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Título

EVIDENCE OF ACCELERATED CHEMICAL WEATHERING DURING DEPOSITION OF CAP DOLOMITES AFTER MARINOAN GLACIATION IN THE BAMBUÍ GROUP (EDIACARAN-CAMBRIAN)

Texto do resumo

The Bambuí Group comprises a carbonate-siliciclastic sequence deposited during the Ediacaran-Cambrian in the context of marine transgression and greenhouse conditions after the Marinoan global glaciation event (ca. 636 Ma). The cap dolomites of the Pedro Leopoldo Member, Sete Lagoas Formation, Bambuí Group were deposited in a post-glacial context. It is globally correlated and can record biogeochemical conditions of importance for the emergence of complex Ediacaran life. In this study, sedimentological, stratigraphic, chemical and isotopic data are reported from the basal section of stratigraphic hole 1-PSB-14, drilled in the upper part of Januária, north of Minas Gerais state. The geochemical data were analyzed under two digestion methods: 1) partial digestion with 0.5M HNO3; 2) total digestion with HNO3+HCl+HClO4+HF. Partial digestion reflects only the elements incorporated in the carbonate fraction. The total digestion includes detrital components that are widespread in the dolomites. Four lithofacies are recognized in this section (from base to top), namely: laminated dolostone with peloids and siliciclasts (LD), dolomitized mudstone (DM), reddish calcitic microbialite (RCM) and laminated mudstone (LL) with the presence of pseudomorphs of aragonite at the top of the section. δ13C isotopic data showed negative values with progressively decreasing values, from -3‰ reaching up to -4‰. The δ18O curve showed a stable behavior with values in the order of -6‰ during the deposition of the LD and DM facies and suggests the preservation of primary seawater values. The RCM facies presented δ13C values of -5‰ and δ18O of -10‰. An abrupt rise in the δ13C curve with values of 0‰ is recorded in the LL facies. It is associated with δ18O values in the order of -6‰. Total chemical digestion showed a contribution of detrital elements with K values between 0.56 and 3.25%; Al between 0.83 and 4.29% and Ti between 0.04 and 0.28%. Nevertheless, there is a strong statistical correlation between detrital elements (K, Al and Ti), redox-sensitive elements (RSE) and bionutrients such as V, U, Zn and P (r2 > 0.7) in the LD facies, which suggests a continental contribution during the deposition of the cap dolomites. The distribution of rare earth elements (REE) in partial digestion indicates a bell-shaped pattern, that is, enrichment of medium REE in relation to light and heavy ones. This characteristic signature of the Marinoan cap dolomites can be explained by the incorporation of medium REEs through minerals rich in Mn and P. This pattern reinforces the global correlation of the dolomites of the Bambuí Group with other chronocorrelated units. However, the total digestion shows a distribution of REE characteristic of clay minerals, reinforcing the interpretation of detrital input during the deposition of cap dolomites. The initial data presented here reinforce the nutrient input from the source area and the alkaline ocean model during the termination of the Marinoan glaciation.

Palavras Chave

Cap carbonate; alkaline ocean; Marinoan Glaciation