ABSTRACT:

Mississippi Delta platform sediments are incipiently unstable because of extremely high sedimentation rates, relatively steep bottom slopes, and the formation of biogenic methane. Thus, sediment failure is easily initiated and large quantities of sediment may be moved down slope en masse. The region is a source of large quantities of oil and gas, however, the recurring mass movements adversely affect recovery of those resources. To better understand these failures and, thereby, improve resource management, a simplified analytical model that treats the failure mechanism as a sedimentation and slope over steepening process and sediment motion as a propagating kinematic wave is developed. The model allows estimation of sedimentation rates necessary to initiate slope failures for a range of observed depths of basal shear planes along which the failures occur. Model results indicate that slope over steepening is a viable slope failure mechanism.

INTRODUCTION

The subaqueous portion of the Mississippi Delta platform annually receives approximately 2 × 1011 kg of sedimentary materials from the Mississippi River (Milliman and Meade, 1983). A large proportion of those sediments are deposited during the few months of the year when the river is in flood. Estimates of point accumulations of sediment range from about 70 cm during a seasonal flood event (Adams and others 1987) to more than 4.0 m during extreme floods (Coleman and others 1980). Because of those extremely high sedimentation rates that result in under consolidation, the relatively steep bottom slopes, and the formation of biogenic methane, sediments of the delta front are highly unstable, slope failure is easily initiated, and highly mobile deltaic sediments may be moved down slope under the influence of gravity (Coleman and others 1980). The mass movement transport system is represented, in part, by an extensive system of cross-shelf trending elongate mudflow gullies.

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