A tide influenced braided delta in a flexural rift margin, the Mesoproterozic Lenheiro Sequence, São João del Rei basin, southern São Francisco Craton, Brazil

Abstract

The Mesoproterozoic Tiradentes Formation incorporates three shallow marine sequences followed by a deltaic to fluvial sequence, the Lenheiro Sequence. Facies analysis, grain size, bed thickness and stacking patterns, along with paleoflow data reveal that the deltaic section of the Lenheiro Sequence includes seven facies associations distributed in two intervals. The lower interval is a ca. 20 m thick succession consisting of pelite and minor sandstone that comprises the facies association FA1. This facies association is interpreted as distal shoreface-offshore deposit in a retrogradational stacking pattern. The upper interval includes at least 160 m of pelite, sandstone, pebbly sandstone and minor conglomerate with a coarsening and thickening upward stacking pattern. Six facies associations (FA2 to FA7) were recognized: prodelta, distal and proximal delta front, distributary channels of the lower and upper delta plain, and abandoned braid-channel in the upper delta plain. Facies analysis allows interpretion of the succession as a tide influenced braid delta. Tide influence is weak in the distal delta front, increases in the upper delta front, decreases in the lower delta plain, and does not affect the upper delta plain. Paleoflow data indicate progradation towards south. Northward directed paleoflow records the flood tide that reworks deposits of the delta front. The distal shoreface-offshore facies association (FA1) of the Lenheiro Sequence covers abruptly and conformably the lagoon-tidal flat facies association of the underlying shallow marine sequence. The gap between facies associations records a rapid creation of accommodation space related to rift tectonics. The sharp boundary between the two sequences is a flooding surface that in turn is overlaid by a thin transgressive system tract (FA1) formed in response to the sudden generation of accommodation. The upper interval, interpreted as a tide influenced braided delta system (Lenheiro Delta), was developed as a highstand system tract in a period of relative tectonic quiescence. The data reveal that the studied interval is a syn-rift sequence located in a flexural margin of a Mesoproterozoic rift.