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

Abstract

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

Site C0018

“…In MTD 6, resistivity increases with depth to 178 m CSF and decreases below that to the base of the MTD. The elevated resistivity observed over most of the MTD interval is consistent with observations from IODP Expedition 308 that sediments in MTDs tend to be overconsolidated (Sawyer et al, 2009). Below MTD 6, within the interval dominated by turbidites (see "Lithology"), the trend of resistivity with depth is more variable than above MTD 6.…”

Expedition 308 synthesis: overpressure, consolidation, and slope stability on the continental slope of the Gulf of Mexico

“…F2). Log and core data demonstrate that MTDs are more dense than over-or underlying undeformed material (Sawyer et al, 2009;Strong, 2009). They are present at all three sites; however, Site U1322 has many more MTDs than Site U1324 (purple bedding in Figs.…”

“…The base of each MTD is marked by an abrupt decrease in density and increase in porosity with depth (Fig. F5C); this is recorded with a prominent negative (black) seismic reflection (Sawyer et al, 2009). …”

“…In the sandstone-and siltstonerich zones beneath 300 mbsf, the caliper log has high values, indicating borehole enlargement. High porosities are measured with the LWD logs in these intervals; Sawyer et al (2009) At Site U1322, porosity decreases from 80% to 54% within the uppermost 50 mbsf; between 50 and 120 mbsf, porosity gradually declines to 44% (Fig. F5D).…”

“…We did core and make in situ pressure measurements at Sites U1322 and U1324. Sawyer et al (2009) described the stratigraphy of the drilling transect (Fig. F2).…”