The Sao Tome deep-sea turbidite system (southern Brazil Basin):: Cenozoic seismic stratigraphy and sedimentary processes

The Sao Tome deep-sea turbidite system, elongated parallel to the rise of the south Brazilian continental margin, was first interpreted as a channel-levee system resulting from contour-current activity. Study of new seismic data permits the proposal of a stratigraphy for the system and a new interpretation of depositional processes. Three major depositional units have been recognized that are separated by major erosive discontinuities. The basal unit seems to be Paleocene to lower or middle Eocene, and the second one, subdivided into two subunits, is probably upper Oligocene to middle Miocene. Both units show superimposed north-to-south-channelized turbidite systems, with supply provided directly from a channel network that crosses the upper margin in the north. The third unit is upper Miocene(?) to Pliocene or Quaternary and is still under predominantly gravity processes: turbidite processes in the lower and upper subunits, and major mass-flow processes in the median subunit. The sediment sources are located either in the north or in the south, with sediment provided by major deep-sea channels. The base of the upper subunit is well marked by an erosive discontinuity (late Pliocene or Pliocene-Quaternary boundary). Impact of the contour currents is mainly recorded as widespread erosive surfaces (seismic discontinuities) correlated to global hydrological events and transparent or wavy deposits. Because this system contains a significant amount of upper Quaternary sands, it suggests the occurrence of petroleum reservoirs along the rise and the Sao Paulo Plateau in the lower continental slope.