NEW INSIGHTS TO HIGH-RESOLUTION STRATIGRAPHY ON THE AMAZON CONTINENTAL-SHELF

The surficial stratigraphy (upper 20 m) of the Amazon subaqueous delta has been well documented, but the high-resolution record of underlyin g strata was unknown prior to this study. A Geopulse seismic system wa s utilized to obtain new information about the preservation potential of sedimentary deposits on the Amazon shelf. Widespread distribution o f methane gas was documented on the inner shelf, and prevented deep pe netration of the seismic signal. On the outer shelf, high-resolution ( <1 m) records were obtained with penetration to depths of similar to 1 00 m. Repetitive sequences (25-30 thick) were observed and interpreted as containing layers of transgressive sand (similar to 8 m thick) and muddy regressive sediment (similar to 20 m thick) formed as lower-for eset and bottomset deposits. This interpretation suggests that net acc umulation of sediment on the Amazon shelf (through cycles of sea-level change) selectively preserves a large fraction of sand (relative to t hat delivered by the Amazon River), and that the muddy strata preserve d are primarily from the lower portion of the subaqueous deltaic seque nce.