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Channel-Levee Evolution in the Pleistocene of the Amazon Fan

Silva, Carlos M.1; Paton, Douglas 1; McCaffrey, William D.1
1 University of Leeds, Leeds, United Kingdom.

The aim of this study was to better understand the way channel-levee systems interact in space and time with other slope deposits in order to establish histories of channel-levee evolution and to interpret the genetic relationships among constituent architectural elements deposits. The data base comprises a 3D seismic survey covering an area of 2300 km2 in the Upper Fan. The channel-levee systems are the volumetrically most important depositional components of the slope in the Pleistocene of the study area. In this area, three channel-levee systems were developed in sequence. The lowest one only clips the area while the upper two can be studied in some detail. The present work is focused on the intermediate one, the Middle Channel-Levee System (MCLS).

The MCLS had a very complex evolution, starting with an erosive channel, deviation of the channel course, transition to aggradational channel-levee system, levee collapse, avulsion, sand inundation in adjacent lows and channel infill until spill out. The erosional base is characterized by a longitudinal cut that bifurcates downstream with width varying from approximately 1.6 to 4.2 km. This erosive feature cuts through high amplitude reflection packages (HARPs) and an underlying unconformity. In the aggradational portion, the channel became wider and less sinuous upward, with width ranging from160 to 2650 m and with levee thickness up to 460m.

The transition from erosive/graded to aggradational channel is characterized by the downstream migration of the channel-levee over the erosive base which is represented by the downstream migration of the transition point from a single thread channel to multiple thread channels. An elevation of the base level and also an upstream uplift due to folding can explain the progressive creation of downstream accommodation necessary to make the transition point migrate.

At some point in the channel-levee evolution, an upstream levee collapse may have triggered the MTC displacement into the low adjacent to the MCLS and caused a partial avulsion of the channel. This avulsion may respond for most of the subsequent sand inundation forming the continuous and sub-parallel HARPs. However, the top portion of the HARPs is also formed by the overspill of the channel fill which indicates that their deposition may represent the end of this channel-levee system instead of the initiation of the next one as has been proposed in models of channel-levee evolution of the modern Amazon Fan.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009