Slope sedimentary facies associated with Pleistocene and Holocene sea-level changes, Campos Basin, southeast Brazilian Margin

Abstract

The upper Quaternary section of Campos Basin continental slope was sampled by 56 piston cores and 5 HPC boreholes totalling 750 m of core. Muds comprise 77% of the cored section and were classified according to colour, which results mostly -from their carbonate and sulphide contents. In the uppermost part of the section (0.5–15 m), where less reducing conditions prevail, slope muds are brown and olive in colour and, respectively, pelagic and hemipelagic in origin. This packet shows a coarsening-upward pattern in the upper slope with Upper Pleistocene olive grey mud at the base, olive grey mud/silty-sand intercalations in the middle and Holocene muddy-sand at the top. As this packet thins considerably downslope, average sedimentation rates range from 26 to 2.0 cm/ka. The contact between olive and the underlying greenish muds is diachronous, occurring at 26 ka in the lower slope and 40 ka in the upper slope. The greenish muds are characterized by a wider range of carbonate content (3–50%) and disseminated sulphide content, including siliciclastic muds of greenish-black and dark greenish-grey colour, hemipelagic greenish-grey muds and pelagic light greenish-grey marls. Intercalated with the siliciclastic muds, shelf-spillover sands (bioturbated muddy sands and thin-bedded turbidites) occur in the upper slope and hemipelagic/pelagic muds in the lower slope. Upper-slope gullies and channels are locally filled with medium- to thick-bedded turbidite sands. Based on current measurements, and on the distribution and sedimentary structures in the bioturbated muddy sands, a reworking or sweeping action by bottom currents is inferred. During Pleistocene lowstands, darker siliciclastic muds, richer in sulphide, are more common denoting a higher terrestrial influence. Mass movement deposits, identified in the southern part of the study area, were also associated with eustatic lowstands. Four discrete slump units, with an average thickness of 50 m, cover an area of 1600 km'. Between these slump deposits, seismically well-layered units are formed by an alternation of dark greenish-grey and greenish-black muds. Palaeontological data and numerical modelling of excess pore pressures generated from water and sediment loading suggest that the emplacement of the youngest slump unit occurred over a 15-ka period from 68 ka to 53 ka. This deformation is inferred to result from accelerated creep triggered by the decrease in hydrostatic pressure during sea-level fall. During the last Pleistocene highstand, upper/middle slope sedimentation was depleted in plant debris denoting a weaker terrestrial influence and, on the lower slope, hemipelagic/pelagic sedimentation was more common.