SEDIMENT MASS-FLOW PROCESSES ON A DEPOSITIONAL LOBE, OUTER MISSISSIPPI FAN

SeaMARC 1A sidescan-sonar imagery and cores from the distal reaches of a depositional lobe on the Mississippi Fan show that channelized mass flow was the dominant mechanism for transport of silt and sand during the formation of this part of the fan. Sediments in these Bows were r apidly deposited once outside of their confining channels. The deposit ional lobe is formed of a series of long, narrow sublobes composed of thin-bedded turbidites (normally graded siliciclastic sand and silt, 2 0 cm thick on average), debris-flow deposits (soft clay clasts up to 5 cm in diameter in a siliciclastic silt matrix, 48 cm thick on average ), and background-sedimentation hemipelagic muds. The mass flows most likely originated from slope failure at the head of the Mississippi Ca nyon or on the outer continental shelf and flowed approximately 500 km to the distal reaches of the fan, with debris flow being the dominant Bow type, An analysis that uses the geometry of the confining channel s and-strength properties of the debris flow material shows that these thin debris Bows could have traveled hundreds of kilometers on extrem ely small sea-floor slopes at low velocities if the flowing medium beh aved as Bingham fluids and were steady-state phenomena.