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Active fluid venting and carbonate formation occur in the accretionary prism in the Cascadia subduction zone off Vancouver Island and Oregon. Methane-derived authigenic carbonates are precipitated in the uppermost terrigenous sediments by subduction-induced dewatering. They can be divided into cemented silts and sands containing less than 10% carbonate cements and concretions composed partially or wholly of carbonate. Textures, chemical compositions, and carbon and oxygen isotope signatures of 64 specimens of mainly carbonate concretions from Ocean Drilling Program Sites 889/890, 891, and 892 were studied using different methods. Mineralogically, the carbonate cement consists of high-magnesian calcite, dolomite, and calcite with varying amounts of detrital constituents. Results from x-ray fluorescence, x-ray diffraction, and electron microprobe analyses suggest dominance of high magnesian calcite in most of the carbonate concretions. Complex carbonates and protodolomite to pure dolomite are found in minor amounts. Cemented sediments and concretions taken in the vicinity of a prominent fault zone at Site 892 show an almost pure calcitic composition and the most intense deformation of all material drilled in the Cascadia Margin. Concretion fragmentation is believed to be caused by both brecciation and hydrofracture from mechanical stress and lowered lithostatic load during upward migration of rocks and fluids along the fault plane. Strain evaluation based on distortions of initially homogeneous marker particle distributions (the Fry technique) obtained aspect ratios (Rf) between 1.2 and 1.71 reflecting uniaxial shortening in the sediment as a response to compaction prior to carbonate formation. Compactional strains (ez, elongation parallel to core axis) are found to range from -0.17 to -0.42 and give evidence for significant settling of the sediment prior to concretion formation. Evidence for post-formational compaction, displacement of carbonate, or recrystallization incipient diagenesis could not be observed. Stable carbon and oxygen isotope signatures identify distinct groups of methane-derived carbonate concretions. Almost half of the examined concretions show variable carbon isotope signatures, ranging from -12%c to -52%c PDB (Peedee belemnite) with no systematic trend downhole. These carbonate concretions evidently reflect precipitation from thermogenic methane, probably with contributions of biogenic methane in minor amounts. A second set of samples is characterized by δ 1 C values between -6%0 to +24%o PDB. Values between -4%o and 0%o reflect marine carbonate precipitated from seawater. The substantial enrichment in 1 C is most likely caused by supply of CO2 evolved from fermentation in the sediment pile. Oxygen isotope ratios largely scatter around δ 1 8 θ values for modern ocean water (-l%c to +6%c PDB). Exceptional "light" δ 1 8 θ signatures with values ranging from -ll%o to -17%e PDB were only found in the vicinity of a prominent fault zone at Site 892. That depletion in O may be explained by upward migration and channeled expulsion of warm fluids along the fault zone that are probably in isotopic disequilibrium with interstitial waters. Zonation patterns of C and O isotope ratios partly occur, showing depletion in C and O towards the centers of concretions. Isotopic gradients may have been caused by episodic supply of methane-rich fluid of different reservoirs, isotopic compositions, and/or

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures