Rapid shut-off and burial of slope channel-levee systems: new imaging and analysis of the Rio Grande submarine fan

The continental slope of the western Gulf of Mexico is host to a major depositional system, the Rio Grande Fan. Unlike many submarine fans, the surface of the Rio Grande Fan lacks large submarine channels and associated levees. Prior analysis of continental shelf stratigraphy has identified the presence of past extensive shelf-edge delta systems, when the Rio Grande River system flowed across the modern shelf and delivered high volumes of sediment to the shelf/slope break. A major gap in understanding this system is how large volumes of sediment, particularly sands, are transported from the shelf edge systems down the slope and onto the basin-floor fan without constructional channel-levee systems. Over 500km of new high-resolution 2D multichannel seismic (MCS) and CHIRP echosounder data were collected over the shelf edge and upper slope of the Rio Grande fan. These new data provide unprecedented imaging of the shelf-edge delta systems and associated slope deposits. Our preliminary observations indicate that while the modern seafloor morphology of the fan is dominated by mass-transport deposits, slumps and minor inactive channels, buried below thick mud deposits are very large aggradational channels-levee systems. These systems have channel belts almost 1km wide, with confining levees that approach 10km in width. The main body of the fan is built from these channel complexes, which appear to have then rapidly buried in mud. We document the evolution, from initial channelization to burial, of these massive slope systems. Regional correlation suggests that this most recent episode of channel-levee growth and shutoff occurred very rapidly, and could indicate drastically higher sediment flux through the paleo-Rio Grande River than that of the modern. Our results highlight an example of a slope-channel system that is subject to significant variations in sediment supply. Such systems can apparently build large late Pleistocene submarine fan deposits that can be difficult to recognize in modern seafloor morphology.