The Amazon Fan contains sedimentary/acoustic sequences characteristic of many large and small modem mud-rich fans. Analyses of high-resolution single-channel seismic-reflection profiles and 3.5-kHz profiles suggest that fan growth is in part related to sea-level fluctuations and in part related to events such as channel bifurcations and large debris flows that appear unrelated to sea-level position. Sinuous fan channels are perched on top of lens-shaped overbank deposits to form channel-levee systems in the upper and middle fan. Individual channel-levee systems overlap and coalesce to build levee complexes that also stack and overlap, but that are bounded by large debris-flow deposits. Because both channel-levee systems and debris flows can be active at the same time, this depositional pattern does not necessarily develop as a result of sea-level change. The sinuous fan channels appear to be nearly at grade because channel sinuosity varies downfan to keep the along-channel gradient uniformly decreasing downfan. Flat-lying, high-amplitude reflection packets that underlie a channel-levee system and extend downfan to form part of the lower fan may develop when new, oversteepened channels are created as a result of avulsion on the middle fan. This suggests that portions of the lower fan are formed concurrently with channellevee systems. Piston cores from near the most recently active channel suggest that the locus of sedimentation shifted landward as sea level rose at the end of the last glaciation.
Sediments cored along the southwestern Iberian margin during Integrated Ocean Drilling Program Expedition 339 provide constraints on Mediterranean Outflow Water (MOW) circulation patterns from the Pliocene epoch to the present day. After the Strait of Gibraltar opened (5.33 million years ago), a limited volume of MOW entered the Atlantic. Depositional hiatuses indicate erosion by bottom currents related to higher volumes of MOW circulating into the North Atlantic, beginning in the late Pliocene. The hiatuses coincide with regional tectonic events and changes in global thermohaline circulation (THC). This suggests that MOW influenced Atlantic Meridional Overturning Circulation (AMOC), THC, and climatic shifts by contributing a component of warm, saline water to northern latitudes while in turn being influenced by plate tectonics.
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 discontinuities 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 lowstands 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.
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