Biostratigraphical dating of the early history of the South Atlantic Ocean

Abstract

The southern Atlantic has always been a favoured testing ground for the hypothesis of continental drift. Apart from the remarkable agreement in the geographical shape of the coast of western Africa and eastern South America, considerable attention has been paid to the origin of the Mid-Atlantic ridge and these factors have provided a basis for testing the concept of drift. Detailed studies of the geology of NE Brazil and Gabon indicate that both areas had been basins of non-marine sedimentation almost continuously from the Upper Palaeozoic through to the Neocomian. During the Neocomian it would appear that both areas were parts of a large freshwater lake which may have been situated in a zone of subsidence produced by an initial phase in the separation of the two land masses. This structure may have been similar to the Great Rift Valley system of today in East Africa. It would seem that the rift continued to widen during the Neocomian and made connexion with the open ocean during the Aptian, thus developing into a ‘protoatlantic’ similar in configuration to the present day Red Sea. During the latter part of the Aptian, salt deposits began to accumulate in the narrower parts of the elongated bays. The deposits in Gabon, Angola and Brazil are large and of economic importance. About this time South America seems to have begun a relative clockwise rotational motion, which in its later stages may have resulted in a fracturing and tearing movement of the crystalline basement rocks in the area bounded by the Ivory Coast and Maranhão. The point in time at which northern and southern arms of the protoatlantic became united may be ascertained by means of a biostratigraphical analysis, based mainly on the evidence provided by the ammonites of the critical sequences. The crucial area lies in a zone formed by the states of Rio Grande do Norte, Pernambuco, Alagoas, Sergipe and Bahia in Brazil, and the Ivory Coast down to Angola and Gabon in West Africa. The analysis of the Albian to Turonian invertebrate associations, in particular the dispersion of the genus Elobiceras and the vascoceratid, pseudotissotiid, mammitid and benueitan faunas shows that the final break between South America and Africa may be dated as upper Lower Turonian. Furthermore, the geographical dispersion of Turonian invertebrates shows that the rifting apart was accompanied by a periodic pattern of regressions and transgressions possibly brought about by oscillatory movements of the continental block.