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doi:10.2204/iodp.proc.308.210.2009

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 308 was dedicated to understanding fluid flow, in situ pressure, and slope stability in rapidly deposited sediments of the northern Gulf of Mexico. The Ursa region was one of the focus areas for Expedition 308 because of high-quality multichannel seismic data illuminating depositional architecture (Sawyer et al., 2007), previous industry drilling that documented overpressure in shallow sand and mud (Ostermeier et al., 2001; Pelletier et al., 1999), and a series of stacked mass-transport complexes (MTCs) (see the “Expedition 308 summary” chapter). The Ursa region is ~210 km southeast of New Orleans, Louisiana (USA) in ~1000–1300 m water depth (Fig. F1). Sites U1322 and U1324 are separated by 12 km along a west-southwest–east-northeast transect (Fig. F1).

Three-dimensional seismic data over the Ursa region show a basin floor fan (Blue Unit) overlain by mud-dominated levee deposits (see the “Expedition 308 summary” chapter). These levee deposits are thick near Site U1324 and thin toward Site U1322. The stacked MTCs are confined within these levee deposits. Drilling was proposed in this region to understand how flow within the Blue Unit affects pressure and stability in the bounding mud. The overpressure profiles at Sites U1324 and U1322 have been characterized through direct measurements (Flemings et al., 2008) and laboratory experiments (Long et al.). We present constraints on the strength of sediments from these sites based on triaxial experiments.

Whole-round samples were used in detailed triaxial experiments to evaluate the stress-strain behavior, undrained strength, friction angle, and modulus. These results complement grain size analyses (Sawyer et al.), consolidation and hydraulic conductivity analyses (Long et al.) of sediment, and permeability analyses (Schneider et al.) of sediments from this region. Together these studies provide a full suite of geotechnical parameters which will help us understand coupling between flow, deformation, and stability and can be used for inputs into basin models.

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