Upper slope sand deposits: the example of Campos Basin, a latest Pleistocene-Holocene record of the interaction between alongslope and downslope currents

Abstract

Abstract Upper slope sand deposits constitute large accumulations of well-sorted very fine to coarse sand that are of great economic interest. A depositional model is here proposed based on hydrographic, physiographic and sedimentological characteristics of the modern Campos Basin margin, SE Brazil. Data comprise long-term bottom-current measurements, long and short cores, high-resolution seismic lines and side-scan sonar records. Results indicate that offshelf bed-load transfer of sand is driven by shelf bottom currents forced by combined oceanographic and meteorological factors, among them the ‘sea-floor polishing effect’, the rotatory sweeping of the sea bed by mesoscale eddies. The main conditions for development of upper slope sand accumulations are: (1) convex outer shelf-slope morphology; (2) sandy sediment available at the shelf edge; (3) net offshelf transport of shelf sands induced by shelf-edge bottom currents; (4) the presence of a relatively strong slope boundary current. Transfer to the slope occurs as successive, short-distance-flowing sand fluxes. The latter occur mainly through shelf-edge incisions and lead to elongated and narrow sand lobes on the upper slope. The convex morphology of the upper margin controls the Brazil Current (BC) flow and two main zones can be identified: a BC ‘funnelling zone’ where the current is accelerated, and a BC ‘expansion zone’ where the current has its speed decelerated. Reworking by the southward-flowing Brazil Current develops gravel lag deposits, erosional scours, and interchannel sand waves on the upper slope (between 200 and 350 m water depth). Downslope, the reduction of slope current intensity enhances the development of flat sand sheets, controlled by the northward flow of the Brazil Counter-Current. The upper Quaternary vertical facies succession is there characterized by a coarsening-to fining-up asymmetric sequence. Upper Pleistocene sand-mud intercalations are overlain gradually or with a sharp or erosive contact by lower Holocene coarse to finegrained clean sands that grade to upper Holocene fine-grained muddy sands toward the top.