ABSTRACT: Proposed Terminology for Sedimentatry and Acoustic Deep-Sea Fan Elements Based on Studies of Amazon Deep-Sea Fan

DAMUTH, JOHN E., Mobil Research and Development Corp., Dallas, TX, ROGER D. FLOOD, Marine Sciences Research Center, S.U.N.Y., Stony Brook, NY, PATRICIA L. MANLEY, Middlebury College, Middlebury, VT, and CARLOS PIRMEZ, Lamont-Doherty Geological Observatory, Columbia University, New York, NY

Our studies of the Amazon deep-sea fan have utilized a variety of high-resolution acoustic techniques to define and understand the sedimentary units that comprise the fan. We have utilized a number of terms to describe certain sedimentary and acoustic units that form distinct seismic and architectural elements of the fan or to describe depositional processes. Because similar units and processes are pervasive on other fans of all sizes, we propose that the terminology used for Amazon fan be considered for incorporation and use in any standardized terminology that may be developed for fan nomenclature. Some of the more important terms are as follows.

A channel-levee system is the basic element of the fan and includes a single distributary channel and its associated aggradational deposits. These deposits include the sediments deposited within the channel floor (including structures produced by downcutting related to downstream avulsion), as well as the associated natural levees formed by overbank deposition. On seismic, zones of high-amplitude reflections (HARs) often mark the aggrading channel axis and may be related to either sand deposition within the channel or acoustic artifacts (side-swipe) from the channel floor. Channel-levee systems are the main architectural elements of the upper and middle fan; however, younger channel-levee systems cut into older systems on the upper fan, whereas the younger channel-levee systems succes ively onlap the flanks of the older systems on the middle fan and do not erode them. Middle-fan channels generally show high sinuosities (up to 2.6) and are termed meandering if the sinuosity exceeds 1.5.

An active channel-levee system shifts to a new position by avulsion. Near the lower portion of the middle fan, avulsion of a channel creates a distinctive, flat-lying high-amplitude reflection packet (HARP) as a result of rapid deposition immediately following the avulsion event. The HARP extends downfan where it merges with high-amplitude reflections on the lower fan and forms the base of the new channel-levee complex created by the avulsion event. The lower

fan is characterized by the absence of large levees associated with channels; however, most channels still meander. Slope-sinuosity relationships on the middle fan suggest that the meandering channels are at grade; however, this relationship changes on the lower fan and suggests a possible change in depositional processes. Thus the middle-to-lower-fan boundary shifts with time, moving upfan when HARPs develop, then moving back downfan as the channel-levee system builds progressively downslope.

Through time, channel-levee systems successively formed through avulsion processes will onlap, overlap, and stack to form a channel-levee complex. Channel-levee complexes are especially important architectural elements of the middle fan, where they are often separated from one another by extensive mass-transport deposits (debris flows), which tend to bury previous channel-levee complexes and level the fan relief. The subsequent development of new channel-levee systems across this surface initiates the formation of a new channel-levee complex.

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