A Study of Channel Patterns and Morphology of Amazon Deep-Sea Fan Using Long-Range Side-Scan Sonar (GLORIA): ABSTRACT

Abstract

The Amazon Cone is a large deep-sea fan off northeast Brazil. Previous studies utilizing 3.5-kHz echograms, seismic profiles, and piston cores show that the modern fan is characterized by a complex of leveed distributary channels which are bounded to the east and west by large slump/debris-flow deposits. Seismic data show that older channel-levee complexes extend beneath these debris flows. We recently conducted a detailed study of the modern distributary-channel system of the fan using the Institute of Oceanographic Sciences (England) long-range side-scan sonar instrument (GLORIA). We were able to continuously insonify a 20 to 30-km wide swath of sea floor along each ship track; thus we were able to continuously map many of the distributary channels of the upper and middle fan for distances of a few tens to 150 km. These sonographs suggest that the central leveed channel on the uppermost fan arises within the floor of the lower Amazon Canyon near the 1,500 m isobath. This central channel leads into at least four leveed channels between 1,500 and 2,500 m. On the middle fan between 2,500 and 4,300 m, these channels divide into additional distributaries. Differences in acoustic reflectivity between channels as well as discontinuity of some channels suggest that certain channels have become inactive and abandoned, and were subsequently partly buried by overbank deposits. This is confirmed by seismic and 3.5 kHz data. The apparent branching of some channels observed on the sonographs may thus be due in part to channel abandonment and avulsion. Most channels on the middle fan are characterized by continuous, highly developed meanders, much like those characteristic of late-maturity rivers on land. The number, tightness, and intricacy of the meanders generally increase from upper to middle fan. Meander migration scars, abandoned meanders, ch tes, neck cutoffs, and breaching of channel levees to form additional distributaries are observed. Portions of the large slump/debris-flow deposits which flank the distributary-channel complex are also visible on the side-scan sonographs. At some locations these debris flows appear to have crossed distributary channels and buried, obliterated, or diverted them. End_of_Article - Last_Page 561------------