The Sedimentary Response to Climate Change During the Paleocene-Eocene Thermal Maximum, Southeastern Bighorn Basin, Wyoming, U.S.A.

摘要

The Paleocene Eocene Thermal Maximum (PETM) was a rapid episode of dramatic global warming at ~55 MA. This event is recognized by a negative carbon isotope excursion (CIE). In the Bighorn Basin, the lowest ~40 m of the Willwood Formation contains the CIE and represents the sedimentary response to PETM climate change. In the southeastern Bighorn Basin, the PETM interval can be divided into three lithologic units on the basis of alluvial architecture and presence of calcium carbonate nodules. This thesis focus’ on the interval which likely represents the response to peak PETM temperatures.A detailed study of the facies and facies associations of the middle yellow-grey interval reveals that it is sedimentologically distinct from the other PETM intervals. Thick avulsion deposits and weakly developed paleosols dominate this interval. These features indicate that sediment accumulation rates were relatively rapid (and sediment yield was high) and that channel avulsion was frequent. The paleosols and facies architecture of the underlying and overlying lithologic units show slower rates of sediment accumulation. Grain-size analysis reveals that floodplain sediment was coarsest in the yellow-grey interval.Paleoclimate proxies demonstrate the nature of climate changes which occurred during the yellow-grey interval. Carbon isotopes show that the yellow-grey interval corresponds to the main body of the CIE and warmest PETM temperatures. Paleofloral analysis and the presence of pedogenic nodules reveal that conditions became drier and precipitation more seasonal during deposition of the yellow-grey interval.The yellow-grey interval represents the sedimentary response to PETM climate change including changes to temperature and precipitation. This thesis hypothesizes that changes to vegetation, storm frequency and storm intensity occurred during the PETM. Changes to storm patterns likely raised sediment yield and the frequency of avulsion by impacting stream discharge, drying soils, and increasing the erosional capacity of storms. Vegetation decreased and evolved, increasing sediment yield and the occurrence of avulsion by decreasing ground surface protection, and floodplain stability. In this study, the links between a high resolution climate record and sedimentary record suggest that the yellow-grey interval represents the sedimentary response to climate change.