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Climate and landscape reconstruction of the Arroyo Chijuillita Member of the Nacimiento Formation, San Juan Basin, New Mexico: Providing environmental context to early Paleocene mammal evolution

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... The Arroyo Chijuillita and Ojo Encino Members are confined to the southern portion of the basin. The Arroyo Chijuillita Member is composed of drab mudstones and small lenticular sandstone beds (Davis et al., 2016). The Ojo Encino Member contains variegated red and drab mudstones, large sheet and channel sandstone units, and three persistent intervals consisting of numerous "black mudstone" paleosol horizons referred to as the "lower," "middle," and "upper black" (Leslie et al., 2018b). ...
... The Ojo Encino Member contains variegated red and drab mudstones, large sheet and channel sandstone units, and three persistent intervals consisting of numerous "black mudstone" paleosol horizons referred to as the "lower," "middle," and "upper black" (Leslie et al., 2018b). Both members have been interpreted to represent meandering fluvial systems deposited in a subtropical climate, and depositional energy increases from the Arroyo Chijuillita to Ojo Encino Member (Tidwell et al., 1981;Williamson, 1996;Davis et al., 2016;. The Kutz Member is equivalent to the "main body" of Williamson (1996) and is primarily exposed in Kutz Canyon. ...
... First, there are dramatic sedimentological differences between the Ojo Alamo Sandstone and the Nacimiento Formation. Sediment accumulation rates were likely much higher during deposition of the Ojo Alamo Sandstone, which is a massive, multistoried channel complex (e.g., Baltz et al., 1966;Cather, 2004;Chapin and Cather, 1983;, than during deposition of the Nacimiento Formation, which is comprised of paleosols, floodplain, overbank, back swamp, ponded deposits, and channels of varying sizes and dimensions (e.g., Cather et al., 2019;Davis et al., 2016;Williamson, 1996). Second, there is an erosive basal contact between the Ojo Alamo Sandstone and Naashoibito Member, which indicates that there is an unconformity between the Paleocene Ojo Alamo Sandstone and the Cretaceous Naashoibito. ...
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The lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation from the San Juan Basin (SJB) in northwestern New Mexico preserve arguably the best early Paleocene mammalian record in North America and is the type location for the Puercan (Pu) and Torrejonian (To) North American land mammal ages (NALMA). However, the lack of precise depositional age constraints for the Ojo Alamo Sandstone and lower Nacimiento Formation has hindered our understanding of the timing and pacing of mammalian community change in the SJB following the Cretaceous−Paleogene mass extinction. Here we produced a high-resolution age model for the Ojo Alamo Sandstone and lower Nacimiento Formation combining magnetostratigraphy and 40Ar/39Ar geochronology spanning the first ∼3.5 m.y. of the Paleocene. Mean sediment accumulation rates during C29n were relatively low (<50 m/m.y.) and equalized from basin center to basin margin indicating an accommodation minimum; sediment accumulation rates approximately double (>90 m/m.y.) during C28r and are highest in the basin center and lowest on basin margin, which indicates high accommodation and an increase in basin subsidence near the C29n/C28r boundary (ca. 64.96 Ma). Puercan fossil localities were restricted to C29n, Torrejonian 1 localities to C28n, and lower Torrejonian 2 localities to C27r. Our revised age model for the SJB suggests that the first appearance of To1 mammals may have been diachronous across North America, with the Torrejonian 1 mammals first appearing in the north (Montana and North Dakota) during C29n, then in middle latitudes (Utah) in C28r, and lastly in southern North America (New Mexico) in C28n.
... The Ojo Alamo Sandstone overlies the Maastrichtian Naashoibito Member of the Kirtland Formation and underlies the early Paleocene Nacimiento Formation (Baltz et al. 1966;Lindsay et al. 1978;Williamson 1996;Mason 2013;Peppe et al. 2013). The contact between the Ojo Alamo Sandstone and Nacimiento Formation is complex and regionally varies from being conformable with the basal paleosols of the Nacimiento weathering into the upper Ojo Alamo Sandstone to unconformable with evidence for the contact being erosive (e.g., Williamson and Lucas 1992;Williamson 1996;Davis et al. 2016). In the BDNZ, the contact between the Ojo Alamo Sandstone and Nacimiento Formation appears to generally be conformable (Williamson 1996;Davis et al. 2016). ...
