Multiple Outer Shelf Deltas and Downslope Massive Mass-Wastings Characterize the Mississippi Canyon, northern Gulf of Mexico
Allen Lowrie1, Carol B. Lutken2, and Thomas M. McGee2
1 Consultant, Picayune, MS
2 Mississippi Mineral Resources Institute, Mississippi University, MS
Detailed published analyses of debris along the shelf break at the head of the Mississippi Canyon describe repeated mass-wasting events between 29,000-35,000 and 15,000 years ago, a time including the last Glacial Maximum with sea level at its lowest.
A single channel seismic line collected along an east west trend over the lower Mississippi Canyon province reveals that similar and older mass-wasting events occurred prior to the penultimate Sangamon highstand. Seismic data, of approximately one-second penetration, are marked by intermittent and irregularly spaced concave-upward reflectors, indicative of cut-and-fill. Despite poor quality of data, there are suggestions of basal unconformities that mark the initial major erosional phase of a single, massive, multi-phased, mass-wasting event.
An interpretation is that several consecutive outer shelf deltas were developed each in turn collapsing onto the slope below. At present, greater climatic and sea level complexities during the glacial advance and related sea level lowstand make deciphering erosional and depositional conditions more difficult. Conventional thinking assumes that mass wasting commenced as sea level lowered. Thus up to 20 separate episodes of wasting per lowstand during the Upper Pleistocene can be discerned.
This is exemplified by a “birdsfoot” at the termination of a mighty river and a deltaic periodic debauching downslope. The evolution of shelf-edge deltas appears to be much more complex than previously described. Regional and local sedimentary strength may be important because both temperature and pressure vary along the outer shelf and the impact of hydrocarbons, including hydrates, is felt.
AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004