Estratigrafia do Grupo Bambuí e coberturas fanerozóicas no vale do rio São Francisco, norte de Minas Gerais

Abstract

This work presents the results of extensive geological mapping over 40.000 km² in northwestern Minas Gerais. Basement rocks (mostly Archean gneiss) crop out locally. The most important rocks ocurring in the study area are: carbonate and terrigenous rocks belonging to the Neoproterozoic Bambuí Group, Cretaceous sandstones of the Urucuia Group and Cenozoic sediments. The distribution of the Bambuí Group in the area is the result of the paleogeography of the basement and also of tectonic processes. Basement exposures were identified in paleo-highs (Alto de Januária) and local depocenters in the southern and eastern portions of the study area. The thickness of the Bambuí Group increases substantially in these areas. The Sete Lagoas Formation, represented by a shallow platform succession, is made up of limestones and dolomites. This unit crops out, exclusively, in the western margin of the São Francisco River. Medium units of the Bambuí Group, Serra de Santa Helena formation (siltstones and dark limestones lenses) and Lagoa do Jacaré formation (oolitic/intraclastic limestones, siltstones and marlstones) are widely distributed in the study area. A detailed petrologic study of carbonates of the Lagoa do Jacaré formation around Lontra region (southern part of the study area) showed an important facies variability for this unit. Serra da Saudade Formation (siltstones and shales) and Três Marias Formation (sandstones, arkoses and siltstones) crop out only in the eastern and southern borders of the study area. Both units present different local depocenters as compared to the lower formations.