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

Cristian Guacaneme,Antonio Carlos Pedrosa-Soares,Gustavo Macedo de Paula-Santos,Marly Babinski

doi:10.1590/2317-4889201720160126

Cristian Guacaneme, Antonio Carlos Pedrosa-Soares + Show 2 more

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https://doi.org/10.1590/2317-4889201720160126

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Abstract

ABSTRACT: High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambuí Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal δ13C values between -1.0 and 0‰, and δ18O values between -12.0 and -8.0‰. However, some dolostones demonstrate δ13C 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 δ13C values between +8.3 and +12.8‰, δ18O 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.

Highlights

Neoproterozoic carbonate sequences overlying glacial deposits, known as cap carbonates, have been widely used as regional and global chemostratigraphic correlation tools due to their characteristic negative carbon isotopic signature and lack of reliable biostratigraphy (Kennedy, 1996; Kaufman et al, 1997; Hoffman et al, 1998; Jacobsen & Kaufman, 1999; Hoffman & Schrag, 2002)
Two samples of the lower Sete Lagoas Formation were analyzed for Sr isotopes, the IN‐08 and Linha Verde (LV)‐06 samples from IN and LV sections, respectively
Carbonate rocks from Sete Lagoas Formation show lithological, geochemical, and isotopic variations, representing both depositional and post‐depositional processes on the carbonate platform

Summary

Introduction

Neoproterozoic carbonate sequences overlying glacial deposits, known as cap carbonates, have been widely used as regional and global chemostratigraphic correlation tools due to their characteristic negative carbon isotopic signature and lack of reliable biostratigraphy (Kennedy, 1996; Kaufman et al, 1997; Hoffman et al, 1998; Jacobsen & Kaufman, 1999; Hoffman & Schrag, 2002). It is identified as a cap carbonate of Sturtian age (Babinski et al, 2007; Vieira et al, 2007b) This unit shows strong negative δ13C values of ‐5‰ at the base that increase to 0‰ in the intermediate part of the sequence, and abruptly change to very positive values as high as +16‰ at the top, accompanied by alterations on the sedimentary facies. This enables the subdivision of Sete Lagoas Formation into lower and upper sequences separated by a positive carbon isotopic shift (Vieira et al, 2007b). This is consistent with the U‐Pb age of 560 Ma obtained from detrital zircon grains of the upper sequence, interpreted as the maximum depositional age (Paula‐Santos et al, 2015)

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