The Cretaceous Sanfranciscan basin, eastern plateau of Brazil

Abstract

The Cretaceous Sanfranciscan Basin, located in the eastern plateau of Brazil, is one of the most impressive continental basin of the country, because of its complex and exceptional association of sedimentary and volcanic rocks. The presence of macro and microfossils, large diamonds and a surprising occurrence of a radiolarian fauna present in chert layers intercalated in eolian sandstones form very interesting topics on the tectonic and paleogeography aspects of the Gondwana during the Cretaceous. The basin has a south-north elongated shape, from the west of Minas Gerais State (latitude 20° S) to the southern Tocantins, Maranhao and Piaui States (latitude 10° S). It is approximately 1100 km long and has a maximum width of about 270 Km (Fig 1). It occurs in a corridor between 44° and 47° of longitude W. It was filled with Areado and Mata da Corda Groups, and lies over, from south to north, low-grade metamorphic sediments of the Bambui Group (Upper Proterozoic), Permo-Carbonifeous glaciogenic sediments of Santa Fe Group, Archean granitoids of the regional basement and sedimentary rocks of the Paleozoic Parnaiba Basin, in its outermost northern border. The early Cretaceous Areado Group has 200 m of maximum thickness. It lies over older rocks along a regional disconformity and comprises a basal conglomerate with transported ventifacts deposited in a desert environment, associated to fine-grained lacustrine sediments and eolian and fluviodeltaic sandstones of the Tres Barras Formation. The late Cretaceous Mata da Corda Group has 230 m of maximum thickness. It lies over the Areado Group along a local unconformity, and is composed by ultramafic alkaline rocks and volcaniclastic rocks of the Patos Formation. Above the latitude 17°S until the north of the basin, occur the Urucuia Formation, considered by some authors as part of this formation (Braun 1970, Grossi Sad et al. 1971). It consists of a basal conglomerate with transported ventifacts, lacustrine, eolian and fluvio deltaic sediments. This sequence is very similar to those of the Areado Group and, as will be shown later, there is the possibility of this formation to be part of the early Cretaceous Areado Group and not part of the Mata da Corda Group.