... The contact between the Ojo Alamo Sandstone and Nacimiento Formation is complex and regionally varies from being conformable with the basal paleosols of the Nacimiento weathering into the upper Ojo Alamo Sandstone to unconformable with evidence for the contact being erosive (e.g., Williamson and Lucas 1992;Williamson 1996;Davis et al. 2016). In the BDNZ, the contact between the Ojo Alamo Sandstone and Nacimiento Formation appears to generally be conformable (Williamson 1996;Davis et al. 2016). ...
... Additionally, the climate of the SJB was likely better suited to pteridophytes than the climate of the more northern basins (Mehltreter et al. 2010; Table 5). There is evidence that the SJB, in addition to being warmer and wetter than other basins, experienced seasonal precipitation (e.g., Wheeler et al. 1995;Davis et al. 2016). The modern relatives of the most common pteridophyte morphotype found in the SJB (SJ-57, cf. ...
Article
Earliest Paleocene megafloras from North America are hypothesized to be low diversity and dominated by long-lived cosmopolitan species following the Cretaceous/Paleogene (K/Pg) mass extinction. However, megafloras used to develop this hypothesis are from the Northern Great Plains (NGP) of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of an earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprising 53 morphotypes, was dominated by angiosperms, with accessory taxa composed of pteridophytes, lycophytes, and conifers. Diversity analyses indicate a species-rich, highly uneven, and laterally heterogeneous flora. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and relatively high precipitation consistent with a modern tropical seasonal forest. When compared with contemporaneous floras from the Denver Basin (DB) of Colorado and the Williston Basin (WB) of North Dakota, the SJB flora had significantly higher species richness but lower evenness. Paleoclimate estimates from the SJB were 7–14°C warmer than the estimates for the DB and WB, indicating a shift from a temperate forest in the NGP to a tropical forest in the SJB. These results demonstrate the presence of a latitudinal floral diversity and paleoclimatic gradient during the earliest Paleocene in western North America. We hypothesize that the warm, wet conditions in the earliest Paleocene SJB drove rapid rates of speciation following the K/Pg boundary, resulting in a diverse and heterogeneous flora.
... We restrict the upper Arroyo Chijuillita Member to the area from Mesa de Cuba to about ten kilometers west of the continental divide ( Fig. 4), where it interfingers with thick channel sandstones of the Tsosie Member (cf. Williamson, 1996;Davis et al., 2016). Williamson and Lucas (1992) as the type section of the Tsosie Member. ...
... Variegated Ojo Encino floodplain mudstone displays hues of gray, red, green, and black. Black horizons in the Nacimiento Formation represent poorly drained paleosols (Davis et al., 2016;Hobbs, 2016) and are suggestive of episodes of high water tables. The basal part of the member is the Penistaja Bed (Williamson and Lucas, 1992), a prominent, slightly pebbly sandstone that disconformably overlies the Arroyo Chijuillita Member near, and east of, the continental divide (Fig. 9). ...
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Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 40Ar/39Ar sanidine age data. In contrast to the dominantly southerly or southeasterly paleoflow exhib­ited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast- flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin. In the eastern and northern parts of the San Juan Basin, paleoflow was gen­erally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ~15 km south of Dulce, New Mexico. There, an ~130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 40Ar/39Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin. Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin.
