Mass transport deposit and turbidite interaction in the Mio-Pliocene Gulf of California : Fish Creek-Vallecito Basin, Salton Trough
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Mass transport deposits represent a significant component of modern and ancient deep-water depositional systems. However, geophysical data lack the resolution needed to identify meso-scale (meters to tens of meters) interactions between mass transport deposits and the underlying and overlying sediment. An exposed section of supra- detachment rift basin sedimentary deposits containing subaerial and subaqueous debris flows and coarse-grained turbidites provides an opportunity to examine both the variability related to debris flow emplacement and the unique type of debris flow known as a sturzstrom. The Fish Creek -- Vallecito Basin, part of the larger Salton Trough region of southern California, contains a late Miocene to Pliocene stratigraphic section that records the opening of the rift basin, marine flooding by the Gulf of California, and the arrival of the Colorado River. The lower Split Mountain Group debris flow (up to 50 m thick) was deposited subaerially and was extensive enough to partially cover the previously deposited alluvial fans. At this time it is likely that the subaerial basin floor had subsided below sea-level, much like the floor of the Salton Sea today. Breaching of the basin walls then led to a rapid marine incursion into an already deep basin, such that the lower debris flow was immediately and conformably overlain by gypsum, mudstones, and coarse- grained Lycium member turbidites of the Imperial Group. A second major debris flow (up to 45 m thick), this one a subaqueous flow, severely deformed, scoured, and truncated the underlying Lycium member turbidites, and profoundly impacted the routing and deposition of the overlying Wind Caves member turbidites that signal the arrival to the basin of the Colorado River. Each of the turbidite successions thus follows an event of catastrophic mass transport. This thesis describes and documents the variable meso-scale erosion of the underlying turbidite deposits by the younger debris flow, the impact of variable upper debris flow surface bathymetry on subsequent turbidite deposition and connectivity, and links these findings to observations made by other workers studying similar deep-water deposits in outcrop and in seismic-reflection data. Building on the efforts of previous workers, this thesis also describes the characteristics of the subaerial lower and subaqueous upper sturzstroms, investigates emplacement mechanism implications of micro-scale features, and provides the first paleogeographic reconstruction of the basin during the time of subaerial to marine transition and the arrival of the Colorado River into the region.