ABSTRACT: Sequence Stratigraphy of Eocene Shelf and Slope Deposits, San Diego Embayment, California

John M. Lohmar, Scott R. Morgan

Analysis of Eocene shelf and canyon deposits of the San Diego embayment illustrates the applicability of sequence stratigraphic principles in tectonically active regions. Formations of the La Jolla Group are separated by widely correlateable sequence boundaries produced by sea level fluctuations. Sequence boundaries 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. Contemporaneous tectonism produced regional subsidence, resulting in gradual innundation of the shelf, and rapid, fault-related subsidence accompanied by deposition of a localized sedimentary wedge in the Ardath. Consequently, the thickness and paleobathymetry of the Ardath wedge exceed that expected from regional subsidence, erosional accommodation, and eustatic rise alone.

Due to shelfal subsidence, the Delmar, Torrey, and Ardath form a transgressive succession of marsh/intertidal, subtidal, and submarine canyon deposits bounded by sequence boundaries, expressed as erosional unconformities. Facies successions across these surfaces generally deepen upward, with basinward facies shifts restricted to the updip limit of these

depositional systems. Although falling sea level initiates these sequences, subsidence limits exposure of the shelf and accompanying basinward shift of facies to a narrow band along the shoreline. In contrast, overlying units exhibit a regressive stacking pattern indicative of reduced subsidence and/or increased sedimentation. Facies successions across sequence boundaries between these units exhibit a more widespread basinward shift.

The stratigraphy of the La Jolla Group illustrates that high-frequency sea level fluctuations produce depositional sequences that are fundamental chronostratigraphic units of basin fill on both active and stable margins. Although tectonism can influence facies distribution, it does not obscure the sequence framework.

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