Climatic, geographic, and structural controls on the stratigraphic architecture of a mixed siliciclastic-carbonate succession from the Pennsylvanian subtropics (Amsden and Tensleep Formations, Northern Wyoming, USA)

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Blanchard, Sebastien
thesis / dissertation description
As our best opportunity to study the anatomy of an icehouse period from onset to demise, the Late Paleozoic Ice Age has been a popular subject in recent years. Studies have investigated the imprint of different scales of Pennsylvanian climate change on glacial, tropical, carbonate, and alluvial records, but overlooked subtropical, arid records. This bias results from our poor understanding of marine erg-margin systems and their stratigraphic architecture. The Wyoming Shelf preserves a mixed siliciclastic-carbonate series deposited in the Pennsylvanian subtropics (Amsden and Tensleep Formation). To explore whether this succession recorded different scales of climate and sea-level changes, this series was investigated using a combination of surface exposure descriptions, petrography, isotope geochemistry, and subsurface data. ^ The long-term (30 Myrs) climatic signal is strongly controlled by Pangea’s northward motion through climate belts. Shorter-term deviations from this arid to humid, then humid to arid trend can be explained by superimposed changes in climate. The Ranchester Limestone Member (RLM) and the Tensleep Formation, deposited in arid coastal settings during high-frequency sea-level fluctuations, make for an outstanding analog to improve the sequence stratigraphic frameworks of marine erg-margins. Lateral changes in the preservation of thick (> 10 m) eolian sandstones in the upper Tensleep Formation are explained by the rejuvenation of a basement lineament, whose seismicity is the preferred trigger for fluidized sand injection through planar fractures in dolomite intervals. ^ The RLM contains an unconformity evidenced from petrographic, biostratigraphic, and geochemical data. The basal RLM (middle Bashkirian) is characterized by negative δ13C values (-3.6 to -1.8‰; avg. = -2.9‰). Similar facies in the upper RLM and Tensleep Formation (middle to late Moscovian) have higher δ13C values (-2.5 to 1.7‰; avg. = -0.2‰). Lower values suggest interaction with meteoric waters. The hypothesis of a prolonged exposure of the shelf during the early Moscovian coincides with prolonged periods of relative sea-level lowstands in other paleotropical records, suggesting a potential link with a glacial epoch. These insights will aid in future studies on the significance of subtropical, mixed successions, and further our understanding of the low-latitude response to sea-level and climatic changes during the Phanerozoic’s longest icehouse period.^