--> 3-D, Characterization of Deposits from a Supercritical Fan: Fish Creek-Vallecito Basin, CA (USA)

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3-D, Characterization of Deposits from a Supercritical Fan: Fish Creek-Vallecito Basin, CA (USA)


Models, experiments, and direct observation suggests that turbidity currents commonly reach Froude supercritical states over moderately steep slopes, impacting how and where sediments are deposited. This occurrence is reflected in seafloor topography marked by bedforms that observed or inferred to migrate upstream. However, the stratigraphic recognition of the resulting deposits and their implications for process morphodynamics, system evolution, and reservoir properties remain incompletely understood. This work analyzes seismic-scale outcrop exposures in the Fish Creek-Vallecito Basin deposited along the steep margins of the early Gulf of California in south-central California. A detailed 3-D outcrop model describes the architecture of supercritical bedforms and spatiotemporal evolution of the deposits within the context of an evolving supercritical fan. The 100+ m-thick, strongly aggradational slope deposits of the Late Miocene (~6.3-5.3 Ma) Lycium Member represent a world class outcrop example of a preserved, reservoir-scale deepwater supercritical fan. For this work, a 3D outcrop model is generated from photogrammetry and interpreted at both at an outcrop and bed-by-bed scale to describe the internal architecture of representative bedforms as well as the seismic-scale regional stratigraphic hierarchy. Additional field data including measured sections and grain size analysis enable detailed characterization of key facies and first-order estimates of depositing flow properties. Early results indicate that supercritical flows were responsible for depositing most if not all of the observed >2 km2, high net-to-gross, lobe-dominated fan. Upstream accreting, lensoid-to-sigmoidal turbidite sandstone beds with intercalated fines are interpreted as deposits along undulating supercritical bedforms with predictable grainsize trends. Resulting bedsets possess tabular dip geometries and lensoid-to-lobate strike geometries. Composite bedsets compose fining up units interpreted as retreating lobe elements that compensationally fill topography. Lateral and vertical trends in bedform type, geometry, and facies point to systematic trends in flow confinement and energy depicting spatiotemporal evolution of a prograding supercritical fan filling antecedent topography of a young rift basin.