Stratigraphy of the Lower Bambuí Group in the Arcos Region (MG): a Contribution from Boreholes

This study presents the fi rst 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 detailan approximately 175 meter-section, which shows the basement and 10 sedimentary units. The basement is represented bydark green to gray fi ne- to medium-grained granodiorite. At the base of the sedimentary sequence, Unit 1 is representedby a decimetric layer of polymictic diamictite and is overlain by the impure limestones of Unit 2. Unit 3 is carbonatic andcontains calcilutite and layers of carbonaceous shale. This unit gradually changes to Unit 4, which is composed of marland mudstone. Unit 5 is composed of a thick sequence of massive calcarenites, which begins to show lamellar layers andmicrobial structures in Unit 6. Unit 7 is similar to the previous one, but contains several intraclastic layers. The numberof 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. Thisstratigraphic arrangement allows the identifi cation 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 fi lling of the basin.