Abstract

Samples of carbonate-cemented sedimentary rocks were collected during 11 Alvin dives in two regions along the Cascadia margin, a northern strike-slip fault zone and a southern thrust-fault zone (“second ridge”). We characterized 35 samples petrographically and chemically. Northern-area samples are dominantly sandstones; second-ridge samples are all mudstones. Vugs and high percentages of carbonate cement in the mudstones suggest that these rocks were never deeply buried. The abundance of veins and brecciation in samples from both regions attests to their proximity to faults. Northern-area cements are dominantly calcite or magnesian calcite, whereas second-ridge cements generally are more dolomitic.

Oxygen and Sr isotopic values of carbonate cements indicate the involvement of two very different fluid reservoirs during cementation. One reservoir generally has low δ18O, very low δ13C, and high 87Sr/86Sr ratios, but the other has high δ18O, moderately negative δ13C, and 87Sr/86Sr ratios lower than modern seawater values. In the northern area, diagenetic carbonates are subdivided into two populations of coupled oxygen and carbon isotopic values. Population I has δ18OPDB values of +3‰ to +5‰ and δ13CPDB values of −55‰ to −45‰. Population II has δ18O values of −13‰ to −4‰ and δ13C values of −25‰ to −1‰. Such common low δ18O values in carbonate cements have not been observed at any other accretionary wedge. 87Sr/86Sr ratios of northern-area samples range from 0.7128 to 0.7088. Oxygen and Sr isotopic values show a correlation between increasing 87Sr/86Sr ratios and decreasing δ18O values. Oxygen and carbon isotopic values of most diagenetic carbonates from the second ridge range from +3‰ to +10‰ and −55‰ to −38‰, respectively. Enrichments of 18O probably result from a combination of dolomitic mineralogy, cold bottom-water temperatures, and the presence of decomposing gas hydrate in the subsurface. 87Sr/86Sr ratios of carbonates from the second ridge range from 0.7086 to 0.7091.

The geochemical signatures of diagenetic cements from the northern area suggest that fluids were derived from greater than 2 km depth, perhaps from the decollement. Prominent gullies throughout the northern area are probably underlain by strike-slip faults that provide conduits for upward fluid migration. Cements at the second ridge precipitated from shallowly derived fluids. The difference in fluid source depth is related to proximity to the fluid conduits and different orientations of minimum principal stress and resultant hydrofractures in the two fault regimes.

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