Record of palaeoclimate across the Cretaceous–Palaeogene boundary from palaeosols in the west‐central San Juan Basin, New Mexico, USA

The mass extinction at the Cretaceous–Palaeogene boundary is widely attributed to sudden and severe climate changes forced by bolide impact and/or flood basalt volcanism. In terrestrial depositional settings, these changes may potentially be recorded by palaeosols. To test the ability of pedogenic features to record both long‐term climate and shorter‐term changes preceding and following the Cretaceous–Palaeogene extinction event, palaeosols in the Upper Cretaceous (Maastrichtian) Naashoibito Member of the Ojo Alamo Formation and the lower Palaeocene (Danian) Nacimiento Formation in the San Juan Basin of north‐western New Mexico, USA, were examined, including data from previous studies. The fine‐grained facies of the Naashoibito Member comprises grey to greenish‐grey and red‐banded mudstones displaying pedogenic features including colour mottling, root traces, cutans, ped fabrics, pedogenic slickensides and calcareous nodules. Aside from a high‐chroma horizon at the formation base, palaeosols in the lower Nacimiento Formation are broadly similar to those observed in the Naashoibito Member. Lateral and vertical variability of the pedogenic features between correlated sections suggest that soil hydrology varied spatially and temporally from very saturated to seasonally well‐drained, with temporal variations controlled by basin sedimentation rates. Abrupt and/or catastrophic climate events precisely at the Cretaceous–Palaeogene boundary are not recorded due to an unconformity at the top of the Naashoibito Member. However, the presence of kaolinite in the clay mineral assemblage of the Nacimiento Formation, particularly near the formation base, but not in the Naashoibito Member, indicates episodic warmth and short (104 years) intervals of more intense weathering conditions during the very early Danian as compared to the late Maastrichtian. Aside from short warm intervals, the overall palaeoclimate during deposition of both formations was warm and consistently subhumid to humid and seasonal, suggesting no substantial long‐term (105–106) climate change took place across the Cretaceous–Palaeogene boundary in the San Juan Basin.


Cretaceous-Palaeogene boundary
The apparent severity and abruptness of the mass extinction at the end of the Cretaceous Period has engendered considerable interest in identifying the precise mechanisms responsible for this event.Both the Chicxulub bolide impact and Deccan Traps flood basalt eruptions are now generally accepted as producing potential forcing mechanisms that operated at the time of the extinctions, although the precise proportions and respective environmental effects of each event remain under investigation.Current and recent research mainly focusses on such diverse environmental disruptions as pCO 2 -induced warming from the Deccan eruptions, which were initiated demonstrably prior to the extinctions (Hernandez Nava et al., 2021;Keller, 2014;Lesko et al., 2021;Tobin et al., 2017;Zhang et al., 2018), as well as sudden soot and/or sulphate aerosol-driven atmospheric cooling in the immediate aftermath of the Chicxulub impact (Kaiho et al., 2016;Morgan et al., 2022;Schulte, 2010;Vajda et al., 2015;Vellekoop et al., 2016) as extinction-driving climate effects.
At some locations, abrupt fluctuations of short-term (10 4 -10 5 years) climate across the Cretaceous-Palaeogene (K-Pg) boundary have been recorded against a much longer-term (10 6 -10 7 years) trend of cooling extending from the Late Cretaceous into the Early Palaeogene (Barrera & Savin, 1999;Friedrich et al., 2012;Linnert et al., 2014;O'Brien et al., 2017).Zhang et al. (2018), using clumped isotope analysis of palaeosol carbonate from the Songliao Basin of northern China, found evidence of substantial pCO 2driven temperature swings preceding and across the K-Pg boundary.A marked cooling episode is recorded from 67.5 to 66.5 Ma, followed by an abrupt warming interval beginning ca 66.4 to 66.3 Ma, coinciding with the C30n-C29r palaeomagnetic boundary.The earlier temperature decrease has been recorded elsewhere, for example in Montana, where an estimated decrease of as much as 8°C was reported by Tobin et al. (2014).Zhang et al. (2018) attributed the pre-K-Pg boundary warming to a pCO 2 increase of ca 500 ppmv and noted the temporal coincidence of this increase with the onset of the Deccan eruptions (Renne et al., 2015;Schoene et al., 2015).Deccan volcanism is known to have proceeded in multiple pulses (Chenet et al., 2009), with the strongest warming associated with the initial eruptive pulse (Self et al., 2006;Wilf et al., 2003) The onset of this warming is estimated to have led to a 5°C increase in summer temperatures in North America (Tobin et al., 2014).
To date, the majority of palaeoclimate proxies across the K-Pg boundary derive from sea-surface temperature proxies, such as δ 18 O of planktic and benthic foraminiferans and carbon isotope excursions from organic and inorganic carbonate (Friedrich et al., 2012;Hull et al., 2020;Nauter-Alves et al., 2023;Petersen et al., 2016;Wilf et al., 2003;Woelders et al., 2018).Nonetheless, a wide variety of palaeoclimate proxies have been utilised to quantify the timing and magnitude of the temperature changes in terrestrial environments leading up to and immediately following the boundary events.These include: leaf-mass analysis and megafloral species richness, which have identified transient temperature spikes at and above the K-Pg boundary (Flynn & Peppe, 2019;Lyson et al., 2019;Wilf et al., 2003); analysis of leaf stomatal indices to track pCO 2 variations, which have suggested an intense pCO 2 spike at the boundary, but a strong decrease in the early Danian (Beerling et al., 2002;Steinthorsdottir et al., 2016); changes in palynomorph assemblages that demonstrate short warming events in the early Danian (Lyson et al., 2019); isotopic analysis of pedogenic carbonate, which suggest pCO 2 -driven temperature swings across the K-Pg boundary (Nordt et al., 2003;Zhang et al., 2018); and variations in clay mineral assemblages, such as the smectite/illite ratios in terrestrial depositional systems that record multiple warming and cooling episodes across the boundary (Gao et al., 2021).Unfortunately, most of these proxies rely on geological features with a discontinuous vertical distribution, unlike many of the proxies from the marine record, and are therefore unable to produce a high-resolution palaeoclimate record.Consequently, conclusions are sometimes based on widely spaced data points (cf.Beerling et al., 2002;Steinthorsdottir et al., 2016;Zhang et al., 2018).Because soils generally form over time scales of 10 3 to 10 5 years (Schaetzl & Thompson, 2015), individual palaeosols represent a time-averaged record of environmental conditions, including palaeoclimate.Sequences of alluvial deposits, where palaeosols are commonly preserved, can potentially provide a near-continuous record of environmental conditions, although with a restricted temporal resolution.

