HR: 0800h
AN: PP11B-0569    [Abstracts]
TI: Does rapid climate change affect sedimentation in a fluvial system? Documenting the PETM using pedogenic carbonate in the Wasatch Formation of Western Colorado
AU: * Blecha, A M
EM: ablecha@mines.edu
AF: Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 United States
AU: Gardner, M H
EM: mgardner@mines.edu
AF: Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401 United States
AB: While the Paleocene/Eocene Thermal Maximum (PETM) has been identified in numerous terrestrial systems across North America, little work has been done relating landform evolution to this event. The Wasatch Formation of western Colorado is an ideal setting for this investigation, with extensive exposures and conspicuous lithological variability straddling the Paleocene/Eocene boundary. In addition, comparisons between the basin axis and the western margin provide conflicting environmental interpretations. Data collected from an isotopic study of pedogenic carbonate from the western margin of this outcrop attempts to identify the PETM in relation to these lithologic discrepancies. Pollen data constrain the PETM to the conspicuous increase in sand volume within the basin axis of the Wasatch Formation. Tectonic forcing has been attributed to this change, but we are investigating the possible influence of climate on this system. The integration of an isotopic record will provide a high-resolution time scale, allowing for direct comparison between lithologic shifts and this global climatic event. The 200 meter thick Wasatch Formation in the western margin study area consists of an upward increase in sand, transitioning from clay dominated mudstone to pebbly sandstone, then returning to a sand poor environment. This sand increase also mirrors the general drying trend of the system. The sand poor organic-rich mudstones located at the basal unconformity contains algal laminae of a shallow lacustrine origin. This wet, poorly drained environment grades both laterally and vertically into more arid and better drained conditions evidenced by red mudstones with carbonate nodules. These mudstones show abundant rooting, burrowing, and slickenlines indicative of soil forming processes. This red bed sequence, averaging 70-90 meters thick, is characterized by isolated point bars and splays increasing upsection in abundance. Capping the red beds are sheet sands with conglomeratic lags. A sequence of minor channelized sandstones follow, interbedded with purple and grey mudstones. These mudstones do not contain carbonate, indicating a return to the previous poorly drained conditions. Carbonate nodules have been collected through the red beds in the western reaches of the outcrop belt. Sampling was at a 0.5 to 8 m resolution, with an average of 2 meters throughout the section. These nodules are currently being analyzed for carbon isotopes to attempt to determine if the negative \delta $^{13}$C shift indicative of the PETM in other basins can be detected in this formation. Assuming the PETM occurs within the red bed interval, lateral tracing to the basin axis correlates these strata to the thick (100 meter) fluvial sandstones of the Molina Member. Whereas the red mudstones along the basin margin indicate drying conditions, the contemporaneous aggradational fluvial system in the basin axis suggests an increase in discharge. This conundrum may reflect the limitations of one dimensional analysis when evaluating the signature of climate in landform evolution.
DE: 9350 North America
DE: 9604 Cenozoic
DE: 3344 Paleoclimatology
DE: 1040 Isotopic composition/chemistry
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting