Author: Brady Z. Foreman

Publisher:

Published: 2013

Total Pages: 506

ISBN-13: 9781303049613

The Paleocene-Eocene boundary ca. 56 million years ago is characterized by an extreme global warming event, known as the Paleocene-Eocene Thermal Maximum (PETM). The event is linked to the massive exogenic release of isotopically-light carbon into Earth's oceans and atmosphere, and is recognizable in the geologic record by an abrupt negative carbon isotope excursion in both organic and inorganic proxy records for duration of approximately 200,000 years. Previous studies indicate the PETM instigated massive changes in ocean and atmospheric circulation, which perturbed both terrestrial and marine environmental conditions and biotic systems. This study exploits the PETM to examine the effects of abrupt climate change on fluvial stratigraphy. The negative carbon isotope excursion associated with the PETM allows the timing and duration of the climate change to be identified independent of lithostratigraphic markers. Local climate shifts are constrained using circulation models, soil geochemistry, and paleobotanical records. Two areas are studied in detail, the Piceance Creek Basin of Colorado and the northern Bighorn Basin of Wyoming. In both areas anomalously thick and laterally persistent fluvial sand-bodies correlate with the PETM interval. In the Piceance Creek Basin the shift in fluvial deposition directly correlates with the onset of the PETM and persists beyond the carbon isotope excursion, whereas in the northern Bighorn Basin the shift appears to lag the isotope excursion by 10-20 thousand years and ends prior to the return to background climatic conditions. In the Piceance Creek Basin the change in sand-body geometry is associated with a shift to deeper paleoflow depths, wider channels, greater preservation of upper flow regime structures, prevalent crevasse splay deposits, and poorer drained floodplain soils. In contrast, within the Bighorn Basin there are no such changes and, apart from greater amalgamation, fluvial deposition appears to be largely unaffected by the PETM. When combined with other PETM terrestrial localities, the records demonstrate the PETM had substantial, but spatially diverse effects on basin-scale grain-size partitioning, discharge regimes, and river-floodplain dynamics. Aspects of the responses in the various basins are reminiscent of those predicted by two-dimensional basin-fill models, however, key differences highlight the role non-linearities, feedback loops, relaxation times, basin geometry, seasonality of precipitation, and vegetation factors play in determining large-scale depositional patterns. Consequently, it is concluded that short-term climatic events such as the PETM hold the potential to strongly alter basin sedimentation patterns, but that the sedimentologic-recorded climatic signal cannot be used to directly reconstruct paleoclimatic conditions. Instead a more appropriate approach is advocated that uses fluvial stratigraphy in concert with geochemical and other proxies to iteratively produce a more robust image of paleolandscape dynamics.