South-Central Section - 50th Annual Meeting - 2016

Paper No. 9-8
Presentation Time: 4:05 PM


MALONEY, Jillian1, BENTLEY, Samuel J.2, OBELCZ, Jeff3, XU, Kehui2, GEORGIOU, Ioannis4, MINER, Mike5 and KELLER, Gregory2, (1)San Diego State University, Department of Geological Sciences, 5500 Campinale Drive, San Diego, CA 92182, (2)Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, (3)Department of Oceanography and Coastal Sciences, Louisiana State University, 2151 Energy, Coast and Environment Building, BATON ROUGE, LA 70803, (4)Dept. of Earth & Environmental Sciences and Pontchartrain Institute for Environmental Sciences, University of New Orleans, 2000 Lakeshore Dr, New Orleans, LA 70148, (5)Marine Minerals Program, Bureau of Ocean Energy Management, Gulf of Mexico Region, New Orleans, LA 70123,

Sediment accumulation rates and patterns play an important role in governing the stability of sediment on the subaqueous Mississippi River delta front. High sedimentation rates result in underconsolidation, slope steepening, and increased biogenic gas production, which are all known to decrease stability. Sedimentation rates are highly variable across the MRDF, but are highest near the mouth of Southwest Pass. Previous work has shown that sedimentation rates near Southwest Pass can exceed 1 m/yr with distributary mouth bar progradation at >70 m/yr (between 1764-1979). However, since the 1950s, the suspended sediment load of the Mississippi River has decreased by ~50% due to dam construction upstream. In order to examine the impact of this decreased sediment load on subaqueous delta sedimentation patterns and mudflow dynamics, we compiled bathymetric datasets, including historical charts, industry and academic surveys, and NOAA data, collected between 1764 and 2009.

Analyses of these datasets show significant changes to delta front growth during the last century. Progradation of Southwest Pass (measured at 10 m depth contour) has slowed from ~67 m/yr between 1764 and 1940 to ~26 m/yr between 1940 and 1979, with evidence of further deceleration from 1979-2009. The 10 m contour at both South Pass and Pass A Loutre has begun retreating at rates >20 m/yr between 1979-2009. The data reveal that advancement of the delta also decelerated in deeper water across many areas of the delta front. Furthermore, over the area offshore from Southwest Pass, the sediment accumulation rate decreased by ~82% between 1940-1979 and 1979-2005. The subaqueous delta front appears to be entering a phase of decline, and we expect these sedimentation trends to impact the spatial and temporal patterns of subaqueous mudflows. Nevertheless, despite the decreased sediment input and accumulation, we still observe advancement of some mudflow lobes during the past ~30 years, and mudflows have recently been documented associated with Hurricanes Ivan (2004) and Katrina (2005). New geophysical and sedimentological data will be required to assess potential mudflow hazards associated with changing sedimentation rates and patterns.