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GEOSPHERE A physical and chemical sedimentary record of Laramide tectonic shifts in the Cretaceous-Paleogene San Juan Basin, New Mexico, USA GEOSPHERE, v. 17, no. 3 Kevin M. Hobbs1 and Peter J. Fawcett2 1New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, USA https://doi.org/10.1130/GES02324.1 2Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, New Mexico 87131, USA

15 figures; 3 tables ABSTRACT kilometers-wide sand bodies and limited overbank allow for updated interpretations of the deposi- CORRESPONDENCE: [email protected] mudstones throughout most of the outcrop area, tional history, lithostratigraphic relationships, and Fluvial siliciclastic rocks bracketing the Cretaceous- are difficult to reconcile with accepted models of tectonic significance of the Ojo Alamo Sandstone, CITATION: Hobbs, K.M., and Fawcett, P.J., 2021, A physical and chemical sedimentary record of Lara- Paleogene (K-Pg) boundary in the San Juan Basin, aggradation and avulsion in large fluvial systems, which we present here. mide tectonic shifts in the Cretaceous-Paleogene San New Mexico (USA), provide records of regional fluvial but available age and lithologic data make difficult The K-Pg boundary in the SJB of northwest- Juan Basin, New Mexico, USA: Geosphere, v. 17, no. 3, and tectonic evolution during the Laramide orogeny. a complete understanding of Paleocene San Juan ern New Mexico is located between the Upper p. 854–875, https://doi.org/10.1130/GES02324.1. Petrographic analyses of sandstones from the Upper Basin fluvial systems and basin evolution. Here, we Cretaceous Kirtland Formation and the Paleocene Cretaceous Fruitland Formation and Kirtland Forma- present new lithologic, petrographic, and thickness Ojo Alamo Sandstone. The exact position of the Science Editor: David E. Fastovsky Associate Editor: Cathy Busby tion and the Paleocene Ojo Alamo Sandstone and data from San Juan Basin K-Pg fluvial siliciclastic boundary was subject to conflicting interpretations Nacimiento Formation show that the rivers depos- units and interpretations of their origins. throughout the 20th century (e.g., Bauer, 1916; Ree- Received 27 July 2020 iting these sediments were sourced in areas where side, 1924; Dane, 1936; Baltz et al., 1966; Lindsay et Revision received 23 December 2020 unroofing of crystalline basement rocks took place, al., 1981; Fassett, 1985). Though still debated (e.g., Accepted 23 February 2021 introducing an increasing proportion of immature ■■ INTRODUCTION Lucas et al., 2009; Fassett et al., 2011), the K-Pg detrital grains into the fluvial system through time. boundary is generally accepted to lie at the base of Published online 21 April 2021 After the Cretaceous-Paleogene boundary, rivers Since the 1970s, Ojo Alamo Sandstone (New the Ojo Alamo Sandstone sensu Baltz (1967), i.e., at deposited an increasing amount of microcline and Mexico, USA) research has focused primarily on the base of the Kimbeto Member of the Ojo Alamo orthoclase feldspar relative to plagioclase feldspar, the interpretation of the formation’s position at Sandstone sensu Powell (1973) and Cather et al. suggesting a growing source in unique crystalline or near the Cretaceous-Paleogene (K-Pg) bound- (2019). We use that position throughout this paper basement rocks. Geochemical analyses show sig- ary (e.g., Fassett et al., 2002; Fassett, 2009; Lucas and note that the exact position of the boundary nificant differences between Al- and K-poor Upper et al., 2009; Flynn et al., 2020), its potential eco- does not affect our interpretations of basin sedi- Cretaceous sandstones and Al- and K-rich lower nomic role as a hydrocarbon reservoir or uranium mentary architecture and tectonics. Paleocene sandstones in the San Juan Basin. source (e.g., Vizcaino and O’Neill, 1977), and its The intricacies of spatial and temporal rela- The high proportion of sand-sized material in importance as a marker of regionally extensive tionships between aggradation-progradation, the Ojo Alamo Sandstone suggests that it was Laramide tectonic activity (e.g., Baltz, 1967; Dick- degradation, system quiescence, accommodation, deposited in a basin with a low ratio of sediment inson et al., 1988; Cather, 2004; Blum and Pecha, and extrabasinal controls lead to complex sedi- supply to accommodation. However, magnetostrati- 2014; Cather et al., 2019). Few studies have offered mentary records in fluvially dominated terrestrial graphic age constraints suggest it had a relatively basin-scale sedimentation, tectonic, and paleo- basins. Recent work shows that many fluvial silici- high sedimentation and/or subsidence rate of as environmental interpretations for the Ojo Alamo clastic packages preserved in modern continental much as 0.38 m/k.y. The sediment supply must have Sandstone. Recent advances in understanding sedimentary basins are the deposits of prograd- been high in order to deposit a basin-wide coarse of fluvial sedimentary processes and preserva- ing distributive fluvial systems (DFSs) or megafans sand-dominated package, suggesting rapid creation tion potential (Owen et al., 2015a, and references (Hartley et al., 2010; Weissmann et al., 2010, 2013, of topographic relief in the San Juan uplift, the pro- therein), Laramide tectonic deformation history of 2015; Kukulski et al., 2013). Other workers highlight posed source area of the Ojo Alamo fluvial system. the southern Rocky Mountains (Heller et al., 2012, the abundance of both modern and ancient fluvial The observed sedimentary architecture and age and references therein), and radiometric and mag- deposits that do not meet the criteria for a DFS, constraints of the Ojo Alamo Sandstone, including netostratigraphic age controls for Cretaceous and suggesting that tributary fluvial systems are at least This paper is published under the terms of the Paleogene rocks of the San Juan Basin (SJB) (Dona- as likely as DFSs to be preserved in the rock record CC‑BY-NC license. Kevin Hobbs https://orcid.org/0000-0002-0607-2967 hue, 2016; Cather et al., 2019; Flynn et al., 2020) (Fielding et al., 2012; Latrubesse, 2015). In either

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type of system, incision, reworking of sediment, systems that deposited sediment in the SJB during ■■ GEOLOGY and downcutting occur when the rate of sediment the Laramide orogeny and allow for increased volume supply is greater than the rate at which sed- understanding of regional paleogeography. In addi- The SJB is located in the Four Corners region iment can be accommodated by subsidence in the tion, we report on the fluvial sedimentology of the of New Mexico and Colorado (Fig. 1) in the Navajo basin (Shanley and McCabe, 1994; Holbrook et al., Ojo Alamo Sandstone, which comprises the first physiographic section of the Colorado Plateau 2006). Conversely, when the creation of accommo- record of SJB sedimentation during and after the physiographic province (Fenneman and Johnson, dation is greater than the rate of sediment volume major Laramide reorganization of paleogeographic 1946). The SJB is an asymmetrical broken-fore- supply, the fluvial system experiences aggradation. and paleofluvial features. This formation shows land structural basin formed during the Laramide Aggrading and/or prograding fluvial systems can- that basin-wide fluvial sedimentation can occur rap- orogeny, which deformed Paleozoic through early not uniformly deposit sediment throughout a basin idly and does not always produce the sedimentary Cenozoic strata (Dickinson et al., 1988; Cather, at any one point in time. Rather, deposition occurs architecture expected in an avulsive fluvial system. 2004). The axial trace of the SJB is arcuate, located in and around individual active channels occupying a small portion of the total basin area (Shukla et al., 2001; Nichols and Fisher, 2007). Over time, as the 110°W 105°W 100°W 95°W Figure 1. Major Laramide foreland basins of west- active channels change position, most or all of the ern North America and Laramide uplifts pertinent to basin area experiences deposition and sediment this study. WB—Williston Basin; BB—Bighorn Basin; accumulation. Basin-wide lithologically uniform PRB—Powder River Basin; WRB—Wind River Basin; beds or deposits therefore are assumed to be dia- 49°N GRB—Green River Basin; DB—Denver Basin; UB— Uinta Basin; RB—Raton Basin; SJB—San Juan Basin; chronous, though the duration of their diachroneity WB SJU—San Juan uplift; D—Defiance uplift; Z—Zuni up- can vary (Nichols and Fisher, 2007). lift; N—Nacimiento uplift; SdC—Sangre de Cristo uplift; Sedimentation in Laramide basins in the North M—Mogollon Rim; GOM—Gulf of Mexico alluvial plain. American Cordillera from the Late Cretaceous to Inset shows a simplified bedrock geologic map of the San Juan Basin, study areas pertinent to this study, the Eocene was simultaneous with uplift in the and the axial synclinal trace of the San Juan Basin regions surrounding the basins (Dickinson et 44°N BB PRB (thick black arcuate line with opposing arrows). Kkf— al., 1988; Clinkscales and Lawton, 2018; Lawton, Fruitland and Kirtland Formations; Toa—Ojo Alamo 2019), and these sediments commonly provide WRB Sandstone–McDermott Formation–Animas Formation; the highest-resolution records of local to regional Tn/Tsj—Nacimiento Formation–San Jose Formation; GRB BDNZ—Bisti/De-Na-Zin Wilderness Area. Tan area deformation and associated syntectonic responses. surrounding the SJB represents units older than the Colorado DB Laramide basin sediments have been investigated Fruitland Formation. for insights into paleogeography, paleotectonics, 39°N UB SdC 108°W 107°W and mantle processes (Dickinson et al., 1988; Heller et al., 2003; Yonkee and Weil, 2015; Heller and Liu, SJU Durango RB 2016). These investigations share the premise that Toa regional flexural response was responsible for D SJB and contemporaneous with creation of uplifts and Colorado N 37°N New Mexico Aztec Dulce basins that led to (1) erosion of materials in the 34°N Z M Farmington uplifted areas surrounding basins, (2) transport Area depicted in Shannon Blu s/ downgradient toward basins, and (3) contempo- Figs. 11, 12, and 13 Head Canyon New Mexico t f

raneous and subsequent deposition and storage i p l

within basins as the basins subsided. As such, the u o t

fluvial deposits preserved in Laramide basins are Kkf n Tn/Tsj e m i directly related to the fluvial, magmatic, climatic, i

29°N BDNZ a c and tectonic conditions present during their ero- Cuba N N M O 36°N sion, transport, and deposition. G Escavada Mesa Wash Here, we document lithologic and petrographic 1000 km Portales changes in SJB K-Pg fluvial sedimentary rocks. N 0 20 40 60 These changes record evolution of the fluvial Kilometers

near the northern and eastern basin margins with sediment of the San Jose Formation (Smith, 1992, et al. (1978, 1981), Klute (1986), and Sikkink (1987). steeply dipping northern and eastern limbs and 1988; Milner et al., 2005). Episodic deposition likely Baltz et al. (1966) designated the lower conglom- shallowly dipping southern and western limbs. continued through the Oligocene (e.g., Cather et al., erate and dinosauriferous middle shale of Bauer Like most of the sedimentary cover of the Colorado 2008), but SJB bedrock units younger than Eocene (1916) to the Kirtland Formation and restricted the Plateau, the central, southern, and western SJB is are not preserved within the basin. A generalized name Ojo Alamo Sandstone to the upper conglom- relatively undeformed, with regional dips of <5°. stratigraphic column showing units relevant to this erate and pebbly sandstone. Fassett (1966) extended The eastern basin margin is highly deformed along paper is presented in Figure 2. this designation to include the medium- to thickly the Nacimiento uplift, with Mesozoic and Cenozoic interbedded shales and channel sandstones over- units steeply dipping to overturned (Woodward et lying the shales of the Kirtland Formation at Mesa al., 1972). The northern basin margin is moderately ■■ PREVIOUS WORK Portales in the southeasternmost outcrop area near deformed along the San Juan uplift, Archuleta anti- Cuba, New Mexico (these facies are absent in other clinorium, and Hogback monocline, with Paleozoic Throughout the 20th century, the issues of age portions of the SJB). The debate over these designa- through Cenozoic units moderately to steeply dip- and nomenclature surrounding the K-Pg units in the tions has continued for half a century and is fueled ping (Steven et al., 1974). The western and southern SJB were addressed by Sinclair and Grainger (1914), by contradictory absolute age estimates, geograph- basin margins are slightly deformed along the Defi- Reeside (1924), Dane (1936), Baltz (1953, 1967), ically limited field studies, varying interpretations of ance upwarp and Zuni uplift, respectively, with Anderson (1960), Baltz et al. (1966), O’Sullivan et al. conformable versus disconformable contacts, and Mesozoic and Cenozoic units gently dipping (Baltz, (1972), Clemens (1973), Fassett (1973, 1974), Lindsay nomenclature obfuscation. 1967). The structure of the SJB leads to a bullseye pattern in map view, with Eocene sedimentary units in the central basin surrounded by rings of outcrop- NW SJB SE SJB ping Paleocene and Cretaceous units. (Farmington) (Cuba) The SJB was located on the western margin of the Western Interior Seaway during its maximum Eocene San Jose Formation San Jose Formation inundation from ca. 95 Ma (Cenomanian) until ca. 74 Ma (Campanian) and accumulated as much as 56.0 Ma 1900 m of marine sands and muds during three major transgressive-regressive episodes (Leipzig, Animas Nacimiento Nacimiento Formation Formation Formation 1982; Klute, 1986; Roberts and Kirschbaum, 1995). Following the retreat of the Western Interior Seaway Figure 2. Generalized stratigraphic e o g n at ca. 74 Ma, the SJB accumulated as much as 625 m Paleocene column of Upper Cretaceous and Pa- a l leogene units in the northwestern of fluvioclastic sediment during the Campanian and P and southeastern San Juan Basin Maastrichtian (Klute, 1986; Sikkink, 1987; Cather, McDermott Ojo Alamo Ojo Alamo (SJB) near the towns of Farmington 2004; Donahue, 2016). These deposits include the Formation Sandstone Sandstone and Cuba, New Mexico, respectively Fruitland Formation and the Kirtland Formation. (see Fig. 1). Zigzag lines represent in- During the onset of the major Laramide regional terfingering contacts. Dashed lines 66.0 Ma and question mark in right-hand deformation in the Paleocene, the SJB accumulated Naashoibito Member Maastrichtian column represent the unknown du- 72.1 Ma as much as 700 m of fluvioclastic sediment (Sik- ration of the Cretaceous/Paleogene De-Na-Zin Member ? kink, 1987; Williamson and Lucas, 1992; Williamson, Kirtland unconformity in the SE SJB. 1996; Russell, 2009). These deposits in the northern Formation Farmington Sandstone s Member SJB include the McDermott Formation and Animas Kirtland Formation Formation, and in the central and southern SJB, Hunter Wash Member e o u

the Ojo Alamo Sandstone and Nacimiento Forma- c a Campanian Fruitland Formation Fruitland Formation tion. The contemporaneous relationships of these t e

units are illustrated in Figure 2. Fluviatile deposition r continued into the Eocene after major Laramide C uplifts occurred adjacent to the SJB, resulting in the Pictured Cli s Sandstone Pictured Cli s Sandstone accumulation of as much as 650 m of fluvioclastic

Powell (1973) provided measurements of thereafter to a source in a basement-cored uplift measured sections previously published by Baltz et >7000 paleocurrent-direction indicators through- near the present-day central to eastern San Juan al. (1966), Baltz (1967), Powell (1973), Leipzig (1982), out the Ojo Alamo Sandstone outcrop area. His Mountains. Sikkink suggested that K-Pg McDermott Klute (1986), Sikkink (1987), and Wegert and Parker interpretation of these data suggests a mean flu- Formation and Animas Formation conglomeratic (2011). We analyzed publicly available electric well vial transport azimuth of 138°. Given the direction fluvial and debris-flow deposits in the northernmost logs from the New Mexico Oil Conservation Divi- and consistency of paleocurrent indicators, Powell SJB were shed from these emerging highlands sion in order to estimate subsurface thickness of suggested a single sedimentary source area to the while the finer-grained fluvial deposits, including pertinent units. Petrographic data were collected northwest of the SJB in the vicinity of the present the Ojo Alamo Sandstone, represent the distal with thin section point counts using the Folk method western San Juan Mountains or La Plata Moun- meandering stream counterparts. (Folk, 1957). Whole-rock bulk geochemical analyses tains (Colorado). Powell’s interpreted depositional In the early 21st century, several studies focused of sandstones and mudstones were performed on a environment for the Ojo Alamo Sandstone is an on the Ojo Alamo Sandstone because of isolated Rigaku ZSX Primus II X-ray fluorescence spectrom- alluvial plain that resulted from the downgradient dinosaur fossils found within it (e.g., Fassett et al., eter by the Analytical Chemistry Laboratory in the transport of sands and gravels from alluvial fans 2002, 2011; Fassett, 2009; Sullivan et al., 2005; Lucas Department of Earth and Planetary Sciences at the located proximal to the source area. et al., 2009). These studies were focused in the geo- University of New Mexico (Albuquerque) in order An investigation of outcrops of Ojo Alamo graphic and stratigraphic vicinity of the dinosaur to investigate geochemical trends among K-Pg for- Sandstone by Klute (1986) included sedimento- fossils in question and focused new attention on mations preserved in the SJB. logic, petrographic, and paleocurrent analyses and the age of what was variously interpreted as the interpretations. Klute interpreted the depositional Ojo Alamo Sandstone (sensu Baltz et al., 1966) or environment of the Ojo Alamo Sandstone as South the Kimbeto Member of the Ojo Alamo Sandstone ■■ FIELD OBSERVATIONS AND RESULTS Saskatchewan– or Platte-type sandy braided rivers (sensu Powell, 1973). Of interest to this study is that sensu Miall (1981). The sandy braided river models the magnetostratigraphy utilized by the above stud- Ojo Alamo Sandstone of Miall (1981, 1996) best fit most of the Ojo Alamo ies suggests that the unconformity below the Ojo Sandstone sedimentary features described by Klute Alamo Sandstone represents a deposition hiatus of The 5–120-m-thick Ojo Alamo Sandstone contains (1986). However, none of the modern river analogs 2–4 m.y. (Lindsay et al., 1981; Butler and Lindsay, pebbly arenite and wacke, mud-clast conglomerate, in Miall (1981, 1996) or Klute (1986) match all of the 1985; Lucas et al., 2006). However, paleomagnetic claystone, and siltstone. The unit’s thickness varies features observed in the Ojo Alamo depositional and geochronologic evidence from Flynn et al. both in outcrop and in the subsurface throughout system. Klute attributed the relative homogeneity (2020) suggests the duration of the sub–Ojo Alamo the basin. Generalized stratigraphic columns with of the Ojo Alamo Sandstone throughout most of Sandstone unconformity is much shorter. Further- major lithologic designations and their locations are the study area to the avulsion of active channels more, radiogenic isotope age estimates derived presented in Figure 3. In most areas, the formation is through the basin and the subsequent reworking of from detrital zircons and sanidines in the Kirtland characterized by pebbly medium- to coarse-grained sediment during channel shifting and flood events. Formation, Ojo Alamo Sandstone, and Nacimiento arenite with local lenses and horizons of claystone. Klute’s petrographic analyses show that the Ojo Formation suggest that the hiatus between these Beds range in thickness from 0.3 to 10 m. Sand- Alamo Sandstone is enriched in potassium feld- formations was as short as 1.5 m.y. (zircon) and stones are typically light brown to yellow brown spar and sedimentary lithic grains relative to the 0.4 m.y. (sanidine) (Mason et al., 2013; Donahue, and in some places have developed reddish brown underlying Kirtland Formation fluvial sandstone. 2016), further decreasing the length of the K-Pg weathering surfaces or desert varnish. Claystones Sikkink (1987) analyzed lithofacies in the Ojo hiatus in the SJB. and siltstones are typically light brown, brown, or Alamo Sandstone and adjacent units in order to pro- dark brown and are commonly covered in colluvium. vide interpretations of depositional history near the Sandstone beds commonly overlie scour surfaces K-Pg boundary in the SJB. Sikkink interpreted the ■■ METHODS in claystones, while claystones conformably overlie Ojo Alamo Sandstone to be synchronous with both gradational boundaries. Sand grains are angular to the Animas Formation and the Nacimiento Forma- Stratigraphic section sites were selected on the subrounded and consist of quartz, feldspar, sedi- tion; interfingering of the Ojo Alamo Sandstone and basis of correlation of the Ojo Alamo Sandstone mentary and volcanic lithic fragments, chert, and McDermott Formation is noted in the northwestern with other units, amount of exposure, complete- mafic minerals. Medium- and coarse-grained SJB near Farmington, New Mexico. The interpreta- ness of section, and accessibility. Stratigraphic sandstones are predominately clay cemented with tions proposed by Sikkink (1987) include a source sections were measured with Jacob’s staff and tape. minor silica cements. Less common fine-grained area in an early Laramide magmatic center near the In addition to our own measurements, we utilized sandstones are cemented with roughly equal parts present-day La Plata Mountains that shifted shortly petrographic data and lithologic descriptions from clay and silica. Bedforms include both trough and

