HR: 0800h
AN: OS21A-1201    [Abstracts]
TI: Petrologic and Geochemical Evidence for Biologic Contributions to Carbonate Formation in the Panoche-Tumey Hills Paleoseep, California
AU: * Sample, J
EM: James.Sample@nau.edu
AF: Dept. of Geology, Box 4099 Northern Arizona Univ., Flagstaff, AZ 86011 United States
AU: Schwartz, H
EM: hildes@es.ucsc.edu
AF: Dept. Earth & Marine Sciences, Univ. Calif., Santa Cruz, CA 95064 United States
AU: Moore, C
EM: casey@emerald.ucsc.edu
AF: Dept. Earth & Marine Sciences, Univ. Calif., Santa Cruz, CA 95064 United States
AB: The Panoche-Tumey Hills (PTH) paleoseep is a Paleocene seep system that extends greater than 20 km nearly continuously along strike along the western edge of the Great Valley. The seep fauna is dominated by tubeworms but includes bivalves, gastropods, and solitary corals. The entire system, including feeder sandstone intrusions, is 800 m thick. Carbonate deposits occur in the upper 150 m, and are most concentrated in the upper 50 m. Authigenic carbonates include several generations of cements and veins. The carbonates are mainly calcite, but there is evidence for calcite replacing aragonite in pipe structures. Textures indicate the seep system evolved from an open, relatively permeable sedimentary matrix to one of increasingly focused flow. Late stages of cementation included formation of pipe structures with flow velocities vigorous enough to allow precipitation of carbonate relatively free of silt and sand. \delta$^{13}$C$_{PDB}$ values as low as -45 permil are consistent with a component of biogenic methane during carbonate formation. Pronounced laminated structures in the carbonates suggest microorganisms played an important role in carbonate formation, by binding cements and/or varying the P$_{CO2}$ in distinct microenvironments. Biomarker analysis will help to constrain the importance of bacterially mediated carbonate precipitation. During the fluid evolution of the seep organic and inorganic structures served as conduits, and isolated fluid reservoirs at small scales. Carbon isotopes differ outside and inside tubeworms and pipe structures by 18 permil and 45 permil, respectively, but in opposite directions. Clearly fluid sources, as characterized by carbon isotope signatures, can vary substantially over short distances and presumably over short time scales. The geochemical data along with the cement textures indicate that individual seep sites within the system were doomed by their own carbonate- precipitating success. As one seep site became clogged and less permeable, seep activity migrated to a new location within the system. The fickle nature of fluid seepage in modern systems is well documented at convergent plate boundaries. The PTH system provides a particularly large and well-exposed ancient analog for understanding the evolution of these types of seeps in four dimensions.
DE: 8045 Role of fluids
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting