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Characterizing the Response of Fluvial Systems to Extreme Global Warming During the Early Eocene Climatic Optimum: An Analysis of Sandstone Petrofacies in the Wasatch and Green River Formations in the Uinta Basin, UT

Evan Rhys Jones; Piret Plink-Björklund

The Wasatch and Green River Formations in the Uinta Basin, UT contain fluvial sandstones that record changes in terrestrial sedimentation coincident with Paleocene-Eocene Thermal Maximum (PETM) and at least six post-PETM hyperthermal climate change events. The aim of this project is to investigate the compositional and textural maturity of these sandstones through quantitative petrographic analysis to provide insights into the response of chemical weathering rates, physical erosion rates, and the seasonality of sediment and water discharge to multiple global warming events. While proxies for chemical weathering rates during the PETM have been developed using the marine osmium isotope record, to date there has been little research on chemical weathering rates in proximal terrestrial depocenters. This work is one facet of a multi-proxy research effort combining quantitative petrographic analysis, the stable carbon isotope record, and a high-resolution stratigraphic and sedimentologic framework across the southern margin of the Uinta Basin. This detailed stratigraphic and sedimentologic framework will provide a direct linkage of changes in sandstone petrofacies to changes in sedimentary facies that can be correlated across the basin and throughout multiple pulses of hyperthermal climate change during the Early Eocene Climatic Optimum (EECO). The relative tectonic quiescence and gradual basin subsidence in the Uinta Basin during the Early Eocene suggests that changes in sandstone petrofacies were dominantly controlled by changes in climatic rather than tectonic forcing, which further supports the validity of comparisons made between different climate change events.

Terrestrial records of PETM climate do not support a humid climate with increased precipitation as previously suggested from marine proxies of climate change. Instead, terrestrial records of the PETM climate show evidence of prolonged drought punctuated by intense terrestrial flooding events in mid-latitude continental interiors. Increases in chemical weathering rates during the PETM due to increased temperature and average precipitation is cited as a key carbon sink to initiate a recovery phase where atmospheric CO2 returned to normal concentrations. If terrestrial records of chemical weathering rates differ substantially from marine proxies the carbon-cycle dynamics active during the EECO must be reconsidered. It is the hypothesis of this study that sediment deposited at these peak hyperthermal climate change conditions in the Uinta Basin will be more compositionally and texturally immature due to extremely high erosion and deposition rates, and subdued chemical weathering rates. Further, sedimentary facies from post-PETM hyperthermal events suggest a gradually less severe response in the seasonality throughout the EECO. This gradual decrease in seasonality of sediment dispersal is expected to correspond to an increase in the compositional and textural maturity of sandstone petrofacies corresponding to peak hyperthermal conditions in the last stages of the EECO.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013