Physical Properties of Mass Transport Complexes in the Ursa Region, Northern Gulf of Mexico (IODP Expedition 308) Determined From Log, Core, and Seismic Data
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AbstractMass transport complexes (MTCs) in the Ursa region (northern Gulf of Mexico) are identified by their low seismic reflectivity, increased resistivity, increased bulk density, high shear strength, and decreased porosity. We integrate seismic, logging while drilling (LWD), and core data to identify MTCs and to characterize their physical properties. The seismic signature of MTCs includes high amplitude reflections at their bases and low amplitude, and often non-continuous, internal reflections. Resistivity increases in MTCs (up to 10%) creating a contrast with overlying and underlying, nondeformed sediments. Increased resistivity correlates with increased bulk density and visual observations of increased deformation. We interpret that increased density results from consolidation associated with shear deformation. This creates different resistivity-density relations for MTCs and non-failed sediments. Undrained shear strength measured on cores increases by as much as 20% in MTCs where resistivity and density are high. Consolidation during shear deformation also decreased permeability of mudstones in this basin. The combination of rapid deposition and low permeability may have contributed to shallow overpressure in the Ursa region.IntroductionMass transport complexes (MTCs) are sedimentary bodies that have experienced downslope migration and various degrees of internal deformation. Previous studies have evaluated the geometry and stratigraphy of MTCs using 2D and 3D geophysical data [e.g., 1,2]. Other studies have related the distribution and size of MTCs to natural disasters associated with submarine slope failures. In few instances natural disasters have been observed as direct result of slope failures. The 1929 Grand Banks tsunami was caused by an earthquake-induced submarine landslide that damaged oceanic infrastructure and caused deaths onshore [3]. In 1998, an earthquake-triggered MTC produced a tsunami that killed <2000 people on Papua New Guinea [4]. In addition to human fatalities, submarine landslides have also caused direct damage to coastal facilities (e.g., the Nice airport, France [5], and Finneidfjord, Norway [6]). Detailed physical and sedimentological characterizations of MTCs have allowed the oil industry to better identify hydrocarbon reservoirs and seals [e.g., 7] and has been used to evaluate physical properties that may impact shallow infrastructure [8].
