Abstract
Facies analysis, aided by X-ray diffraction, QemScan analysis, U-Pb detrital ages and field geological data are used in a detailed sedimentary characterization of the Arkose Level, an immature sedimentary unit that occurs abruptly and discontinuously in the upper Paranoá Group, a sequence of mature marine rhythmites deposited in a shallow cratonic margin basin. This basin was developed during the Mesoproterozoic in the western margin of the São Francisco Craton, central Brazil. The study reveals three facies associations in the Arkose Level: (1) Facies Association I (C), (2) Facies Association II (MC) and (3) Facies Association III (FM). The lithofacies are characterized by plane-parallel lamination and planar cross-bedding, trough cross-stratification, channel-fills, overturned cross-stratification, and convoluted structures. The sedimentological and paleocurrent data acquired in this study leads to the interpretation that this unit represents a record of braided-river deposit, which was influenced by marine reworking in the uppermost portion. The depositional model comprises a transitional continental-marine environment, with channels in the continental domain interpreted as a high-energy braided river system around the margins of a shallow intracratonic basin during the Mesoproterozoic. Compiled and newly acquired LA-ICP-MS detrital zircon U-Pb ages reveal a new source area contribution of 1.1 Ga in the Upper portion of the Paranoá Group. The abrupt deposition of braided-river deposits amid the mature Paranoá basin is most probably associated with local faulting, responsible for exposing the crystalline basement and the sedimentary rocks of previous basins, providing new and near-source areas of crystalline basement rocks. The inferred faulting can be associated to a Stenian unsuccessful rifting event, or to the early stages of opening the Goiás-Pharusian ocean, in the Tonian.
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