Geology and Geochemistry of Cold Seepage and Venting-related Carbonates
CHEN Duo-fu CHEN Xian-pei CHEN Guang-qian Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Guangzhou 510640)
Cold seepage and venting is a fluid seepage and venting mainly composed of water, hydrocarbon(gas and crude oil), hydrogen sulfide and fine sediments derived below seafloor. Its temperature is similar to sea water and widely occurs on the seafloor of continental slope of active and passive margins. Cold seepage and venting-related chemosynthetic communities are highly in organism density, mainly mussels, clams, tube worms and bacterial mats. The sediments in cold seepage and venting are mostly maed up of carbonates and gas hydrates with minor sulfides and sulphates. The chemosynthetic carbonates occurred as buildup, chimney, cement, nodule, hardgroud and fine vein, and generally as buildup of chemosythetic bioclasts and multi-stage authigenic cements. The carbonate source, sedimentary environment and sedimentations are different with normal carbonates,of which carbon is from sea water. Therefore, using term "Chermoherm" represents the chemosynthetic carbonate buildup to distinguish from the terms of bioherms, lithoherms, pseudobioherms, biostromes of carbonate buildups in normal sea environment,in which the carbon is from sea water. In strata, Chermoherm frequently occurred in abyssal sediment strata as a carbonate buildup containing a lot of fossils of benthic fauna. There are disrupted sedimentary phase and environments in longitudinal and cross strata section, so much as turnover. Carbonate minerals in chemosynthetic carbonates are primarily Mg-calcite, aragonite, and dolomite,which are not different with normal carbonates. Carbon isotope values in chemosynthetic carbonates are very low due to bacteria mediation. Cold seepage and venting occurred along fault zone or permeable beds and at the surface expression of mud volcano and salt diapirism. Cold seepage and venting fluids are derived from formation fluids driven by a combination of (1)pore-space reduction of rapid sedimentation and tectonic compaction, deformation and cementation, and (2)increased buoyancy due to a decrease in pore-fluid density related diagenesis and catagenesis, and gas hydrate decomposition at depth. The sedimentation processes of cold seepage and venting-related carbonates are cementation, filling action and biochemical sedimentation. The hydrocarbon mainly of methane into cold seepage and venting is changed to carbon dioxide by bacteria mediation and deposited chemosynthetic carbonates.