Active fluid venting and its surface manifestations (unique animals and carbonates) occur over the accretionary prism in the Cascadia subduction zone located off central Oregon. A large variety of authigenic carbonate deposits and unique carbonate structures have been observed from submersibles and remotely operated vehicles and recovered with aid of submersibles and bottom trawls from the outermost continental shelf and lower continental slope. The carbonate deposits range from relatively thin crusts and slabs to irregular edifices and well-formed circular chimneys that rise from 1 to 2 m above the seafloor. Mineralogically, the carbonate cement consists of aragonite, calcite, Mg-calcite, or dolomite with varying amounts of detrital constituents. Stable carbon and oxygen isotope data identify four distinct subgroups of methane-derived carbonates from several different vent sites and different fluid source zones. Subgroup I represents one vent site on the lower slope and is characterized by oxygen isotope values ranging from +6.8% to +4.7% PDB. Subgroup II represents another vent site about 1 km away and exhibits oxygen values of +3.4% to +4.9% PDB. Carbon isotopic values range from -40.96 to -30.23% versus -44.26 to -53.44% PDB, respectively, for the two vents. An irregular edifice from the outer shelf has the same isotopic composition as subgroup II. A companion study shows that the expelled fluids contain largely biogenic methane and methane-derived dissolved carbonate; a shallow fluid source zone (<1 km) in indicated.

The isotopic carbon values of the subgroup I and II carbonates are consistent with the carbon composition of the expelled fluids and apparently represent a historical record of the composition of these fluids. In subgroup III, strong 18O enrichment and heavier carbon values characterize the dolomitic chimneys from the outer continental shelf. Cemented sandstones from a ''window'' in the accretionary complex of the lower slope (subgroup IV) are characterized by extreme Δ18O (-5.9 to -5.98%) and moderate Δ13C (-18.7 to -12.67%)-depleted carbonates. This ''light'' oxygen isotope composition most likely originated from the upward migration of warm hydrothermal fluids along the main d¿collement, which tapped the warm subducting basaltic slab, during the early stages of formation of the accreted complex. Well-defined plumbing tubes within some carbonate chimneys on the shelf infer a single well-defined subsurface conduit with a fairly energetic fluid flow. The majority of the chimneys probably formed above the seafloor as long as the rate of carbonate precipitation exceeded the rate of detrital input during their formation. We calculate a minimum of one conduit for each 35 m2 at one vent site on the shelf. A less energetic flow is suggested by the chaotic plumbing network of an irregular edifice and by the widespread occurrence of the carbonate slabs and crusts at numerous vent sites. ¿ American Geophysical Union 1990