Systematic Spatial Variations in the Fabric and Physical Properties of Mass-Transport Deposits in the Ursa Region, Northern Gulf of Mexico

We clarified the interrelations between deformation and spatial variations in the physical properties and fabric of mass-transport deposits (MTDs) within the Ursa Basin in the Gulf of Mexico, based on measurements of the anisotropy of magnetic susceptibility (AMS) and porosity, and observations of the rock fabric. Drillholes sunk as part of the Integrated Ocean Drilling Program (IODP) Expedition 308 (Sites U1322, U1323, and U1324) penetrated clay-rich MTDs in the Ursa Region within the uppermost 600 m below the seafloor. The soft-sediment deformation within the Ursa MTDs is characterized by contorted sediments and a homogenized appearance. Site U1322 shows evidence of major mechanical and strain decoupling at the base of the MTDs; i.e., the intensity of the magnetic lineation, porosity, and shear strength show gradual changes in the upper parts of the MTDs, with a sharp offset in these parameters at the contact with the underlying unit. At the grain scale, these MTDs are characterized by strong shearing-induced textures (S–C structures) and bedding-oblique foliations. However, at Site U1324, MTDs show only minor variations in fabric and physical properties despite being correlatives of those at Site U1322. Site U1324 is located near the headwall areas of MTDs, whereas Site U1322 lies near the toe areas. Therefore, at Site U1324 the MTDs are close to their pre-failure condition, whereas the sediments at Site U1322 record high strain and long-distance transport. The obtained values of differential porosity and magnetic lineation (indicative of grain alignment) show a systematic relation between the sites, indicating that the physical properties of sediments in MTDs are controlled by the fabric.