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Paleosols can represent intervals of nondeposition in sedimentary packages and are used with increasing frequency as proxies for paleoenvironmental conditions during basin filling. However, the complexities of factors both internal and external to pedogenesis require consideration of paleosols in any basin or sedimentary package in a broader context than is often assumed. With this in mind, we measured and analyzed pedogenic features, stratigraphic position, geochemical composition, and petrography of paleosols in the Nacimiento Formation to gain insight into the paleoenvironmental conditions of the early Paleocene in the San Juan Basin. During this interval, the San Juan Basin was located in middle northern latitudes (∼40°N) and saw rapid terrestrial siliciclastic sedimentation related to Laramide tectonism. Evidence from earlier researchers suggests that prevailing climate conditions during Nacimiento Formation deposition were warm and humid. We used morphological properties of paleosols to categorize paleosols into pedotypes indicative of distinct pedogenic conditions. The general stratigraphic distribution of these pedotypes shows an up-section increase in soil drainage conditions through the Nacimiento Formation that cannot be correlated with known climate changes. We suggest that fluvial system evolution was the major control on pedogenic conditions. We investigated Nacimiento Formation paleosols with widely used paleosol geochemical climate analyses, which provided paleoclimate estimates that are in disaccord with independently derived estimates. We show that in alluvial depositional systems with source areas in weathered sedimentary rocks, these analyses can be difficult to interpret and likely lead to estimates that do not reflect true pedogenic conditions during the postdepositional near-surface alteration of sediments. Petrographic analysis of Nacimiento Formation paleosols showed that some likely formed under semiarid to subhumid conditions that allowed pedogenic accumulation and illuviation of smectite clays yet did not substantially chemically alter primary detrital plagioclase feldspar grains in paleosol B horizons. The paleosols of the Nacimiento Formation, when analyzed at the basin scale, show that sedimentary aggradational processes can overpower climate processes in creation of a sequence of paleosols in a stratigraphic section. In addition, the incorporation of clays from sedimentary rocks in a basin’s source area can drastically skew the geochemical signatures and therefore interpretations of paleosols in that basin.
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Lower Paleocene deposits in the San Juan Basin document one of the best records of mammalian change and turnover following the Cretaceous-Paleogene extinctions and are the type section for the Puercan (Pu) and Torrejonian (To) North America Land Mammal age biozones (NALMA). One of the largest mammalian turnover events in the early Paleocene occurs between the Torrejonian 2 (To2) and Torrejonian 3 (To3) NALMA biozones. The Nacimiento Formation are the only deposits in North America where the To2-To3 mammalian turnover can be constrained, however the precise age and duration of the turnover is poorly understood due to the lack of a precise chronostratigraphic framework. We analyzed paleomagnetic samples, produced a 40Ar/39Ar detrital sanidine age, and developed a detailed lithostratigraphy for four sections of the upper Nacimiento Formation in the San Juan Basin, New Mexico (Kutz Canyon, Escavada Wash, Torreon West and East) to constrain the age and duration of the deposits and the To2-To3 turnover. The polarity stratigraphy for the four sections can be correlated to chrons C27r-C26r of the geomagnetic polarity time scale (GPTS). Using the local polarity stratigraphy for each section, we calculated a mean sediment accumulation rate and developed a precise age model, which allows us to determine the age of important late Torrejonian mammalian localities. Using the assigned ages, we estimate the To2-To3 turnover was relatively rapid and occurred over ~120 kyr (-60/+50 kyr) between 62.59 and 62.47 Ma. This rapid duration of the mammalian turnover suggests that it was driven by external forcing factors, such as environmental change driven by the progradation of the distributive fluvial system across the basin and/or changes in regional or global climate. Additionally, comparisons of the mean sediment accumulation rates between the sections that span from the basin margin to the basin center indicate that sediment accumulation rates equalized across the basin from the end of C27r through the start of C26r, suggesting an accommodation minima in the basin associated with the progradation of a distributive fluvial system into the basin. This accommodation minimum also likely led to the long hiatus of deposition between the Paleocene Nacimiento Formation and the overlying Eocene San Jose Formation.
