Origin of Amazon mudbanks along the northeastern coast of South America

Abstract

Seismic profiles, sediment cores, and water column measurements were collected along the northeastern coast of Brazil to examine the origin of mudbanks in the Amazon coastal mud belt. These 10–60-km-long, shore-attached features previously had been observed to migrate along the 1200 km coast of the Guianas in response to wave forcing. CHIRP (3.5 kHz) seismic profiles of the shoreface and inner shelf located two mudbanks updrift of the previous eastern limit in French Guiana. 210Pb geochronology shows that these two banks are migrating to the northwest over a relict mud surface in 5–20 m water depth. The mudbanks are 3–4 m thick and are translating over a modern shoreface mud wedge deposited by previous mudbank passage in <5 m water depth. Initial mudbank development is taking place on the intertidal and shallow subtidal mudflats at Cabo Cassipore, associated with an alongshore-accreting clinoform feature. Sediment trapping in this area is controlled by the nearshore presence of strong water column stratification produced by the enormous Amazon freshwater discharge on the shelf and by proximity to the Cassipore River estuary. Seasonal and decadal periods of sediment supply and starvation in this area likely are controlled by variations in northwest trade wind intensity.