Stratigraphy and U–Pb geochronology of the basal units of the vazante group: A lateral correlation with the glaciogenic Jequitaí formation (Minas Gerais, Brazil)

Abstract

The basal units of the Vazante Group, the Santo Antônio do Bonito and Rocinha formations, present different stratigraphic records and younger ages than the rest of the group. These units are characterized by diamictites, sandstones, pelites and rare carbonates originated from subaqueous gravitational sedimentation. U–Pb geochronological data in detrital zircon indicate a Tonian maximum depositional age for both formations. The youngest zircon grains yielded ages ranging from 903 to 935 Ma. The main source for zircon is Paleoproterozoic (mainly Rhyacian and Orosirian) to Mesoproterozoic (Calymmian and Ectasian) areas being probably the São Francisco paleocontinent the main area. Due to the complete absence of ice-rafted debris within fine-grained sediments in the Santo Antônio do Bonito Formation, we interpret a gravitational reworking as the main process of diamictites deposition, probably during a period of ice retreat and sea level rise. The U–Pb data corroborate a lateral correlation between the Santo Antônio do Bonito and Rocinha formations with the glaciogenic Jequitaí Formation (Bambuí Group). From east to west, glacial sediments of the Jequitaí Formation were reworked probably by a slope-apron system during a late-glacial to post-glacial stage on the western margin of the São Francisco paleocontinent. Therefore, the Vazante Group can be divided into two successions: the upper Lagamar-Serra do Garrote formations and the lower Santo Antônio do Bonito-Rocinha formations. The Lagamar-Serra do Garrote sedimentation comprises a late Mesoproterozoic mixed carbonate-siliciclastic deposition in passive margins. The second succession is a gravitational slope-apron resedimentation and probably correlates with the Neoproterozoic glacial rocks of the Jequitaí Formation from the Bambuí Group. Finally, the collisional phase of the Brasília Belt was responsible for stratigraphic inversion, placing the older Lagamar-Serra do Garrote formations over the younger Santo Antônio do Bonito-Rocinha formations.