EARLY PALEOCENE COLD SEEP DEPOSITS, WESTERN SAN JOAQUIN VALLEY, CA: I. FLUID SOURCE AND TRANSPORT SYSTEM

Most cold-seep deposits do not expose their underlying "plumbing". In the Panoche Hills, of the western San Joaquin Valley, California, outstanding exposures of a Cretaceous-Tertiary cold-seep system reveal its source, subsurface transport path, and surface development.

The Panoche Hills occur along the western margin of the Great Valley forearc basin in central California. The base of this seep-system is defined by a network of interconnected sandstone intrusions propagating upward through the shale-rich Moreno Formation as dikes and sills. Previous investigations have indicated that the source of these intruded sands is the underlying Panoche Group sandstones. The sandstone intrusions cut through approximately 500-600 meters of section and are interpreted as permeable pathways for fluids that fed the overlying seep-related carbonates and chemosynthetic organisms. Coeval dikes and sills have a wide variance in attitude and suggest similar magnitudes of principal stresses. Sills are slightly preferred in the lower portion of the section and are statistically bedding-parallel, dominantly dipping NE. Dikes are slightly preferred in the upper part of the section. Seep related-organisms and carbonates are concentrated within a 50 meter-thick stratigraphic horizon near the upper boundary of the Moreno Formation. This stratigraphic horizon begins generally less than 50 meters up-section from the terminus of the sandstone intrusions.

These exposures define a fluid system that is, in total, about 800 m thick and that uniquely ties subsurface sandstone intrusions to seafloor cold seeps. Fluids were derived from the underlying section that thickens eastward towards the center of the forearc basin. The fluids migrated to the Panoche Hills site due to loading by the thicker sediments in the basin center and due to buoyancy force from hydrocarbons. Intrusive sill formation is indicative of lithostatic fluid pressures whereas the dikes may have formed at less than lithostatic fluid pressures. The tipping-out of the dikes ten’s of meters below the seeps suggests that the fluid pressure sufficient to hydrofracture the sediments dissipated in the moderate permeability (est. 10-0 to 10-1 md) silty mudstone occurring just below the cold seep horizon.