tabular crossbeds, convolute bedding, horizontal plane bedding, lenticular bedding, wavy bedding, Mudstone and fluid-escape structures. Silicified stumps, wood fragments, and logs as much as 34 m long are common in the sandstones of the Ojo Alamo Sandstone. Crossbedded and plane-bedded sandstone Leaf fossils are present but uncommon in the forma- Tn tion’s claystones (Flynn and Peppe, 2019). Toa Pebble conglomerate Conglomerates and conglomeratic sandstones of the Ojo Alamo Sandstone are either pebble Crossbedded pebbly sandstone conglomerate or mud-clast conglomerate. Pebble conglomerates contain clasts from 2 to 100 mm (−1 to −7 ϕ) in diameter and include subrounded Tn to well-rounded pebbles of chert, alkali feldspar 15 meters granite, quartzite, silicified wood, trachyandesite, Toa sandstone, limestone, and metapelite. Mud-clast conglomerates contain clasts from 30 to 500 mm (−5 to −9 ϕ) in diameter of subangular to very angular claystone. The mud-clast conglomerate Tn lithofacies is restricted to cut-and-fill structures Toa that represent scouring in both intraformational claystones as well as the underlying Cretaceous claystones at the base of the Ojo Alamo Sandstone. Toa Toa Toa Kk Kk Kk Descriptions of Ojo Alamo Sandstone Facies Head Canyon BDNZ Mesa Portales We categorize the Ojo Alamo Sandstone into three broad facies associations: (1) amalgamated Figure 3. Generalized stratigraphic sections and major lithofacies at three representative Ojo Alamo Sand- channel belt deposits, (2) overbank floodplain stone outcrop areas. Head Canyon is in the northwestern San Juan Basin (SJB); Bisti/De-Na-Zin Wilderness deposits, and (3) isolated channel-fill deposits, Area (BDNZ) is in the west-central SJB; Mesa Portales is in the southeastern SJB (see Fig. 1). Kk—Kirtland described below and in Table 1. Formation; Toa—Ojo Alamo Sandstone; Tn—Nacimiento Formation.

TABLE 1. DESCRIPTIONS AND INTERPRETATIONS OF FACIES ASSOCIATIONS OF THE OJO ALAMO SANDSTONE Facies Description Structures Bed thickness Interpretation Amalgamated channel Poorly to moderately well-sorted medium- Planar horiontal beds, tabular crossbeds, 0.1–1.5 m Deposited in vertically and laterally belt deposits to coarse-grained pebbly sands. Beds trough crossbeds, tangential crossbeds, amalgamating fluvial channels with contain trace to 5 pebbles. Some beds cut-and-fill structures, contorted bedding high sediment supply exhibit weak grading. Overbank floodplain Sandy and silty claystones with organic Horiontal tabular beds, discontinuous Mudstones: Deposited on fluvial floodplains in a deposits shale lenses. Minor moderately sorted shale lenses, discontinuous sand lenses 1–5 cm; vertically accreting aggradational fine- to medium-grained sandstone lenses. sandstones: system Rare weakly developed paleosols. 0.1–0.4 m Isolated channel-fill Poorly to moderately sorted medium- to Symmetric and asymmetric channel forms; 0.1–1.0 m Deposited during low-stage deposits coarse-grained sandstones and pebbly beds typically mimic cross-sectional episodes when systems assumed sandstones. Clay rip-up clasts common morphology of channel cut Fig. 4 anastomosing forms, or in cross-bar where cut into mudstones. channels

Amalgamated Channel Belt Deposits Figure 4. Cut-and-fill structures The amalgamated channel belt facies associ- in the Ojo Alamo sandstone. ation contains poorly to moderately well-sorted (A) Complex 1.2-m-thick medium- to coarse-grained pebbly crossbedded channel structure cut into and plane-bedded sandstones. These lithological intraformational mudstone 2 km southwest of Farming- compositions and sedimentary structures, along ton, New Mexico. Dashed line with abundant petrified wood and lack of marine marks the base of the channel or estuarine fossils, suggest deposition in fluvial 1 m structure. (B) Simple 1.1-m-thick channel environments (Miall, 1978; Robinson and channel structures cut into, filled with, and overlain by McCabe, 1998; Bridge, 2003; Gibling, 2006). Tabu- medium-grained sandstone, lar and trough crossbeds represent downstream Mesa Portales, New Mexico. and oblique bar-face deposits (Best et al., 2003) Dashed lines mark the base of and range from 0.3 to 1.5 m in thickness. The channel structures. Dotted line marks the base of the overlying abundance of tangential crossbeds suggests high stratum. Scale bars are approx- sediment transport rates (Bagnold, 1954; Sallenger, imate given that photographs 1979; Bridge, 2003). Planar horizontal beds repre- are slightly oblique to outcrops. sent vertically accreted bar-top deposits and range Coordinates of Figure 4A = from 0.2 to 1.5 m in thickness. These beds suggest 3 m 36.699°N, 108.227°W. Coordi- nates of Figure 4B = 35.949°N, high bed shear stress and sediment transport rates 107.034°W. (Bridge and Best, 1988, 1997; Bridge, 2003). These facies commonly exhibit highly contorted bedding, especially near the tops of stories. Cut-and-fill structures are found both at the base of and within these deposits (Fig. 4). Where these facies overlie mud- Overbank Floodplain Deposits aggradational settings. These facies conformably stones, clay clasts from the underlying deposits are overlie sandy channel deposits and are overlain by found within the sandstones and conglomerates. The overbank floodplain facies association amalgamated channel belt deposits and cut-and- The deposits of this facies association form sim- contains sandy mudstones with interbedded fill structures. ple and multi-story bodies as much as 11 km wide fine-grained sandstones. This facies association’s Deposits of this facies association form later- and as much as 19 m thick. Individual channel belts laterally discontinuous sand lenses, organic-rich ally extensive complexes with widths of >1 km are impossible to recognize in the field. In the south- fossil hash layers, and leaf fossils (Flynn et al., 2014; and thicknesses of as much as 7 m. These depos- ern and western SJB, these facies contain multiple Flynn and Peppe, 2019) suggest deposition in flu- its are commonly truncated by channel sandstones stories that represent erosion within former chan- vial floodplain environments (Kraus and Gwinn, (Fig. 5A). In the southern SJB at Mesa Portales nel deposits, suggesting reworking of material and 1997; Alexander and Fielding, 2006). Paleosols are and the western SJB at Head Canyon, these facies a low ratio of accommodation to sediment supply uncommon; where present, they lack horizonation separate as many as four thick, laterally extensive (Kjemperud et al., 2008; Owen et al., 2015b). Multiple and are identified based solely on root traces. Bed- amalgamated channel belt complexes, leading to stories, lateral continuity, and the lack of identifiable ding is difficult to observe in most outcrops but is the bluff-and-slope topography common in the Ojo channel-belt margins collectively suggest that ver- thin and tabular to laminar where observed. Mud- Alamo outcrop area at those locations (Fig. 5B). tical and lateral amalgamation of channel deposits stones in these facies contain considerable sand was a widespread process occurring throughout and silt, suggesting deposition in close proximity to both the temporal and the spatial range of deposi- channels, high sediment supply, or high flood mag- Isolated Channel-Fill Deposits tion of fluvial units (Friend et al., 1979; Wang et al., nitude (Guccione, 1993; Pizzuto, 1987; Owen et al., 2011). While there is no significant downcurrent thin- 2015b). The sandy and poorly sorted nature of these The isolated channel-fill facies association ning of these facies, they do exhibit a downgradient deposits is similar to that described in the proximal contains poorly to moderately sorted medium- to reduction in mean and maximum grain size from overbank deposits of Slingerland and Smith (2004) coarse-grained sandstones and pebbly sandstones coarse sand and cobbles in the northwest to medium and Hajek and Edmonds (2014), which they show to that fill simple channels with well-defined channel sand and pebbles at Mesa Portales in the southeast. be associated with vertically accreting systems in geometry in cross-section. Their geometry (Fig. 4A)

suggests incision of a simple channel into overbank Figure 5. Amalgamated channel or channel deposits followed by deposition from a belts in the Ojo Alamo Sand- stone. (A) Overbank floodplain non-migrating channel. These deposits commonly deposits of the Ojo Alamo contain clay rip-ups where overlying mudstones. Sandstone (outlined in white Both symmetric and asymmetric channel forms dashed line; partially obscured are present. Wings associated with levee and/or by boulder talus) truncated by medium-grained channel sand- crevasse-splay deposits are absent. Some isolated stone, Mesa Portales, New channel-fill deposits are found in close proximity Mexico. Arrows mark the tops to one another in the same stratigraphic interval of three stratigraphically dis- (Fig. 4B), suggesting possible anastomosing fluvial tinct channel belts. Note the wide shelf at left formed by form. Given the lithologic similarity to the sand- retreat of sandstone overlying stones of the amalgamated channel belt facies mudstone; the shelf is absent through which they incised, these isolated chan- at right, where the vertically nel-fill deposits might represent possible episodes stacked sandstone bodies form a continuous subvertical expo- of low stage during which the depositional rivers sure. Scale varies due to photo assumed an anastomosing form that was not pres- angle; sandstone bodies are ent during episodes of higher stage (Miall, 1981, 5–9 m thick. (B) Succession of 1996). Alternatively, these deposits could represent vertical faces of channel sand- cross-bar channels (Sambrook Smith et al., 2009). stone and vegetated slopes of overbank mudstones within the This interpretation does not require a stage-depen- Ojo Alamo Sandstone, Mesa Portales, New Mexico. Arrows mark the tops of four stratigraphically distinct channel belts. dent variation in fluvial form and is in accord with Scale varies due to photo angle; sandstone bodies are 4–12 m thick. Coordinates of Figure 5A = Looking north from inferred reworking of sediment arising from the 35.926°N, 107.034°W. Coordinates of Figure 5B = Looking southeast from 35.930°N, 107.032°W. observed lithologic homogeneity between isolated channel-fill deposits and amalgamated channel belt deposits. Sedimentary Architecture Sandstone Petrography Deposits of this facies association form lens- shaped bodies of 5–90 m width and 1–9 m thickness The sedimentary architecture of the Ojo Alamo Petrographic data from SJB Upper Cretaceous that cut into underlying sandstones and mudstones. Sandstone varies across the outcrop area. In and lower Paleogene sandstones are presented in These deposits are overlain by sandstones along some sections, such as the Shannon Bluffs south Figure 6. Sandstones from the Fruitland Formation distinct boundaries. Most isolated channel-fill of Farmington (Fig. 1), the Ojo Alamo Sandstone are well-sorted fine- to medium-grained subrounded deposits in the Ojo Alamo Sandstone exhibit no forms a single 12–70-m-thick amalgamated com- to rounded quartz arenites. Kirtland Formation sand- appreciable upward-fining trends, suggesting that plex channel belt sandstone cliff with no observed stones show significant variation in mineralogical they are not abandoned main channels (Miall, 1996; mudstones. These multi-story sand bodies are composition; they are moderately to well-sorted Bridge, 2003). Flynn and Peppe (2019) reported laterally continuous for at least 3 km. In other loca- fine- to medium-grained subrounded to subangular 15–25-m-wide lenticular carbonaceous shales tions, such as Head Canyon and Mesa Portales, the arkosic arenites and lithic arenites. Sandstones from they interpreted as abandoned channel fills analo- Ojo Alamo Sandstone consists of laterally continu- the Ojo Alamo Sandstone are poorly to moderately gous to oxbow lakes in modern meandering fluvial ous sand bodies separated by mudstone interbeds. sorted medium- to coarse-grained subrounded to systems, an interpretation with which we agree. Where mudstones are present, they are commonly very angular arkosic arenites; two of the 39 sam- These deposits are difficult to observe except where laterally truncated by overlying channel deposits, ples are lithic arenites. Sandstones from the lower exposed in vertical cliff faces, leading to a probable making mudstones less laterally extensive than Nacimiento Formation (Arroyo Chijuillita Member underestimation of their abundance. Given their sandstones. In the Bisti/De-Na-Zin Wilderness Area of Williamson and Lucas [1992] and Kutz Member relative thinness and narrowness when compared and Escavada Wash (Fig. 1), the Ojo Alamo Sand- of Cather et al. [2019]) are moderately to well-sorted to geometries of amalgamated channel belt depos- stone has a 6–10-m-thick single-story form. These fine- to medium-grained angular to well-rounded its, we do not interpret the channel dimensions thinner single-story forms contain abundant very feldspathic arenites and arkosic arenites cemented recorded in the isolated channel-fill facies as rep- coarse sand and pebbles, perhaps representing with clays and amorphous silica. resentative of the main channels of the Ojo Alamo high-energy bar-head or confluence scouring and Detrital grains in SJB K-Pg sandstones include Sandstone fluvial depositional system. minimal deposition of relatively coarse material. quartz, plagioclase feldspar, potassium feldspar,

Q Q

F L F L Fruitland Formation, n = 5 Kirtland Formation, n = 28

Q Q

F L F L Ojo Alamo Sandstone, n = 39 Nacimiento Formation, n = 11

= QFL composition of sandstone sample LEGEND = Formation average QFL composition = Compositional eld of sands from continental block provenance (Dickinson et al., 1983) = Compositional eld of sands from recycled orogen provenance (Dickinson et al., 1983) = Compositional eld of sands from magmatic arc provenance (Dickinson et al., 1983)

Figure 6. Ternary diagrams showing composition and tectonostratigraphic provenance of detrital grains of San Juan Basin Cretaceous-Paleogene sandstones. Q—quartz; F—feldspar; L—lithic fragments. Provenance compositional fields are from Dickinson et al. (1983).

chert, lithic fragments, hornblende, micas, amphi- by sericitization and vacuolization (Fig. 7), with These data show that while the total feldspar con- bole, and opaque minerals assumed to be oxides. chemically altered feldspars becoming less com- tent in both formations is not significantly different, Both monocrystalline and polycrystalline quartz mon upsection. there is significant variation in the ratios of pla- grains are present. The wide range of angularity in Relative proportions of plagioclase feldspar gioclase feldspar to potassium feldspar in the SJB quartz and chert grains in the Kirtland Formation, and potassium feldspar for sandstones from the across the K-Pg boundary, with the Ojo Alamo Ojo Alamo Sandstone, and Nacimiento Formation Kirtland Formation and Ojo Alamo Sandstone are Sandstone being richer in potassium feldspar. suggests that rivers depositing all three of these presented in Figure 8. Mean values for 25 Kirtland In both formations, potassium feldspars include formations were reworking sands from older Formation sandstones are 62% plagioclase feld- orthoclase and microcline. Phanerozoic units upstream. Detrital feldspars in spar and 38% potassium feldspar. Mean values the Ojo Alamo Sandstone display a wide range for 39 Ojo Alamo Sandstone sandstones are 37% of physical weathering and chemical alteration plagioclase feldspar and 63% potassium feldspar. Geochemical Analyses

The bulk geochemical compositions of 57 Q sandstones from the Ojo Alamo Sandstone and immediately adjacent formations are presented in L Table 2. SiO2 and Al2O3 compose 60%–89% of the Q sandstones by mass. The Ojo Alamo Sandstone is S Qp richer in K and Al than the sandstones of the Kirt- S land Formation (Figs. 9 and 10). The geochemical M L composition of the Nacimiento Formation sand- M stones is more variable and overlaps with that of S Q the Kirtland Formation and Ojo Alamo Sandstone. Q L F Q S ■■ DISCUSSION Q F Changing Sediment Sources across the K-Pg Q Boundary Q Q As shown in Figure 6, significant petrographic S S F Q variation exists among the sandstones of the Fruit- land Formation, Kirtland Formation, Ojo Alamo Q Sandstone, and Nacimiento Formation. The quartz Q Qp arenites of the Fruitland Formation (Leipzig, 1982) suggest that the rivers that deposited the Fruitland L Formation during middle to late Campanian time F L were sourcing mature sediments, likely reworked L from underlying Cretaceous marine and shoreface sands. The scarcity of feldspars and lithic grains is Figure 7. Photomicrographs of detrital grains in sandstones of the Ojo Alamo Sandstone. Coordinates of sample indicative of either very distal deposits of a mature in Figure 7A and 7B = N35.9505°, W107.0191°. Coordinates of sample in Figure 7C = N35.8903°, W107.0386°. Coor- dinates of sample in Figure 7D = N35.9290°, W107.0322°. (A) Unaltered microcline (M) surrounded by sericitized fluvial system or a quartz-rich source area, such as feldspars (S) and quartz (Q). (B) Unaltered microcline (M), polycrystalline quartz (Qp), monocrystalline quartz (Q), a broad coastal plain underlain by beach deposits lithic grains (L), and untwinned feldspars (F). Note variation in rounding in quartz grains. (C) Angular plagioclase of the ultimate Western Interior Seaway regression, feldspar (F) showing albite twinning and minor sericitization, subangular plagioclase feldspar showing relict now preserved in the SJB as the Pictured Cliffs albite twinning and sericitization (S, upper left), and subangular sericitized feldspar (S, right of center), all with Sandstone. The quartz-feldspar-lithic (QFL) compo- iron-oxide staining on grain boundaries. (D) Angular unaltered untwinned feldspar with exsolution lamellae (F), polycrystalline quartz (Qp), monocrystalline quartz (Q), and lithic grains (L). Eight-digit symbols at the bottom sition of the Fruitland Formation plots in the craton right of each photomicrograph (e.g., OJAL-101B) are specimen identifiers. interior–continental block provenance categories of

of Cather et al. (2019), indicating southeastward direction of transport, and paleogeographic infor- mation from Baltz (1967), Cather (2004), Heller et O

j al. (2012), and Lawton (2019), we suggest that this o

l relief might have been created in the first stages a m

o of the Laramide San Juan or Defiance uplifts to

S a

n the northwest of the SJB or the regional uplift and d s

t o unroofing of the Mogollon Rim to the southwest n e sandstones of the SJB (Fig. 1). Sandstones in the Ojo Alamo Sandstone show an increase in both the total proportion of lithic

Figure 8. Relative proportions of pla- grains and in the ratio of potassium feldspar to n

= gioclase feldspar and potassium plagioclase feldspar relative to the underlying Cre- 3 9 feldspar from sandstones of the Ojo Al- taceous sandstones. The increase in the ratio of amo Sandstone and Kirtland Formation. potassium feldspar to plagioclase feldspar and the Plagioclase feldspars comprise 62% of Kirtland Formation sandstone feldspars proportions of hornblende, mica, and heavy mineral but only 33% of Ojo Alamo Sandstone grains in the Ojo Alamo Sandstone suggest devel- feldspars. Data are not displayed in opment of a sediment source in unique crystalline stratigraphic order because intraforma- igneous or metamorphic terrains, potentially the

K tional stratigraphic position is difficult i r t l to trace across the San Juan Basin. Needle Mountains (Colorado) in the western San a n

d Juan uplift ~125 km north of the SJB or the Mogollon

F o

r Rim ~300 km southwest of the SJB. The paleo m a

t i current analyses of Powell (1973) and Sikkink (1987) o n

both support a northerly source area for the Paleo- s a n

d cene fluvial system that deposited the Ojo Alamo s t o

n Sandstone. The Needle Mountains in the western e s

San Juan Mountains contain the nearest and largest

exposures of crystalline basement rocks and, in light 2 5 of the above paleocurrent analyses, were upstream 0% 50% 100% of the Ojo Alamo Sandstone depocenter, making Plagioclase feldspar Potassium feldspar them the likely source for basement-derived detri- tus in the Ojo Alamo Sandstone. Donahue (2016), Bush et al. (2016), and Pecha et al. (2018) presented data showing a significant population of detrital zir- Dickinson et al. (1983). The increase in feldspar and lithic grains in Kirtland Formation sandstone as the cons from the Ojo Alamo Sandstone with ages as lithic grains in sandstones of the Kirtland Forma- result of evolving upland source areas related to young as 68.0 ± 1.4 Ma, suggesting a young igneous tion could be indicative of unroofing of basement the early phase of Laramide localized uplift. The source within the Ojo Alamo drainage area, most rocks in the sediment source area, an increase in sand-dominated fluviatile Vermejo Formation in likely the Colorado Mineral Belt–associated La Plata volcanic activity in the drainage basin, a significant the Raton Basin ~250 km east of the SJB (Fig. 1) magmatic center ~100 km north of the SJB. This reorganization of regional drainage patterns lead- is coeval with the Kirtland Formation and contains interpretation agrees with the paleocurrent analyses ing to a different sediment source area, or some detrital grains indicative of as much as 1 km of and the presence of minerals in both the La Plata combination of these. Given the conformable char- localized uplift in the headwaters of its depositional magmatic center and the more proximal McDermott acter of the Fruitland Formation–Kirtland Formation fluvial system (Cather, 2004). Similar-scale uplift Formation (O’Shea, 2009; Gonzales, 2010; Wegert contact (Baltz, 1967; Klute, 1986), significant reor- and fluvial response likely occurred in the source and Parker, 2011). ganization of regional drainage patterns between areas of rivers in the SJB, leading to the increase in Sandstones in the lower Nacimiento Forma- the deposition of these two formations is unlikely. feldspathic and lithic detrital grains in the Kirtland tion have similar petrographic compositions to Instead, we interpret the increase in feldspar and Formation. In light of the paleocurrent synthesis the Ojo Alamo Sandstone. Unaltered detrital

grains of plagioclase feldspar, potassium feld- deposit of the same fluvial system(s) that depos- fluvial deposits in this study area record rivers flow- spar, and hornblende suggest a source area in ited the Ojo Alamo Sandstone. ing from highlands in the southwest toward the crystalline bedrock, likely shared with that of the coastal plain of the retreating Western Interior Sea- Ojo Alamo Sandstone. Sericitized feldspars and way (Potochnik, 1989; Cather et al., 2012; Dickinson, well-rounded quartz and chert grains suggest Regional Landscape Evolution Recorded in 2013) (Fig. 11). Detrital zircons in these sediments reworking of sediment from older sedimentary SJB K-Pg Deposits are predominately Phanerozoic in age, suggesting units. The >5-km-thick exposed section of Paleo- a limited source of Proterozoic crystalline basement zoic and Mesozoic sedimentary rocks between the The relative homogeneity, lateral surface and rocks during this depositional period (Dickinson et San Juan uplift and the SJB depocenter provides subsurface stratigraphic continuity, and improved al., 2012; Bush et al., 2016; Donahue, 2016; Pecha abundant potential sources for the variety of grain age constraints from the Ojo Alamo Sandstone et al., 2018). The orientation of these rivers was compositions and textures in Nacimiento Forma- and adjacent units provide a unique opportunity similar to the north-northeast-flowing Turonian riv- tion sandstones. The petrographic composition, for interpretation of the sedimentary, tectonic, and ers proposed by Blum and Pecha (2014), indicating interfingering contact with the underlying Ojo fluvial factors at work during deposition across the that the lower Kirtland Formation provides the final Alamo Sandstone, and earliest Paleocene age of K-Pg boundary, shown in paleogeographic maps in record of sedimentation in the study area prior to the Nacimiento Formation all suggest that it is the Figures 11–13. Campanian and lower Maastrichtian significant regional drainage reorganization caused

TABLE 2. BULK MAJOR ELEMENT GEOCHEMICAL COMPOSITION OF SANDSTONES SAMPLED FROM THE KIRTLAND FORMATION Kk, OJO ALAMO SANDSTONE Toa, AND NACIMIENTO FORMATION Tn

Formation Sample name Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Total Latitude Longitude mass mass mass mass mass mass mass mass mass mass mass N Kk KISH-001 1.4 1.11 13.8 45.4 0.15 1.84 25.0 0.7 1.03 7.13 8.8 35.8880 107.0380 Kk KISH-002 1.75 1.38 18.8 .4 0.25 3.20 1.5 1.18 0.04 4.1 .5 35.8880 107.0380 Kk KISH-003 1.52 1.12 18.5 8.3 0.04 2.73 1.3 1.00 0.02 4.1 .5 35.8880 107.0380 Kk KISH-004 1.08 1.3 17.8 5.3 0.22 4.30 2.3 1.05 0.21 . 7. 35.8880 107.0380 Kk KISH-011 1.35 0. 11.8 3. 0.14 1.8 34.80 0.78 1.2 .2 .2 35.8880 107.0380 Kk KISH-012 1.88 1.35 1.3 5.1 0.27 3.10 2.33 1.42 0.04 4.5 .4 35.8880 107.0380 Kk KISH-013 1.48 1.23 1.1 7.5 0.04 2.1 1.0 0. 0.02 5.4 .7 35.8880 107.0380 Kk KISH-023 1.47 1.1 18. 8.1 0.04 2.1 1.70 0.0 0.02 4.3 .2 3.3184 108.0478 Kk KISH-024 1.12 1.44 18.3 0.4 0.21 4.21 2.58 1.02 0.12 .17 8. 3.3184 108.0478 Kk KIFA-001 1.28 0. 14.8 5. 0.10 2.4 12.80 0.81 1.12 5.0 .0 3.151 107.888 Kk KIFA-002 1.07 1.11 17. 2.1 0.21 2.1 10.70 0.0 0.0 2.3 .3 3.151 107.888 Kk KIFA-003 1.32 1.1 14.5 4.2 0.04 1. .33 1.00 0.43 4.1 8. 3.151 107.888 Kk KIFA-004 1.31 1.20 1.1 0.1 0.11 2.33 .0 0.8 0.0 .3 8. 3.151 107.888 Kk NASH-02 2.35 1.13 15.8 5.0 0.04 2.11 1.23 0. 0.04 8.05 0.5 3.3248 108.0577 Toa OJAL-102 0.55 0.5 25.8 5.7 0.0 1.48 1.3 1.23 0.00 7.2 .3 35.505 107.011 Toa OJAL-104 0.41 1.05 23.5 0.5 0.07 3.4 1.51 1.03 0.0 8.15 .8 35.803 107.038 Toa OJAS-001 0.72 1.43 20.7 2.2 0.10 4.3 1.18 1.12 0.0 7.15 . 35.20 107.0322 Toa OJAS-002 0.35 1.13 20.1 2.4 0.08 4.05 1.58 0.8 0.17 8.8 .8 35.20 107.0322 Toa OJAS-003 1.21 0.32 13.3 74.1 0.11 .4 0.58 0.47 0.04 2. .3 35.20 107.0322 Toa OJAS-005 0.88 1.21 1. 3.0 0.13 4.83 1.03 1.1 0.11 7.32 . 35.20 107.0322 Toa OJAS-011 0.8 1.43 21.3 2.1 0.11 4.7 1.18 1.1 0.05 .8 .7 3.3203 108.0215 Toa OAES-01 1.01 1.11 21.5 5.5 0.12 7. 2.51 1.01 0.10 5.25 .8 3.1755 107.13 Toa OAES-02 0.1 0.80 20.4 3.4 0.10 5.1 1.4 1.21 0.07 4. .0 3.1755 107.13 Toa OALP-02 0. 0.5 22. 2. 0.08 .0 1.1 0. 0.04 4.10 . 3.11 108.110 Toa OALP-03 0.3 1.08 23.1 1.1 0.0 .1 1.1 1.08 0.07 4.8 .7 3.11 108.110 Toa OALP-04 0.5 0.42 20.0 .7 0.11 .8 0.8 0.88 0.04 2. .1 3.11 108.110 Toa OADN-01 0.7 1.01 22.7 2.1 0.10 .1 1.5 0.2 0.10 3.4 . 3.701 108.228 Toa OADN-03 0.5 1.13 23.3 1.5 0.12 5.73 1.37 1.10 0.13 3.5 .3 3.701 108.228 continued

TABLE 2. BULK MAJOR ELEMENT GEOCHEMICAL COMPOSITION OF SANDSTONES SAMPLED FROM THE KIRTLAND FORMATION Kk, OJO ALAMO SANDSTONE Toa, AND NACIMIENTO FORMATION Tn continued

Formation Sample name Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Total Latitude Longitude mass mass mass mass mass mass mass mass mass mass mass N Tn CUME-02 0.75 0.0 1.7 1.8 0.10 2.22 7.1 1.32 0.78 4.12 .0 35.85 107.034 Tn CUME-0 1.0 0.1 11.0 7.7 0.02 1.55 0.31 0.1 0.02 2.75 4. 35.84 107.035 Tn CUME-08 0.70 0.07 1.2 2.0 0.07 2.17 .71 1.10 0.58 5.0 8.5 35.84 107.035 Tn CUME-14 1.23 1.0 15.0 57. 0.05 2.03 0.5 0. 0.05 13.5 2.2 35.85 107.034 Tn CUME-18 0. 0.73 21.4 4.5 0.08 2.3 1.3 1.15 0.03 5.2 .1 35.84 107.0345 Tn CUME-1 0.10 0.7 1.2 4.8 0.07 2.1 4. 0. 0.85 4. 8. 35.84 107.0345 Tn BLAN-05 0.87 0. 1. 5. 0.0 2.11 5.81 1.1 1.00 7.3 .0 3.3022 107.81 Tn MEPO-001 0.57 0.77 20.4 5.3 0.07 1.8 1.34 1.24 0.03 8.22 .8 35.35 107.0058 Tn MEPO-002 0.7 0.53 18.5 .7 0.04 2.42 0.08 1.38 0.03 5.3 8. 35.35 107.0058 Tn MEPO-003 0.2 0.53 15. 47. 0.0 2.08 2.40 0.4 1.52 3.52 8. 35.35 107.0058 Tn LIPL-01 1.18 1.31 22. 0.1 0.21 4.53 1.11 0. 0.0 7.32 .7 3.012 108.00 Tn LIPL-02 1.01 1.10 23.4 4.1 0.10 5.0 0.0 1.01 0.0 3.1 . 3.012 108.00 Tn LIPL-03 0.5 0. 21.2 0. 0.08 .1 2.2 1.0 0.10 3.1 7.3 3.035 108.100 Tn LIPL-04 0.3 1.13 22.5 1.7 0.13 5.8 1.1 1.01 0.08 4.55 8. 3.8552 108.113 Tn LIPL-07 0.85 0.87 1. 0. 0.02 3.8 5.1 0.0 0.05 .54 .8 3.8550 108.118 Tn FK-001 1.8 0.7 15.0 58. 0.0 2.51 1.8 0.0 0.02 3.80 85.4 3.2217 107.8731 Tn BIPO007 0.2 2.28 17.5 5.3 0.07 2.43 1.1 0.82 0.03 .82 2.1 3.228 107.742 Tn FK-002 2.28 0.8 1.5 .0 0.0 2.57 1.75 0.58 0.04 4.45 5.0 3.2217 107.8731 Tn KFT08 1.55 1.40 1. 4.8 0.02 0.82 0.7 1.10 0.03 5.72 3.0 3.0173 107.378 Tn CRO05 0.13 0.1 7.2 81.8 0.01 0.07 0.11 0. 0.01 0.83 1.3 3.5513 107.821 Tn CRO04 0.32 0.43 20.1 3.3 0.02 0.10 0.55 1.03 0.01 3.8 8.8 3.5513 107.821 Tn JACA05 1.28 1.0 20.4 0.0 0.01 1.10 0.88 0.7 0.03 5.1 0.7 3.331 107.3 Tn BETS-04 1.33 0.80 1. 3.8 0.21 2.1 1.28 0.5 0.02 .8 4.1 3.1750 107.731 Tn BDN-53 1.18 1.02 1.7 5.1 0.04 1.2 0.54 0.7 0.02 .33 0. 3.3284 107.1 Tn BDN-50 1.07 0. 17.7 3.3 0.04 1.47 0.55 0.72 0.01 4.5 0.8 3.3284 107.1 Tn BDN-35 1.38 1.05 22.1 5.2 0.02 0.50 1.08 0.78 0.04 5.78 8.0 3.3243 108.014 Tn BDN-41 1.52 1.1 14. 58.8 0.10 2.23 0.5 0. 0.04 .13 8. 3.3257 108.0012 Tn ANGE-0 1.17 1.0 15. 1.3 0.05 2.15 1.1 0. 0.02 .37 2.5 3.5244 107.008 Tn ANGE-05 1.17 1.07 15. 1.7 0.05 2.1 1.17 0. 0.02 .40 3.1 3.5244 107.008

by the Laramide orogeny. The relatively large areal (Fassett, 1974, 1985; Newman, 1987), resulting in changes associated with the onset of Laramide expanse of these Turonian through Maastrichtian preservation of younger pre-erosion rocks in the tectonism. The late Maastrichtian age of this dep- deposits suggests limited localized basin subsid- northwest than in the southeast. This differential ositional hiatus (Williamson and Weil, 2008) is in ence. Farther west in the North American Cordillera, erosion is perhaps due to uplift in the southeast accord with proposed regional- (Mackey et al., 2012; Campanian deposits record evidence of broad flex- related to the incipient Nacimiento uplift, passage Bush et al., 2016; Cather et al., 2019) to continen- ural responses to the beginning of Laramide-style of a flexural high associated with the transition tal-scale (Galloway et al., 2011; Cather et al., 2012; deformation (Aschoff and Steel, 2011; Leary et from broad-wavelength Sevier tectonism to short- Blum and Pecha, 2014) drainage reorganization epi- al., 2015), but no such evidence is yet recognized er-wavelength Laramide tectonism beneath the SJB sodes. Given the lack of sedimentary rocks in the in the SJB. during this interval, or increased subsidence to the SJB from this interval, it is unclear whether or not After deposition of the Fruitland Formation east of the SJB leading to lowering of local base the La Plata magmatic center or the San Juan uplift and the majority of the Kirtland Formation by level. More work is needed to increase understand- contributed sediment to the SJB via south-south- north-northeast-flowing rivers, there was a period ing of the timing and effects of tectonism in the east-flowing streams during this interval. As shown of non-deposition and erosion in the SJB (Fig. 12). area. We propose that this period of non-deposition in Figure 12, we interpret the erosive Maastrich- This erosion removed more sediment from the and erosion represents the interval of SJB drainage tian fluvial systems to have had the same general southeastern SJB than from the northwestern SJB reorganization resulting from regional landscape orientation as those that deposited the Kirtland

65 Figure 9. Si versus Al for sandstones from Kirt-

%) land Formation, Ojo Alamo Sandstone, and s s Nacimiento Formation. Kirtland Formation sand- a 60 Kirtland Formation

( m stones are depleted in Al relative to Ojo Alamo

i Ojo Alamo Sandstone

S Sandstone sandstones. Nacimiento Formation Nacimiento Formation 55 sandstones exhibit Si/Al ratios that overlap with those of sandstones from both the Kirtland For- 50 mation and the Ojo Alamo Sandstone.

35 5 10 15 20 25 30 Al (mass %)

65 Figure 10. Si versus K for sandstones from Kirt-

%) land Formation, Ojo Alamo Sandstone, and

s s

a 60 Kirtland Formation Nacimiento Formation. Kirtland Formation sand-

( m stones are depleted in K relative to Ojo Alamo i Ojo Alamo Sandstone S Sandstone sandstones. Nacimiento Formation 55 Nacimiento Formation sandstones exhibit Si/K ratios that overlap with those of sandstones from both the Kirtland For- 50 mation and the Ojo Alamo Sandstone.

35 0 1 2 3 4 5 6 7 8 K (mass %)

109° W 108° W 107° W 106° W paleoflow direction for the Ojo Alamo depositional system (Fig. 13). The accommodation of Laramide 0 50 100 km uplift–derived sediment in the SJB represents two significant changes in regional landscape evolution: 38° N first, a >90° shift in regional drainage directions N resulting from reorganization of regional-scale topography; and second, a preferential storage of sediment in isolated subsiding basins rather than on wide coastal plains. The first shift is recorded Alamosa nearly simultaneously in other basins (Heller et Durango Pagosa Springs Trinidad al., 2012). The second shift is in accord with K-Pg

Utah sedimentary records on the western Gulf Coastal Colorado 37° N Plain in Texas (USA) that show a lack of earliest Paleocene sediments there (Galloway et al., 2011; Mackey et al., 2012). The south-southeast–flowing Arizona Farmington Paleocene depositional system persisted through

New Mexico the deposition of the Nacimiento Formation.

ay aw Se r Avulsion and Preservation during Ojo Alamo Cuba rio te Sandstone Deposition 36° N n I n er st Fluvial depositional systems are affected by Santa Fe e W Gallup external (allogenic) boundary conditions and processes such as climate, base level, subsidence, and ca. 72 Ma paleogeography uplift, as well as internal (autogenic) processes (Fruitland Formation/Kirtland Formation deposition) and variability such as avulsion, channel mor- Albuquerque 35° N (modi ed from Cather, 2004) phometry, roughness, and bank strength. It is not always apparent whether a signal observed in the Fluvial Kirtland Fmtn. rock record is the result of allogenic or autogenic Upland areas Marine areas depositional depocenter variability. Wang et al. (2011) suggested that over areas shorter time scales, fluvial depositional systems Figure 11. Paleogeographic interpretation of the San Juan Basin (SJB) and surrounding areas during the Cam- aggrade sediment somewhat randomly, whereas panian. Low-gradient streams flowing from the Mogollon Highlands in the southwest toward the retreating over longer time scales, systems preferentially Western Interior Seaway deposit well-sorted sands and muds across much of the region. The thickest ac- cumulations are preserved in the northwestern SJB. Modified from Cather (2004). Dashed line indicates the aggrade via compensation, the tendency for dep- location of the base of the modern Ojo Alamo Sandstone outcrop belt. ositional systems to fill topographic lows. The duration of time above which allogenic variability seems to overpower autogenic variability is differ- Formation. The variation in the thickness of the Ojo latest Maastrichtian and earliest Paleocene, begin- ent for each system and difficult to constrain (Paola Alamo Sandstone across the SJB (Fig. 14) might be ning with the Naashoibito Member of the Kirtland et al., 1992; Sheets et al., 2002; Covault et al., 2010) explained by accommodation of earliest Paleocene Formation and the Ojo Alamo Sandstone. Detrital but is affected by roughness and aggradation rate (i.e., Ojo Alamo Sandstone) sediment in incised val- zircon (Donahue, 2016; Pecha et al., 2018), volcani- (Wang et al., 2011). In the northern and central SJB, leys left by north-northeast-flowing Maastrichtian clastic grain petrography (O’Shea, 2009; Gonzales, the uppermost Naashoibito Member of the Kirt- erosive streams. 2010; Wegert and Parker, 2011), and paleocurrent land Formation, the entire Ojo Alamo Sandstone, The Maastrichtian hiatus ended with the (Powell, 1973; Sikkink, 1987) evidence suggests and the lowermost Nacimiento Formation are all deposition of the first basin-filling sediments by a source area in the La Plata magmatic center contained within geomagnetic polarity chron C29r south-southeast–flowing fluvial systems in the and San Juan uplift, requiring a south-southeast (Peppe et al., 2013; Williamson et al., 2014; Flynn et

109° W 108° W 107° W 106° W Sandstone deposition in the northern and central

t SJB. In order to better understand relationships

0 50 100 km i

u between allogenic processes, autogenic variability,

i and deposition of the Ojo Alamo Sandstone, we

e used age constraints of chron C29r from Sprain et

38° N d

Incipi e

ent San Ju r al. (2018), observed outcrop and well-log forma- a g

n upli

N ft n a

S tion thickness data (presented in Table 3), and the

n e

i methods of Wang et al. (2011) to produce a range p ? i c of compensation time scales ( ). estimates rep- n TC TC

I Alamosa resent the sedimentation durations above which Durango Pagosa Springs Trinidad alluvial sediments present forms influenced purely

Utah by allogenic processes and is defined as: e

Colorado n li c 37° N o n l o T = , m C r

Arizona Farmington

k ? c where l is a roughness length scale in meters, and

a b

g Incipient San New Mexico o Juan r is the basin-wide sedimentation rate in meters H Ba sin per thousand years (Wang et al., 2011). Sedimen-

tation durations below TC will produce sediments ? ? that record both autogenic and allogenic variations. 36° N Cuba For roughness length scales, we use the depth of the thickest channel deposits we observe in the Ojo Alamo Sandstone of 10 m. Sedimentation rates are Santa Fe Gallup calculated from available age constraints and formation thickness data (Table 3). Our TC estimates ca. 67 Ma paleogeography (SJB depositional hiatus; range from 26 k.y. (using the highest sedimentation erosion of youngest rate) to 43 k.y. (using the lowest sedimentation rate). Albuquerque Cretaceous units) 35° N These TC values essentially represent the duration required for the fluvial system to have filled its Erosional areas; Fluvial Incipient La Plata deepest channels with sediment. These estimates Upland areas reds indicate depositional magmatic center are an order of magnitude less than the duration of greater erosion areas sedimentation for the entire Ojo Alamo Sandstone Figure 12. Paleogeographic interpretation of the San Juan Basin (SJB) and surrounding areas during the late estimated above. Therefore, it is likely that the Ojo Maastrichtian. Deposition of the lower members of the Kirtland Formation had ceased, and erosional streams Alamo Sandstone’s sedimentary architecture is the had removed lower Maastrichtian and possibly upper Campanian deposits. Erosion was greatest in the south- result of predominately allogenic processes. This eastern SJB, potentially due to relatively greater uplift in that area associated with the incipient Nacimiento conclusion is supported by the formation’s wide uplift. In the central SJB, northeast-flowing streams became entrenched, creating a non-planar erosion surface onto which uppermost Maastrichtian and Paleocene sediments would be deposited. Maastrichtian-aged detri- geographic distribution, which would have been tal zircons in upper Maastrichtian SJB deposits (Naashoibito Member of the Kirtland Formation) suggest that unlikely to have occurred in a system controlled shallow intrusive igneous rocks of the La Plata magmatic center were uplifted and eroded shortly after cooling by autogenic processes. (Donahue, 2016). Dashed line indicates the location of the base of the modern Ojo Alamo Sandstone outcrop belt. Estimated Ojo Alamo Sandstone sedimentation rates are 1.1–6.2 times greater than estimated sedimentation rates of the overlying Nacimiento al., 2020), which had a maximum duration of 587 area. The erosional unconformity between the Ojo Formation of Williamson (1996), suggesting a rap- ± 53 k.y. (66.311–65.724 Ma), the Paleogene por- Alamo Sandstone and the underlying Naashoibito idly growing sediment source, rapidly subsiding tion of which was 328 ± 15 k.y. (66.052–65.724 Ma) Member of the Kirtland Formation precludes con- basin, or combination of the two. The Nacimiento (Sprain et al., 2018), thus the Ojo Alamo Sand- stant deposition through chron C29r, therefore Formation estimated sedimentation rates of Wil- stone represents <~328 k.y. of deposition in that shortening the assumed duration of Ojo Alamo liamson (1996) are minimum estimates because

109° W 108° W 107° W 106° W Ojo Alamo fluvial system in as little as ~328 k.y. 0 50 100 km is paradoxical. With the exception of minor muds preserved at Mesa Portales, a large proportion of the fine sands and muds must have bypassed

38° N San Ju through the SJB. The downgradient depocenter is an uplif N ? t not known and likely was removed by post-deposition uplift and erosion. By the late Paleocene, fluvial systems were routing sediment from the southern Rocky Mountains Laramide highlands to the west- Alamosa ern Gulf of Mexico coastal plain (Galloway et al., ? Pagosa Springs Trinidad Colorado Durango 2011; Mackey et al., 2012), but Paleocene sediments along the presumed paleoriver paths between the Utah 37° N SJB and the Gulf of Mexico coastal plain are not preserved. In light of these considerations, the

Arizona Farmington Ojo Alamo Sandstone in the SJB highlights the

New Mexico ? problems associated with assuming that the pres- t

f t i f l i ent structural basin in which fluvial sedimentary l p p u

? u

e t rocks are preserved can be directly equated to the o c f

i ? t

l n s i

a p

r u fluvial basin in which those sediments were depos-

C e ?

o e

D t d

n ited. The lithologic composition (e.g., predominant

e e Cuba i r

36° N m i coarse sandstones and scarcity of mudstones) and n

c a

a S

N the proximity to major Laramide tectonic features (e.g., <1 km from the Nacimiento fault, a Laramide Santa Fe reverse fault with ~2 km of vertical offset; Wood- Gallup ward et al., 1992) suggest that the present outcrop To Gu Zu lf of Mexico n ? and subsurface area of the Ojo Alamo Sandstone i u pl ? ift ca. 65.96 Ma paleogeography represents a small portion of the area of the 35° N Albuquerque (Ojo Alamo Sandstone deposition) entire fluvial system in which the sediments were deposited. Recent studies of well-preserved intact Fluvial ca. 67 Ma La Plata paleofluvial basins have shown that the deposits of Upland areas Laramide uplifts depositional volcanic center areas a single fluvial system show significant lithologic and stratigraphic variation over spatial scales of Figure 13. Paleogeographic interpretation of the San Juan Basin (SJB) and surrounding areas during the earli- hundreds of kilometers (Klausen et al., 2014, 2015; est Paleocene. Early Laramide uplift of the Defiance, Zuni, and San Juan uplifts reorganized regional drainage patterns, causing area streams to flow south-southeast. Subsidence of the SJB provided accommodation Owen et al., 2015b). Deposits of such systems are for the sediment these streams eroded from the emerging highlands. Volcanism in the La Plata magmatic subject to post-deposition deformation (Klausen center contributed sediment via erosion of shallow intrusive and volcanic rocks in the source area and via et al., 2014) and partial exhumation and removal tephra fall. Uplift of the Sangre de Cristo highlands separated the fluvial systems of the Raton Basin from SJB (Owen et al., 2015b), making interpretation of the fluvial systems for the first time since the retreat of the Western Interior Seaway (Bush et al., 2016), leading to sediment accumulation and storage in isolated Laramide basins rather than on the retreating Gulf of Mexico remnants of the system subject to inherent prob- coastal plain (Galloway et al., 2011; Blum and Pecha, 2014). Dashed line indicates the location of the base of lems associated with incomplete stratigraphic the modern Ojo Alamo Sandstone outcrop belt. records. While there is no evidence for major vertical downcutting or erosion after deposition of the Ojo Alamo Sandstone, some portion of it has been they do not take into account the numerous sed- associated with mudstones because a subsiding removed around the basin margins. imentary hiatuses evidenced by well-developed basin creates accommodation for storage of even Several workers have addressed the topic of paleosols throughout the formation (Hobbs and fine-grained easily transportable sediment (Blair, recognizing avulsion deposits in fluvial sedimen- Fawcett, 2014). In fluvial-dominated continen- 1987; Valero et al., 2017). Therefore, the deposi- tary packages (e.g., Smith et al., 1989; Kraus and tal basins, high sedimentation rates are usually tion of as much as 120 m of coarse sands by the Wells, 1999; Slingerland and Smith, 2004; Jones

