Previous   |    Next

doi:10.2204/iodp.proc.308.205.2008

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 308 was aimed at understanding how geology, pressure, and stress combine to control overpressure and fluid flow on the Gulf of Mexico continental slope (see the “Expedition 308 summary” chapter). We focused on two areas: a reference site, Brazos-Trinity Basin IV, and an overpressured area, the Ursa Basin. The Ursa Basin is located ~200 km southeast of Louisiana (USA) in ~1000 m of water directly downdip of rapid Pleistocene sedimentation from the Mississippi River system (Fig. F1). In the uppermost 1000 meters below seafloor (mbsf) in the Ursa Basin, a sand-rich permeable unit, the Blue Unit, was buried rapidly and asymmetrically by an eastward-thinning mud-rich overburden (Fig. F2) (Winker and Booth, 2000; Winker and Shipp, 2002; Sawyer et al., 2007b; see the “Expedition 308 summary” chapter).

At Site U1324 in the Ursa Basin, we cored a 608 m thick succession, including the eastern levee of the Southwest Pass Canyon, overlying distal turbidites, and hemipelagic drape (see the “Site U1324” chapter). At Site U1322, we cored a stacked succession of mass transport deposits, distal turbidites, and hemipelagic drape (see the “Site U1322” chapter). In general, these sediments were identified by shipboard sedimentologists as predominantly fine grained (silt and clay) with little sand (see the “Expedition 308 summary” chapter). However, shipboard analyses could not quantify the relative proportion of clay, silt, and sand, which also prevented proper lithologic classification.

Most lithologic classification schemes for clastic sediments rely on determining the relative percentage of clay-sized particles, silt-sized particles, and sand-sized particles (Wentworth, 1922). These textural data are then used to name the rock/​sediment type, for which numerous conventions exist (Shepard, 1954; Krumbein and Sloss, 1963; Folk, 1968; Blatt et al., 1980; Potter et al., 1980). “Mud” is a generic term referring to sediments composed predominantly of particles finer than 63 µm, which includes silt- and clay-sized particles (Aplin et al., 1999; Potter et al., 2005; Yang and Aplin, 2007). In the Ursa Basin, we encountered sediments that were almost exclusively composed of particles finer than 63 µm (see the “Expedition 308 summary” chapter). To avoid naming all samples “mud” in this report, we adopted the Shepard (1954) classification (Fig. F3).

Our objective was to create a downcore profile of particle size at both sites, which is critical for permeability constraints in numerical fluid flow models, and which will help illuminate the stratigraphic history of the Ursa Basin. We analyzed 340 samples (Tables T1, T2), which were primarily from 10 cm³ plug samples taken shipboard and whole-core trimmings from constant-rate-of-strain (CRS) consolidation tests (see Long et al.). We employed standard wet-sieve and hydrometer techniques to measure particle size distribution and the relative percentages of sand, silt, and clay in each sample. Here we describe our experimental background and procedure and present the overall results. Individual data files for each experiment can be found in “Supplementary material.”

Top of page   |    Previous   |    Next