The Andrews Mountain Member of the Campito Formation consists of complexly interbedded fine sands, siltstones, and mudstones that accumulated in an offshore, shallow-shelf setting. Deposition of these units was punctuated by short duration, high-energy flows that deposited crudely graded, hummocky cross-stratified sand beds. These flows are inferred to be the product of the ebb of large storm surges. The sedimentology of these units indicates a five-stage scenario for their formation. (1) Initial erosion and suspension of muddy fairweather substrates occur in response to long-period storm waves. (2) As an intense low-pressure system moves onshore, a sediment-charged return bottom flow is generated by a storm-surge ebb. This flow moves across the shelf and further erodes a d entrains material. (3) As the competence and capacity of the flow are exceeded, fine sand is deposited as parallel laminated and hummocky cross-stratified units. (4) Rapid attenuation of the flow then leads to low-flow regime deposition of climbing ripples and small-current ripples. Where deposition occurs within storm wave base, upper parts of the beds are reworked into complex wave-current ripples and flaser bedding. (5) Soft-bodied, benthic communities recolonize muddy substrates with the resumption of fairweather conditions.

Rather than depositing sand sheets over extensive areas, multiple storm-surge flows are inferred to have incrementally moved sand across the shelf. In addition, thinning/fining-upward and thickening/coarsening-upward stratigraphic sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source.

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