Tracking connection and restriction of West Gondwana Sao Francisco Basin through isotope chemostratigraphy

C and Sr isotope compositions of carbonate rocks from the intracontinental Sao Francisco basin can track ocean connections and restriction. The lower three formations of the Bambui Group can be grouped into three chemostratigraphic intervals (CI), recording different evolution stages of the basin. Lowermost CI-1 comprises the basal cap carbonates of the Sete Lagoas Formation displaying an initial C negative excursion, followed by a coeval C and Sr positive excursions (delta C-13 values from -5 to 0% and Sr-87/Sr-86 ratios from 0.7074 to 0.7082) in 10 m of stratigraphic record. It marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression. CI-2 comprises carbonates of the middle portion of the Sete Lagoas Formation with delta C-13 values around 0% and Sr-87/Sr-86 ratios around 0.7082 that matches those observed worldwide for the Late Ediacaran. It records the onset of a Gondwana sea pathway connecting several epicontinental basins, allowing migration of index-fossil Cloudina sp. Uppermost CI-3 starts after a major positive excursion in the delta C-13 values reaching + 16% and a steepened decrease of Sr-87/Sr-86 ratios to 0.7075 which are lower than those expected for the Ediacaran-Cambrian boundary. This interval comprises the upper Sete Lagoas, Serra de Santa Helena and Lagoa do Jacare formations and records the end of the connection of the Sao Francisco basin to the Gondwana sea pathway setting a restricted epeiric sea. Restriction was probably caused by Late Ediacaran uplifting of orogenic belts surrounding the basin. Other West Gondwana Cloudina bearing units also display the same mismatch in the Sr isotope ratios, suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.