ABSTRACT: Growth of a Channel-Levee System, Amazon Fan

PIRMEZ, CARLOS, Lamont-Doherty Geological Observatory, Palisades, NY, and ROGER D. FLOOD, S.U.N.Y. at Stony Brook, Stony Brook, NY

The Amazon Channel extends continuously from the shelf down to 4260 m of water depth for at least 900 km. Results from a detailed survey using SeaBeam bathymetry and water-gun seismic reflection profiles are used to investigate the processes of channel sedimentation and fan growth. Channel gradient gradually decreases down slope from 12 m/km on the canyon to 1 m/km, revealing a relatively smooth longitudinal depth profile. Sinuosity and channel relief are positively correlated with valley gradient, suggesting that the equilibrium of the channel system is maintained by changes in channel length and relief. For valley gradients between 7 and 2-3 m/km, sinuosity reaches values up to 3.5. Outside of this valley gradient range the channel is less sinuous (less than 1.5). Equilibrium of the system was disrupted in the past during avulsion events. A nick point is introduced in the longitudinal depth profile because the channel thalweg is perched above the regional fan surface. The nick point and associated base-level drop may be particularly significant on the middle fan where the channel thalweg is highly perched. Equilibrium adjustment is attained by headward migration of the nick point and downstream aggradation. Initial deposits of the newly formed channel appear as flat-lying high-amplitude reflections that onlap the abandoned and pre-existing channel-levee systems. Levee growth, characterized by downlap onto the initial sheet-flow type deposits, apparently resumes once an equilibrium profile is reestablished. Sections of high sinuosity along the present-day channel are located up slope of avulsion sites, indicating channel gradient readjustment after the avulsion events. Avulsions are the result of an imbalance in the delicate equilibrium between channel progradation/aggradation and the resulting channel morphology, and pulsating turbidity current activity. This could be an important process in reworking and transporting coarse-grained sediments in a mud-rich fan.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)