| Objectives
Given the limited temporal resolution of the pedogenic record, relatively few studies have focussed on palaeosols in formations bracketing the K-Pg boundary for the specific purpose of comparing the pre-boundary (i.e.Maastrichtian) and post-boundary (Danian) climate trends.The San Juan Basin of north-western New Mexico contains an extensive sedimentary record that extends above and below the K-Pg boundary.In contrast to the lower Palaeocene palaeosols of the Nacimiento Formation, which have been well-studied in the west-central San Juan Basin (Davis et al., 2016;Hobbs, 2016;Hobbs & Fawcett, 2022), the late Maastrichtian palaeosols of the Naashoibito Member of the Ojo Alamo Formation have been described in only general terms (Lehman, 1985).This study provides new, detailed descriptions of palaeosols of latest Maastrichtian age in the San Juan Basin for the specific purpose of interpreting general palaeoclimate conditions prior to the K-Pg boundary event.Additionally, palaeosols from the previously studied early Palaeocene (Danian) Nacimiento Formation in the study area are described and interpreted.The rationale for measuring a section of this formation is twofold.First, it allows comparison of the palaeosol descriptions and interpretations presented here with those in previously published studies of the same stratigraphic unit in order to assess the validity of the methodology used in this study.Second, and more significantly, it assists in making a direct comparison of the palaeoclimate during the late Maastrichtian to that of the early Danian with a view toward identifying transient climate events and comparing these with the longer-term background climate of this critical time interval in the San Juan Basin.

| Palaeosols as a record of palaeoclimate
Regardless of forcing mechanism, significant changes in mean annual temperature (MAT) and/or precipitation have the potential to create a sedimentary record through their impacts on weathering, sedimentation rates, vegetation growth, surface and subsurface hydrology and soil formation.Pedogenic processes, such as rooting, ped and cutan formation, seasonal shrinking and swelling of clays, mineral weathering, red colouring, eluviation and illuviation, operate at the mercy of ambient climate (Schaetzl & Thompson, 2015).Large variations in temperature impact mineral weathering rates in soils, as well as the depth, density and activity of plant roots and soil fauna.Changes in mean annual precipitation (MAP), or of its seasonal distribution directly affect the processes of eluviation and illuviation as well as the seasonal shrinking and swelling of the clays in soils.Hence, the utility of the study of palaeosols for tracking long-term climate trends is clear, although with the caveat that the record of short-term disruptions lasting from months to decades may be erased subsequently during longer intervals of climate stability.
Key pedogenic features that provide evidence of soil hydrologic conditions include colour, the presence (or absence) of pedogenic slickensides and the presence of clay cutans.Red colouring, or rubefaction, in soils results from the formation of Fe-oxides and hydroxides under oxidising conditions during pedogenesis, followed by dehydration and crystallisation of hematite during burial.Because oxidation of the soil minerals requires substantial periods of low saturation, red colouring in palaeosols is commonly accepted as an indication of moderately to well-drained soil conditions.Rubefaction, therefore, may occur in Protosols, Vertisols and Argillisols.Additionally, Argillisols are evidence of moderately to well-drained soil conditions as the translocation and accumulation of clay minerals to the B horizon is the defining characteristic of this order.Gley colours in the soil matrix materials are the result of reducing conditions, most notably causing reduction and grey-green (drab) colouration of iron compounds (Kraus & Hasiotis, 2006;Noble & Berkowitz, 2016;Tabor et al., 2017).This reduction may be pervasive when it is due to an elevated water table.Thus, Gleysols record extended anoxic to dysoxic conditions due to high water saturation in the soil environment.These conditions are not necessarily perpetual, but exist at least for a majority of the time the horizon occupies the soil-forming environment.Saturated soil conditions are common on low-gradient floodplains in moist temperate climates, but spatially controlled by floodplain topography (Hartley et al., 2013;Lane et al., 2017;Sendek et al., 2021;Weissmann et al., 2013Weissmann et al., , 2015)).Low areas of abandoned channels, floodplain ponds and wetlands adjacent to active channels all may be present at elevations that intersect the water table at least seasonally, or very nearly so, promoting high saturation soil conditions.Unlike the gley colouring of an entire soil horizon, drab mottles surrounding rhizoliths are not an indication of excess soil moisture.Drab haloes are often localised around organic matter, which produces localised reducing conditions in well-drained soils during decomposition.
Vertic fractures, commonly with pedogenic slickensides, and wedge-shaped peds are formed by shrinking and swelling of the soil materials attributed primarily to expandable 2:1 phyllosilicates (Tabor et al., 2017) responding to episodic wetting and drying soil conditions.Such conditions are commonly but not exclusively associated with a strongly seasonal distribution of precipitation.Soils forming under these conditions can be described as moderately to seasonally well-drained.In soils forming over a shallow water table, the seasonality may be reflected in seasonally saturated soils, resulting in the formation of gley colouring in a soil displaying vertic features.In these circumstances, the resulting soil (or palaeosol) is a gleyed Vertisol.
The composition of the clay mineral assemblage in palaeosols can be useful in the study of palaeoclimate.The formation of clay minerals from the weathering of silicate minerals is subject to both the mineralogy of the source material and climate.Chemical weathering reactions, hydrolysis in particular, are both water-dependent and endothermic; silicates break down faster and more completely under warm and wet climates (Al-Hawas, 1989;Berner & Holdgren Jr., 1979;Carnicelli et al., 2015;Tan, 1982).In simplest terms, soils dominated by minerals such as illite and/or smectite are considered less weathered, that is, formed under cooler or drier conditions, than soils dominated by kaolinite.This basic guideline comes with the caveat that the clay mineralogy of the soil may reflect the composition of the source rocks for the sediment rather than the weathering conditions in the depositional basin (cf.Chamley, 1989;Chesworth et al., 2016;Evans, 1992;Spinola et al., 2017;Velde & Meunier, 2008).Thus, immature palaeosols, for example, Protosols, may inherit a substantial portion of their clay mineralogy from the source area due to the shorter period of residence for mineral grains in the A horizon, whereas mature palaeosols, in which the soil materials spend longer periods in the active soil layer, may better reflect climate conditions in the depositional basin.

