Seismic Facies and Late Quaternary Growth of Amazon Submarine Fan

Abstract

The Amazon Fan contains sedimentary/acoustic sequences characteristic of many large and small modern 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 channel-levee 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.