Submarine Landslides In The Mississippi River Delta

Abstract

ABSTRACT Systematic side-scan sonar and high-resolution seismic records from the shallow-water offshore areas of the Mississippi Delta have revealed widespread sub aqueous slope failures in bottom sediments. These failures have resulted in damage and loss to offshore structures and pipelines. The features occur on slopes with very low inclinations (ranging from 0.2° to 1.5°) and in water depths of 5-100 m. The types of features include collapse depressions, bottleneck slides, elongate slides and slumps, mudflow gullies, and overlapping mudflow lobes. Although movements include both vertical and rotational displacements, the bas1c mechanism can be approximated as down slope translation of shallow slabs of debris. Although movement rates of up to several hundred meters/year have been documented, it is postulated that large-magnitude surged may be inherent in these features. These submar1ne landslides result from complex temporal and spatial combinations of wave-induced stresses, sediment loading, and generation of high pore water and methane gas pressures. INTRODUCTION Numerous detailed marine surveys (high-resolution geophysical, side-scan sonar, and bathymetric data) have been conducted in the area offshore of the Mississippi River. These surveys have revealed the presence of a large number of distinct types of sub aqueous slope instabilities that can be generally classed as submarine landslides. Most of these bottom sediment mass movements are of sufficient magnitude to severely endanger bottom-emplaced petroleum facilities such as offshore drilling and production platforms, wellheads, and pipelines. The modern bird-foot (Balize) delta of the Mississippi River displays three major distributaries (Fig. I) and has formed within the past 600-800 years.1 Seaward progradation rates of the distributaries vary from in excess of 100 m/yr to less than 50 m/yr, depending upon the specific distributary monitored. Sedimentation rates seaward of the delta shoreline vary spatially as well as temporally. Near the mouths of the distributaries, accumulation rates in excess of 1 m/yr have been documented, whereas in adjacent interdistributary bays accumulation rates rarely exceed a few centimeters per year. In offshore waters, in front of the delta, accumulation rates vary from a few tens of centimeters per year in 50-m-depth water to a few millimeters per year in water depths approaching 200 m. Offshore slopes of the entire delta front are extremely low, rarely exceeding 1.5°. In the interdistributary bays, bottom slopes are generally less than 0.5° and are rarely greater than 0.2°. In water depths of approximately 5-80 m, bottom slopes range from 0.7° to 1.5°, and in depths of 80-150 m the slopes are less than 1°. In general, hydrographic maps indicate extremely irregular topography, the bottom displaying a large number of radial trending submarine gullies 2 in water 20-80 deep and broad, flat terraces seaward to water depths as great as 200 m (Fig. 1).