| Geological setting, stratigraphy and age
The study area is located in the San Juan Basin, a Laramide (Late Cretaceous-Eocene) structural depression located in north-western New Mexico and south-western Colorado, USA, that encompasses over 20,000 km 2 of Late Cretaceous (late Campanian through Maastrichtian) and Palaeogene (primarily Palaeocene-early Eocene) aged rocks (Figure 1).The San Juan Basin was one of a series of intra-foreland basins and uplifts formed by fragmentation of the larger Western Interior foreland basin during the initial stages of the Laramide orogeny during the early Campanian (reviewed by Cather, 2004).The best-studied portion of the basin is the west-central San Juan Basin, from which almost all of the vertebrate fossils, magnetostratigraphic analyses and radioisotopically dated rocks relevant to the Cretaceous-Palaeogene boundary in the San Juan Basin are derived.The area of the study herein is about 47 km south-southeast of Farmington, New Mexico, in the Bisti/De-Na-Zin Wilderness Area, within the westcentral San Juan Basin.Specifically, this study focussed on outcrop sections of the Upper Cretaceous-Palaeocene Ojo Alamo and lower Palaeocene Nacimiento formations in the drainages of Hunter Wash, Alamo Wash and De-na-zin Wash, three of the large tributary arroyos of the Chaco River (Figure 1).This area includes the type sections of the formations that adjoin the K-Pg boundary.
During the late Campanian, retreat of the Western Interior Seaway in the San Juan Basin was followed by deposition of the coastal plain facies of the Fruitland Formation, followed by latest Campanian to early Maastrichtian deposition of the Kirtland Formation (Fassett & Hinds, 1971;Hunt & Lucas, 1992;Lucas et al., 2006).Erosional bevelling across the basin created an unconformity prior to deposition of the Ojo Alamo Formation.Locally, the Ojo Alamo Formation is a lithologically composite unit.In the west-central San Juan Basin, it consists of a basal, intermittent conglomerate or sandstone and a middle mudstone-siltstone dominated unit, collectively termed the Naashoibito Member, and an upper conglomerate/sandstone sheet named the Kimbeto Member (Baltz et al., 1966;Powell, 1973; Figure 2).The 'fine-grained facies' of the Naashoibito Member, defined herein as all strata between the lower conglomerate of the Ojo Alamo Formation and the Kimbeto Member, is the target stratigraphic unit of latest Cretaceous age in this study.The Naashoibito Member is laterally equivalent to sandstonedominated strata that characterise the Ojo Alamo Sandstone elsewhere in the San Juan Basin (Baltz et al., 1966).The Late Cretaceous age of the Naashoibito Member in the west-central San Juan Basin is confirmed by dinosaur and other vertebrate fossils (Jasinski et al., 2011).More specifically, a late Maastrichtian age derives from an Ar/Ar sanidine date of 66.5 Ma reported by Mason et al. (2013) for the base of the unit, as well as biostratigraphic evidence that indicates an apparent Lancian Land Mammal Age (Williamson & Weil, 2008).
In the west-central San Juan Basin of north-western New Mexico, the Naashoibito Member is erosionally truncated by the Kimbeto Member of the Ojo Alamo Formation (the Ojo Alamo Sandstone in the usage of Flynn &Peppe, 2019 andFlynn et al., 2020).A Palaeocene age of the Kimbeto Member, where it overlies the Naashoibito Member, is confirmed by palaeobotanical macroflora and palynomorphs (Anderson, 1960;Baltz et al., 1966;Flynn & Peppe, 2019;Newman, 1987;Williamson & Weil, 2008).Williamson et al. (2008) andFlynn et al. (2020) noted that hypothetically the base could predate the boundary based on calculated sedimentation rates for the Nacimiento Formation; however, the authors consider this possibility very unlikely.
The Kimbeto Member has a gradational and intertonguing upper contact with the overlying Palaeocene Nacimiento Formation (Baltz et al., 1966;Lucas & Rigby Jr., 1979;O'Sullivan et al., 1972;Rigby Jr., 1981).Radioisotopic and palaeomagnetic geochronology place the base of the Kimbeto Member very near the K-Pg boundary, perhaps as close as 66.03 Ma (Flynn et al., 2020;Flynn & Peppe, 2019), establishing a probable Palaeocene age for the base, whereas the uppermost strata of the Naashoibito Member appear to be exclusively latest Maastrichtian in age.As there is no K-Pg boundary bed in either member, the boundary is assumed to occur in the unconformity separating the two.Hobbs (2016) described the duration of the K-Pg hiatus in the San Juan Basin as 'indeterminate', noting that it has been estimated at as much as 2 to 4 Myr (Lucas et al., 2006) or conversely as little as 0.4 Myr (Donahue, 2016;Mason et al., 2013).its thickest exposures hypothetically could be <100 kyr prior to the K-Pg.The geochronology of Flynn and Peppe (2019) places the top of the Kimbeto Member (their Ojo Alamo Sandstone) in the study area about 8 m below the C29r-C29n boundary, which is dated at 65.69 Ma (Flynn & Peppe, 2019).This suggests an age for the base of the Nacimiento Formation of ca 65.88 Ma.

| Sedimentology
The lower conglomerate of the Naashoibito Member is a laterally persistent bed of variable thickness that contains abundant clasts of quartz, quartzite, volcanics and granite (Baltz et al., 1966).The basal contact with the De-nazin shales is conformable to locally disconformable.The bed is overlain conformably by the finer-grained facies of the Naashoibito Member, where the latter are present.At Alamo Wash the fine-grained facies of the Naashoibito Member consists of variegated shales and mudstones with interbedded sandstones with a total thickness ranging from about 2 to over 20 m (Lehman, 1985;Powell, 1972).Lehman (1985) interpreted the banded mudstones in only general terms as palaeosols with leached (pale grey) and spodic (purple) horizons that formed on a well-drained floodplain with a seasonally dry climate.Klute (1986) described the upper Naashoibito facies in detail, noting well-sorted, ripple-laminated to massive sandy claystones, and thin, discontinuous coal and lignite beds.Klute (1986) interpreted the depositional setting of the Naashoibito Member as a low-gradient alluvial plain dotted by swampy areas and traversed by high-sinuosity (meandering) streams with associated splays and chute fills.The Kimbeto Member overlies the Naashoibito Member disconformably with locally substantial relief at the bases of channel scours that commonly contain clasts of the underlying Naashoibito and large shale rip-ups (Baltz et al., 1966).In some localities, the fine-grained facies of the Naashoibito Member and the lower Ojo Alamo conglomerate are scoured out by the Kimbeto Member, which Lehman (1985) described as a soft, smectite-rich sandstone deposited by low-sinuosity, sandy streams.Although dominated by sandstone and minor conglomerate, the Kimbeto Member contains lenses of grey to olivegreen shale.
The base of the Nacimiento Formation is placed at the basal mudstone overlying the uppermost of these interbedded sandstones.The Nacimiento Formation consists mainly of pale blue to greenish-grey (drab) coloured mudstones, sandy mudstones and muddy sandstones with lesser, variegated olive-green to purple and mottled red-grey mudstones (Baltz et al., 1966;Davis et al., 2016;Hobbs, 2016;Hobbs & Fawcett, 2022;Rains, 1981;Williamson & Lucas, 1992).The lower part of the Nacimiento Formation is designated the Arroyo Chijuillita Member (Williamson & Lucas, 1992).Sandstone bodies typically display lateral accretion surfaces and are thus interpreted as recording the transition to high-sinuosity fluvial systems, in contrast to the low sinuosity of the underlying Kimbeto Member streams (Davis et al., 2016).One of the more distinctive features of the Nacimiento Formation is the occurrence of numerous thin, resistant ledges that are siliceous in composition (Davis et al., 2016;Hobbs, 2016;Rains, 1981;Williamson & Lucas, 1992).These beds are termed silcretes, the term used to describe silica-cemented sedimentary layers in a broad sense by most authors.If pedogenic in origin, these beds are relevant to the interpretation of early Palaeocene palaeoclimate.Silcretes aside, palaeosols in the conventional sense are common in the Nacimiento Formation.Hobbs and Fawcett (2022) described multiple pedotypes in the lower Nacimiento Formation, including drab, poorly horizonated profiles, smectitic, well-horizonated profiles and profiles containing mottled red and drab horizons.

