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Master of Science (MS)
Hajek, Elizabeth A.
The Paleocene-Eocene Thermal Maximum (PETM) is an extensively studied global warming event occurring approximately 56 Ma and lasting around 200 kyr. Marked by a negative 13C excursion from a massive influx of CO2 to the atmosphere, the PETM caused environmental alterations including increases in global temperature, changes in hydrology and ocean chemistry, and floral and faunal overturns. Evidence of these alterations during the PETM is found within both marine and continental basins. During the early Paleogene, the Laramide Orogeny formed a series of nonmarine basins within the Western Interior of the United States. Three of these basins, the Bighorn and Hanna Basins in Wyoming and the Piceance Basin in Colorado, are known to hold continuous depositional records spanning the PETM. In each of these three basins, coincident with the main body of the PETM, there occurs at least one unusually large, amalgamated fluvial sand body that is distinct from sand bodies in the underlying and overlying stratigraphy.
Comparatively thicker and wider PETM sand bodies necessitate both greater river aggradation and lateral mobility during the PETM, but the causative geomorphic mechanisms and environmental changes are poorly constrained. Herein, I test the hypotheses that the wider, thicker PETM sand bodies were generated by: (1) deeper PETM rivers; (2) enhanced sand preservation via transitory increases in paleoslope; and (3) reductions in clay fraction deposition in floodplains and commensurate loss of overbank cohesiveness. I present new and previously collected grain-size data from the sand bodies to estimate changes in river paleoslope, and overbank grain-size data to compare potential changes in floodplain cohesion spanning the PETM within the three basins. I find little to no change in river flow depths and no change in paleoslopes spanning the PETM in any basin studied, nor does the clay content of floodplain strata significantly change spanning the PETM. Thus, the unusually large sand bodies during the PETM were likely not generated by unusually large river channels, nor increased steepening of depositional slope, nor a reduction in floodplain cohesion. When combined with other studies, this lack of variation supports an alternative hypothesis that increased avulsion frequency drove the generation of large fluvial sand bodies by removal of floodplain fines. Greater avulsion frequency could have been facilitated by the attainment of channel-forming discharge more frequently due to a greater recurrence of extreme rainfall events during the PETM. Additionally, lateral mobility was possibly enhanced by the reduction in paleovegetation density during the PETM. This reduction in vegetation could result in two modifications to river behavior: increased rate of lateral movement or a greater avulsion frequency. Finally, I propose this phenomenon, whereby nonmarine basins preserve a greater proportion of coarse sediment, may be facilitating a buffering of sediment flux signals generated by the PETM and causing the observed time lags between the geochemical onset of the PETM and increases in siliciclastic sedimentation rates marginal marine settings.
Geology, Stratigraphy, Paleogene
Western Washington University
Subject – LCSH
Global Warming--Research; Sedimentary basins--Wyoming; Sedimentary basins--Colorado; Geology, Stratigraphic--Paleocene
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Todd, Delaney, "Grain size variability spanning the Paleocene-Eocene Thermal Maximum in Laramide Basins: Reconstructing paleoslopes and overbank erodibility" (2021). WWU Graduate School Collection. 1061.