff for scale). B) Representative outcrop of both coarse- and fine-grained lithofacies (~1.7 m geologist for scale). C) Typical fluvial story sequence with lower scour surface and basal conglomerate, fining upward sequence (~30 cm rock hammer for scale). D) Representative mudrock and siltstone lithofacies representing overbank and alluvial ridge deposition (~1 m geologist for scale).

57 Figure 5. Photographs illustrating described lithofacies in the Cuchara Formation. A) Distinctly pink multi-story sand-bodies are coarser-grained and more latterly extensive than sand-bodies in the Poison Canyon Formation. B) Typical fluvial story delineated at its base by large mudrock and siltstone rip-up clasts transitioning upwards to amalgamated coarse-grained sandstone. Individual rip-up clasts are outlined by white dashed lines and commonly exceed 1 m diameter (~1.7 m geologist for scale). C) East-dipping multi-storied sand-body. Story breaks weather recessively along zones of rip-up clasts and are vegetated (~1.7 m geologist and dog for scale). D) Fine-grained facies with gradational lower contact and erosive upper contact with overlaying sand-body. This lithofacies is exceedingly rare in the Cuchara Formation (~30 cm rock hammer for scale).

58

Figure 6. (Caption on p. 60)

59 Figure 6. Petrofacies classification of early Paleogene sandstones in the Huerfano Basin. A) QFL ternary diagram for sandstone compositions sensu Folk (1980). Q— total quartzose grains, F— total feldspar grains, L— total lithic fragments. See Table 1 for details of grain types and ternary poles. Previous petrographic studies by Cullers (1995) and Boggs (1978) are plotted for reference. B) QFL ternary diagram for provenance classification sensu Dickinson (1985). See Table 1 for details of grain types and ternary poles. Previous petrographic studies by Cullers (1995) and Boggs (1978) are plotted for reference. C) Pie plots of cobble censuses by formation. See Table 2 for details of subcategorization of cobble lithologies. Sedimentary— sedimentary + metasedimentary + chert, Volcanic— felsic volcanic + intermediate volcanic + mafic volcanic. Individual fragments of quartz and feldspar are not tabulated within these composition estimations.

60 Figure 7. (Caption on p. 62)

61 Figure 7. Comparative U–Pb detrital zircon age spectra of early Paleogene strata in the Huerfano Basin. Coordinates of samples localities are given (also mapped in Fig. 2) and stratigraphic level are given in Figure 3A. A-C) Age density (black crosses), histogram (grey rectangles), probability density function (PDF; red lines), and kernel density estimate (KDE; blue lines) functions of detrital zircon samples are plotted in relative stratigraphic order from oldest (C) to youngest (A). Major age populations determined using “AgeCalc” (details in Gehrels and Pecha (2014)) are noted next to each peak. D-F) U–Pb concordia plots for corresponding age spectra in A-C. Age determination error ellipses (green ellipses) are plotted at the 2 sigma level.

62 TABLES

63

APPENDICES

Appendix 1. (digital) U–Pb age determinations, spot analyses, isotope ratios, discordance Appendix 2. (digital) Bed-by-bed sedimentology log and paleocurrent measurement

64