Strength History of Sediments from IODP Expedition 308, Northern Gulf of Mexico: Slope (In)Stability and Mass Transport Complexes

Experiments on core samples from Integrated Ocean Drilling Program (IODP) Expedition 308 in the Ursa region of the northern Gulf of Mexico suggest a complex strength and deformation history within sediments of mass transport complexes (MTCs) – weakening during shallow, uniaxial consolidation associated with burial followed by strengthening during shear consolidation and MTC emplacement. Triaxial strength experiments completed on specimens from IODP Sites U1322 and U1324 document internal friction angles range from 29 to 17 degrees and normalized undrained shear strength ranges from 0.36 to 0.21. Both parameters show a general decrease with increasing vertical consolidation stress up to 1.8 MPa. These results indicate a weakening of mud-dominated sediments during early burial in the Ursa region. This decrease in frictional strength coupled with overpressure that exceeds 60% of the reduced lithostatic stress likely control to the initiation of slope failures in the region. Core measurements of undrained shear strength (vane shear tests) show enhanced strength within MTCs relative to the hemipelagic drape that bounds them. These measurements mark a transition from weakening to strengthening based on deformation mechanism, uniaxial versus shear consolidation. We explore the progressive decrease in strength followed by shear-induced strengthening to understand the thickness and distribution of slope failures. We integrate this strength work with seismic and logging data that provide detailed information on the distribution and physical properties of numerous of stacked mass transport complexes that have been emplaced over the last 60 kyr. We focus on the mud-dominated sections of the Ursa region. This mud is primarily smectite and is the host material to numerous MTCs in the Ursa region.