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Three-Dimensional Characterization of Complex Depositional Architectures of Seismic Scale High Energy, Unconfined Aggradational Slope Turbidites Deposited Along Narrow Margins of the Fish Creek-Vallecito Basin, South-Central California – Outcrop Example of a Supercritical Fan?

Abstract

Froude supercritical sediment gravity flows are a growing area of focus in deepwater sediment transport and slope evolution as continued exploration reveals architectures that challenge common models of slope depositional environments. While increasingly studied in modern and experimental environments, the potential for and nature of preserved supercritical flow deposits in outcrop and their role in slope development is less well understood. This work characterizes in three dimensions (3D) the seismic scale high energy Lycium Member turbidite outcrops in the Fish Creek-Vallecito Basin in south-central California. The Lycium Member is ~100 m thick and exposed over ~8 km2. Outcrops reveal ~3-7 m thick units of locally sourced, interbedded, ~3-7 m thick sand and mud bodies interpreted to have deposited on a steep (4-5 degrees) slopes off narrow, high relief axial rift margin along an early extension of the Gulf of California. Individual sand units are composed primarily of sharp to erosional based structureless, poorly sorted, normally graded 5-50 cm thick medium-grained turbidites with thin (1-3 cm) bioturbated mud caps and 1-3 m thick, amalgamated, coarse sands interspersed with mud clasts and 20-100 cm diameter boulders. Mud-rich units are dominated by heavily bioturbated muds and interbedded thin (<5 cm) fine-grained sands. Previously interpreted as laterally accreting channel sands, detailed examination reveals few channel-form geometries. Rather, sand units extending hundreds of meters in all directions show undulating, sigmoidal to lenticular streamwise geometries with boulder beds located at inflection points of broader sand body geometries. While successive sand bodies do obliquely truncate underlying strata locally, these slope sands are largely aggradational, accreting vertically downgradient with individual beds building transverse to the paleoflow direction. This work proposes that such unique geometries might be best explained by a model of slope deposition in which narrow, steep slopes are actively built out by aggradational, coarse-grained, relatively unconfined, high energy (possibly supercritical) turbidites, and provides a 3D characterization that can inform reservoir models to fit this evolving understanding of slope depositional environments.