108° W 107° W well-defined avulsion deposits requires interpretation of the processes and products of the Ojo Alamo fluvial system. One possible cause, illustrated in Figure 15, is that the Ojo Alamo fluvial system did not undergo random avulsion in the typical sense. Fassett (1985) suggested that the 37° N Ojo Alamo Sandstone depositional system shifted

er v eastward through time due to development of i R s accommodation in that direction during the devel- a r Dulce im ive n n R opment of the Laramide foreland basin and uplifts A Jua San (Fig. 15). This could explain the fining-eastward lithologic changes noted by Sikkink (1987). If over- Farmington bank muds were stored preferentially at any one time on the eastern side of the fluvial system due to increased accommodation there, then perhaps they were subsequently removed when the channel belt shifted eastward. In this scenario, the only remnants of the overbank muds are the thin discontinuous mudstones observed among the channel belt deposits. The abundant channel scours into and through muds (Fig. 4A) are evidence of mud removal during Ojo Alamo Sandstone deposition; perhaps this process was more important 92 m (300 ft) 76 m (250 ft) than has been previously considered. Flynn et al. 61 m (200 ft) (2020) reported paleomagnetic evidence for the Ojo 46 m (150 ft) N Alamo Sandstone–Nacimiento Formation contact 30 m (100 ft) being time transgressive in nature, with the contact 36° N Cuba Ojo Alamo Sandstone younging to the south. Those authors interpreted outcrop area that time transgression as the result of down- 39 km (24 miles) stream progradation of a distributive fluvial system. Similar paleomagnetic and geochronologic investigations in the subsurface and along the southern Figure 14. Isopach map of Ojo Alamo Sandstone. Modified from Russell (2009). and eastern margins of the Ojo Alamo Sandstone outcrop area are necessary to elucidate whether the formation records an eastward younging. Bet- and Hajek, 2007). Though the specific type of avul- system difficult. The formation is spatially wide- ter constraints on west-to-east age relationships sion affects the exact sedimentary signature of the spread, exhibits subparallel paleocurrent indicators of the Ojo Alamo Sandstone are needed in order event, avulsion deposits are generally recognized throughout its outcrop area, and appears to have a to develop this hypothesis more fully, but the Ojo as having (1) a lower coal or paleosol overlain by common source area in the La Plata magmatic cen- Alamo Sandstone presents a case in which the pre- a general coarsening-upward sequence of hetero- ter–San Juan uplift area, which all suggest that it vailing accepted explanations for processes and lithic fine sandy deposits and channel sandstones was deposited by a fluvial system characterized by products of avulsion in continental fluvial systems belts, and (2) an upper paleosol representing aban- migrating channels. Avulsion is a requisite process are problematic. donment of the developed channel system (Kraus for such systems to form. However, the sedimen- An alternative interpretation of the Ojo Alamo and Wells, 1999). The Ojo Alamo Sandstone lacks tary architecture of the Ojo Alamo Sandstone does Sandstone is that it represents the deposits of paleosols, abundant overbank mudstones, and not provide the record of avulsive processes out- roughly parallel coalescing alluvial systems drain- heterolithic fine sandy deposits, making interpre- lined above. Reconciling the seeming necessity of ing the La Plata magmatic center, San Juan dome, tation of the avulsion history of its depositional avulsion in the system with the observed lack of and/or Colorado Mineral Belt uplifts during an

episode of rapid uplift. This interpretation is sup- TABLE 3. SEDIMENTATION RATE ESTIMATES FOR THE OJO ALAMO SANDSTONE ported by the subparallel paleocurrent indicators Thickness of Oo Alamo Sandstone Inferred duration of sedimentation Sedimentation rate of Powell (1973) and Sikkink (1987), perpendicu- m k.y. m/k.y. lar-to-paleoflow lithologic changes of Sikkink (1987), 120 maximum thickness 313 minimum estimate of Sprain et al. 2018 0.38 and thickness variation (Fig. 14) of the formation. 120 343 maximum estimate of Sprain et al. 2018 0.35 The rapid uplift of a northerly source area would 5 average thickness 313 0.21 have provided an abundance of coarse material for 5 343 0.1 south-flowing streams that rapidly filled accom- minimum thickness 313 0.02 343 0.02 modation space and bypassed most fines through the SJB. By the time of deposition of the overlying Nacimiento Formation, the ratio of sediment supply to accommodation was lower, leading to accumu- La Plata-San Juan uplift lation of more fines.

■■ CONCLUSIONS Nacimiento NORTH uplift Petrographic and paleocurrent evidence from De ance uplift- fluvial siliciclastic rocks bracketing the K-Pg bound- Hogback monocline ary in the SJB provides records of evolution of paleorivers’ source areas through the Late Creta- ceous and Paleocene. The first rivers depositing sediment in the SJB after the retreat of the Western Interior Seaway flowed north-northeast and deposited the quartz sands of the Fruitland Formation. By the Maastrichtian Age, the same north-northeast– flowing rivers deposited the arkosic sands of the Kirtland Formation, suggesting unroofing of crys- San Juan Basin talline bedrock in their source areas southwest of the SJB. Reorganization of regional fluvial systems after deposition of most of the Kirtland Formation in the Maastrichtian resulted in removal of some early Maastrichtian and possibly upper Campan- ian fluvial sediments and a basin-wide erosional unconformity. This reorganization is likely related to the onset of Laramide tectonism at ca. 67 Ma. By the beginning of the Paleocene at 66 Ma, SJB paleorivers flowed south-southeast from the La Plata magmatic center–San Juan uplift and through Figure 15. Schematic diagram showing eastward progression of the Ojo Alamo fluvial the SJB, depositing the arkosic sandstones and system depositional area due to increased uplift in the west and/or increased subsid- conglomerates of the Ojo Alamo Sandstone. The ence and accommodation in the east. (A) Inception of Ojo Alamo Sandstone deposition, coincident with creation of accommodation in the San Juan Basin after basin-wide Ojo Alamo Sandstone’s relative enrichment in Maastrichtian erosion. Exact chronologic relationships among subsidence, uplift, and potassium feldspar relative to plagioclase feldspar sedimentation are unknown. (B) Increased accommodation to the east due to increased suggests a source area rich in potassium feldspar, subsidence in the east, increase uplift in the west, some combination of these two, or the nearest known and most likely such area being unknown factors. Major axis of the Ojo Alamo fluvial system shifts to the east, depositing younger sediments (here represented by a darker shade) eastward. (C) Further increase the Needle Mountains of the western San Juan in accommodation and shift of the fluvial system to the east. Ojo Alamo Sandstone uplift. Early to middle Paleocene sandstones of deposition continues, here represented by the darkest shade.

the Nacimiento Formation show similar paleocur- migrating fluvial system. A potential explanation Baltz, E.H., 1967, Stratigraphy and regional tectonic implications of part of Upper Cretaceous and Tertiary rocks, east-central rent indicators and mineralogic composition to the that reconciles the absence of expected avulsion San Juan Basin, New Mexico: U.S. Geological Survey Pro- Ojo Alamo Sandstone. This petrographic informa- stratigraphy with the necessity of a spatially migrat- fessional Paper 552, 101 p., https://doi.org/10.3133/pp552. tion, combined with the interfingering depositional ing fluvial system is that the system did not avulse Baltz, E.H., Ash, S.R., and Anderson, R.Y., 1966, History of nomenclature and stratigraphy of rocks adjacent to the Cre- contact between the Ojo Alamo Sandstone and with the typical compensation-driven processes taceous-Tertiary boundary, western San Juan Basin, New the Nacimiento Formation, suggest that the latter associated with rivers in unconfined basins. Rather, Mexico: U.S. Geological Survey Professional Paper 524-D, records the continuation of the same fluvial sys- the Ojo Alamo fluvial system potentially was driven 23 p., https://doi.org/10.3133/pp524D. tem(s) that deposited the former in an incipient and unidirectionally eastward across the incipient SJB Bauer, C.M., 1916, Contributions to the geology and paleontology of San Juan County, New Mexico: 1. Stratigraphy of a evolving broken foreland basin. by relatively increased uplift in the west, relatively part of the Chaco River Valley, in U.S. Geological Survey, In the northern and central SJB, the Ojo Alamo increased subsidence in the east, or some com- Shorter Contributions to General Geology, 1916: U.S. Geo- Sandstone occurs entirely within the Paleogene bination of the two during the onset of Laramide logical Survey Professional Paper 98-P, p. 271–278, https:// doi.org/10.3133/pp98P. portion of chron C29r, which gives it a maximum deformation in the earliest Paleocene. The observed Best, J.L., Ashworth, P.J., Bristow, C.S., and Roden, J., 2003, deposition duration of as 328 ± 15 k.y. and a sedi- lack of avulsion stratigraphy throughout most of Three-dimensional sedimentary architecture of a large, mentation rate of as high as 0.39 m/k.y., indicating the Ojo Alamo Sandstone alternatively could be mid-channel sand braid bar, Jamuna River, Bangladesh: significant subsidence during the early Paleocene. Journal of Sedimentary Research, v. 73, p. 516–530, https:// explained by the interpretation that the formation doi.org/10.1306/010603730516. Because the wide sandstone bodies that compose represents not the deposits of one fluvial system Blair, T.C., 1987, Tectonic and hydrologic controls on cyclic allu- the majority of the Ojo Alamo Sandstone are associ- but rather the coalesced deposits of multiple alluvial fan, fluvial, and lacustrine rift-basin sedimentation, Jurassic–lowermost Cretaceous Todos Santos Formation, ated with low ratios of accommodation to sediment vial systems draining a rapidly uplifted northerly Chiapas, Mexico: Journal of Sedimentary Research, v. 57, supply, the Ojo Alamo Sandstone must represent source area. Further work is needed regarding the p. 845–862, https://doi.org/10.1306/212F8C83-2B24-11D7 a period of high sediment supply during which timing of uplifts adjacent to the SJB in order better -8648000102C1865D. predominately coarse sand and pebbles were to constrain SJB depositional history and to inform Blum, M., and Pecha, M., 2014, Mid-Cretaceous to Paleocene North American drainage reorganization from detrital zir- deposited in the SJB. Given the presence in the Ojo understanding of relationships between tectonics cons: Geology, v. 42, p. 607–610, https://doi.org/10.1130 Alamo Sandstone of Maastrichtian-aged detrital and sedimentation. /G35513.1. zircons from the La Plata magmatic center and the Bridge, J.S., 2003, Rivers and Floodplains: Forms, Processes, abundance of potassium feldspar presumably from and Sedimentary Record: Oxford, UK, Blackwell Publish- ing, 504 p. crystalline basement rocks in the San Juan uplift, ACKNOWLEDGMENTS Bridge, J.S., and Best, J.L., 1988, Flow, sediment transport and this high sediment supply is likely related to the Thorough and constructive comments from Associate Editor bedform dynamics over the transition from upper-stage rapid development of positive topographic relief Cathy Busby and reviewers Chris Clinkscales and Andrew Flynn plane beds: Implications for the formation of planar lami- allowed for considerable improvements to an earlier version of nae: Sedimentology, v. 35, p. 753–763, https://doi .org /10 .1111 in the source area of the Ojo Alamo fluvial system. this manuscript. 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