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Inception, Development and Demise of Supercritical Bedforms in a Channel-lobe Transition: Green Canyon, Abyssal Gulf of Mexico


We present a study based on 3D seismic data describing the inception, evolution and demise of a supercritical bedform field formed at a channel-lobe transition at the base of continental slope. We also constrain the range of flow conditions using 1D turbidity-current model that accounts for lateral expansion associated with flow onto an unconfined surface. The study area is located on the northern Gulf of Mexico continental rise near the Green Canyon reentrant, about 2km south of Sigsbee Escarpment. This area is underlain by a salt diapir, and sediment accumulation is strongly affected by halokinetic movement. The channel-lobe transition occurs at an area of abrupt steepening of seafloor gradient at the head of a submarine slide. The steepening of sea floor gradient by the slide event appears to have initiated the development of the bedforms. As long as sediment supply by turbidity currents along the channel continued, the bedforms grew and migrated upstream, healing the slump-scar topography. However, turbidity currents through the channel appeared to have ceased after a period, resulting in demise of the bedforms, even before the complete healing of the topography is achieved. The bedforms are strongly updip-migrating, near symmetrical, with crescentic to near-straight crest lines. The height of the bedforms range between 15m and 40m, and the wavelength range between 170m and 310m. The dimensions symmetrically increase away from the mouth of the channel for approximately 3km and then decrease. In our simple numerical model, we systematically vary the current depth or thickness, velocity, and volumetric suspended-sediment concentration in order to determine the most likely set of properties for the currents that produced the studied train of submarine bedforms. Upstream-migrating nature of the bedforms, and the transport conditions estimated by numerical modeling as part of this study support the supercritical nature of the bedforms, and the interpretation that the studied bedforms are antidunes.