MICROBIALITES IN THE MORENO FORMATION PALEOSEEP CARBONATES, PANOCHE-TUMEY HILLS, SAN JOAQUIN VALLEY

It is widely recognized that microbial degradation of methane at seafloor cold seeps can result in the precipitation of carbonate ‘microbialites' characterized by extreme d¹³C depletion, distinctive microbial textures, cements and biomarkers and in some cases microbial body fossils. Hundreds of paleoseep carbonates in the lower Paleocene portion of the Moreno Formation in the Panoche-Tumey Hills (PTH) appear to have formed largely through such microbial activity. These carbonates, ranging in scale from centimeters to tens of meters, occur as anomalous mounds, slabs and concretions in siliciclastic basinal sediments. They crop out along 20 km and over 150 to 200 m of section. Invertebrate fossils are invariably associated with carbonates in this stratigraphic interval, and reflect a typical cold seep biota dominated by tube worms and lucinid bivalves. Each of the PTH carbonate masses formed by localized early cementation of the siliciclastic matrix with Mg-calcite, dolomite and calcite and minor amounts of ferrous hydroxides, pyrite and barite. The typical paragenetic sequence is 1) micrite, 2) yellow calcite, 3) fibrous and botryoidal calcite and 4) equant calcite spar, interrupted by episodes of corrosion and pyrite precipitation. Most primary cement phases show evidence of microbial activity. Wavy (stromatolitic) laminae (0.1 to 0.8 mm) commonly form rinds up to several cm thick around conduits and tube worms in phase 1 micrites, and have d¹³C values as light as -47 permil, consistent with a component of biogenic methane. Phase 1 micrites also commonly encase tube worms other seep fossils, and in places contain pyrite framboids, microbial peloids and possible bacterial coccoids (~1mm in diameter) and sheaths (to 5 mm in diameter). Clotted microfabrics are common within phase 1 microvugs, and are locally associated with the phase 2 and 3 calcites as linings within fluid conduits, shells and tubes. Regionally, microbial textures are commonest where paleoseep carbonate volume is greatest and in mound-shaped carbonates which contain numerous pipe-like structures and are inferred to represent sites of focused rather than diffuse fluid seepage. The interplay between rate/style of fluid flow and microbial activity controlled chemical gradients and carbonate precipitation in the PTH seep system.