Presenter: Thomas Bjerstedt (Bureau of Ocean Energy Management)

Description:
Nearly two miles below sea level in the eastern Gulf of Mexico, a 300+ mile long relict channel-levee system once rivaled the scale of the modern subaerial Mississippi River. The meandering system was first imaged using side-scan sonar in 1987 by the U.S. Geological Survey’s GLORIA EEZ-Scan 85 Scientific Staff project. The U.S. Bureau of Ocean Energy Management visualized the channel-levee system with modern 3D seismic surveys and visualization software. A 1,000-ft-deep canyon attributed to the ancestral Pearl River incised the shelf edge just west of the modern birdfoot delta. The relict system’s defining characteristics include: 1) a 100 ft deep by 2,000 ft wide meandering channel axis, as measured from levee crest-to-crest, 2) a system width of 25 mi, as measured from levee toe-to-toe, with maximum sediment thickness approaching 1,000 ft, 3) steep levee walls prone to point failure along meander cut banks with resulting cone-shaped splays of coarser overbank deposits that are revealed by seismic water-bottom amplitudes, and 4) long-lived, stable channel slip faces within the meander plain. The system became inactive following up channel avulsion and 30 to 200 ft of hemipelagic clay slowly draped the complex. The age of this channel-levee system is not firmly established because direct dates from channel or levee facies have yet to be reported. The stratigraphic context and physical morphology of this remarkable, continental-scale system clearly shows a mid- to late Pleistocene lowstand to highstand couplet. Lowstand initiated canyon-cutting and slope failure with chaotic mass movements, followed by discontinuous high-amplitude acoustic events interpreted as channelless levees deposited on a braid plain.  Highstand allowed a fine-grained, well ordered channel-levee system to form with inverted topography caused by compaction within the confining levees before abandonment.