Contribuicao ao estudo paleoecologico da bacia da foz do amazonas

Abstract

Paleoecological studies of the Amapa Basin, based principally on foraminifers, have shown the evolution of the depositional environments of the area. Statistical methods applied to the microfossils present in side-wall cores and conventional cores yielded the paleoecological parameters used in the reconstruction of the depositional history of the basin during Tertiary time. The experience gained from the quantitative study of the cores was applied to cuttings. Although the latter were studied by qualitative methods only, the results were excellent. The paleoecological parameters permitted an environmental interpretation in terms of water depths and physiographic features, such as the location of the shelf edge, lagoons, etc. The qualitative analyses showed the presence of four faunistic facies, here called Paleoenvironmental Units. They are the following: Unit 1, whose fauna is dominated by foraminifers of the genera Liebusella, Textulariela, Rotalia and Quinqueloculina, besides bryozoans and corals; Unit 2, characterized by, i.a., Uvigegerina, Bulimina, Eponides and Lenticulina; Unit 3, characterized by, i.a., Amphistegina, Miogypsina and Nummulites; and Unit 4, with, i.a., Cyclammina, Clavulina and Ammodiscus. Also the relative abundance of planktonic foraminifers was used in the definition of the paleoenvironmental units. Sections and paleoenvironmental maps of chronostratigraphic units have shown that in the period of time from the Late Paleocene to the Middle Miocene the area was dominated by a calcareous platform, with the subenvironments of inner and middle shelf and outer shelf/continental slope, the mutual limits of which oscillated because of small variations in the level of the sea. At the end of Miocene time a regressive phase began, caused by a considerable increase in the influx of terrigenous sediments prograding continually over the carbonate platform. This phenomenon is attributed to the beginning of the Andean orogeny.