1 Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the 2 Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan 3 Basin, New Mexico, USA 4 5 Andrew G. Flynn1*, Adam J. Davis1,2, Thomas E. Williamson3, Matthew Heizler4, C. William 6 Fenley IV1, Caitlin E. Leslie1, Ross Secord5, Stephen L. Brusatte6, and Daniel J. Peppe1* 7 1Terrestrial Paleoclimate Research Group, Department of Geosciences, Baylor University, Waco, 8 Texas, 76706, USA; *Corresponding authors: [email protected]; 9 [email protected] 10 2Wood PLC, Novi, Michigan, 48377, USA 11 3New Mexico Museum of Natural History and Science, Albuquerque, New Mexico, 87104, USA 12 4New Mexico Bureau of Geology & Mineral Resources, New Mexico Tech, Socorro, New 13 Mexico, USA, 87801 14 5Department of Earth and Atmospheric Sciences and University of Nebraska State Museum, 15 University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, USA 16 6School of GeoSciences, University of Edinburgh, Grant Institute, James Hutton Road, 17 Edinburgh EH9 3FE, UK 18 Page 1 of 78 19 ABSTRACT 20 The lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation from the San Juan Basin 21 (SJB) in northwestern New Mexico preserve arguably the best early Paleocene mammalian 22 record in North America and is the type location for the Puercan (Pu) and Torrejonian (To) North 23 American Land Mammal ages (NALMA). However, the lack of precise depositional age 24 constraints for the Ojo Alamo Sandstone and lower Nacimiento Formation has hindered our 25 understanding of the timing and pacing of mammalian community change in the SJB following 26 the Cretaceous-Paleogene mass extinction. Here we produced a high-resolution age model for 27 the Ojo Alamo Sandstone and lower Nacimiento Formation combining magnetostratigraphy and 28 40Ar/39Ar geochronology spanning the first ~3.5 Myr of the Paleocene. Mean sediment 29 accumulation rates during C29n were relatively low (<50 m/Myr) and equalized from basin 30 center to basin margin indicating an accommodation minimum; sediment accumulation rates 31 approximately double (> 90 m/Myr) during C28r and are highest in the basin center and lowest 32 on basin margin indicating high accommodation and an increase in basin subsidence near the 33 C29n/C28r boundary (~64.96 Ma). Puercan fossil localities were restricted to C29n, Torrejonian 34 1 localities to C28n, and lower Torrejonian 2 localities to C27r. Our revised age model for the 35 SJB suggests that the first appearance of To1 mammals may have been diachronous across North 36 America, with the Torrejonian 1 mammals first appearing in the north (Montana and North 37 Dakota) during C29n, then in middle latitudes (Utah) in C28r, and lastly in southern North 38 America (New Mexico) in C28n. Page 2 of 78 39 INTRODUCTION 40 The Ojo Alamo Sandstone and the Nacimiento Formation from the San Juan Basin (SJB) 41 in northwestern New Mexico and southwestern Colorado (Fig. 1) preserve a nearly continuous 42 succession of lower Paleocene terrestrial deposits (e.g., Baltz et al., 1966; O’Sullivan et al., 43 1972; Williamson, 1996; Williamson et al., 2008; Cather et al., 2019). The SJB also preserves 44 one of the most complete records of early Paleocene mammalian evolution following the 45 Cretaceous-Paleogene (K-Pg) mass extinction and has been extensively studied for over a 46 century (e.g., Granger, 1917; Matthew, 1937; Simpson, 1959; Williamson and Lucas, 1992; 47 Williamson, 1996; Williamson et al., 2015). The early Paleocene Puercan and Torrejonian North 48 American Land Mammal ages (NALMAs) were defined using SJB fossil mammalian faunas 49 (Wood et al., 1941). These land mammal ages were subsequently divided into biochrons, based 50 largely on the SJB record, and are used for early Paleocene correlation across North America 51 (Lindsay, 2003; Lofgren et al., 2004). 52 Two intervals of potentially rapid mammalian turnover, between Puercan 2 (Pu2) and 53 Puercan 3 (Pu3), and between Torrejonian 2 (To2) and Torrejonian 3 (To3) faunas, were 54 documented by Williamson (1996). Additionally, the boundary between Pu3 and Torrejonian 1 55 (To1) records the near total replacement of mammalian communities with new species and 56 potentially represents another time period of high mammalian turnover, but this pattern is 57 somewhat unclear due to low sampling intensity in To1 (Williamson, 1996). Although Leslie et 58 al. (2018b) were able to develop a high-resolution age model for the To2-To3 transition, no high- 59 resolution age models exist for the Pu2-Pu3 and Pu3-To1 transitions in the SJB, limiting our 60 understanding of the timing and rate of faunal change through this crucial time interval. Page 3 of 78 61 Previous work has used magnetostratigraphy to develop an age model for the Ojo Alamo 62 Sandstone and lower Nacimiento Formation with the goal of identifying the K-Pg boundary and 63 evaluating the chronology of mammalian evolution within the SJB (e.g., Butler et al., 1977; 64 Butler and Taylor, 1978; Lindsay et al., 1978, 1981; Taylor and Butler, 1980; Butler and 65 Lindsay, 1985; Fassett, 2009). However, sample spacing was relatively large and mammal 66 localities were not always precisely correlated to magnetostratigraphic sections, which has meant 67 that there is not a high-resolution age model for Ojo Alamo Sandstone or lower Nacimiento 68 Formation. Additionally, the age and duration of type section of the Nacimiento Formation at 69 Mesa de Cuba (Cope, 1875) has never been determined. 