ABSTRACT: Fan Sedimentation on Continental Margins: A Comparison of Miocene Gulf Coast Systems with the Middle Eocene Cozy Dell Formation of Southern California

W. C. Rusty Riese, M. S. Clark

Recent seismic and subsurface stratigraphic studies of the Miocene System, offshore Texas resulted in the development of a depositional model that documents and explains the deposition of sand-prone fans on the outer shelf during times when these areas were still covered by several hundred feet of water. This model suggests that sediment transport was by turbid flow and that sedimentation resulted in the development of leveed channel systems. The geometries and scales of these fan systems have been documented by three-dimensional seismic analysis and field-wide well penetrations in the Matagorda 668 field. It further advances the hypothesis that global eustatic levels were generally higher and basin water budgets greater during the Miocene than during later times in at le st the Gulf Coast area.

Translation of this model to outcrop areas is hampered in the Gulf Coast, and detail verification and refinement of the model has been restricted to subsurface work alone. This has also hampered our attempt to carry this model to non-Miocene-age systems.

Work on the middle Eocene Cozy Dell Formation exposures in the Topatopa and Santa Ynez Mountains of southern California has revealed that this formation has a depositional history which may not be different than that interpreted for the Gulf Coast Miocene. This formation was deposited by turbidity currents in what has been interpreted to be an upper

slope setting. The fans deposited by these currents have well-developed channel-levee complexes and display scales of geometry similar to those seen in the Gulf Coast Miocene.

This formation displays seismic-sequence scale stratal geometries that suggest that it was deposited during lowstands of sea level. This interpretation implies deposition of sand-prone systems in upper-slope settings during lowstands, but the outcrops of these strata display internal geometries that suggest that they ought to have been deposited on a lower gradient surface than thi--one more accommodating to the development of leveed channels. This upper slope setting may therefore be misinterpreted by too strict application of Vail models and by misinterpretations of benthic foraminiferal data. The upper bathyal water depths that have been interpreted from these data for the associated highstand shales may be reflecting colder water rather than deeper water--water moving south along he western margin just as it is today. If this can be demonstrated, the full comparison of these systems to those from the Gulf Coast can be accomplished and the model that has been developed there extended into the Paleogene of southern California.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990