| Field measurements
The strata of the Naashoibito Member was measured at four locations spaced 1 to 1.5 km apart, named here (from north to south) Willow Wash, South Mesa North (the north end of south Mesa), South Mesa South (the south end of South Mesa) and Barrel Springs (Figure 1).Each section was described and measured from the unconformable upper contact of the De-na-zin Member of the Kirtland Formation with the base of the lower conglomerate of the Ojo Alamo Formation, through the entire exposure of the Naashoibito Member to its upper contact with the Kimbeto Member.Unit numbers are assigned for every distinct bedding unit as distinguished by lithology, grain size or colour from the surrounding units.Additionally, a ca 35 m section of the lower Nacimiento Formation was measured in De-na-zin Wash from the basal contact with the Kimbeto Member of the Ojo Alamo Formation.Beds in each section that display distinctive pedogenic features were described by assessment of features such as pedogenic fabric, presence of root traces or rhizoliths, colour mottling, pedogenic carbonate and slickensides, as well as assignment of hue, value and chroma using Munsell® soil colour charts.Names were assigned to palaeosol orders, with modifying adjectives that describe the most prominent secondary characteristic, using the palaeosol nomenclature established by Mack et al. (1993) with modifications by Tabor et al. (2017).
In outcrop, both the Ojo Alamo (Naashoibito Member) and Nacimiento formations consist of heavily weathered badlands slopes of mudstones and sandstones (Figure 3A,B) that required excavation to expose fresh rock.On steep slopes, exposure of laterally or vertically continuous sections was not possible, although it was possible to expose sufficient fresh rock to determine colour, grain size and observe pedogenic features for each lithologic unit.In fresh (i.e.excavated) exposures the mudstones are primarily 0.5 to 1.5 m thick layers that are light grey (e.g.5Y 8/1) to greenish grey (7.5YR 6/1) to dusky red (7.5R 4/3) in colour and laterally continuous at outcrop scale.Most, but not all of the mudstone layers contain pedogenic features that are recognisable in excavated exposures.These features include: carbonised roots (Figure 4A) and/or red mottling in grey mudstones; drab mottling in red mudstones, commonly following root traces (Figure 4B,C); calcareous nodules (Figure 4D); blocky to wedge-shaped peds; clay cutans on ped surfaces (Figure 4E); and arcuate pedogenic slickenside surfaces (Figure 4F).Grey mudstone layers with evidence of pedogenesis that gradationally overlie mudstone layers with a different hue (e.g.reddish) and pedogenic features are regarded as the A horizons in profiles containing both A and B horizons.The B horizons are classified based on specific pedogenic features such as Bw (structural horizon underlying a recognisable A horizon), Bt (illuviated horizon enriched in clay), Bg (gley horizon containing evidence of low oxygen conditions, such as drab, that is, grey or green soil matrix colour), Bss (horizon containing pedogenic slickensides) or Bk (calcite-enriched) horizons (Tabor et al., 2017), or combinations of these features, for example, Bssg or Btk horizons.
It is worth noting here that Lehman (1985) reported the occurrence of nodular baryte in the lower portions of some of the palaeosol horizons, which he attributed to pedogenic alteration of volcanic ash.Baryte concretions associated with the palaeosols, as described by Lehman, were not identified.However, abundant concretions with a radiating crystal fabric (Figure 4D) similar to that described by Lehman (1985) were found, but these consisted of sparry calcite.The composition was confirmed by X-ray diffraction, which failed to detect baryte.
Not all horizons appear as A/B pairs; layers with pedogenic features and abrupt upper boundaries are probably soils truncated by erosion in which the A horizon was removed.Compound profiles (sensu Kraus & Bown, 1986;Kraus, 1999) were identified in a few locations where B horizons with different pedogenic features are gradationally superimposed.Assignment of palaeosol orders follows the usage of Mack et al. (1993) and is based mainly on features of the B horizon described above.Palaeosols identified in this study include the following.Immature palaeosols lacking distinct horizonation, but commonly displaying root traces are Protosols (Bw horizon).

| Clay mineral analysis
The mineralogy of the clay assemblage in 20 samples selected to represent the various palaeosol types in both formations was determined by X-ray diffraction analysis of a <2 μm fraction separated by settling tube.Analyses of these separates were conducted on replicates that were oven-dried at 60°C, solvated in a desiccator with an ethylene-glycol atmosphere at 60°C for 8 h or baked at 550°C for 1 h.Diffraction patterns were measured with a Bruker Phaser D2 X-ray diffractometer using a Cu-anode tube operating at 30 kV and 10 mA and solid state detector.Samples were scanned from 4° to 40° 2θ in 0.002° steps recording at 0.5 s per step.Clay mineral identifications are based on the criteria of Moore and Reynolds (1997) and Poppe et al. (2001).Illite was identified by the presence of a well-defined 001 reflection at 10.01 Å in air-dried, glycolated and heated samples.Kaolinite was characterised by the 001 reflection at 7.16 Å, which is unaffected by glycolation and disappears on heating, and the 002 peak at 3.58 Å.The presence of smectite was confirmed by the presence of the 001 peak that expands from 12.8 Å when air-dried to 17.2 Å on glycolation and collapses to 10 Å on heating.The relative proportions of clay minerals were calculated semi-quantitatively from weighted peak areas using methods described by Poppe et al. (2001).

