Physical Properties of Stacked Slumps in the Ursa Region, Northern Gulf of Mexico (IODP Expedition 308) Determined from Log, Core, and Seismic Data

Slumps identified in multi-channel seismic (MCS) data in the Ursa region of the northern Gulf of Mexico are divided into two types based on consolidation state and shear strength. The MCS signature of slumps includes high amplitude reflections at their bases and tops and semi-transparent internal reflectors. Logging while drilling (LWD) measurements and core data (IODP Expedition 308 Sites 1322, 1323, 1324) constrain variations in bulk physical properties of non-deformed and slumped intervals. The dominant LWD signature of slumps is an increase (up to 10%) in resistivity relative to the overlying and underlying, non-slumped sediments. Two types of slumps are characterized by their resistivity signature: (1) increasing resistivity with depth to the base of the slump and an abrupt decrease at the base of the slump and (2) constant resistivity with abrupt resistivity changes delineating the top and base of the slump. Increased resistivity correlates with increased bulk density and with visual observations of increased deformation. We interpret increased density results from consolidation associated with shear deformation. The two separate resistivity anomalies provide insights to where deformation occurred during slumping: increasing resistivity with depth is interpreted as localized consolidation and densification near the base of the slump during mobilization, whereas constant resistivity indicates distributed deformation. Undrained shear strength measured on cores increases by as much as 20% in slumps where resistivity and density are high. Increased consolidation during shear deformation and slumping can be related to increased pore pressure and active fluid flow documented in the Ursa region.