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Modern sequence-stratigraphic theory has its foundation in the work of L.L. Sloss and W.C. Krumbein (1940s–1960s) and several Exxon researchers (1970s–1990s). This work largely focuses on the nature and origin of sedimentary cycles within marine stratal successions. More recently, sequence-stratigraphic concepts have evolved to include the analysis of terrestrial strata. Historically, the recognition of unconformity-bounded cyclic stratal units (such as sequences) has relied upon the geometric relationships of strata (i.e., onlap, toplap, truncation, and downlap) within two- and/or three-dimensional outcrop or subsurface successions. Oftentimes, however, outcrops or boreholes are isolated and do not preserve these diagnostic stratal relationships. In such instances, documentation of changes in the vertical, rather than lateral, succession of strata may allow reconstruction of the cyclic accommodation history and placement of associated bounding discontinuities. This technique, referred to as “stacking pattern” analysis, was originally developed for shallow-marine carbonate successions. More recently, the stacking pattern methodology has been similarly applied to alluvial successions and takes into account the unique processes of terrestrial deposition and pedogenesis. The most conspicuous and fundamental cyclic stratal units recognized within alluvial settings are fluvial aggradational cycles (FACs). Fluvial aggradational cycles are meter-scale, typically fining-upward successions that have a disconformable lower boundary and an upper boundary that either has a paleosol weathered into it or is disconformably overlain by the succeeding FAC without a paleosol. Fluvial aggradational cycles are thought to represent sediment accumulations during channel avulsion events that are subsequently weathered during the following period of channel stability. A thick succession of FACs indicates sediment accumulation during a prolonged episode of accommodation gain. Variations in the rate of accommodation gain (and loss) are interpreted to result in the organization of FACs into alluvial sequences and longer period composite sequences. Episodes of base-level rise result in relatively rapid rates of alluvial aggradation and less developed and more poorly drained paleosols. Associated FACs are thicker than average and transition from initially lower sinuosity, higher competence alluvial systems to comparably higher sinuosity, lower competence channel deposits. As base-level rise decelerates and initially falls, paleosols become increasingly well developed and better drained, and FACs are thinner than average and transition to even lower competence, higher sinuosity channel sandstones that are more extensive as a result of prolonged channel migration under low accommodation conditions. During base-level fall, the incisement of alluvial valleys produces sequence boundaries that are infrequently flooded across interfluve areas. Fluvial aggradational cycles across interfluve positions are much thinner than average and are characterized by the most well-developed and best-drained paleosols. Application of the alluvial stacking pattern methodology is demonstrated within three case studies. Case study 1, from Big Bend National Park, Texas, considers a latest Cretaceous to earliest Tertiary passive margin and coastal plain succession and correlates alluvial sequences and associated climate and ecosystem changes to eustatic sea-level oscillations. Case study 2, from northern and northeastern New Mexico, documents a Late Triassic foreland basin succession in which tectonically induced accommodation events are correlated between isolated outcrop successions that are located 200 km apart. Case study 3, from central New York, demonstrates how stacking pattern analysis allows correlation of a Middle Devonian alluvial composite sequence with equivalent regressive–transgressive marine strata along a convergent plate boundary.
Conference Paper
The San Juan Basin (SJB) in New Mexico, USA contains one of the best early Paleocene records of mammalian evolution, making it an ideal location to examine ecosystem response following the Cretaceous-Paleogene (K-Pg) extinction. The mammalian record is punctuated by faunal change and turnover represented by the type Puercan and entire Torrejonian North American Land Mammal Age biozones. However, relatively little is known about the early Paleocene plant communities, paleoenvironment, or paleoclimate. This makes it difficult to assess if changes between the mammalian biozones are driven by climate change or other factors, such as rapid evolution following the K-Pg extinction. Here we reconstruct early Paleocene fossil plant communities, paleoclimate, and paleoenvironment in the SJB to assess possible drivers behind mammalian turnover. Collections of fossil leaves from Puercan and early Torrejonian strata indicate that early Paleocene plant communities were relatively diverse, heterogeneous across the landscape, and dominated by angiosperms. Interestingly, the Puercan floras are considerably different than Torrejonian floras, suggesting the possibility of synchronous turnover of plant and animal communities. Analyses from fossil leaves, paleosols, and stable carbon and oxygen isotopes from mammalian tooth enamel suggest little variability in Paleoenvironment or paleoclimate between the Puercan and Torrejonian. Isotopic data from tooth enamel through all of the Puercan and Torrejonian biozones indicates relatively little variability in paleoclimate through the early Paleocene succession. These results suggest that early Paleocene climate in the SJB was relatively stable, and the similar climatic reconstructions for the Puercan and Torrejonian suggest that mammalian turnover is not related to environmental or climate change. Instead the documented faunal turnover through the Puercan and Torrejonian is more likely the result of factors intrinsic to the San Juan Basin mammalian communities, such as rapid evolution, and/or faunal migration in and out of the basin. Grant Information We acknowledge grant support from the National Science Foundation (EAR-1325544, EAR-1325612, EAR-1325552) and the Petroleum Research Fund (ACS PRF 52822-DN18).
