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Hydrodynamics of Overpressure

Rapid sediment loading (>1 mm/y) drives overpressure (P*, pressure in excess of hydrostatic) in basins around the world (Fertl, 1976; Rubey and Hubbert, 1959). Sedimentation is so rapid that fluids cannot escape, the fluids bear some of the overlying sediment load, and pore pressures are greater than hydrostatic (Fig. F1).

Recent work has focused on how sedimentation and common stratigraphic architectures couple to produce two- and three-dimensional flow fields. For example, if a permeable sand is rapidly loaded by a low-permeability mud of varying thickness, fluids flow laterally to regions of low overburden before they are expelled into the overlying sediment (Fig. F2). This creates characteristic distributions of rock properties, fluid pressure, effective stress, temperature, and fluid chemistry in the aquifers and bounding mudstones (Fig. F2). This simple process can cause slope instability near the seafloor (e.g., Figs. F1, F2, F3) (Dugan and Flemings, 2000; Flemings et al., 2002); in the deeper subsurface, this process drives fluids through low-permeability strata to ultimately vent the seafloor (Fig. F3) (Boehm and Moore, 2002; Davies et al., 2002; Seldon and Flemings, 2005).

Expedition 308 will document the spatial variation in pressure, stress, and rock properties in a flow-focusing environment. We will compare our observations to the model predictions. We will first establish rock and fluid properties at a reference location (Brazos-Trinity). We will then drill multiple holes along a transect in the overpressured Ursa system to characterize spatial variation in rock properties, temperature, pressure, and chemistry.

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