Eustatic Controls on Stratification and Facies Associations in Deep-Water Deposits, Great Valley Sequence, Sacramento Valley, California
Scott R. Morgan, Kirt M. Campion
The Great Valley sequence consists of submarine fan deposits that are divided into laterally persistent sandstones and conglomerates separated by thick shaly intervals. The frequency of sandstone-shale successions in the Great Valley closely corresponds to the occurrence of major eustatic falls observed elsewhere in the world during the Upper Jurassic and Cretaceous. This close correspondence between the number of observed fans and sea level cycles has implications for the timing of fan development and facies models of deep-water deposits.
On the basis of seismic expression, deep-water deposits from various basins have been divided by Mitchum into a sand-prone lower fan, which has a sharp basal contact, and a younger upper fan, which exhibits downlap onto and over the lower fan. Sand-prone members of the Great Valley (e.g., Venado and Forbes) are sharp-based, fining-upward units that have an aggradational or retrogradational stacking pattern of fan lobes. Massive sandstone, pebbly sandstone, conglomerate, pebbly mudstone, turbidites, and lenticular turbidites compose the fan lithologies. These rocks are typically referred to as inner fan channel or midfan lobes. In contrast, shale-dominated sections with thin-bedded turbidites (e.g., Boxer and Yolo) that have been variously described as basin plain, outer fan, inner fan levee, and slope correspond to the upper fan.
Sharp basal fan contacts, textural contrasts between the lower and upper fans, and encasement of sand-prone fans in thick shaly sections indicate that fan development is an episodic rather than a continuous process. Rapid eustatic fall causing stream incision and shelf bypass is a likely mechanism for basin-wide and interbasinal fan development. Lithofacies encountered in fan deposits are related to grain size in the source area; specific lithologies in Great Valley fans (e.g., conglomerate) may be absent in other basins. As a general model for sediment distribution, however, the coarsest clastic sediment available to the system is concentrated in the lower fan.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.