Sedimentation, Climate Change and Tectonics: Dynamic Stratigraphy of the Pliocene-Pleistocene Fish Creek-Vallecito Basin, California
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In order to better understand the interactions between climate change, landscape erosion and sedimentation, a detailed study was conducted on Plio-Pleistocene non-marine deposits of the Palm Spring Group in the Fish Creek-Vallecito basin, California, USA. Three inter-related studies focused on (1) local response to global climate change in late Pliocene-early Pleistocene time, (2) large-scale evolution of lithofacies architecture, and (3) climate modulation of late Pliocene sediment flux on Milankovitch time scales. Stable isotopes and paleosol classification reveal that between ~4.0 and 0.75 Ma, aridity increased in the study area concurrent with a shift towards a less intense and more winter-dominated precipitation regime. These changes are interpreted to reflect the long-term waning of summer monsoon precipitation in southern California. A dramatic and enigmatic reorganization of basin strata occurred at 2.9 Ma. Detailed basin analysis shows that locally-derived sediment was supplied by the predecessors of two modern drainages, Vallecito and Carrizo creeks. Initial progradation of alluvial deposits from these two sources across the Colorado River delta plain began between 4.0-3.4 Ma. At 2.9 Ma, rapid progradation of these two deposystems was coeval with emplacement of a megabreccia and transgression of Borrego Lake. My data indicate that tectonic realignments at both local and regional scales drove this reorganization. Time series analysis of rock magnetic data from a densely-sampled stratigraphic section of the lacustrine Tapiado Formation reveals that between 2.9 and ~2.75 Ma landscape denudation in the Carrizo Creek catchment was partly modulated by orbital obliquity. Peaks in landscape denudation implied by my data correspond to obliquity highs. More frequent high intensity precipitation events (i.e. monsoons and tropical storms) probably drove increased erosion during these time periods relative to obliquity lows. The breakdown of this relationship at around 2.75 Ma corresponds to a dramatic increase in northern hemisphere glaciation and may reveal a reduction in monsoonal influence in southern California. A geologic map of the Fish Creek-Vallecito basin is included as a supplemental file to this dissertation. This dissertation contains previously published and unpublished coauthored material.