MARINE GEOLOGY OF THE BRAZILIAN CONTINENTAL MARGIN AND ADJACENT OCEANIC BASIN BETWEEN THE LATITUDES OF 23 DEGREES AND 37 DEGREES

The Sao Paulo Plateau is a broad marginal plateau underlain by thick evaporite deposits. A comparison of the sedimentary facies of the plateau with those of the Santos Basin, an adjacent shelf basin, indicates that the two regions are structurally continuous. During the Aptian, the Sao Paulo Ridge and Florianopolis High profoundly affected the marine circulation within the newly created oceanic basin, open marine conditions prevailed south of these features; whereas, to the north of this barrier thick evaporite layer was deposited. The Sao Paulo Ridge, the Florianopolis High and the ridges and troughs to the north of the Rio Grande Rise, define the Rio Grande Fracture Zone, an important tectonic trend in the South Atlantic. The location of the Rio Grande Fracture Zone was probably controlled by the existence of a former zone of weakness in the continent which was reactivated during the separation of South America and Africa.;The sedimentary evolution of the Vema Gap and southern Brazil Basin was reconstructed by the analysis of seismic-reflection data. A major erosional event, marked by a 20-25 my hiatus, occurred between the early Eocene and late Oligocene, and appears to mark the initiation of flow of the Antarctic Bottom Water (AABW) through the Vema Gap and Brazil Basin. In the late Oligocene, large amounts of sediments were deposited in these areas by the AABW and the Vema Channel was delineated by the infilling of the Vema Gap. Sediments that bypassed the Vema Gap, mainly through the Vema Channel, formed a large fan-shaped deposit at the southern part of the Brazil Basin.;The Rio Grande Rise is a complex aseismic rise consisting of distinct units separated by a narrow abyssal plain. The western portion of the rise is an area of oceanic crust older than 80 my on which younger intraplate volcanism occurred during the Eocene. In contrast, the eastern portion of the rise is a linear feature trending N-S, parallel to the axis of the Mid-Atlantic Ridge, and is inferred to be an abandoned spreading center.