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doi:10.2204/iodp.proc.308.215.2012

Summary of hydrogeologic findings and plans for future work

Expedition 308 documented high overpressures. At Site U1324, overpressures are 70% of the hydrostatic effective stress to a depth of 200 mbsf (λ* = ~0.7). Beneath this, overpressures drop significantly and are approximately equal to 1 MPa to a depth of 600 mbsf. The shallow overpressures are present in very rapidly deposited, low-permeability mudstone, whereas the deeper zone where the overpressure is lower is composed of interbedded siltstone, mudstone, and sandstone. At Site U1322, the overpressures are 60% of the hydrostatic effective stress (λ* = 0.6) to 200 mbsf through a uniform mudstone section.

Permeability and compressibility of Ursa Basin mudstones decline significantly with depth near the seafloor. As a result, the coefficient of consolidation (hydraulic diffusivity) is approximately constant at 1 × 10^–8 m²/s over the depth range drilled. Given the sedimentation rates in the Ursa region, sedimentation alone can account for the severe overpressures observed at Site U1324. However, one-dimensional modeling cannot explain the high pore pressures observed at Site U1322. In this location, modeling shows that the more rapid sedimentation at Site U1324 than at Site U1322 is driving flow laterally within the permeable Blue Unit. This elevates the pore pressures at Site U1322 and reduces them slightly at Site U1324.

Two-dimensional models also show that the deeper zones at Site U1322 were unstable during times of rapid sedimentation, soon after deposition of the Blue Unit; these zones correlate to locations where MTDs are present (Stigall and Dugan, 2010). The largest MTD (MTD-2 in Fig. F2B) may have resulted from the combination of moderate overpressure from rapid sedimentation coupled with horizontal earthquake acceleration of a magnitude 5 earthquake within 140 km of the Ursa region (Stigall and Dugan, 2010).

MTDs have a unique signature in log, core, and seismic data. We interpret that these deposits are remolded during mass transport and as a result they follow a unique compression curve that results in lower porosity at a given effective stress.

An original goal of Expedition 308 was to measure and monitor pore pressure within the Blue Unit at multiple locations to confirm the pore fluid pressure distribution in this aquifer. Future work will focus on long-term monitoring of pressure in both the Blue Unit and in over- and underlying mudstones. These measurements will allow us to constrain whether significant lateral flow is occurring within the Blue Unit, what the hydraulic connectivity of the Blue Unit is, and what the permeability of the Blue Unit is. Pressure monitoring may also allow us to image ongoing deformation occurring due to submarine landslides.

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