SUBAQUEOUS MUDSLIDES ON THE MISSISSIPPI RIVER DELTA FRONT: IS SEAFLOOR MOVEMENT DRIVEN EXCLUSIVELY BY KATRINA-SCALE HURRICANES?

Cyclic seafloor wave loading associated with the 1969 passage of Hurricane Camille triggered subaqueous mudslides across the Mississippi River delta front (MRDF), which caused the destruction of several offshore oil platforms. Subsequent geophysical and geotechnical studies of the MRDF 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 due to in-situ biogenic gas production, and (3) the frequent passage of tropical cyclones. Most MRDF geohazards research to date has focused on hurricane-triggered mass failures. We seek to test the hypothesis that MRDF seafloor instability is intrinsically linked to major hurricane events. Three spatially overlapping bathymetric datasets were utilized: (1) October 2005 (J.P. Walsh, East Carolina University), (2) February 2009 (Fugro Geoservices, Inc.), and (3) June 2014 (Louisiana State University, Bureau of Ocean Energy Management, and University of New Orleans). Resultant bathymetric digital elevation models (DEMs) were delineated into one of three geomorphic facies, based on elevation relative to the regional trend: gully (bathymetric depression), lobe (bathymetric rise), and undisturbed seafloor. Comparison of facies classification between datasets indicates the seafloor “footprints” of gullies and lobes are relatively stable between surveys, with downslope lobe progradation and gully widening via sidewall failure generally limited to <100 m/5 years. However, vertical surface differencing of the DEMs shows elevation change of up to 5 meters between surveys (~1 m/year), most of which is confined to the gully/lobe complexes. Deepening is generally associated with the shallow (20-50 m) reaches of the gullies, while accretion mostly occurs in deeper (50-80 m) water depths. These results indicate that although massive seafloor changes probably do not occur without major hurricane forcing, the MRDF is not in an equilibrium state of no net seafloor movement on a yearly timescale. Knowledge regarding regional seafloor movement is important as the MRDF enters the declining phase of the deltaic cycle; this effort represents a first step towards a MRDF seafloor stability monitoring program.