The narrow, shallow, low-accommodation shelf of central Brazil: Sedimentology, evolution, and human uses

Abstract

The continental shelf off the coast of central Brazil, extending from 10 to 16°S, is unusually narrow (~20km) and rests on the São Francisco craton. The shelf break is located between the 45 and 50m isobaths and coincides with major hinge-lines of the marginal basins. The shelf was exposed for most of the Quaternary period, particularly during the last 1my, when the average sea level was −62m. Submarine geomorphology is strongly influenced by this extended sub-aerial exposure and reduced subsidence, resulting in widespread incisions on the shelf. During the limited episodes of shelf inundation, as is the case today, a few meters of non-framework grain assemblages dominated by coralline algae accumulated on the outer shelf, while quartz sands were restricted to water depths of less than 10–15m. Mud accumulation on this unusually shallow shelf is aided by additional accommodation space provided by incisions and canyon heads indenting the shelf. Artisanal fisheries, targeting high-value commercial species associated with hard bottoms located on the outer shelf and shelf break, are the most important human use of this shelf. Data used in this study have been compiled from theses and previously conducted surveys and consist of four piston cores, 509km of chirp subbottom profiles and side scan recordings, and 711 bottom grab samples that have been analyzed for various textural and compositional aspects.