Coastal and Marine Environments and Their Biota

Abstract

The southern Brazilian continental margin is characteristic of low intensity tectonic evolution processes (Villwock 1987), which levelled the basement, formed thick sedimentary layers, and led to the development of an extensive coastal plain (Fig. 6.1.1). The gently sloping continental shelf (2 m km^-1) between Santa Marta Grande Cape (28° 40’ S) and Chuí (34° 40’ S) has a width of 100–180 km (Martins et al. 1972) and reaches its largest extension between 31° and 33° 30’ S. Remnant valleys of ancient fluvial channels typically cut the shelf in this region. Characteristic morphological features of the shelf include marine terraces (Koswmann et al. 1977) and beachrock outcrops (Calliari and Abreu 1984), whilst linear sand ridges typically occur on the inner shelf (Figueiredo 1975). The transition between shelf and upper slope starts at 160–190 m depths at a rate of 20 m km^-1. The considerable width of the slope (up to 250 km near Cabo Santa Marta) as well as the gradual drop to approximately 3000 m depth suggest the dominance of depositional processes during its formation. At the central part of the Pelotas Basin, the slope resembles the shape of a cone, which extends from the outer shelf to about 4000 m depth. A well-developed continental rise with a maximum width of 400 km (30° S) extends between 2700 and 4000 m depth (Zembruscki et al. 1972). An important feature of the oceanic basin is the Rio Grande Rise, which extends in an east-west direction between the continental rise and the flanks of the Mid-Atlantic Ridge and separates the ocean basins of Brazil and Argentina (Palma 1984).