Abstract:
Seismic, core, and logging data from Integrated Ocean Drilling Program (IODP) Expedition 308 record multiple Mass Transport Deposits (MTDs) within the upper 600 meters below seafloor (mbsf) of the Ursa Basin, northern Gulf of Mexico. The most prominent, MTD-2, is 35 to 100m thick, spans all three drill sites - U1324 U1323 and U1322 - and is located approximately 100mbsf. MTD-2 is seismically imaged with a positive, low-amplitude top reflection, and negative, high-amplitude basal reflection. MTD-2 is identified in core and logging data as a low porosity, high bulk density, zone. At U1324, the basal sediments of MTD-2 are 7 porosity units less than those immediately below, while at U1322, the basal sediments are 10 porosity units less. This decline in porosity corresponds to a 5% increase in bulk density from overlying bounding sediments to MTD-2. We hypothesize that MTD densification is a result of sediment remolding during debris flow. Remolding destroys the original chaotic fabric, resulting in shear-aligned grains that have lower porosity. We also suggest that the arrested flow is buried and consolidated under uniaxial strain. Initial high resolution x-ray texture goniometry (HRXTG) fabric analysis shows greater basal plane alignment of smectite and chlorite in a MTD specimen, compared to a non-MTD specimen. Consolidation curves from initial uniaxial experiments show that at a given porosity, a synthetically remolded specimen has on average a 2 MPa lower vertical effective stress than the original intact specimen. Pre-consolidation effective stresses measured for Constant Rate of Strain (CRS) consolidation tests on 11 MTD and 13 non-MTD Ursa specimens suggest that the pre-consolidation stresses are approximately the same whether within or outside of an MTD. We will further explore these hypotheses through HRXTG fabric analysis and CRS tests on natural and synthetically remolded Ursa core specimens. If our hypotheses are correct, we expect (1) weaker consolidation curves for MTD vs. non-MTD specimens; (2) Similar consolidation curves for MTD and remolded specimens; (3) No distinction in linear pre-consolidation stress trend between MTD and non-MTD specimens; and (4) Chaotic fabric in non-MTDs, and shear-aligned fabric in remolded and MTD specimens. Ultimately, MTDs pose a hazard because it takes longer for suction anchor piles and jetted conductors - installed for production platforms - to penetrate MTDs relative to bounding sediment.