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Storm-Surge-Ebb Origin of Hummocky Cross-Stratified Units of the Andrews Mountain Member, Campito Formation (Lower Cambrian), White-Inyo Mountains, Eastern California
Jeffrey F. Mount
The Andrews Mountain Member of the Campito Formation consists of complexly interbedded and regionally metamorphosed feldspathic quartzites, siltstones, and mudstones of latest Precambrian and Early Cambrian age. Extensive outcrops of the member occur in the White-Inyo Mountains of eastern California. The Andrews Mountain Member accumulated in a shallow water, offshore depositional system within the broadly subsiding Cordilleran miogeocline. The sedimentology of the member suggests that both storm and fairweather processes were involved in its formation.
The following idealized sequence of sedimentary structures is recognized within individual beds: 1) an erosional base with localized flute and load casts; 2) fine-grained sands containing parallel and low-angle cross-lamination, hummocky cross-stratification, rare penecontemporaneous deformation, and internal channels and discontinuities; 3) these free sands grade into silts and very fine sands containing climbing ripples, wave-current ripples, flaser bedding, and locally abundant bioturbation; and 4) the uppermost portions of the bed grade into bioturbated muds with Cruziana ichnofossil assemblages and local occurrences of linguoid, symmetrical, and interference ripples.
The sedimentary structures in these sequences reflect deposition by high-energy storm events. The flows generated by these events are interpreted to be the product of the ebb of large storm surges. An inferred scenario of storm deposition can be reconstructed for the Andrews Mountain Member shelf: 1) initial erosion and suspension of fairweather sediments in response to long-period storm waves; 2) as a low pressure system moves onshore and dissipates, a sediment-charged, return bottom flow is generated that flows across the shelf and further erodes and entrains material; 3) fine sands are deposited as parallel-laminated and hummocky cross-stratified units as the competence and capacity of the flow decreases; 4) attenuation of the surge leads to low-flow-regime deposition of climbing r pples and, where deposition is within storm wave base, wave-current ripples and flaser bedding; 5) with the resumption of fairweather conditions, softbodied, benthic communities recolonize muddy substrates which are locally reworked by tidal and oceanic currents.
Rather than depositing sand sheets over extensive areas, multiple storm surges are inferred to have moved sand incrementally across the Andrews Mountain Member shelf. Thinning/fining-upward and thickening/ coarsening-upward sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source.
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