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Abstract

Apaleoseep system consisting of hundreds of sand injectites and authigenic carbonate structures crops out in the Panoche and Tumey Hills, central California. This paleoseep system developed on the western margin of the Great Valley forearc basin and is contained within the uppermost, early Paleocene part of the dominantly siliciclastic Moreno Formation. It is 20 km (12 mi) long and is distributed over more than 700 m (2296 ft) of stratigraphic section. Injectites appear in the lower 600 m (1968 ft), thinning upward from 3 m (9.8 ft) to less than 1 cm (0.4 in.), and cooccur with the paleoseep carbonate structures in the uppermost 200 m (660 ft) of section. The paleoseep slab, mound, and concretionary carbonates are 13C depleted (to −46‰ Vienna Peedee belemnite) and commonly contain pipelike structures and the remains of chemosynthetic macroinvertebrates, including tube worms and lucinid bivalves. Their diverse morphologies likely reflect different rates and styles of fluid flow, but most show a similar paragenesis beginning with biologic colonization and pervasive micrite authigenesis and concluding with sparite precipitation in vugs and conduits. The close stratigraphic and compositional associations of paleoseep carbonate structures with injectites suggest that they were contemporaneous, and that injectites controlled the location of the seeps. Variations in abundance, morphology, and geochemistry of the authigenic carbonates, fossils, and injectites across the outcrop area indicate considerable variability in seep venting rates locally, regionally, and throughout the nearly 2-Ma duration of the seep system. Thus, the Panoche and Tumey Hills locality offers a four-dimensional view of the nature and evolution of a large, injectite- driven, cold seep in a forearc setting.

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