ABSTRACT

The deposition of the Sheep Pass Formation and correlative units of east-central Nevada records a critical geologic history of the Sevier hinterland during Late Cretaceous to Late Eocene time. This study provides a critical test to a variety of models pertaining to the kinematic role of the Sevier hinterland through time, how hinterland processes relate to the foreland, and how orogenic plateaus may evolve from the culmination of shortening to extensional collapse. In particular, questions pertaining to the timing and depositional setting of megabreccia deposits at the base of the Sheep Pass Formation relate directly to whether the Sevier hinterland experienced extension related to mid-crustal flow, or out-of-sequence shortening indicating that the upper crust was effectively decoupled from the mid-crust during the latest Cretaceous to Paleocene. Additionally, this study tests whether megabreccia deposition and the development of major angular unconformities overlying the Sheep Pass Formation indicate a major shift from shortening to extension within the Sevier hinterland during the Middle to Late Eocene. As part of this project, we are conducting detailed geologic mapping, stratigraphic measurements, and structural analyses to determine the tectonic setting of megabreccia deposits and syndepositional faults within the Sheep Pass Formation type section. Our new Argon-Argon dating provides the first absolute age control for the Sheep Pass Formation, brackets major angular unconformities and megabreccia at the top of the Sheep Pass Formation, and allows for regional correlations of tectonic events. Newly obtained fossil data also provides new opportunities for relative dating. We are currently compiling stratigraphic measurements, fossil data, geochronology, provenance and paleodispersal data from additional localities within the Sheep Pass Formation throughout east-central Nevada to reconstruct the Late Cretaceous to Eocene geologic history and paleogeography of the region. Additionally, our Uranium-Lead detrital zircon dating in collaboration with the University of Arizona provides opportunities to reconstruct the regional unroofing history, to identify the timing of initiation of Cenozoic volcanism, and the links between volcanism and tectonics within the Sevier hinterland. Similarly, our collaboration with Stanford University in measuring the oxygen isotope ratios of lacustrine carbonates within the Sheep Pass Formation will help determine how isotopic shifts relate to tectonic events. Although our focus is on what happened in one part of the Sevier orogenic system, our results have implications for ancient orogens in other regions, as well as modern orogenic systems. Our improved understanding of the tectonic and paleoelevation history of the hinterland plateau allows us to make a more comprehensive comparison to the well-determined evolution of the external fold-thrust belt. Research findings related to this project are being shared with the research community through presentations at national and regional conferences and through the eventual publication of maps and peer reviewed scientific journal articles. Results will both benefit and be of interest to workers within the fields of tectonics, structural geology, sedimentology, stratigraphy, geochronology, igneous petrology, paleontology and isotopic geochemistry. This project facilitates the professional development of Peter Druschke, a Ph.D. student, and two undergraduate students. Druschke in particular is receiving exceptional multidisciplinary training of great breath in basin analysis, and preparation for competitive research in an academic career, through study and interaction with a sedimentologist-stratigrapher, a structural geologist, stable isotope geochemist, two geochronologists, and a paleontologist. The undergraduate students additionally will work closely with the Druschke and the PI's and will benefit immensely from opportunities for investigative field research, exposure to a variety of advanced analytical techniques, and data analysis. In addition, students and collaborators involved with this project will benefit from cooperation and exchange between multiple academic institutions. This project is being supported with funds from the NSF Tectonics and EPSCoR programs.

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