Reports: DNI855161-DNI8: Middle to Late Jurassic Paleogeography, Western Colorado

Sally Potter-McIntyre, PhD, Southern Illinois University

This project has produced exciting preliminary results that are detailed below. Additionally, two students (one Ph.D. student and one M.S. student) have been supported this year. Two papers have been published, and three other papers are in prep. Both the students and the PI have conducted field and laboratory investigations that resulted in the following presentations: * John Ejembi, PhD candidate, presented a poster at the Applied Geoscience Conference in March, 2017, and he will present a poster at the American Geophysical Union fall meeting, and the fall meeting of the Geological Society of America. * Jason Williams, M.S. 2017, presented an oral presentation at the Geological Society of America meeting in Fall, 2016. * PI Potter-McIntyre presented a paper at Goldschmidt, 2017. The purpose of this investigation is to better understand the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico to further our understanding of the development of the Piceance and Paradox Basins. Additional investigation has gone into the depositional environment of the Wanakah Formation, and the Morrison Formation, particularly focusing on the Tidwell Member and the upper part of the Brushy Basin Member that contains a restricted saline lake environment in southeastern Utah. Paleosol investigations are augmenting the climate interpretations and paleomagnetics are being used to understand diagenetic fluid flow patterns and related tectonics. One of the major discoveries resulting from this research is a presentation of the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the Bell Mountain Complex on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not present in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across southwestern Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment. Continued research supports these early conclusions and it is fairly clear that the Bell Mountain Complex (that include the McClure Mountain Syenite) is the source area for a large amount of input suddenly during the Middle to Late Jurassic. Another important outcome of this research are revised interpretation of the region’s paleoenvironmental history. Paleosol investigations are suggesting the region stayed arid, rather than becoming more temperate – the explanation usually endorsed for the changing from dominant eolian depositional regimes to hypersaline lacustrine environments. Paleomagnetic investigation is ongoing and interpretations have not been solidified as yet; however, this will likely result in a fourth paper from this research. This grant has supported an M.S. students this past year who completed his degree in summer, 2017. The PhD candidate has passed his comprehensive exams and is on schedule for a 2019 graduation. The career of the PI has been positively impacted as well, by providing new data with which to write larger grants. Additionally, she has presented this research at a large international conference this past summer, where networking opportunities were abundant and she made invaluable connections with researchers around the world. Other new collaborations have begun – most notably between the PI and another faculty at SIU in the field of paleomagnetics, with a faculty member at the University of Arkansas, Glenn Sharman, who is skilled at detrital zircon geochronology statistical analyses, and with a researcher at Illinois State University, David Malone, with whom a manuscript is in prep from a previous master’s student whose analyses were funded by this grant during 2015.