ABSTRACT

Grain-size distribution on the continental shelf adjacent to the Amazon River reveals three detrital sedimentary deposits: an outer-shelf sand deposit (1.5-3.0 , well sorted, negatively skewed); an inner-shelf mud deposit (6-9 , poorly sorted, positively skewed); and a corridor extending from the river mouth across the inner shelf, in which mud is interbedded with sand. Modern mud covers most of the inner shelf, and surface sediment does not reveal along-shelf or across-shelf sorting of discrete particles. Size-sorting of particle aggregates (floccules, fecal pellets) may occur at the time of deposition but is difficult to evaluate. A lack of shoreward fining (reverse grading) c ntradicts the sedimentation model of Gibbs (1976). The model most consistent with textural observations is that of a modern subaqueous delta prograding over a relict basal sand (the outer-shelf sand). Modern sand in surface sediment is restricted to the corridor extending from the river mouth across the inner shelf. Ambient physical processes are not competent to transport this sand along-shelf. Subsurface sand layers within the inner-shelf mud deposit (i.e., outside the corridor) are similar in texture to surficial sand in the corridor of interbedded mud and sand. These observations may suggest that the location and/or size of the corridor has changed with time.