70 In this study, we developed a high-resolution magnetostratigraphic age model for the Ojo 71 Alamo Sandstone and lower Nacimiento Formation spanning the Puercan and early Torrejonian 72 (To1-To2) interval from seven measured sections encompassing the first ~3.5 Myr of the 73 Paleocene. These sections, from northwest to southeast, are (1) Kutz Canyon, (2) Gallegos 74 Canyon, (3) Chico Springs, (4) De-Na-Zin, (5) Kimbeto Wash, (6) Betonnie Tsosie Wash, and 75 (7) Mesa de Cuba (Fig. 1). Local polarity zones from each section, constrained by 40Ar/39Ar ash 76 and detrital sanidine ages, were correlated to the global geomagnetic polarity time scale (GPTS) 77 (Ogg, 2012). Sediment accumulation rates were calculated for each section and/or magnetic 78 chrons within the sections and used to develop an age model for the Ojo Alamo Sandstone and 79 the base of the Nacimiento Formation. This age model was then correlated with the 80 magnetostratigraphic sections of Leslie and others (2018b) to develop a basin-wide 81 geochronological framework for the entire lower Paleocene in the SJB to assess basin evolution 82 during the early Paleocene. The age model was then used to constrain the age of Puercan and 83 early Torrejonian (To1 – To2) fossil localities across the SJB. These revised ages for the Puercan Page 4 of 78 84 and Torrejonian mammal sites have important implications for understanding the tempo of 85 mammalian turnover after the end-Cretaceous mass extinction and the timing of key events in 86 early mammal evolution. 87 88 PREVIOUS STUDIES 89 Geologic Background 90 The SJB is a Laramide foreland basin in northwest New Mexico and southwest Colorado 91 that preserves a nearly continuous succession of Upper Cretaceous (Campanian) to lower Eocene 92 terrestrial deposits (Baltz et al., 1966; Chapin and Cather, 1983). Cather (2004) argued for three 93 distinct subsidence phases in the SJB: (1) an early phase during the late Campanian – early 94 Maastrichtian, (2) a middle phase during the late Maastrichtian – early Paleocene, and (3) a late 95 phase during the Eocene. The middle phase of subsidence was hypothesized to allow for 96 deposition of lower Paleocene sediments in the basin (Cather, 2004). 97 The SJB preserves two lower Paleocene formations: (1) the Ojo Alamo Sandstone and (2) 98 the Nacimiento Formation (Baltz et al., 1966; O’Sullivan et al., 1972). The Ojo Alamo 99 Sandstone unconformably overlies the Maastrichtian Naashoibito Member of the Kirtland 100 Formation (e.g., Baltz et al., 1966; Powell, 1973; Chapin and Cather, 1983; Lehman, 1985; 101 Cather, 2004; Sullivan et al., 2005; Williamson and Weil, 2008a; Williamson et al., 2008; Flynn 102 and Peppe, 2019) and is composed of gold to yellow colored, cross-bedded, medium to coarse- 103 grained sandstone with interbedded sandstone and siltstone deposits, and localized carbonaceous 104 shale beds and local accumulations of large, nearly complete trees suggesting high depositional 105 energies. These deposits have been interpreted to represent an alluvial plain in a seasonally dry, Page 5 of 78 106 sub-tropical climate with one or more sediment sources in the southern Rocky Mountains ( Baltz 107 et al., 1966; O’Sullivan et al., 1972; Powell, 1973; Tidwell et al., 1981; Chapin and Cather, 1983; 108 Sikkink, 1987; Cather, 2004; Flynn and Peppe, 2019). Differences in stratigraphic terminology 109 related to the Ojo Alamo Sandstone and underlying Naashoibito Member have created confusion 110 about the relationship between these two units (for example see discussions in Powell, 1973; 111 Williamson and Weil, 2008a, 2008b). Previous authors recognizing the Naashoibito Member and 112 Ojo Alamo Sandstone as unique stratigraphic units representing different formations (e.g., Baltz 113 et al., 1966; Williamson, 1996; Williamson and Weil, 2008a; Williamson et al., 2008) or as 114 different members of the same formation (Powell, 1973; Sullivan et al., 2005; Fassett, 2009; 115 Cather et al., 2019). Herein we recognize the Ojo Alamo Sandstone as a separate stratigraphic