Retrogressive failures recorded in mass transport deposits in the Ursa 3 Basin , Northern Gulf of Mexico

6 [1] Clay-rich mass transport deposits (MTDs) in the Ursa Basin, Gulf of Mexico, record 7 failures that mobilized along extensional failure planes and transformed into long runout 8 flows. Failure proceeded retrogressively: scarp formation unloaded adjacent sediment 9 causing extensional failure that drove successive scarp formation updip. This model is 10 developed from three-dimensional seismic reflection data, core and log data from 11 Integrated Ocean Drilling Project (IODP) Expedition 308, and triaxial shear 12 experiments. MTDs are imaged seismically as low-amplitude zones above continuous, 13 grooved, high-amplitude basal reflections and are characterized by two seismic facies. A 14 Chaotic facies typifies the downdip interior, and a Discontinuous Stratified facies typifies 15 the headwalls/sidewalls. The Chaotic facies contains discontinuous, high-amplitude 16 reflections that correspond to flow-like features in amplitude maps: it has higher bulk 17 density, resistivity, and shear strength, than bounding sediment. In contrast, the 18 Discontinuous Stratified facies contains relatively dim reflections that abut against intact 19 pinnacles of parallel-stratified reflections: it has only slightly higher bulk density, 20 resistivity, and shear strength than bounding sediment, and deformation is limited. In both 21 facies, densification is greatest at the base, resulting in a strong basal reflection. Undrained 22 shear tests document strain weakening (sensitivity = 3). We estimate that failure at 23 30 meters below seafloor will occur when overpressure = 70% of the hydrostatic effective 24 stress: under these conditions soil will liquefy and result in long runout flows.