| Naashoibito Member
Observations of the palaeosols in the Naashoibito Member reported here accord in part with those of Lehman (1985), although horizons that appear greyish-purple on the heavily weathered slopes are typically dusky to weak red in fresh exposures.In the four sections measured, a crossbedded, sandstone-dominated unit ranging from 3.5 to 7 m in thickness within the fine-grained facies was observed (Figure 5).The top of this sandstone is consistently 3.5 to 5 m below the Kimbeto Member base, so this is taken as a correlative unit that subdivides the Naashoibito finegrained facies into a thicker lower portion and a thinner upper portion.This division facilitates comparison of palaeosols between presumably correlative portions of the measured sections above and below the sandstone.The main pedogenic features of the four sections (from north to south) measured in the fine-grained facies of the Naashoibito Member are described below (Figure 5; Table 1A through D).
The Willow Wash section is the northernmost and thinnest of the four Naashoibito sections measured, with 15 m of strata measured from the base of the lower conglomerate to the base of the Kimbeto Member (Figure 5).These strata consist of 12 sedimentary units, seven of which contain pedogenic features (Table 1A).All of the palaeosol horizons display gley features, but the additional presence of slickensides in four of these (Bssg horizons) categorises them as gleyed Vertisols (Figure 6; units 6, 8, 11 and 14).Gleyed Protosols are identified where rootlets and gley features are both present (Bwg horizons in units 4, 10 and 13).The lower portion of the Willow Wash section (below the middle sandstone) comprises units 5 through 8 (3.2 m thick).This portion of the section contains four palaeosols, including gleyed Protosol, gleyed Argillisol (Btg horizons) and gleyed Vertisols.The upper portion of the Willow Wash section comprises units 10 through 14 (4.8 m).The three palaeosols here constitute a gleyed Protosol and gleyed Vertisols.
The Naashoibito Member at the north end of South Mesa (labelled South Mesa North in the figures) is 27 m thick, the thickest of the four sections measured (Figure 5).From the 33 sedimentary units comprising the member at this location, 22 were identified as containing pedogenic features (Table 1B).Eleven of the units represent five distinct palaeosol profiles containing paired A and B horizons, typically differentiated by a lighter A horizon transitioning to a darker B; one of these profiles appears to be a compound profile with a B1 horizon (unit 15) F I G U R E 5 Sections of Naashoibito Member measured from uppermost Kirtland Formation to lowermost Ojo Alamo Formation at Willow Wash, South Mesa North, South Mesa South and Barrel Spring codes for identified palaeosol horizons.Informal stratigraphy of the member referred to in the text is shown (lower conglomerate, lower section, middle sandstone, upper section).Units refer to unit numbers described in text.
T A B L E 1 Properties of pedogenic horizons identified in the measured sections.The vertical distribution of the palaeosol types here is complex.The lower portion of the section (i.e.below the correlative middle sandstone unit) at this location comprises units 5 through 22 (14.8 m thick).The basal part of this section (unit 5) is a Gleysol, but the remainder of the lower section contains Protosols, calcic Protosols (Bwk horizons), Vertisols and calcic Vertisols (Bssk horizons; Figure 6).The upper portion of the section (above the middle sandstone unit), comprising units 26 through 33 (5 m thick), contains Vertisols and Argillisols, which are reddened, and a gleyed Vertisol near the top of the member.

Unit
At the south end of South Mesa (South Mesa South in the figures) 26 m of strata were measured comprising 17 sedimentary units in the Naashoibito Member.Of these, T A B L E 1 (Continued) 11 contain pedogenic features similar to those observed at South Mesa North (Table 1C).Only one of these appears to be a profile containing both A and B horizons.
The vertical distribution of these palaeosol types here again is complex (Figure 5).Units 4 through 13 constitute the lower portion (below the middle sandstone), with a thickness of 9.8 m.The lowermost palaeosols are Vertisols and gleyed Vertisols (units 4-6), and the remainder of the lower portion alternates between gleyed and calcic Protosols and calcic Vertisols and Vertisols.The upper portion of this section (3.7 m) consists of a Protosol and a Gleysol (Figure 6; units 16-18).The southernmost section studied is at Barrel Spring, where the Naashoibito Member measures 26 m.Here 22 sedimentary units were described of which 13 contain pedogenic features allowing characterisation as palaeosol horizons (Figure 5; Table 1D).Two profiles display both A and B horizons; one of these is interpreted as a compound palaeosol with B1 (unit 16) and B2 (unit 15) horizons.The lower portion of the Barrel Spring section (below the middle sandstone) comprises units 4 through 20 (20 m thickness) and is dominated by Protosols, including both gleyed and calcic Protosols, and Argillisols, including calcic Argillisols, with no stratigraphic bias to the distribution of the Protosols or Argillisols (Figure 6).One Gleysol (unit 19) occurs near the top of the lower portion.The upper portion of the Barrel Spring section comprises units 22 and 23 (3 m thick) and contains just a single palaeosol, a Gleysol (unit 22).
The clay mineralogy of selected palaeosols in the South Mesa North (SMN), South Mesa South (SMS) and Willow Wash sections was determined by the methods described above (Table 2).The clay mineral assemblages are dominantly smectite, with minor illite fractions (20% or less) recorded in only two samples, SMN unit 14 (smectite 80%/ illite 20%) and SMS unit 12 (smectite and <5% illite).

