High-resolution stratigraphy of peritidal microbial carbonates from the Lagoa do Jacaré Formation, Bambuí Group, north of Minas Gerais state, Brazil

Abstract

High-resolution stratigraphic analysis was carried out on tidal microbial carbonates from the middle part of the Lagoa do Jacaré Formation from the Ediacaran-Cambrian Bambuí Group in the Ubaí area. In a 41 meter-thick section, sixteen lithofacies and three facies associations were interpreted as deposits of supratidal, intertidal, and subtidal settings. The tidal cycles are asymmetric and shallow upward, the high-resolution stratigraphic approach allowed inferences as to short-term and long-term fluctuations of sea level during deposition. The metric tidal ranges here described suggest strong tidal currents, which is incompatible with the giant enclosed sea model as an analog for Bambuí deposition, given that tidal ranges in suc seas are null or less than a few decimeters. Therefore, we suggest that the Ubaí carbonates were deposited in a semi-enclosed epicontinental sea subject to strong tidal forces when the formerly resticted Bambuí Basin reopened probably during deposition of the upper half of the Lagoa do Jacaré Formation, as suggested by previous isotopic data and now reinforced by our sedimentologic and stratigraphic field data.