Reports: DNI852822-DNI8: Climate, Tectonics and the Ecosystem Impact: Exploring the Relationship Between Extrinsic Controls and Cyclic Alluvial Stratal Accumulations within the Paleocene Nacimiento Formation, San Juan Basin, New Mexico

Daniel J. Peppe, PhD, Baylor University

Our team has made excellent progress towards developing a sequence stratigraphic model of deposition and a detailed paleoclimate and paleoenvironmental record for the early Paleocene Nacimiento Formation in the San Juan Basin, New Mexico, USA.  Our efforts during project year 2 have included:

Fieldwork. A team of researchers (the PI and PhD student Adam Davis) conducted 5 weeks of fieldwork in the San Juan Basin in summer 2015.  Our work included describing and measuring lithostratigraphic sections through the basal Nacimiento Formation at Kimbeto Wash and Mesa de Cuba to compare to our previsously analyzed sections in the Bisti/De-Na-Zin Wilderness area (Figure 1).  We recorded all sedimentologic, biologic, and pedologic features of the rocks and collected samples from all paleosols for more detailed laboratory analyses. 

During these field season, two sections were measured at Mesa de Cuba and three sections were measured at Kimbeto Wash.  All sections were sampled at sub-meter resolution.  These sections were chosen to avoid sand channels and grain size trends, paleosol occurrence, thickness, and characteristics, cycle boundaries, and sedimentary and biological structures were collected in the field.  These data will be used to construct composite sections for both locations that can be compared to our existing dataset from the Bisti/De-Na-Zin Wilderness area.   

Figure 1. Paleosols and sandstone beds in the basal Nacimiento Formation in the Bisti/De-Na-Zin Wilderness Area. Photo shows the stratigraphic interval chosen to assess sequence stratigraphic stacking patterns and paleosols.

Sequence stratigraphic, paleoenvironmental, and paleoclimate analyses. PhD student Adam Davis has been working to develop a sequence stratigraphic framework for the early Paleocene Nacimiento Formation in the Bisti-De-Na-Zin Wilderness area during the Puercan 2 land mammal age (Figure 2).  Six stratigraphic sections across on ~1 km transect have been measured and fluvial aggradational cycles (FACs) were documented in each section (Figure 2).  Analyses of these sections indicate the occurrence of eight distinct pedotypes that vary in stratigraphic occurrence across the landscape.  The paleosols and depositional facies change temporally through the section and indicate changes in landscape position relative to a major drainage system up section (Figure 3).    

Samples from paleosols from each section were processed for bulk geochemistry to reconstruct paleoclimate for the sequence.  The elemental concentrations were used to estimate mean annual paleoprecipitation (MAP) and mean annual temperature (MAT) using paleosol-based transfer functions.  MAP estimates for the paleosols range from ~100 – 165 cm/year and relatively invariant through the stratigraphic sequence.  MAT estimates range from ~9.0 – 15 °C.  Based on fossil leaf estimates and the composition of the vertebrate faunas from this stratigraphic interval, the MAT estimates are likely minimum estimates for temperature. 

Our results thus far indicate that while there were changes in environments of deposition, there was no major climatic change between the Puercan 2 (Pu2) and Puercan 3 (Pu3) North America Land Mammal “ages”.  This suggests that climate change is unlikely to be the driver of the mammalian species turnover between Pu2 and Pu3.  Future laboratory analyses of paleosol samples collected from Mesa de Cuba and Kimbeto Wash will help to determine if the changes in environments of deposition noted in the Bisti/De-Na-Zin Wilderness area were regionally significant and the potential role climate and environmental change played in influencing mammalian turnover.

Figure 2. Panorama photographs of outcrop studied in the Bisti/De-Na-Zin Wilderness Area from north (A) to west (B) with measured section transects plotted in red and hill tops emphasized with black lines.

Figure 3. Panorama photographs of Photopan Outcrop (A and C) and interpreted architectural element and facies designations plotted on the outcrop (B and D). Measured section paths are given by red lines and measured section numbers are given above or below.

Professional development. During the second year of the project, this project has provided valuable training and career advancement for the PI and students.  The supported work has advanced the PI’s research into a new direction assessing paleosols and sequence stratigraphy.  PhD student Davis has been engaged in the project in a supervisory role and has gained valuable experience working with and mentoring undergraduates.  PhD student Caitlin Leslie has also engaged in this project and has gained valuable field and laboratory experience.  Two undergraduate students worked on this project and gained valuable laboratory research experience.