Environment and Biota of the Patos Lagoon Estuary

The Patos Lagoon is part of the coastal province which is geologically defined by the Pelotas Basin and the basement (Villwock 1978). The basement is characterized by the Sul-Rio Grandense shield, composed of crystalline Precambrian rock formations, and by Paleozoic and Mesozoic sedimentary and volcanic sequences of the Paraná Basin, both of which provided the sediments for the marginal Pelotas Basin. Owing to successive transgressions and regressions during the Cenozoic, clastic continental, transitional, and marine sediments of about 8000 m thickness accumulated in the Pelotas Basin. The emersed portion comprises the present flat coastal plain between 29 and 35° S which can geomorphologically be divided into an inner alluvial plain, the Lombas Barrier, the Guaíba-Gravataí Lagoon system, a multiple barrier complex, the Patos-Mirim Lagoon system, and the continental shelf (Fig. 4.1.1; Delaney 1965; Villwock 1984). Geomorphologically, the estuary of the Patos Lagoon is surrounded by tertiary and quaternary deposits of the low lands of the Patos-Mirim Lagoon system and the multiple barrier complex, which are characterized by eolian deposits, beach ridges, sand plains, stable and active dune fields, and lagoonal terraces (Fig. 4.1.2; Godolphin 1976; Zeltzer 1976; Paim et al. 1987).