Review of the Timing and Causes of the Amazon-Fan Mass Transport and Avulsion Deposits during the Latest Pleistocene

Abstract

The late Pleistocene Amazon deep-sea fan provides a modern analogue to ancient fan systems containing sandy hydrocarbon reservoirs. Extensive deposits of sand-rich material are found in the Amazon Fan mass-transport deposits (MTDs) and the base-of-channel avulsed sand bodies which underlie the channel-levee systems. These deposits were drilled as part of ODP Leg 155, the results of which form the basis of this review. The hemipelagic sediment above the MTDs and avulsed sand bodies were dated using primarily AMS radiocarbon dating. The dating provides support for the interpretation that the MTDs and avulsed sand bodies were triggered by relatively small, millennial-scale changes in relative global sea level (+/- 5-20 m). Equally controversial has been the suggestion that changes in sea level also control the architecture of the channel-levee distributive systems within the Amazon Fan. For example, Maslin et al. (2006) proposed that prior to 22,000 calendar years BP a tripartite channel system existed while afterwards only one active channel-levee system existed. This switch may have been due to the fall in sea level below the shelf break, providing direct access between the canyon and the sediment supplied to and eroded from the shelf-edge delta front. This would have significantly increased the sediment supply to the Amazon Fan at 22 ka, contributing to channel entrenchment involving channel-floor erosion and the growth of levees within the canyon-channel transition area, promoting the development of a single deep, incised channel. If future work confirms that Amazon deep-sea Fan sedimentation is sensitive to relatively small changes in sea level, this will provide support for the central assumption of the theory of sequence stratigraphy, namely that changes in sea level control basin sedimentation and the emplacement of sand-rich, potential hydrocarbon-bearing, deposits. It is hoped that these controversial suggestions reviewed here will stimulate more investigations into the Amazon Fan and other deep-sea fans.