High-resolution magnetostratigraphy of the upper Nacimiento Formation, San Juan Basin, New Mexico, USA: implications for basin evolution and mammalian turnover

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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Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan Basin, New Mexico, USA

Geological Society of America Bulletin ◽

10.1130/b35481.1 ◽

2020 ◽

Vol 132 (9-10) ◽

pp. 2154-2174 ◽

Cited By ~ 1

Author(s):

Andrew G. Flynn ◽

Adam J. Davis ◽

Thomas E. Williamson ◽

Matthew Heizler ◽

C. William Fenley ◽

...

Keyword(s):

New Mexico ◽

North America ◽

Sediment Accumulation ◽

Community Change ◽

San Juan ◽

Accumulation Rates ◽

San Juan Basin ◽

Early Paleocene ◽

Age Model ◽

Basin Margin

Abstract 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.

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Early Paleocene Magnetostratigraphy and Revised Biostratigraphy of the Ojo Alamo Sandstone and Lower Nacimiento Formation, San Juan Basin, New Mexico, USA

10.31233/osf.io/6mwfy ◽

2019 ◽

Author(s):

Andrew G. Flynn ◽

Adam J. Davis ◽

Thomas E. Williamson ◽

Matthew Heizler ◽

C. William Fenley ◽

...

Keyword(s):

New Mexico ◽

North America ◽

Sediment Accumulation ◽

Community Change ◽

San Juan ◽

Accumulation Rates ◽

San Juan Basin ◽

Early Paleocene ◽

Age Model ◽

Basin Margin

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 Myr of the Paleocene. Mean sediment accumulation rates during C29n were relatively low (<50 m/Myr) and equalized from basin center to basin margin indicating an accommodation minimum; sediment accumulation rates approximately double (> 90 m/Myr) during C28r and are highest in the basin center and lowest on basin margin indicating high accommodation and an increase in basin subsidence near the C29n/C28r boundary (~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.

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Diverse early Paleocene megaflora from the Ojo Alamo Sandstone, San Juan Basin, New Mexico, USA

10.31233/osf.io/rmz87 ◽

2017 ◽

Author(s):

Andrew G. Flynn ◽

Daniel J. Peppe

Keyword(s):

New Mexico ◽

North America ◽

Great Plains ◽

Deciduous Forest ◽

Northern Great Plains ◽

San Juan ◽

Temperate Deciduous Forest ◽

San Juan Basin ◽

Endemic Flora ◽

Forest Biome

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 of North America, and relatively little is known about floras from southern basins. Here, we present a quantitative analysis of a diverse earliest Paleocene megaflora (<350 kyr after K/Pg boundary) from the Ojo Alamo Sandstone in the San Juan Basin (SJB), New Mexico. The megaflora, comprised of 55 morphotypes, was dominated by dicotyledonous angiosperms, with accessory taxa composed of ferns, monocotyledonous angiosperms, and conifers. The majority of morphotypes were restricted to the SJB indicating the presence of an endemic flora in the earliest Paleocene. Diversity analyses indicate that the flora was species rich, highly uneven, and laterally heterogeneous. Paleoclimate estimates using multivariate and univariate methods indicate warm temperatures and high precipitation. When compared to 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, site rarefied richness, and basin rarefied richness, but was significantly less even. Paleoclimate estimates from the SJB were 7-14 °C warmer than the DB and WB, indicating a shift from a temperate deciduous forest biome in the Northern Great Plains to a tropical seasonal forest biome in the SJB. These results demonstrate that the SJB flora is markedly different from floras in the Northern Great Plains, that there were latitudinal differences in species composition and diversity, and that there was a variable floral response to the K/Pg boundary in North America during the earliest Paleocene. 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 heterogenous, endemic flora.

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Early Paleocene tropical forest from the Ojo Alamo Sandstone, San Juan Basin, New Mexico, USA

Paleobiology ◽

10.1017/pab.2019.24 ◽

2019 ◽

Vol 45 (4) ◽

pp. 612-635 ◽

Cited By ~ 2

Author(s):

Andrew G. Flynn ◽

Daniel J. Peppe

Keyword(s):

New Mexico ◽

North America ◽

Tropical Forest ◽

Great Plains ◽

Northern Great Plains ◽

San Juan ◽

San Juan Basin ◽

Floral Diversity ◽

Early Paleocene ◽

High Precipitation

AbstractEarliest 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.

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Laramide fluvial evolution of the San Juan Basin, New Mexico and Colorado: Paleocurrent and detrital-sanidine age constraints from the Paleocene Nacimiento and Animas formations

Geosphere ◽

10.1130/ges02072.1 ◽

2019 ◽

Vol 15 (5) ◽

pp. 1641-1664 ◽

Cited By ~ 4

Author(s):

Steven M. Cather ◽

Matthew T. Heizler ◽

Thomas E. Williamson

Keyword(s):

New Mexico ◽

San Juan ◽

Fluvial System ◽

San Jose ◽

Southwestern Part ◽

San Juan Basin ◽

Southern Rocky Mountains ◽

Basin Scale ◽

Early Paleocene ◽

Depositional Age

Abstract 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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A physical and chemical sedimentary record of Laramide tectonic shifts in the Cretaceous-Paleogene San Juan Basin, New Mexico, USA

Geosphere ◽

10.1130/ges02324.1 ◽

2021 ◽

Author(s):

Kevin M. Hobbs ◽

Peter J. Fawcett

Keyword(s):

New Mexico ◽

Upper Cretaceous ◽

Crystalline Basement ◽

Sediment Supply ◽

San Juan ◽

Fluvial System ◽

Fluvial Systems ◽

San Juan Basin ◽

Basement Rocks ◽

Age Constraints

Fluvial siliciclastic rocks bracketing the Cretaceous-Paleogene (K-Pg) boundary in the San Juan Basin, New Mexico (USA), provide records of regional fluvial and tectonic evolution during the Laramide orogeny. Petrographic analyses of sandstones from the Upper Cretaceous Fruitland Formation and Kirtland Formation and the Paleocene Ojo Alamo Sandstone and Nacimiento Formation show that the rivers depositing these sediments were sourced in areas where unroofing of crystalline basement rocks took place, introducing an increasing proportion of immature detrital grains into the fluvial system through time. After the Cretaceous-Paleogene boundary, rivers deposited an increasing amount of microcline and orthoclase feldspar relative to plagioclase feldspar, suggesting a growing source in unique crystalline basement rocks. Geochemical analyses show significant differences between Al- and K-poor Upper Cretaceous sandstones and Al- and K-rich lower Paleocene sandstones in the San Juan Basin. The high proportion of sand-sized material in the Ojo Alamo Sandstone suggests that it was deposited in a basin with a low ratio of sediment supply to accommodation. However, magnetostratigraphic age constraints suggest it had a relatively high sedimentation and/or subsidence rate of as much as 0.38 m/k.y. The sediment supply must have been high in order to deposit a basin-wide coarse sand-dominated package, suggesting rapid creation of topographic relief in the San Juan uplift, the proposed source area of the Ojo Alamo fluvial system. The observed sedimentary architecture and age constraints of the Ojo Alamo Sandstone, including kilometers-wide sand bodies and limited overbank mudstones throughout most of the outcrop area, are difficult to reconcile with accepted models of aggradation and avulsion in large fluvial systems, but available age and lithologic data make difficult a complete understanding of Paleocene San Juan Basin fluvial systems and basin evolution. Here, we present new lithologic, petrographic, and thickness data from San Juan Basin K-Pg fluvial siliciclastic units and interpretations of their origins.

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