HR: 1340h
AN: B43E-1648    [Abstracts]
TI: Distribution and Geochemistry of Methane-Derived Cold Seep Carbonates Panoche, California
AU: * Csar, A J
EM: ajc98@nau.edu
AF: N. AZ Univ., Department of Geol. Blg. 12, Flagstaff, AZ 86011,
AU: Sample, J
EM: james.sample@nau.edu
AF: N. AZ Univ., Department of Geol. Blg. 12, Flagstaff, AZ 86011,
AB: Isolated authigenic carbonate concretions and pavements occur locally within fine grained siliciclastic rocks of the Tertiary Great Valley Sequence of western California. Outcrops in the Panoche and Tumey Hills region are a record of prolonged expulsion of methane- and H2S- rich fluids from a relict cold seep system at the sea floor of a paleo-forearc basin. The entire outcrop length of the seep horizons is at least 15 km along strike. Sandstone injectites underlie the main seep horizons and may have provided fluid pathways to the sea floor. The concretions found in this locality are commonly rounded and vertically elongate, up to 15 m in height and resembling pillars in current outcrop form. Discrete carbonate pavements crop out continuously for as much as 100 m, are generally less then 3 m thick, and lacking any discernable stratification. The entire surface expression of the cold seep carbonates follows along strike, as a series of discontinuous shale encased mounds. Faunal assemblages (tubeworms, bivalves, and textures suggestive of algal mats) are fossilized, commonly in living position, within the carbonate cements. Growth and cross cutting relations recorded in these carbonate cements provides a chronology of the geochemical evolution of fluid venting at the cold seep. The earliest cement phase typically encasing the fossils and sedimentary structures is generally a high magnesium, detritus rich, finely micritic calcite or protodolomite. Energy dispersive spectrometry indicates that these cements have Ca/Mg ratios ranging from 8:1 to nearly 1:1. Within this hosting matrix are commonly a series of circular or wavy planar precipitation bands indicating sequential cementation. These later cements tend to be low Mg calcite (Ca/Mg below 8:1) which precipitated into void spaces from edge to center as coarsely fibrous crystals as large as 1 mm in width and several mm long. Each of these cement types has evidence of multiple phases of dissolution and precipitation, including coatings of fine euhedral sulfide or sulfate which also tend to grow from edge to center The presence of sulfide or sulfate indicates changes in seep chemistry, possibly related to changing redox conditions, between carbonate precipitation events. The trend with younger cements is towards progressively lower Mg content in calcite. Late-stage fibrous gypsum, commonly containing bitumen, cuts across all previous structures and contacts.
DE: 0460 Marine systems (4800)
DE: 0463 Microbe/mineral interactions
DE: 3004 Gas and hydrate systems
DE: 8104 Continental margins: convergent
SC: Biogeosciences [B]
MN: 2007 Fall Meeting