Growth Processes of a Channel-Levee System on Amazon Fan
Carlos Pirmez and Patricia L. Manley
Cores and well logs, together with seismic reflection and swath bathymetry data reveal the growth dynamics of a meandering channel-levee system on Amazon Fan. The modern channel shows a smooth, concave-up talweg depth profile suggesting that the system maintains a dynamic equilibrium in which slope, channel cross-section and sinuosity are adjusted to prevailing flow conditions. The meandering channel-levee system is aggradational, with the channel perched above the surrounding fan surface atop thick channel and overbank deposits. Overbank deposits within a cycle of channel growth are characterized by an overall thinning and fining upward succession of thin-bedded turbidites, and an apparent stability of channel planform. Multiple levee aggradation cycles are stacked upslope of bifurcation sites. Channel avulsion coincides with an abrupt upward coarsening of the overbank turbidites. During avulsion, channel entrenchment and changes in planform geometry play a role in readjusting the sys em to the increased flux of sediment. Downslope of the bifurcation, sand-rich flows fill the interlevee valleys and form units up to 30 m thick, followed by rapid upbuilding of levees and concentration of coarse material within the channel confines.
Thin-bedded overbank turbidites accumulated at rates up to 2-3 cm/year, whereas avulsions occurred at intervals of 5 ka or less during the last glacial. While overall fan growth is modulated by sea-level changes, these high frequency changes in sediment delivery to the fan suggest that controls on sediment distribution and fan growth by factors other than eustacy are probably important in this large fan.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California