Article
Quartzose sands from continental cratons are widespread within interior basins, platform successions, miogeoclinal wedges, and opening ocean basins. Arkosic sands from uplifted basement blocks are present locally in rift troughs and in wrench basins related to transform ruptures. Volcaniclastic lithic sands and more complex volcano-plutonic sands derived from magmatic arcs are present in trenches, forearc basins, and marginal seas. Recycled orogenic sands, rich in quartz or chert plus other lithic fragments and derived from subduction complexes, collision orogens, and foreland uplifts, are present in closing ocean basins, diverse successor basins, and foreland basins. Triangular diagrams successfully distinguish the key provenance types. -from Authors
Article
Red beds are sediments and sedimentary rocks with hues ranging from 2.5YR to 5R. Strata containing red beds commonly exhibit color mottling and may include layers that are not red. The fine-grained hematite (<2 μm) that pigments red beds can form in the weathering zone and below the zone of soil formation. Many pre-Quaternary red beds contain buried and lithified paleosols that can be identified by micro- and macro-morphologic features. Below the zone of soil formation, hematitic pigment forms by diagenetic processes including: oxidation of Fe following the alteration of ferromagnesian minerals and volcanic ash, and oxidation of Fe sulfides. -from Authors
Article
Field data from various sites in the W. United States suggest that the master stream as base level has an effect only locally and has an influence that extends only a short distance up the tributary. Base level and the profile of the graded stream are not closely related. The vertical position of a river at any reach is the result of aggradation or degradation in the closely adjoining reaches and not to a base level far removed in space. The aggrading or degrading condition is a deviation from grade. This deviation may be expressed as the passing of a threshold of critical power, a concept that recognises the interaction of all hydraulic and morphologic factors.-after Authors
Chapter
The patterns of marine magnetic anomalies for the Late Cretaceous through Neogene (C-sequence) and Late Jurassic through Early Cretaceous (M-sequence) have been calibrated by magnetostratigraphic studies to biostratigraphy, cyclostratigraphy, and a few radiometrically dated levels. The geomagnetic polarity time scale for the past 160 myr has been constructed by fitting these constraints and a selected model for spreading rates. The status of the geomagnetic polarity time scale for each geological period is summarized in Chapters 11–22 as appropriate. PRINCIPLES Magnetic field reversals and magnetostratigraphy The principal goal of magnetostratigraphy is to document and calibrate the global geomagnetic polarity sequence in stratified rocks and to apply this geomagnetic polarity time scale for high-resolution correlation of marine magnetic anomalies and of polarity zones in other sections. The basis of magnetostratigraphy is the retention by rocks of a magnetic imprint acquired in the geomagnetic field that existed when the sedimentary rock was deposited or the igneous rock underwent cooling. The imprint most useful for paleomagnetic directions and magnetostratigraphy is recorded by particles of iron oxide minerals. Most of the material in this chapter is updated from summaries in Harland et al. (1990) and Ogg (1995). Excellent reviews are given in Opdyke and Channell (1996) for magnetostratigraphy and McElhinny and McFadden (2000) for general paleomagnetism.
Article
The Upper Triassic Sonsela member of the Chinle Formation in the Petrified Forest National Park was evaluated using sedimentologic, stratigraphie, paleopedologic, and petrographie criteria along a continuous 0.5-km (0.3-mi) outcrop. The study interval consists of interbedded sandstones and mudstones and is composed of a two-tiered hierarchy of cyclic alluvial deposits with bounding paleosols. The succession is composed of 15 fluvial aggradational cycles (FACs) that comprise two FAC sets (FAC-Sets). The FAC-Sets are composed of architectural elements suggestive of a mixed-load fluvial system that is alternately dominated by bed-load deposits and suspended-load deposits. A thinning and fining stacking pattern within FACSets is accompanied by an upward increase in pedogenic modification, suggesting that cycles systematically stack in response to a longer period decrease in the rate of accommodation gain. Sandstones are classified as litharenites, feldspathic litharenites, and lithic subarkoses, and occur within recycled orogen, dissected arc, and transitional arc provenance fields. Sandstone compositional maturity increases upward through the FAC-Sets. Point counts of intergranular volume (as a proxy for primary porosity) within channel faciès and subsequent transform to syndepositional permeability provide a two-dimensional depiction of the lateral variability in reservoir quality. Paleosols are weakly to moderately developed and have little stratigraphie variation. These characteristics suggest that climatic fluctuations are not responsible for evolving fluvial depositional style or associated reservoir quality. Trends in sandstone compositional maturity suggest that fluvial stacking patterns and depositional style are related to pulses of tectonism. Sandstones are volcanogenic rich and have undergone an almost complete diagenetic loss of porosity caused by the precipitation of authigenic clays. Paragenetic reconstruction suggests that porosity loss occurred contemporaneous with the silicification of fossil logs in channel deposits. Log compaction at the time of silicification averaged 9.1%, suggesting that log silicification and porosity loss occurred soon after deposition. Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved.
