Influence of climate and the expansion of C4 grasses on sequence-scale cyclicity and landscape development during the late Miocene to Pleistocene of West Texas.

Dhillon, Ryan S.
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The Neogene records a period of global climate change and terrestrial ecosystem evolution; however, little is known about whether these changes influenced sedimentary and soil forming processes. The late Miocene to Pleistocene Fort Hancock and Camp Rice Formations from two adjacent isolated basins, southeast Hueco Bolson and northwest Eagle Flat Draw, were analyzed to determine whether changing climate and concomitant C₄ grass expansion influenced fluvial depositional style. Alluvial stacking-pattern analysis of two drill cores reveals sequence-scale trends in paleosol maturity, cycle thickness, and facies proportions. Stable isotope geochemistry of bulk paleosol samples and pedogenic carbonate suggest C₄ expansion at 7.25 Ma and provide a pCO₂ reconstruction of the late Miocene to Pleistocene. C₄ onset coincides with a decreasing sedimentation rate, less frequent channel avulsion events, and an increase in paleosol maturity. Stratigraphic stacking-patterns parallel pCO₂ concentration, suggesting that sedimentation and pedogenesis are influenced by third- to fourth-order global climate cycles.

Alluvial sequence stratigraphy., Stacking-pattern analysis., Paleosol geochemistry., Sedimentology/stratigraphy., Paleoclimatology/paleoecology.