Controls from Channel-Levee Architectures in the Amazon Fan
Submarine channel-levee architecture can yield important evidence on the behaviour of turbiditic flows at the time when the channel was active. This study is based on the interpretation of a 3D seismic survey located in water depths of about 1000 to 2000 m on the upper slope of the Amazon Fan, where three upslope-stacked channel-levee systems of Pleistocene age can be distinguished. Each channel disperses obliquely down the slope, resulting in levee size asymmetry, with the downslope levee is of greatest size. The upslope results from a sequence of avulsions on the upslope levees, with the bathymetric influence of the older channel levee system dictating the slope-oblique orientation of the new channel. The channel levees are architecturally distinct. The oldest, Lower Channel-Levee system (LCLS), clips the Eastern border of the survey. In this system, the channel becomes more sinuous and narrower upward, an upsteam-migrating backfill sequence terminating as the channel became plugged due to clay deposition. In the Middle Channel-Levee System (MCLS), the channel becomes both less sinuous and wider upward. Unlike the back fill of the channel in the lower system, the channel in this system is filled by longitudinally continuous reflectors sub-parallel to the channel axis. These reflectors overspill the levee to form the uppermost layers of the adjacent high amplitude reflection packages (HARPs) - whereas HARPs were absent in the LCLS architecture. The Upper Channel-Levee System (UCLS), is composed of at least four cycles of channel-levee development, sub-vertically stacked, with channels widening upward in each cycle.
The distinct nature of these channel-levee architectures are inferred to relate to different histories of change in flow efficiency. Thus a gradual decrease in flow efficiency would account for the narrowing upward and back fill in the channel and the consequent reduction of net to gross in the LCLS. A gradual increase followed by an abrupt (avulsion-related) fall of the flow efficiency could explain the widening upward and the channel fill with an increase of net to gross in the MCLS. The stacking of channels in the UCLS was possibly favoured by the topographic confinement of the downslope levee combined with upslope folding that together inhibited channel avulsion, in combination with pulses of upstream uplift generating associated increases in accommodation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009