Article
Paleomagnetic samples were collected at over 600 sites (3 samples per site) through the late Cretaceous to middle Paleocene age continental sedimentary sequence in the San Juan Basin. The sampled lithostratigraphic units include the Fruitland, Kirtland, and Nacimiento Formations as well as the Ojo Alamo Sandstone. Alternating field demagnetization to peak fields of 200 to 300 oe was required to remove secondary components in order to reveal the polarity of the primary depositional remanent magnetizm. Unambiguous polarity determinations were possible for almost all sites. The magnetic polarity zonation shows a strong correlation with the magnetic polarity time scale (for example, the sea-floor anomaly sequence) in the interval of anomalies 25 through 31. This correlation is strongly supported by the vertebrate fossil occurrences and by a radiometric date of 64.6 + or - 3.0 Ma from an ash layer in the late Cretaceous Kirtland Formation. -Authors
Conference Paper
Upper Cretaceous and lower Paleocene rocks in the San Juan Basin, New Mexico, contain a robust record of dinosaur and mammal evolution, diversification, and extinction. Despite this rich fossil record, the ages and durations of the Upper Cretaceous Naashoibito Member of the Kirtland Formation and the lower Paleocene Ojo Alamo Sandstone and Nacimiento Formation have been relatively poorly constrained. These poor age constraints have limited the ability to accurately correlate these vertebrate records to others across North America and to assess rates of speciation and extinction of dinosaurs in the Cretaceous and mammals before and after the Cretaceous-Paleogene (K-Pg) mass extinction. Further, the ages of the Naashoibito and the Ojo Alamo are contentious and have been interpreted to range from Campanian to early Paleocene. Here we present new geochronologic results that combine magnetostratigraphy and 40Ar/39Ar dating of detrital sanidine from sedimentary units and sanidine phenocrysts from a volcanic ash to constrain the ages of the Naashoibito Member, the Ojo Alamo Sandstone, and the lower Nacimiento Formation. Coupled detrital sanidine dates and magnetostratigraphy indicate that the Naashoibito correlates to chrons C31n – C29r, suggesting a protracted depositional history with multiple disconformities. Further, our results indicate that the youngest Cretaceous sedimentary rocks in the San Juan Basin were likely deposited within the last 300 kyr of the Cretaceous. A 40Ar/39Ar sanidine date of 65.59±0.01 Ma (1S, analytical error only) from an ash within the Nacimiento demonstrates that biozone Pu2 (2nd biozone of the Puercan Land Mammal “age”) began within ~440 kyr of the K-Pg boundary. A probable volcanic ash coincident with the first occurrence of Pu3 mammals yielded an age of 65.43±0.04 Ma, tentatively suggesting that Pu2 was only ~150 kyr long. These dates and our magnetostratigraphy indicate that the Ojo Alamo was deposited in chron C29r and the lower Nacimiento in chrons C29r – C28r. This new geochronology helps to constrain the ages of the first occurrence of the Pu2 and Pu3 faunas in the San Juan Basin and indicates that deposition of basal Paleocene strata in the basin began <300 kyr after the K-Pg boundary. All argon dates are relative to FC-2 sanidine at 28.201 Ma and 40K decay constant of 5.463e-10 /a.
