SUBMARINE MASS FAILURE WITHIN THE DELTAIC-MARGINAL MARINE DOMENGINE FORMATION (EOCENE), SAN JOAQUIN BASIN, CALIFORNIA

Outcrops of the Eocene Domengine Formation within the Vallecitos Syncline (central California) provide an exceptional opportunity to observe submarine mass failure of a well-exposed deltaic to marginal marine sedimentary sequence. The Domengine Formation is a prominent shallow- to non-marine unit that is widely distributed in the Sacramento and San Joaquin Basins and constitutes a locally important petroleum reservoir. Evidence of large-scale submarine mass failure within the Domengine Formation near the abandoned mining town of New Idria includes spectacular folds (>75 m in height) and faults that developed before the strata were lithified. Field investigations and aerial imagery indicate that this large-scale soft-sediment deformation is more extensive than previously described and suggest it was strongly influenced by depositional facies and stratigraphic architecture (e.g., style and scale of folding and faulting). Stratigraphy within the mass-transport deposit (MTD) is interpreted as an assemblage of heterolithic, shallow- to marginal-marine shoreline, deltaic, and estuarine deposits that record marked sea level fluctuations in middle Eocene time. Twenty-four measured MTD-related folds have hinges that trend ~168°-348° and verge towards the west-southwest (258.5° ± 3.7°, 1 s.e.), suggesting that slumping occurred on an overall west- or southwest-dipping paleoslope. Although layer-parallel detachment occurs at numerous stratigraphic levels within the Domengine Formation, the basal detachment surface is positioned at or slightly above a prominent lithologic transition from sand- to mud-dominated facies. In addition, the observed degree of deformation increases upward in the stratigraphy, with the largest folds (and degree of lateral translation) occurring in the uppermost strata. This work provides a rare glimpse into the collapse of a shelfal-deltaic stratigraphic sequence, yielding insight into the linkage between marginal marine facies and propensity for submarine mass movement. Our results also have implications for the tectonic and paleogeographic development of the central Great Valley forearc basin during middle Eocene time.