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Architecture, Lithofacies, and Depositional Model for the Ballarat Sequence: A Mid-Pleistocene Fan-Delta Complex, Panamint Valley, Inyo County, California


A robust three-dimensional stratigraphic framework is required in order to understand the role sediment supply plays in the development of stratigraphic architecture. Such a high-resolution framework is essential for ongoing research into controls on stratigraphy and is the focus of this presentation. Here, we integrate data from measured lithologic sections, photomosaics, and high-resolution dGPS surveys of prominent stratigraphic surfaces to present a stratigraphic framework for this lacustrine fan-delta complex. During the early to mid-Pleistocene, a fan-delta sequence prograded from the Pleasant Canyon catchment in the Panamint Mountains westward into pluvial Lake Panamint, the penultimate endorheic basin in the Owens River system. Subsequent faulting along the Panamint Valley fault zone cut the fan-delta complex, uplifting the proximal segment relative to the subsiding basin, and post-abandonment incision and dissection have created exceptional outcrop exposures measure ∼170 meters in thickness, 1.25 km along depositional strike, and 0.75 km along depositional dip. Measured sections through 170 meters of vertical succession allows for subdivision into three lithostratigraphic units defined by dominant internal lithofacies. Notable stratigraphic intervals are a basal sand- and gravel-dominated unit, a unit containing two light-colored fine-grained layers located approximately in the middle of the succession, overlain by a unit of sand- and matrix-supported gravel-dominated bedsets. The fan surface is composed of a 50-100 cm thick stage IV pedogenic carbonate, and a well-developed desert pavement cap. Thus the fan delta was constructed in at least three phases: 1) Gravel and sand in the base of the sequence accumulated as either subaerial or subaqueous debris flows, 2) a brief interval of paleosol formation followed by abrupt deepening of lake level represented by fine-grained light-colored calcareous strata, and 3) as lake level grew shallower the system responded by coarsening upward to subaerial debris flows near the surface of the sequence. This detailed stratigraphic framework aids ongoing work to quantify sediment flux to the system using a novel cosmogenic nuclide approach.