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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, a detailed paleoclimate and paleoenvironmental record, and a refined age model for the early Paleocene Nacimiento Formation in the San Juan Basin, New Mexico, USA.  Our efforts during project year 1 have included:

Fieldwork. A team of researchers (the PI, Stacy Atchley, Thomas Williamson, PhD student Adam Davis, and MS student Andrew Flynn) conducted 10 days of fieldwork in the San Juan Basin in May 2013.  Our work included scouting localities that best preserved laterally and vertically continuous exposures of the early Paleocene Nacimiento Formation and collecting paleomagnetism samples from the Nacimiento Formation (Figure 1).  Davis and Flynn then returned to the field for 6 weeks of fieldwork in May – June 2013 and again for one week of fieldwork in May 2014 to measure lithostratigraphic sections and collect samples from all paleosols and sandstone beds.

During these field seasons, six stratigraphic sections were measured in the basal Nacimiento Formation and sampled at a sub-meter resolution (Figure 2). Sections were specifically chosen to intersect four channel sandstones contained within the study interval (sections 2, 3, 4 and 6), and two additional sections were measured adjacent to these sandstones to capture lateral variability (sections 1 and 5). Grain size trends, paleosol occurrence, thickness, and characteristics, cycle boundaries, and sedimentary and biological structures were collected in the field and used to construct composite measured sections placed on a conspicuous red and green laminated bed found in all measured sections (Figure 2).  Paleosols in the sections were categorized into 8 pedotypes based on similar attributes and master and subordinate horizon designations and sub-sampled for analyses.  Paleomagnetism samples were also collected from 15 stratigraphic levels in the basal Nacimiento Formation.

Figure 1.   Paleosols and sandstone beds in the basal Nacimiento Formation. 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).  During fieldwork Davis measured 6 stratigraphic sections and identified fluvial aggradational cycles (FACs) in each section.  The results of his analyses indicate the occurrence of eight distinct pedotypes that vary in stratigraphic occurrence across the landscape.  These analyses also indicate that there are likely four fluvial aggradational cycle sets (FAC-sets).  Each probable FAC-set is capped by a distinct laterally extensive paleosol that likely reflects a stable landscape.  The capping paleosol is different for each probable FAC-set suggesting variability in environments of deposition through time. 

Samples from each of the eight pedotypes 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.  Results of this work will be presented at GSA in 2014.

Figure 2. Composite measured sections using a conspicuous red and green laminated bed found in each section as a datum (indicate by blue line). FACs in each section indicated by arrows. Possible FAC-set boundaries indicated by black lines and possible FAC-sets indicated by orange arrows on the left of the figure. Pedotype designation indicated by different colors.

Paleomagnetism analyses. Under the advisement of the PI, undergraduate research C. William Fenley took the lead on the paleomagnetism analyses from the basal Nacimiento Formation.  The results of his analyses demonstrates a direct correlation to the geomagnetic polarity time scale and provide new, refined age constraints for the sequence (Figure 3).  Importantly, this work indicates that the early Puercan 2 land mammal age is considerably older and significantly shorter than previously suggested.  This refined age model for the basal Nacimiento Formation provides important age constraints for our sequence stratigraphic analyses and indicates that we have sampled an ~150 kyr long interval.  This work was the focus of Fenley’s BS thesis and was presented at a Baylor University student research exposition.  These results were presented at the New Mexico Geological Society annual meeting and 2014 and will be presented at the Geological Society of America (GSA) and Society of Vertebrate Paleontology annual meetings in 2014.

Figure 3. Revised geochronology for the latest Cretaceous and earliest Paleocene strata in the San Juan Basin. 

Professional development. During the first 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, and also led to the development of a collaborative NSF grant funded in 2014 focused on understanding the influence of environmental change on mammalian turnover in the early Paleocene in the San Juan Basin.  PhD student Davis has been engaged in the project in a supervisory role and has gained valuable experience working with and mentoring undergraduates.  MS student Flynn gained valuable field experience and while working on this project was able to begin developing his MS thesis research project.  Three undergraduate students worked on this project and gained valuable laboratory research experience.  Additionally, undergraduate Fenley developed this work into his BS thesis project and presented the results at Baylor University URSA Scholar’s Week.