| Nacimiento Formation
A single 35 m thick section of the Nacimiento Formation was measured from the basal contact with the underlying Kimbeto Member of the Ojo Alamo Formation at De-nazin Wash.Of the 29 sedimentary units described in this F I G U R E 6 Palaeosol orders interpreted from the horizons identified in the Naashoibito Member sections shown in Figure 5 with interpretation of overall hydrologic changes.Units refer to unit numbers described in text.
section, 18 display pedogenic features, including four palaeosol profiles with both A and B horizons present (Figure 7; Table 1E).Of the 14 palaeosols identified, more than half (eight) display gley colouring, including Gleysol, gleyed Protosol, gleyed Argillisol and five gleyed Vertisols (Figure 8).These last are the most common palaeosol order in this section, occurring prominently in the lower 10 m of the section (units 6 through 11).Reddened palaeosols, including Protosols, Argillisols and Vertisols, occur clustered near the middle of the section (units 12 through 15) and near the top of the measured section (unit 27).
Notably, the section contains a thin unit near the formation base, ca 2 m above the contact with the Kimbeto Member, that is a high-chroma yellowish-orange (10YR 6/8) sandy mudstone (Figure 9).The unit is about 20 cm in thickness and contains carbonised roots surrounded by orange haloes.The contacts with the overlying and underlying beds are sharp, and the unit lacks horizonation.Therefore, it is designated a Protosol.The clay mineralogy of this horizon consists of illite and kaolinite, with illite more abundant at approximate percentages of 65% illite/35% kaolinite (Table 2).X-ray diffraction analysis of a bulk powder sample of this unit also reveals Fe-enrichment, specifically the presence of goethite, identified from peaks at 4.18, 2.69 and 2.45 Å.The overlying unit (#5) is a Protosol in which the clay assemblage also consists of kaolinite and illite with minor smectite in approximate proportions of 70% kaolinite/20% illite/10% smectite.This is in contrast to the clay mineralogy of the succeeding unit (#6) in which smectite is present but kaolinite is also present, in percentages of ca 80% smectite/20% kaolinite.Palaeosols higher in the section generally contain abundant smectite, but kaolinite is also present in subordinate proportions.
As described above, the Palaeocene Nacimiento Formation in the San Juan Basin hosts numerous thin, resistant ledges that are siliceous (Hobbs, 2016;Rains, 1981;Williamson & Lucas, 1992).The ledges in the lower Nacimiento Formation were examined to assess their pedogenic significance.Individual ledges tend to be laterally extensive, although variable in thickness, and only rarely exhibit intersections and bifurcations.Upper and lower contacts are sharp, but the upper contacts are typically more-or-less flat, whereas the lower contacts are commonly irregular with scoured bases (Figure 10A).As noted by Hobbs (2016), root casts were observed in these beds, as well as modest colour mottling, but other sedimentary structures, such as cross beds or ripple marks, were not observed.Otherwise, evidence of extensive pedogenesis is lacking.Petrographic observations reveal a largely matrix-supported fabric, the presence of abundant, very angular quartz tephra grains, or shards, but also rounded (detrital) quartz, feldspar and detrital mudstone clasts (Figure 10B).The matrix consists of both silica and a minor detrital clay component.Rains (1981) interpreted these beds as silcretes in the pedogenic sense, implying silica cementation through very extensive surface exposure times in a warm, moist climate, drawing an analogy to similar features in Australia.Hobbs (2016) rejected this interpretation, noting a lack of petrographic and macromorphological features that would be associated with pedogenic processes.He interpreted the origin of the silica as via the devitrification of volcanic ash fall deposits that experienced minimal reworking, primarily by aeolian processes.The field observations of the Arroyo Chijuillita Member of the Nacimiento Formation in De-na-zin Wash, particularly the lateral facies relationships, the nature of the upper and lower contacts and the occurrence of volcanic-appearing shards mixed with detrital sediment, including potential mudstone rip-up clasts, suggests that silcretes as described here are largely the fill deposits of shallow channels.These observations are interpreted as indicating a volcanic provenance for much of the sediment, but not all.Therefore, it is suggested that air-fall ash deposits on the floodplain were reworked by precipitation events and transported to fluvial channel systems by overland flow where they were mixed ultimately with preexisting bedload and suspended load sediments.In addition, as noted by Hobbs, no quartz overgrowths or vug-filling silica was found.Pedogenesis and diagenesis were very limited, and most silica cementation was probably the result of devitrification of glassy components of the original ash in near-surface conditions, similar to the conclusion of Hobbs (2016).In summary, the silcretes of the Arroyo Chijuillita Member are not considered primary pedogenic features, and they are eliminated as indicators of palaeoclimate.F I G U R E 8 Palaeosol orders interpreted from the horizons identified in the Nacimiento Formation section shown in Figure 7 with interpretation of overall hydrologic changes.Units refer to unit numbers described in text.
On the contrary, a wide variety of pedogenic features were found (slickensides, clay cutans, calcareous nodules, gley features) that suggest a variety of soil-forming conditions.Hence, the Naashibito palaeosols are classified into four primary palaeosol orders based on their primary pedogenic features-Vertisol, Protosol, Argillisol and Gleysol-with subdivisions of these based on a prominent secondary characteristic, yielding three Vertisol types (including gleyed and calcic), three Protosols (including gleyed and calcic) and three Argillisols (including calcic and vertic).Calcisols, the palaeosol equivalent of calcic Aridisols, are not observed in the Naashoibito Member, although as stated previously, calcareous nodules were found in the calcic Vertisols, Protosols and Argillisols that may be the nodules reported by Lehman (1985) as containing baryte.The radiating crystal structure observed in the nodules does occur in baryte precipitated in sediments, therefore diagenetic replacement of baryte by calcite is not necessarily discounted.However, baryte formation in sediments is generally regarded as occurring almost exclusively in marine environments (Alderton, 2021).It is suggested instead that the nodules observed in the Naashoibito Member formed by epigenetic recrystallisation of pedogenic calcite with an originally micritic texture.The accumulation of calcite in the B horizon of a soil requires sufficient moisture for translocation of Ca 2+ to the B horizon, but insufficient throughflow to remove it from the soil, allowing it to precipitate as CaCO 3 .While commonly associated with semi-arid climates, these conditions frequently occur in subhumid but strongly seasonal environments (Retallack, 2005).
Although varying in thickness, the four sections measured are correlative, with a fine-grained lower interval overlying the Ojo Alamo Formation lower conglomerate, in turn overlain by the middle sandstone package, 3.5 to 7 m thick, and an upper fine-grained interval that lies above the sandstone and extends to the unconformable contact with the overlying Kimbeto Member (Figure 5).As illustrated by Figure 6, there is little lateral continuity of specific palaeosols between locations.For example, much of the lower interval of the South Mesa North section is dominated by reddened Vertisols, particularly in the upper part of the lower portion.Conversely, the lower portion at Barrel Spring is dominated by gleyed Protosols that transition upward to reddened Argillisols and calcic Argillisols, but Vertisols are entirely absent.The lower portion of the Willow Wash section consists entirely of gleyed palaeosols, including gleyed Protosols, Argillisols and Vertisols.The upper portions of the sections (above the middle sandstone package) similarly exhibit a lack of lateral continuity of palaeosol types between sections.The upper portion of the South Mesa North section consists mostly of reddened Vertisols and Argillisols, whereas the upper portions of the sections at South Mesa South, Barrel Spring and Willow Wash are all dominated by gleyed palaeosols, for example, gleyed Vertisols and Protosols and Gleysols.Given the lack of clear continuity of the main palaeosol types between the four, relatively closely spaced sections in both the lower and upper portions of the Naashoibito fine-grained facies, it is possible to conclude that these differences are primarily the consequence of local soil hydrology rather than variations in climate.Specifically, various positions on the floodplain can be related to elevation relative to the water table, and, by extension, soil hydrology (Hartley et al., 2013;Sendek et al., 2021).
Despite the lack of lateral continuity, evidence is found for broad changes in soil hydrology in the Naashoibito Member over time.In all four sections, the basal palaeosols display gley colouring (Gleysol, gleyed Protosol or gleyed Vertisol).In three of the four sections (South Mesa North, South Mesa South and Barrel Spring) the remainder of the interval below the middle sandstone is dominated by non-gleyed Vertisols, Argillisols and Protosols (Willow Wash is the exception to this trend).Above the middle sandstone, the uppermost Naashoibito palaeosols in all four sections consist at least in part of Gleysols or gleyed Vertisols.The widespread occurrence in the basal palaeosols of gley colouring indicates poorly drained soil conditions and potentially moist climate conditions during initial deposition of the Naashoibito Member.In most sections, the palaeosols transition upwards to orders associated with well-drained soil conditions, possibly recording a shift to lower MAP or greater seasonality.But climate trends that are recorded over broad regions, such as the Late Triassic aridification of the Colorado Plateau or the Newark Supergroup basins (Kent & Olsen, 2000;Tanner & Lucas, 2006) are typically the result of plate movements over a much longer time scale, tens of millions of years, than accounted for by the deposition of these strata.Moreover, the near uniform smectitic composition of the clays in the palaeosols (Table 2) suggests that there were no major changes in weathering regime in either the sediment source area or the basin during deposition.Alternatively, the same palaeosol trend could indicate basin-wide changes in sediment accumulation rate independent of climate.Aggradation across the area of the basin driven by episodic activity during the ongoing Laramide orogeny (Donahue, 2016) could have increased the elevation of the active soil layer with respect to the water table.Above the middle sandstone, the palaeosols in all four sections display evidence of at least sporadic, although not synchronous high-water saturation, perhaps indicating a decline in the rate of sediment delivery to the basin, which caused a decrease in the height of the land surface and active soil zone above the water table.
Considering the observed palaeosol types, the general palaeoclimate interpreted during the interval of Naashoibito Member deposition is one of consistently warm, humid to subhumid and seasonal conditions.Significantly, the deposition of the Naashoibito Member overlapped in whole or in part with the Deccan eruptions; according to Zhang et al. (2018) the onset of the end-Maastrichtian warming occurred ca 66.4 Ma, which may have coincided with the initiation of Naashoibito deposition, based on correlation of the magnetostratigraphy (Davis et al., 2016).The interpretation of well-drained soils from correlative measured sections suggests an interval of lower soil moisture during deposition of the middle portion of the Naashoibito Member, followed by a return to higher moisture conditions near the top of the member.However, the occurrence of gleyed palaeosols throughout the section in some locations, such as at Willow Wash, suggests high soil moisture at least locally during the entire interval of Naashoibito Member deposition.Hence, it is suggested here that the observed vertical trend in the palaeosols, from poorly drained to moderately well-drained and back to poorly drained, primarily reflects changes in the dynamics of sediment accumulation and basin aggradation rather than changing climate.This more conservative interpretation is supported by the aforementioned consistency of the clay mineralogy in the member.

