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doi:10.2204/iodp.proc.308.211.2010

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 308 (see Behrmann et al., 2006) was dedicated to the study of overpressure generation and fluid flow focusing at the Gulf of Mexico continental margin. Of the two areas investigated by drilling, Ursa Basin (see, e.g., Winker and Booth, 2000; Winker and Shipp, 2002; or the “Expedition 308 summary” chapter for a detailed description of the geological setting) is the location where severe overpressure was identified (see the “Expedition 308 summary” chapter; Flemings et al., 2008) as a consequence of fine-grained muds being sedimented at very high rates, especially in the uppermost Pleistocene. In this setting, repeated slope failure occurred, with manifestations of mass transport deposits (MTDs) being most frequent where the measured overpressures are highest (see the “Expedition 308 summary” chapter; Sawyer et al., 2007; Flemings et al., 2008). This is clearly seen in Figure F1, which shows the combined lithologic, porosity, shear strength, and pore pressure logs for IODP Sites U1322 and U1324. The upslope site (U1324) shows lower overpressure and a smaller proportion of MTD in the section, which is in line with the predictions of the fluid focusing model for sedimentary prisms (e.g., Dugan and Flemings, 2000; Flemings et al., 2008).

When unconsolidated or poorly consolidated sedimentary rock masses are moved in the form of slumps, debris flows, or mud flows (e.g., Hein, 1985), there are variable degrees of clay realignment by remolding and shearing (e.g., Bohlke and Bennett, 1980). The clay fabric in particular seems to influence the geotechnical properties of the deposits (e.g., Bennett et al., 1981). It is thought that during mobilization the sediments become fluidized through temporary fabric collapse (in the case of sensitive clays) or by grains becoming buoyant during the ingress of externally derived fluids (e.g., Maltman and Bolton, 2003). Especially in the petrophysical core and log data from Site U1322 it is evident (Fig. F1) that porosity decrease and shear strength increase is not a simple function of depth but discontinuities are visible coincident with the boundaries separating units characterized by different modes of sediment transport (suspension and fallout versus MTD). Exceptionally low porosity observed near the bases of some of the MTDs speaks in favor of clay fabric collapse and concurrent fluidization as a consequence of the slumping process.

In order to learn more about the relationship of strength, fabric, and mineralogy of normally sedimented deposits and MTDs in the Ursa Basin, we have started to carry out a comprehensive investigation into the mechanical behavior, microfabrics, and textures of both types of sediment sampled at Sites U1322 and U1324. The results of the study will be published in full elsewhere after termination of the analytical work. To date, we have carried out a series of undrained triaxial shear tests on five whole-round core samples and ring shear tests on four samples. Here, we present these first results with some preliminary discussion. Ring shear testing (e.g., on remolded specimens) has the principal merit of being able to quantify the effect of mineral composition on the strength and frictional behavior of the material over a large range of confining pressures (e.g., Lupini et al., 1981). Also, velocity-dependence of shear strength can be investigated, giving answers to the question of whether the tested sediment reacts to accelerated shearing by weakening or strengthening. If velocity weakening is documented, then the material is thought to be especially prone to develop slumps and slides as result of a mechanical runaway situation. On the other hand, the principal reason to include triaxial testing in the program of strength investigation is that, in contrast to ring shear testing, triaxial testing is performed on undisturbed sediment material under undrained conditions. These conditions provide crucial information on the dependence of strength on fabrics and permit monitoring of excess pore water pressure changes during consolidation and shearing.

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