C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

Abstract

High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambui Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal σ13C values between -1.0 and 0%, and σ18O values between -12.0 and -8.0%. However, some dolostones demonstrate σ13C values varying from +2.8 to -6.8%, highly radiogenic 87Sr/86Sr ratios (>0.7111), and low Sr concentrations (<350 ppm) related to post-depositional processes. In contrast, pure limestones at the top show very positive σ13C values between +8.3 and +12.8%, σ18O values between -10.0 to -6.0%, and 87Sr/86Sr ratios from 0.7073 to 0.7086, with high Sr concentrations (>900 ppm). They are linked to depositional controls on the carbonate platform, such as fluvial and/or submarine water influx, in which carbonates deposited on the proximal sector exhibit significant Sr isotopic variations and those on the distal sector were not subject to such controls, resulting in very homogeneous Sr isotope profiles. However, 87Sr/86Sr ratios of the distal carbonates are less radiogenic than carbonates expected for late Ediacaran (∼0.7085). This discrepancy suggests a restricted marine basin without Sr isotopic homogenization with contemporary oceans and, in this case, global correlations based on Sr isotope stratigraphy are not reliable.