Assessing subaqueous mudslide hazard on the Mississippi River delta front, Part 2: Insights revealed through high-resolution geophysical surveying

Wednesday, 17 December 2014
Jeffrey Obelcz1,2, Kehui Xu1,2, Samuel J Bentley1,3, Ioannis Y Georgiou4, Jillian M Maloney1,3, Michael D Miner5, Kevin Hanegan4 and Gregory Keller1,3, (1)Coastal Studies Institute, Baton Rouge, LA, United States, (2)Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, LA, United States, (3)Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States, (4)University of New Orleans, Department of Earth and Environmental Sciences, New Orleans, LA, United States, (5)U.S. Department of Interior, Bureau of Ocean Energy Management, New Orleans, LA, United States
The northern Gulf of Mexico, including the subaqueous Mississippi River delta front (MRDF), has been productive for oil and gas development since the early 1900s. In 1969 cyclic seafloor wave loading associated with the passage of Hurricane Camille triggered subaqueous mudflows across the MRDF, destroying several offshore oil platforms. This incident spurred geophysical and geotechnical studies of the MRDF, which found that the delta front is prone to mass failures on gentle gradients (<0.5°) due to (1) high rates of fine-grained sedimentation and associated underconsolidation, (2) excess sediment pore pressure attributed to in-situ biogenic gas production, and (3) the frequent passage of tropical cyclones. In June 2014, a geophysical pilot study was conducted 8 km southwest of Southwest Pass, the distributary that currently receives the largest fraction of Mississippi River sediment supply. The resultant dataset encompasses 216 km of subbottom Chirp seismic profiles and a 60 km2 grid of bathymetry and sidescan data. Preliminary interpretation of these data shows the survey area can be classified into four primary sedimentary facies: mudflow gullies, mudflow lobes, undisturbed prodelta, and undisturbed delta front. Subbottom profiles reveal extensive biogenic gas from 20 to about 80 m water depths on the delta front; sidescan data show a variety of bottleneck slides, mudflow gullies and mudflow noses. Previous studies have attempted to constrain the periodicity and magnitude of subaqueous mudslides on the MRDF. However, large age gaps and varied resolution between datasets result in ambiguity regarding the cause and magnitude of observed bathymetric changes. We present high-temporal resolution MRDF bathymetric variations from 2005 (post Hurricane Katrina), 2009 (relatively quiescent storm period), and 2014 (post 2011 Mississippi River flood). These data yield better magnitude and timing estimates of mass movements. This exercise represents a first step towards (1) assembling a comprehensive geologic dataset upon which future MRDF geohazard assessments can be founded, and (2) understanding the dynamics of a massive passive margin deltaic lobe entering a phase of decline.