| Nacimiento Formation
Rather than classifying the palaeosols in the Nacimiento Formation using a palaeosol taxonomy, Hobbs (2016) and Hobbs and Fawcett (2022) assigned six pedotypes based largely on chroma and hue, plus other features; these are broadly equivalent to Argillisols, Protosols and Vertisols.Hobbs (2016) noted that these palaeosols represent a spectrum of water-table conditions, from poorly drained palaeosols more prevalent lower in the formation to welldrained to seasonally variable drainage more dominant in the upper part of the Nacimiento Formation, in the Ojo Encino and Escavada members.These younger members were not examined for this study nor in the study by Davis et al. (2016).Most of the palaeosols observed compare well with pedotypes I (low chroma, drab with roots) and IV (low value grey and red with slickensides) of Hobbs (2016), representing poorly drained and seasonally moderate drainage conditions, respectively.In the sections measured by Hobbs, pedotypes I and IV dominate the lower Nacimiento Formation (Arroyo Chijuillita Member), according well with the observations at De-na-zin Wash.Davis et al. (2016) interpreted similar palaeosol types, describing Inceptisols and Entisols, which are termed Protosols in the classification scheme used herein (Mack et al., 1993), plus multiple types of Vertisols.These authors also attributed differences in soil drainage to position on the floodplain, with Vertisols formed on the better-drained distal floodplain, while the less mature soils formed on the poorly drained floodplain proximal to the stream channels.Davis et al. (2016) thus interpreted climate stability during the depositional interval of the lower Nacimiento Formation.
As discussed above, most of the palaeosols described in the De-na-zin Wash section are gleyed, that is, gleyed Vertisols and gleyed Argillisols and Protosols, with fewer reddened Vertisols, Argillisols and Protosols.These represent a mix of poorly to moderately well-drained soil conditions, similar to those seen in the Naashoibito Member palaeosols.The most notable exception is the presence of a high-chroma horizon near the base of the formation interpreted as an oxidised, Fe-enriched Protosol.The clay mineral assemblage of the palaeosols in the part of the formation examined consists of varying proportions of kaolinite, illite and smectite, with kaolinite and illite dominating in the lower units in the sections, and smectite more prevalent in higher units.Given the influence of climate on clay mineral formation (Evans, 1992;Velde & Meunier, 2008), it is suggested that the variations in the clay mineral assemblage may help inform the discussion of climate during the early Palaeocene, as discussed below.Davis et al. (2016), Hobbs (2016) and Hobbs and Fawcett (2022) used geochemical indices to obtain quantitative estimates of palaeoclimate during Nacimiento deposition.
Hobbs (2016) and Hobbs and Fawcett (2022) utilised the Palaeosol Weathering Index (PWI; Gallagher & Sheldon, 2013) and the S proxy (Sheldon et al., 2002) to obtain MAT estimates and the chemical index of alteration (CIA-K; Sheldon et al., 2002) to estimate MAP.The PWI and S-proxy calculations for MAT in the San Juan Basin produce a range of 9 to 17°C, and the CIA-K calculations produce a MAP range of 890 to 1380 mm.Their calculated MAP S-proxy MAT estimates co-vary widely through the Nacimiento Formation (Hobbs & Fawcett, 2022; figure 5).However, Hobbs (2016) and Hobbs and Fawcett (2022) noted that the calculated MAP and MAT do not correlate well with the well-drained pedotypes they sampled and described, nor are they compatible with the palaeontologic evidence of a largely aquatic lower vertebrate fauna.They concluded that the use of geochemical indices does not fully account for extrinsic factors such as weathering of materials in the source area prior to delivery to the basin, and/or aeolian contributions to the sedimentary environment.Hobbs (2016) and Hobbs and Fawcett (2022) found that the clay mineral assemblage through the entire thickness of the Nacimiento Formation consisted of a combination of smectite, kaolinite and illite in varying proportions, as found here in the Arroyo Chijuillita Member.However, they discounted the importance of the abundance of kaolinite in some palaeosols due to the presence of relatively unaltered feldspars in sandy B horizons and suggest that the kaolinite may have been inherited from the source area, either present in the source rocks or from a previous weathering regime.Davis et al. (2016) conducted similar geochemical investigations of palaeosols in the Arroyo Chijuillita Member, calculating a MAP of 1003 to 1342 mm from the CIA-K calculation.From the CALMAG method, which is considered more applicable to Vertisols (Nordt & Driese, 2010), they obtained a slightly higher calculated MAP range of 1241 to 1514 mm.Davis et al. (2016) noted that the higher CALMAG MAP values are not dissimilar to the MAP of ca 1800 mm obtained by leaf margin analysis of fossils from the underlying Ojo Alamo Formation (Flynn et al., 2014).They concluded that the climate during deposition of the lower Arroyo Chijuillito Member of the Nacimiento Formation was warm and relatively moist, but distinctly seasonal, much as interpreted here for the Naashoibito Member.Further, they found a relatively narrow range of variation of the palaeoclimate proxies across their sampled interval despite the variation in palaeosol types.Accordingly, they concluded that climate was stable through this interval, and that the variation in palaeosol types is strictly a function of soil hydrologic conditions across the floodplain, which was subject to variations in the dynamics of sedimentation in the basin.
Similar to the finding of Hobbs (2016), kaolinite is generally more abundant than smectite near the base of the Arroyo Chijuillita Member of the Nacimiento Formation, where a high-chroma Protosol was observed, and is present in variable concentrations higher in the section.In the Denver Basin, the temporal calibration (palaeofaunal, palaeofloral, palynological, palaeomagnetic) of the strata at Coral Bluffs places the transition from Puercan 1 to Puercan 2 NALMA at ca 65.75 Ma (Lyson et al., 2019).There, a warming pulse of 2 to 3°C is recorded by increases in pollen, leaf mass, and megafloral standing species richness that initiated ca 20 kyr prior to this boundary and continued after it for ca 80 kyr, ending approximately at the C29r-C29n boundary (Lyson et al., 2019).Flynn et al. (2020) place this chron boundary at Den-na-zin Wash at 8 m above the base of the Nacimiento Formation.Therefore, this entire lower interval of the Nacimiento Formation may have been deposited during the warm interval identified by Lyson et al. (2019) in the Denver Basin.The geochemical indices data in most of the sections studied by Davis et al. (2016) demonstrate greater warmth and moisture in the basal 5 to 10 m of the Nacimiento Formation.Therefore, it is suggested here that the kaolinite-enriched strata, including the oxidised Protosol near the base of the section at De-na-zin Wash, record more intense weathering during at least a portion of this warming in the San Juan Basin, thereby supporting the magnetostratigraphic correlation between the basins.Furthermore, Lyson et al. (2019) identified a younger, transient warming spike of ca 3°C near the Puercan 2-Puercan 3 transition.Potentially, this brief episode is recorded by the variations in kaolinite content found higher in the Nacimiento Formation at De-na-zin Wash (Table 2).It is suggested, therefore, that contrary to the interpretation of Hobbs and Fawcett (2022), the variations in kaolinite content in the palaeosols observed by them and herein reflect changing weathering conditions, either in the depositional basin or in the sediment source area, during the depositional history of the formation.

