Abstract: Evolution of Marginal Plateau: DSDP Data from Site 356 on Sao Paulo Plateau (Brazilian Continental Margin)

N. Kumar, L. Gamboa, P. R. Supko

Site 356 was drilled on the southeast corner of the Sao Paulo Plateau, a prominent marginal plateau in the southwest Atlantic. The sequence drilled consists of open-marine Albian limestone, Turonian-Coniacian mudstones, Coniacian-Maestrichtian marly chalks, and Cenozoic pelagic calcareous and siliceous sediments. The Albian limestone is underlain by Aptian evaporites which were not penetrated. Five major reflectors, ranging in age from late Albian to Oligocene-Miocene, were drilled and dated. These reflectors have been mapped across the plateau to the slope. The oldest reflector marks the top of the late Albian limestone sequence. A Coniacian-Santonian reflector marks the top of the mudstone sequence. A middle Maestrichtian reflector marks the top of the marly chalks, and two younger reflectors (early Eocene and Oligocene-Miocene) mark diagenetic changes within the Cenozoic pelagic sequence.

Geophysical data indicate that the plateau is underlain by 2 to 3 km of terrigenous and calcareous sedimentary deposits. This sequence overlies a similar thickness of Aptian evaporites. Since Albian time the plateau has been a major depocenter which was bounded on the south and on the north by fracture zones. The steep escarpment at the southern edge of the plateau is an expression of the southern fracture zone. This fracture zone also marks the northern boundary of the Rio Grande Rise and appears to be related to an arch on the continental shelf. Although the northern fracture zone does not have a topographic expression in the region of the plateau, its location is delimited by offset magnetic anomalies. Landward of the plateau this fracture zone appears to connect with east-west-tre ding faults mapped on the continental shelf and inland areas.

The stratigraphy of the Santos basin, on the Brazilian shelf adjacent to the Sao Paulo Plateau, can be correlated directly to the stratigraphic sequence drilled at Site 356. Aptian evaporites are present in the basin as well as under the plateau. During the Albian, shallow-platform limestones were deposited in the basin, and open-marine limestones were deposited on the plateau. A phase of sedimentary progradation during Turonian-Coniacian time deposited coarse conglomerates in the basin, and distal turbidites on the plateau. A major transgression in the Maestrichtian halted terrigenous sedimentation on the plateau and did not allow terrigenous sediments to go beyond the Santos basin. Cenozoic sediments in both areas are open-marine. This correlation of the sedimentary history of the S ntos basin and Sao Paulo Plateau suggests that the sedimentation in these regions has been controlled by the same major structures.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA