Abstract Mudflows on the Mississippi River Delta Front (MRDF) are recognized hazards to oil and gas infrastructure in the shallow (20–300 m water depth) Gulf of Mexico. Preconditioning of the seafloor for failure results from high sedimentation rates coupled with slope over-steepening, under-consolidation and abundant biogenic gas production. Catastrophic failure of production platforms and pipelines due to seafloor displacement during infrequent large hurricanes such as Camille in 1969 and Ivan in 2004, point to cyclical loading of the seafloor by waves as a primary movement trigger. Due to data limitations, the role of smaller storms and background oceanographic processes in driving seafloor movement has remained largely unconstrained but these are thought to contribute to significant seafloor change. With the aid of new high-resolution multibeam mapping and seismic reflection profiling across sections of the MRDF, several moving features within the deforming delta-front environment are investigated and potential hazards to infrastructure installed and adjacent to the region are discussed. Via repeat mapping surveys of selected areas and records of changing shipwreck locations, we highlight significant seafloor displacement across annual to decadal timescales. For example, individual blocks mapped within mudflow gullies adjacent to Southwest Pass show downslope transport of more than 80 m in a single year, while the SS Virginia , a 153 m-long oil tanker sunk in 1942, has been relocated and found to have moved downslope more than 400 m in 14 years, without a major hurricane (>Category 2) passing through the region.

Abstract Fluid muds are high-concentration near-seabed suspensions of fine sediments, and have been recognized in coastal sedimentary systems for many years. However, in the past decade, fluid-mud development and transport have been identified in an increasing number of settings as important mechanisms for marine dispersal of fluvial sediments. It has been shown that the combination of high-energy benthic hydrodynamics and sufficient fine sediment can result in cross-shelf gravity-driven flows (on very low slopes) that can blanket hundreds of square kilometers to thicknesses exceeding 10 cm. The sedimentary fabric that results from gravity-driven flows consists of a stacked pattern of predominantly fine-grained, fining-upward packages. The resulting morphology of the shelf can be a clinoform, with maximum deposition occurring on the foreset (convex-upward) region. The western Louisiana inner shelf has been experiencing fluid mud deposition in response to increased fine sediment supplied by the Atchafalaya River since ∼1950’s. A review of observations collected during recent studies of the Eel and Fly rivers, and their similarities to the Atchafalaya shelf indicate that wave-enhanced gravity driven flows are responsible for the sedimentary features and clinoform morphology present along the Chenier plain coast of Louisiana.

Abstract Recent studies of fluvial-marine sediment dispersal have demonstrated that, under conditions of adequate sediment supply and intense turbulence in the bottom- boundary layer, wave-enhanced gravity flows can develop on shelves with gradients <0.7°. These conditions can exist seasonally on many continental shelves proximal to river mouths. Therefore, such flows (and resultant deposits) are probably more widespread than previously thought. Examples from a high-energy active-margin setting (Eel Shelf, California) and a lower energy passive margin (Atchafalaya River/Louisiana Inner Shelf) indicate that similar bedding can be produced by high- concentration near bed flows in apparently contrasting depositional settings, due to the occurrence of similar physical processes in the bottom boundary layer. Because such hyperpycnal flows can carry much more sediment mass than can buoyant plumes of suspended sediment, and can move at velocities on the order of cm s -1 , such benthic sediment flows on shelves could represent a significant and previously underestimated transport mechanism for fine sediment across continental shelves.