EARLY PALEOCENE COLD SEEP DEPOSITS, WESTERN SAN JOAQUIN VALLEY, CA: II. AUTHIGENIC CARBONATES AND TUBE WORM-DOMINATED CHEMOSYNTHETIC ECOSYSTEMS

A 20 km-long zone of fossiliferous, authigenic cold seep carbonates is exposed in the early Paleocene portion of the Moreno Formation in the Panoche Hills and adjacent Tumey Hills, western San Joaquin Valley. Depleted carbon isotope values, preserved fluid conduits, distinctive cements, microbial textures and fossils of typical chemosynthetic organisms indicate that these carbonates are predominately methane-derived. They occur as concretions and as discontinuous pavements and mounds (up to 8 m-tall/wide in outcrop) in the southern portion of the field area and as sandstone cements in the north; everywhere they overlie a ‘feeder’ system of clastic intrusions. Most of the seep carbonates are concentrated within a 50 m-thick zone in the upper Dos Palos Shale Member of the Moreno Fm., but dispersed mounds lower in the section delineate a seep-bearing stratigraphic interval that is in total more than 150 m thick. Average sedimentation rates for the Dos Palos Shale suggest that such an interval represents more than two million years. Invertebrate fossils in this interval are invariably associated with carbonates and represent a low diversity biota dominated by large vestimentiferan tube worms (to 2 cm diameter x >1 m long). Tube worm remains occur as dense clusters in large carbonate mounds and as isolated specimens in small mounds and carbonate-cemented sandstones. Associated lucinid and solemyid bivalves and provannid and naticid gastropods are rare, while solitary corals, pholad bivalves and allochthonous wood fragments are relatively common in carbonate pavements at the top of the seep event horizon. Abundant wavy laminae and clotted micrites in carbonates throughout the field area are evidence of ubiquitous microbial activity, including mediation of carbonate precipitation. The distribution of fossils and carbonates in this region suggests that fluid seepage became more vigorous during the early Paleocene. Maximum flux occurred near the end of the lifetime of the system (upper Dos Palos Shale time) and in the southern part of the field area. This site adds new early Paleocene data to the existing record of Jurassic-Eocene fluid expulsion and chemosynthetic ecosystems in the Great Valley forearc and constitutes the largest, best exposed and longest lived paleoseep system known in the Cordillera forearc.