Article
Two kinds of cumulative floodplain paleosols, red and grey paleosols, formed on overbank deposits of the Willwood Formation in the Sand Coulee area of the Bighorn Basin, Wyoming. Although both kinds experienced down-profile iron movement, due to seasonal saturation of a clay-rich A horizon, the red paleosols were better drained and their B horizons were rubified. Various redoximorphic features indicate that the grey paleosols were poorly drained.The red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone, a paleosol/landscape relationship termed pedofacies by Bown and Kraus (1987). Although the grey paleosols show relatively systematic changes in hydromorphy, consistent pedofacies changes were not recognized. Furthermore, no lateral relationship between the red and grey paleosols was observed. These features suggest that, because of the retarding effects of poor drainage on soil weathering, poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures.Results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine-grained facies on which paleosols developed. Although readily observable in Sand Coulee, pedofacies are difficult to recognize in the Elk Creek area, where accumulation rates were approximately half as rapid as in Sand Coulee. The attainment of steady-state conditions is believed to have obscured pedofacies in the Elk Creek area. Finally, pedofacies are only developed on true overbank deposits, which, in the Willwood Formation and probably many other ancient alluvial sequences, make up only a fraction of the fine-grained deposits. Only immature soils formed on the remainder of the fine-grained facies because they were deposited very rapidly, as a result of channel avulsion.
Article
The Upper Triassic Blue Mesa Member and the overlying Sonsela Member of the Chin le Formation consist of paleosol-bearing alluvial strata whose characteristics vary markedly. Strata exposed within a 4 km(2) study area at Petrified Forest National Park were evaluated using sedimentologic, stratigraphic, paleopedologic, and petrographic criteria to formulate potential depositional models that may account for varying alluvial deposition. The study succession records a progressive up-section increase in grain size, increase in channel depth and width, increase in lateral and vertical connectivity of channel deposits, decrease in overbank preservation and crevasse-splay and/or sheetflood deposition, and increase in overbank drainage. Mean annual precipitation from the CALMAG paleosol weathering proxy suggests precipitation averages approximately 1300 mm/yr (1.3 m/yr) throughout the succession despite changing paleosol drainage. These features are consistent with deposition in a progradational fluvial system such as a large fan or splay complex. Sediment accumulation rates in the study interval suggest decreased subsidence in the upper Blue Mesa Member. Reduced rates of accommodation gain may have promoted progradation of the fluvial system. An up-section decrease in sandstone mineralogical maturity may be related to increasing sediment bypass in proximal fan positions or increased rates of erosion and sedimentation. Increasingly better drained paleosols are attributed to deposition in more upland positions during fan or avulsion-complex progradation, rather than changing climate.
Article
The currently used model for alluvial sequence stratigraphy has been criticised (Miall, 1991) and an alternative model is offered here. In this model alluviation mainly takes place during transgressive and not highstand phases, when accommodation space allows sediment storage. The high capacity for some river systems to store floodplain sediments is a critical consideration. The effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple, base-level controlled sequence models, and recognising higher-order, fourth- and fifth-order cycles will be particularly difficult.
Article
Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation-ally overlain by paleosols or are sharply overlain by the coarser-grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad-ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in-terval. From base to top, each sequence is characterized by an asym-metric increase and decrease in FAC thickness, a decrease in the pro-portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi-sodes of channel avulsion and stability, and longer-term avulsive chan-nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous–Tertiary (K–T) transition within the Tornillo Basin may provide an example of me-gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive-equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage-to lowstand-equivalent). Se-quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly-drained paleosols are associated with the transgressive-equivalent por-tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand-and falling stage-equivalent.
Article
Biostratigraphy in combination with magnetostratigraphy yields a stratigraphic-chronologic foundation in which biological and environmental changes can be correlated with an independent time frame. This combination promises greater resolution of chronostratigraphic units than those based on faunal change and superposition. The occurrences of early and middle Paleocene land mammals in the San Juan Basin are bracketed relative to the magnetic polarity time scale: mammals of Puercan (early Paleocene) age occur in rocks deposited during magnetic anomaly 28, and mammals of Torrejonian (middle Paleocene) age occur in rocks deposited during magnetic anomaly 26 and during the reversed polarity interval preceding anomaly 26.
