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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 (Figs. F1, F2A). This will create characteristic distributions of rock properties, fluid pressure, effective stress, temperature, and fluid chemistry in the aquifers and bounding mudstones (Fig. F2B). This simple process can cause slope instability near the seafloor (Fig. F3A) (Dugan and Flemings, 2000; Flemings et al., 2002); in the deeper subsurface, this process drives fluids through low-permeability strata to ultimately vent them at the seafloor (Fig. F3B) (Boehm and Moore, 2002; Davies et al., 2002; Seldon and Flemings, 2005).

Expedition 308 documents the spatial variation in pressure, vertical stress, and rock properties in a flow-focusing environment. We first established rock and fluid properties at a reference location (Brazos-Trinity Basin #4). We then drilled multiple holes along a transect in the overpressured Ursa system to characterize spatial variation in rock properties, temperature, pressure, and chemistry.

Geological Overview: The Gulf of Mexico

The Gulf of Mexico is a type location for a shallow drilling campaign aimed at understanding how sedimentation drives compaction and fluid flow (Fig. F1). Sedimentation, deformation, hydrodynamics, slope stability, and biological communities are interwoven in the Pleistocene strata of the Gulf of Mexico. Rapid sedimentation upon a mobile salt substrate is the driving force behind many of the active processes present (Worrall and Snelson, 1989). Bryant et al. (1990) described the physiographic and bathymetric characteristics of this continental slope (Fig. F4). In the region of offshore Texas and western Louisiana, individual slope minibasins are surrounded by elevated salt highs (Pratson and Ryan, 1994) producing a remarkable hummocky topography. This morphology is obscured in the eastern Gulf of Mexico, where sedimentation has been very rapid and more recent than the region of offshore Texas and Louisiana.

Pleistocene sediments were drilled in Brazos-Trinity Basin #4 and Ursa Basin (Fig. F4). The sedimentation rate in Brazos Trinity Basin #4 was envisioned to be relatively low, whereas the sedimentation rate in Ursa Basin was envisioned to reach rates of at least 1 cm/y. We anticipated hydrostatic pore fluid pressures in Brazos-Trinity Basin #4 and overpressured pore fluids in Ursa Basin.

Geological Setting

Brazos-Trinity Basin #4

Brazos-Trinity Basin #4 is 200 km due south of Galveston, Texas (USA) in ~1400 m water depth (Figs. F4, F5). The basin is one of a chain of five basins that are connected by interbasinal highs. It is a classic area for analysis of turbidite depositional environments because it is used as a modern analog to describe the formation of deepwater turbidite deposits (Badalini et al., 2000; Winker, 1996; Winker and Booth, 2000).

The primary data set used to evaluate the well locations is a high-resolution two-dimensional (2-D) seismic survey shot by Shell Exploration and Production Company to image the turbidite stratigraphy (Fig. F5). The line spacing is ~300 m. Three of the drilled locations are shown on dip seismic Line 3020 (Fig. F6). A strike line through Site U1320 is also illustrated (Fig. F7). Site U1320 (Figs. F5, F6, F7) is located where the turbidite deposits are thickest, whereas Site U1319 (Figs. F5, F6) is along the southern flank of the basin where turbidite deposits are more condensed.

Ursa Basin

Ursa Basin (~150 km due south of New Orleans, Louisiana [USA]) lies in ~1000 m of water (Figs. F4, F8). The region is of economic interest because of its prolific oilfields that lie at depths >4000 meters below seafloor (mbsf). Mahaffie (1994) described the geological character of the Mars oilfield. The Ursa field is in Mississippi Canyon Blocks 855, 897, and 899 and is 11.9 km east of the Mars tension leg platform.

We are interested in the sediments from 0 to 1000 mbsf. Four extraordinary three-dimensional (3-D) seismic data sets are available within Ursa Basin (Fig. F9). Shell and industry partners shot the Ursa exploration survey for exploration purposes. The high-resolution surveys were shot by Shell for the purpose of shallow hazards analysis.

Winker and Booth (2000) described deposition of Pleistocene to Holocene sediments. The Mississippi Canyon Blue Unit is a late Pleistocene, sand-dominated, "ponded fan" that was deposited in a broad topographic low that extended in an east-west direction for as much as 200 km and a north-south direction for as much as 100 km. The Blue Unit is overlain by a leveed-channel assemblage that was mud dominated and had dramatic along-strike variation in thickness. Pulham (1993) described a similar facies assemblage for this region.

Shell made downhole pressure measurements with a pore-pressure penetrometer (piezoprobe) at the Ursa platform (Eaton, 1999; Ostermeier et al., 2000; Ostermeier et al., 2001; Pelletier et al., 1999) (Fig. F10). They also acquired whole-core samples and performed consolidation experiments to evaluate preconsolidation stress and estimate overpressure. Piezoprobe measurements (circles) and maximum past effective stresses interpreted from consolidation experiments (triangles) indicate that (1) overpressure begins near the seafloor and (2) the pore pressure is ~50% of the way between the hydrostatic (Ph) and the lithostatic (v) (Fig. F10). Pressures (both hydrostatic and lithostatic) are calculated from below seafloor and not from sea level.

Seismic Line A–A' (Fig. F11) illustrates the proposed boreholes. The sedimentary section is composed of a 300 m thick overburden that is predominantly mudstone. Beneath the overburden lies the first significant sand: the Blue Unit. The Blue Unit has a relatively flat base. Its upper boundary has relief, which most likely reflects postdepositional erosion. The Blue Unit is composed of interbedded sand and mudstone (Figs. F10, F11). A leveed-channel facies overlies the Blue Unit; it has a sand-cored channel that is flanked by mud-prone levee deposits. A mudstone package that thickens to the west overlies this sand assemblage. This mudstone package has numerous detachment surfaces that record slumping. The overlying mudstone is the eastern margin of a larger levee-channel system formed to the west.

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