Stratigraphy of the lower Bambuí group in the Arcos region (MG): a contribution from boreholes

Abstract

This study presents the first results of the sample analysis of two boreholes drilled in a quarry located in the Arcos region (MG). Petrographic analysis (macro- and microscopic observations) was performed and made possible to study in detail an approximately 175 meter-section, which shows the basement and 10 sedimentary units. The basement is represented by dark green to gray fine- to medium-grained granodiorite. At the base of the sedimentary sequence, Unit 1 is represented by a decimetric layer of polymictic diamictite and is overlain by the impure limestones of Unit 2. Unit 3 is carbonatic and contains calcilutite and layers of carbonaceous shale. This unit gradually changes to Unit 4, which is composed of marl and mudstone. Unit 5 is composed of a thick sequence of massive calcarenites, which begins to show lamellar layers and microbial structures in Unit 6. Unit 7 is similar to the previous one, but contains several intraclastic layers. The number of lamellar layers decreases sharply, giving rise to a thick interval with considerable granulometric variations in Unit 8, which grades to the oolitic calcarenite of Unit 9. At the top of the sequence, Unit 10 is a thick package of stromatolitic dolarenite. This stratigraphic arrangement allows the identification of progradational/retrogradational trends. The sedimentological features, especially those present in the basal portion, suggest that at least part of the carbonatic sequence could represent a cap carbonate, supporting a glacial origin for the diamictite. Other data also collected from the basal units suggest that changes occurred in the source area at the beginning of the filling of the basin.