Seismic Stratigraphy of a Forearc Depositional System: Shelf-Margin Deltas to Basin Floor Fans, Northern San Joaquin Basin, California

Armentrout, John M.¹, David H. Suek², Denise H. Harrison³, Paul J. Harrison³, David B. Codding⁴, Steve D. Jones⁵, Scott Lewis⁶, Steve Graham⁷ (1) Cascade Stratigraphics, Inc, Damascus, OR (2) Black Coral LLC, Denver, CO (3) Fall-Line Exploration, Dillon, CO (4) Yates Petroleum, Artesia, NM (5) Black Coral, Centennial, CO (6) QuantStrat, Denver, CO (7) Stanford University, Palo Alto,

The ‘traditional' sequence stratigraphic depositional model was formulated from passive margin data and subsequent computer simulations. Convergent forearc basin tectonics provide a markedly different setting and yet 2D and 3D seismic observations of the Upper Cretaceous of the Northern San Joaquin Basin provide documentation of similar geomorphic geometries and succession of depositional facies.

Regional 2D reflection profiles down a fault controlled paleo-valley system image a succession of sand-prone, depositionally thick bodies in inner-shelf, outer-shelf, shelf-margin, canyon-mouth, toe-of-slope and basin floor settings. Analysis of a 3D seismic volume demonstrates that each of these depositional elements can be placed into a sequence stratigraphic framework for this tectonically driven system. Each of the depositional elements occurs where dramatic geomorphic transitions provided accommodation space and changes in flow velocity resulting in the sand-prone facies being deposited in a spatially foreshortened configuration when compared to passive margin deposition.

Periods of tectonic emergence of the basin margin repeatedly forced relative lowstands of sea level across the narrow shelf. Sand-prone sediment flux was very high and accumulation was largely restricted to the depositional axis of the forearc basin. The lowstand depositional facies include basin-floor fans, toe-of-slope prograding complexes, canyon-mouth ‘thicks', and shelf-margin prograding complexes. As the basin margin subsided and sediment flux decreased, depocenters transgressed first across the upper slope and onto the outer shelf as retrograde packages and subsequently toward the basin margin where paleo-river systems supplied relative highstand prograding systems.