Mass-transport deposits of the Amazon Fan

Abstract

Seismic reflection profiles show at least four major mass-transport deposits (MTDs) on the Amazon Fan that drilling has shown date from the late Pleistocene. Each deposit extends over an area on the order of 10 4 km 2 and is 50-100 m thick. The entire thickness of individual MTDs was penetrated at Sites 931, 933, 935, 936, 941, and 944, and wireline logs were collected at most of these sites. Most deposits consist of large deformed blocks (meters to decameters) of clayey sediment. A little matrix is recognized between blocks, and some weaker smaller blocks are highly deformed. Thin matrix-rich deposits with small clasts near the top of some units are true debris flows. Properties of clasts in the MTDs show a broadly repetitive character vertically within the deposit, on a scale of meters to tens of meters. There is no evidence that a long time span is represented by disconti-nuities in sediment properties; rather, this repetitive pattern probably represents retrogressive failure from a headwall scarp. Major units 20-50 m thick within the MTDs can be correlated between sites. Sediment properties and microfossils suggest that most sediment was derived from muddy channel-levee deposits on the continental slope, but some sediment (particularly near the base of flows) resembles local deep-water levee sediments. Mass-transport events are inferred to have initiated in slope and upper-fan levee sediments. This sediment was underconsolidated because of rapid prodeltaic deposition during marine low-stands as well as a result of the presence of shallow gas and gas hydrates. Local steepening and weakening by diapiric intrusion may also have facilitated failure. The ages of the mass-transport events may correlate with times of falling sea level, when gas hydrate sublimation could destabilize sediments. MTDs were partly confined by pre-existing channel-levee topography on the fan. In places, high-relief levee deposits were eroded by the mass-transport flow and incorporated in the basal part of the deposit.