The shallow structure of two coastal basins or embayments along the continental margin of southern Brazil and Uruguay was investigated by geophysical techniques during a cruise of the U.S. Coast and Geodetic Survey Ship Oceanographer in late 1966.

Deformation of the continental shelf and slope sediments in the first of these basins, the São Paulo Embayment, situated between Rio de Janeiro and Florianópolis, suggests the presence of diapiric intrusions. Grabens developed above diapirs are reflected as topographic valleys on the continental slope and as zones of contemporaneous faulting on the outer shelf where sedimentation has kept pace with deformation. The São Paulo Embayment subsided sufficiently from Late Cretaceous through Pleistocene time to accommodate a series of three prograded sedimentary wedges with a cumulative stratigraphic thickness of at least 4 km. Each wedge of prograded sediment is presumed correlative with the onset of block-faulting, uplift, and rejuvenation of a mountain complex which borders the landward edge of the São Paulo Embayment. The axis of maximum sediment accumulation has migrated landward during the Tertiary.

In any reconstruction of pre-rift South America and Africa, the São Paulo Embayment lies immediately to the south of the Cuanza Basin of Angola. These two coastal basins have Mesozoic similarities but, beginning in the Tertiary, their patterns of marine sedimentation diverge; this fact is consistent with proposed Cretaceous rifting of this portion of the two continents.

The Pelotas Basin may be traced from beneath the coastal lagoons in southern Brazil and Uruguay part-way across the continental shelf. The continental slope off the Pelotas Basin consists of a large conical apron of sediment; the deeper strata are arched mildly and younger sediments thin over structural highs. The relationship of the cone to the Pelotas Basin is not clear.

Although submarine canyons are numerous and well-developed north and south of the study area, only fault-controlled valleys are known along the southern Brazil margin. Outcrops of continental shelf strata are rare along the slope; progradation and slope-conformable sedimentation are the rule.

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