Linking Seismic Facies, Physical Properties, and Depositional Processes in Mass Transport Deposits, Ursa Basin, Northern Gulf of Mexico

Slumps and debris flows, recorded within mass transport deposits (MTDs), have characteristic seismic facies, sedimentology, and physical properties. In seismic, MTDs are imaged as low-amplitude zones above a high-amplitude basal reflector. Within these zones we identify two distinct seismic facies: 1) locally high-amplitude chaotic, and 2) discontinuous/locally stratified. In cross section, reflections within the locally high-amplitude chaotic facies are high amplitude and discontinuous. In interval amplitude maps, these reflections are sinuous and channel-like. The basal reflector contains grooves. The top reflector is continuous but is lower amplitude than the base. This facies is significantly denser (lower porosity), has higher shear strength, and higher resistivity than bounding undeformed sediment. In core, deformation is substantial; contorted mud containing folds and mud clasts. In contrast, the locally stratified facies contains relatively dim reflectors that abut against pinnacle-shaped islands of parallel stratified reflectors. Each pinnacle protrudes above the surrounding material and is attached to the MTD base. This facies has slightly higher bulk density, shear strength, and resistivity relative to bounding undeformed sediment. Deformation is subtle and recorded as slightly tilted bedding and small-offset faults. In both facies, the higher density creates a strong impedance contrast between bounding undeformed sediment, and thus a strong reflection at the top and base. However, the density contrast is much higher at the base; therefore the amplitude of the basal reflector is higher. We interpret that the locally high-amplitude/chaotic facies records debris flows having long runout and substantial internal deformation; the locally stratified facies records slumps with only a short run-out and minimal internal deformation. We consider shear during movement to be the mechanism that underlies the densification signature in both slumps and debris flows.