Lacustrine fan-deltaic sedimentation in a Proterozoic rift basin: the Sopa-Brumadinho Tectonosequence, southeastern Brazil

Abstract

The Sopa-Brumadinho Tectonosequence (100 to 200 m thick) is the fourth unit in ascending order of the up-to-2000 m thick Mesoproterozoic Espinhaço Supergroup, representing the depositional products of the third and main phase of the rift stage of the Espinhaço basin. The Sopa-Brumadinho Tectonosequence in the study area consists of diamond-bearing lacustrine fan-delta deposits, bounded by prominent angular and erosional unconformities. Detailed lithofacies studies were carried out in the central part of the southern Serra do Espinhatço, southeastern Brazil, to characterize the paleogeography and basin-fill architecture of the Sopa-Brumadinho Tectonosequence. Thirteen lithofacies are present: massive clast-supported conglomerates (lithofacies UGCS), inversely graded clastsupported conglomerates (lithofacies IGCS), normally graded clast-supported conglomerates (lithofacies NGCS), massive to crudely stratified clast-supported conglomerates (lithofacies CSCS), sandy matrix-supported conglomerates (lithofacies S-MS), muddy matrix-supported conglomerates (lithofacies M-MS), talus-breccia clast-supported conglomerates (lithofacies TB-CS), massive sandstones (lithofacies Sm), horizontal stratified sandstones (lithofacies Sh), trough cross-stratified sandstones (lithofacies St), planar cross-stratified sandstones (lithofacies Sp), graded-stratified sandstones (lithofacies Sg), and pelites (lithofacies P). They represent the products of several types of subaerial and subaqueous sediment gravity flows (ranging from turbidity currents to cohesive debris flows) and ephemeral braided streams. The deposits are usually arranged in coarsening- to fining-upward tectono-depositional intervals. Each tectono-depositional interval represents a progradational lobe, which starts at the base with lacustrine pelites and evolves with time from distal (subaqueous) to proximal (subaerial) characteristics. The fining-upward part represents the abandonment phase of the depositional lobes. The origin, characteristics, distribution and spatial arrangement of the various lithofacies, the interbedding of gravityflow and stream-flow deposits, the predominance of debris-flow conglomerates over aqueous current-formed ones, the semi-radial, fan-like dispersion pattern of the paleocurrents, and the recognition of associated subaerial and subaqueous depositional settings are all indicative of a lacustrine alluvial-fan/fan-delta environment, or a fan-delta system. An interplay between several intrabasinal and extrabasinal controls probably determined the fan evolution. The sedimentation of the lacustrine fan-delta systems of the Sopa-Brumadinho Tectonosequence probably took place in a N-S elongated continental rift basin, compartmentalized by N-S trending normal faults and W-E trending transfer faults, under a tropical semi-arid climate. The basin-fill architecture was probably determined by episodic subsidence, clockwise block tilting and asymmetric-graben evolution, as well as by inherited characteristics of the source area. Additionally, a new descriptive scheme for classification of conglomerate lithofacies is here proposed.