Article
Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior of the southwestern United States were evaluated using an adapted marine stacking-pattern analysis methodology. A three-tier cyclic hierarchy is present in the strata at both study areas. Meter-scale fining-upward fluvial aggradation cycles (FACs) comprise fluvial aggradational cycle sets (FACSETs) 4-15 m thick (avg. 8.4 m). FACSETs in turn stack into four fluvial sequences 26-48 In thick (avg. 41 m). Within these sequences, transgressive-systems-tract equivalents (TE) are characterized by channel sands and associated minor overbank deposits and relatively immature paleosols (i.e., high rates of deposition), whereas highstand- to falling-stage-systems-tract equivalents (HFE) are dominated by overbank muds and relatively well-developed paleosols (i.e., lower rates of deposition). These two fluvial successions, which are 200 km apart, contain age-equivalent fluvial sequences that record similar histories of deposition and pedogenesis: Sequence 1 contains only an incomplete HFE; Sequence 2 includes both the TE and HFE; Sequence 3 is an RFE; and Sequence 4 contains only a TE. Fluvial sequences likely accumulated in response to pulses of source area uplift and/or basin subsidence, which resulted in changes in accommodation. Conversely, higher-frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the products of autocyclic processes, such as channel avulsion, floodplain aggradation, and channel migration. These results suggest that regionally significant tectonic episodes may be discernible in suspended-load fluvial deposits that accumulated over a broad area.
Article
Late Cretaceous and Paleogene plant fossils collected at 149 localities in the Denver Basin, Colorado, are placed into a stratigraphic framework based on palynostratigraphy, magnetostratigraphy, vertebrate paleontology, geochronology, sequence stratigraphy, electric well logs, and two cored wells. Between 69 and 54 Ma, the Denver Basin accumulated sedimentary rocks that recorded the withdrawal of a seaway, the uplift of a mountain range, and evidence of the Cretaceous-Tertiary and Paleocene-Eocene boundary events. Fossil floras deposited in the Denver Basin record these events as variations of floral composition, species diversity, and leaf margin and size (used to estimate mean annual temperature and precipitation, respectively). Attention to these details and to the position of the floras relative to the basin margins and sedimentary facies allows for the recognition of six megafloral associations (K-L, K-D1, P-D1-West, P-D1-Central, P-D1-East, and E-D2). Preliminary comparison of these assemblages documents: floral change at the K-T boundary; a strong paleoenvironmental gradient probably associated with increased topographic relief along the basin margin in the early Paleocene; and a warmer, drier Eocene vegetation.
Article
Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na2O, K2O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this finding, we introduce the CALMAG weathering index, defined as Al2O3 / (Al2O3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the well-established CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the fl ux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca2+ and Mg2+. Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.
Article
The thickest part of asymmetric rift, pull-apart, and foreland basin fills commonly consists of large-scale (hundreds to thousands of metres thick), tectonically generated cyclothems of fine-grained marine, lacustrine, or longitudinal fluvial deposits and coarse-grained transverse braid-plain or alluvial-fan deposits. The appearance of coarse clastics in these basin fills is typically noted as the time of tectonically rejuvenated source-area uplift, based on the conceptual tie between relief and coarse grain size, and on the application of the Davis theory of landform development. We propose the opposite interpretation: that the commencement of fine-grained sedimentation above coarse-grained deposits in a tectonic cyclothem is the best indicator of renewed tectonic activity. This reinterpretation is more consistent with (1) modern examples, (2) the consideration of source-area and sedimentary-basin geomorphology, (3) the disparity between the reaction rates of the various environments to subsidence, (4) the disparity between the rates of tectonic uplift and erosion, and (5) the controls on elastic-wedge progradation. In our model, extensive coarse-grained clastic-wedge progradation is an indicator of tectonically quiescent phases.
Article
Microprobe and XRD analysis show that titanohematite is the ferrimagnetic mineral in the 750 m thick sequence of continental deposits straddling the Cretaceous/Tertiary boundary in the San Juan Basin, New Mexico. Demagnetization experiments and magnetic minerals are used to correlate this section with sections of similar age in Italy.-I.M.S. Dept. of Geosciences, Univ. of Arizona, Tucson, AZ 85721, USA.