Controls on the Development of Depositional Sequences and Facies Along an Active Margin

LOHMAR, JOHN M., Exxon Company, U.S.A., Thousand Oaks, CA, and SCOTT R. MORGAN, Exxon Production Research Company, Houston, TX

Sequence stratigraphic analysis of Eocene shelf and slope deposits near San Diego illustrates how eustasy and tectonics control the development of depositional sequences and facies along an active margin. Sequence boundaries produced by sea level fluctuations chronostratigraphically define, in ascending order, the Delmar, Torrey, Ardath, Scripps, and Friars formations of the La Jolla Group, and the base of the overlying Poway Group. The Delmar, Torrey, and Ardath sequences form a transgressive succession of marsh/intertidal, subtidal, and submarine canyon deposits. Submarine canyon, deltaic, and estuarine deposits of the Scripps and Friars sequences are organized in a regressive stacking pattern terminated by widespread fluvial deposition of the overlying Poway Group.

The broad rise and fall of sea level represented by the La Jolla Group follows the general eustatic trend for the time. This trend was interrupted by several brief lowstands, which initiated many of the individual sequences. Facies successions across sequence boundaries are controlled by both eustacy and regional tectonic subsidence. An overall eustatic rise during deposition of the lower La Jolla Group, combined with shelf subsidence, limited subaerial exposure during periodic lowstands, but did not prevent associated erosion of the shelf and slope. Consequently, sequence boundaries are overlain by marine strata in many places, producing an upward deepening of facies across these surfaces. In the upper La Jolla Group, sequence boundaries were produced by lowstands punctuating a gener l

eustatic fall and are associated with a more typical abrupt shoaling of facies. However, subsidence continued to limit subaerial exposure until a major eustatic fall produced downcutting of the fluvial Poway Group across the shelf.

Gault-related subsidence along the shelf edge also controlled the development of a slope facies within the Ardath sequence. However, while both regional and fault-related tectonic subsidence control the succession and distribution of facies, neither obscures the sequence framework produced by eustacy.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)