2019 AAPG Annual Convention and Exhibition:

Datapages, Inc.Print this page

Three Dimensional Outcrop Model of an Evolving Supercritical Fan in the Early Gulf of California

Abstract

Numerical and physical modeling as well as direct observation of turbidity currents indicates that sediment gravity flows commonly reach Froude supercritical states over moderately steep slopes. This occurrence is reflected in seafloor bathymetry data, which reveal that such flow conditions impact how and where sediments are deposited. However, the stratigraphic details of the resulting deposits, their preservation potential, and their significance in sediment transport, basin evolution, and reservoir applications 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. The result is a detailed three-dimensional (3D) characterization and interpreted outcrop model of an evolving supercritical fan, which is used to describe the architecture of supercritical bedforms and spatiotemporal evolution of the deposits as well as provide first order constraints on the depositing flow conditions.

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 a regional 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 a relatively thick but aerially small, high net-to-gross, lobe-dominated fan. Stacks of 10s of upstream accreting, lensoid-to-sigmoidal turbidite sandstone beds with intercalated fines reveal undulating 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, actively subsiding rift basin.