| CONCLUSIONS
Strata of the Naashoibito Member of the Ojo Alamo Formation in the west-central San Juan Basin contain a variety of palaeosol types, including Gleysols, Protosols, Argillisols and Vertisols, and variations thereof.This variety reflects differences in soil hydrologic conditions, but, given the lack of lateral continuity of palaeosol types between the measured sections, the variety of palaeosols are attributed primarily to control by topographic heterogeneity of the floodplain.The active soil layer in low areas, such as abandoned channels and low areas proximal to channels, were closer to or partially intersected the water table, causing high water saturation resulting in redoximorphic soil conditions and gley features.Soils on elevated areas of the floodplain experienced better drainage, at least seasonally, allowing illuviation and oxidation of materials in the solum.Three of the four Naashoibito Member sections measured exhibit similar vertical trends of poorly drained soils transitioning upward to moderately and well-drained soils, and back to poorly drained soils near the top of the Naashoibito Member.The palaeosol clay assemblages are uniformly dominated by smectite, however, suggesting that there were no major variations in palaeoclimate.Therefore, the vertical variations in palaeosol types are attributed to changes in sedimentation and aggradation rate in the basin.The climate in the San Juan Basin during the latest Maastrichtian is interpreted as warm and subhumid to humid, but seasonal.Any changes in climate that did occur during Naashoibito deposition had less of an impact on pedogenic processes than did the local soil hydrology.
The unconformity at the K-Pg boundary in the San Juan Basin precludes the possible preservation of an interval recording the proposed impact-derived, sootinduced cooling episode (Kaiho et al., 2016).The presence of a high-chroma Protosol and the abundance of kaolinite in strata near the base of the Arroyo Chijuillita Member of the Nacimiento Formation records a warmer interval with stronger weathering during initial deposition of this unit, similar to that observed in the Denver Basin.Palaeosols in the lower Nacimiento Formation do not substantially differ morphologically from those occurring in the Naashoibito Member, although the clay mineral assemblages exhibit varying proportions of smectite, kaolinite and illite.Thus, the pedogenic evidence suggests that the long-term palaeoclimate in the early Danian was broadly similar to that of the latest Maastrichtian-warm, subhumid to humid and seasonal-following an initial interval or multiple intervals of higher temperature during the earliest Danian.As in the Naashoibito Member, vertical changes in palaeosol types are attributed primarily to basinal changes in sedimentation rate.Within the time frame of the stratigraphic units studied herein, the long-term climate was broadly stable from Maastrichtian through early Danian time, but with transient warm episodes at the K-Pg boundary and in the earliest Danian.

F
I G U R E 1 Geological map of the west-central San Juan Basin illustrating the location of the study area and the sections measured for this study.Section numbers: 1 = Willow Wash; 2 = South Mesa North; 3 = South Mesa South; 4 = Barrel Spring; 5 = De-na-zin Wash.
Flynn et al. (2020) calculated a sediment accumulation rate for the lower Nacimiento Formation in the study area of ca 43 m/Myr.If this rate is applied to the Naashoibito Member, and further, if the age of 66.5 Ma is accepted for the base of the Kimbeto Member, the top of the Naashoibito Member in F I G U R E 2 Upper Cretaceous-Lower Palaeogene stratigraphy of the westcentral San Juan Basin with locations and ages of principal ash beds.Each * represents the stratigraphic height of a dated ash bed, as indicated by the heading at the top of the column (Ar/Ar dated ash beds).

F
I G U R E 3 Overview photographs of: (A) the Naashoibito Member (NM) of the Ojo Alamo Formation, overlain by the Kimbeto Member (KM) South Mesa North, and (B) Nacimiento Formation (NF), overlying the Ojo Alamo Formation (OAF) at De-na-zin Wash (arrow at the approximate contact).Palaeosols in which the most prominent characteristic is greyish or greenish soil matrix, or gley colouring (Bg horizons), with or without root traces, are called Gleysols.Palaeosols with clay-enriched B horizons (Bt horizons), clay cutans on peds and/or root traces and typically reddish in colour are Argillisols.Palaeosols with distinct lineation on arcuate fracture surfaces (Bss horizons) are called Vertisols.Palaeosol order designations were modified if they display a combination of these characteristics, for example, gleyed Vertisols (Bssg horizons) or calcic Argillisols (Btk horizons).
Clay mineralogy of samples from sections at Willow Wash (WW), South Mesa North (SMN), South Mesa South (SMS) and De-na-zin Wash (DNZ).
described the Naashoibito palaeosols as consisting of alternating pale albic, or leached, and spodic horizons, based on the assumption of significant Fe accumulation in the B horizon.Modern Spodosols typically form in acidic forest soils in cool climates through significant leaching of the upper soil and the accumulation of organics complexed with Al and/or Fe in the subsoil.An examination of the reddish Naashoibito palaeosols fails to confirm the presence of horizons that meet these criteria.

F I G U R E 7
Section measured at De-na-zin Wash from uppermost Ojo Alamo Formation (Kimbeto Member) through lower Nacimiento Formation, with palaeosol horizons for numbered units as discussed in the text.

F
Features of the high-chroma horizon near the base of the Nacimiento Formation.(A) Position of the horizon (arrow) about 2 m above the contact with the underlying Kimbeto Member of the Ojo Alamo Formation.(B) Detail of mottling and rooting (arrow) in the high-chroma sandy mudstone.F I G U R E 1 0 Silcrete in lower Nacimiento Formation (unit 17 at De-na-zin Wash).(A) Overview of silcrete ledge showing irregular base.(B) View of angular quartz shards (arrows) with plagioclase feldspar (F).
Horizon nomenclature as described in the text.UTM coordinates provided for base and top of each section. Note: