Timing of the late Quaternary Amazon Fan Complex masstransport deposits

Abstract

This paper is included in the Special Publication entitled 'Geological evolution of ocean basins: results from the Ocean Drilling Program', edited by A. Cramp, C.J. MacLeod, S.V. Lee and E.J.W. Jones. The Amazon Fan at the mouth of the Amazon River is a large, highly structured muddy deep-sea fan. Sediment recovered by Ocean Drilling Program Leg 155 has allowed us to reconstruct the structure of the late Quaternary Amazon Fan Complex. A significant sedimentary component of the complex are the mass-transport deposits (MTDs) formed by catastrophic failure of the continental slope. There are two sets of MTDs: the near-surface MTDs and the deep MTDs. The near-surface MTDs are divided into the Eastern and Western Debris Flows which are capped by Holocene sediments. The deep debris flows are divided into the Unit R MTD in the western Amazon Fan complex, and the Deep Eastern MTD in the eastern Amazon Fan Complex. The MTDs each cover an area of up to 25 000 km2, reach a maximum thickness of 200 m, and consist of approximately 50 000 gigatonnes of sediment. A characteristic of the MTDs is the inclusion of reworked glacial and interglacial material from both the continental slope and the surrounding fan. The relatively thin interglacial deposits below the MTDs are from the nanofossil zones CN14b and CN15a and thus younger than 460 ka. Bio-, seismic- and magnetostratigraphy and sedimentation rate constraints have been used to date the top of both the near-surface and the deep MTDs. It seems that the near-surface Western and Eastern Debris Flows were last active during Termination I. The Deep Eastern MTD was last active at about 33 ka, while the Unit R MTD was last active at about 45 ka. It is inferred that the interglacial deposits beneath these MTDs were formed during Oxygen Isotope Stages 7, 9 and 11. We speculate that the MTDs were triggered by climatically induced changes in sea level, and that the interglacal deposits may have acted as slip planes for the MTDs.