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Felipe Torres Figueiredo, University of São Paulo (Brazil)
Renato Paes de Almeida, University of São Paulo (Brazil)
Liliane Janikian, University of São Paulo (Brazil)
Cláudio Riccomini, University of São Paulo (Brazil)
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Extreme models for a whole ice-covered Earth has been recently suggested for Neoproterozoic on the basis of a low-latitude glacial hypothesis, supported by presumably coeval sedimentological record in several cratonic areas around the world. This record almost invariably includes a diamictite-carbonate succession of variable thickness interpreted as a paradoxical association of glacial and post-glacial deposits. Nonetheless, this global correlation is currently mainly based on isotope stratigraphy of post glacial cap carbonates of global distribution. The evidence for a low-latitude distribution is only supported by one very reliable paleomagnetic pole obtained for Elatina Formation.
The lack of detailed work on glacial deposits itself makes relevant the study of depositional environment and provenance analysis of the Bebedouro Formation in order to evaluate these models. This unit crops out at the northern portion of São Francisco craton (Una-Utinga, Salitre and Irec sites) and comprises glaciogenic deposits at the base of the Una Group, considered one of the representative units of Neoproterozoic climatic transition events.
Facies and architectural element analysis were carried out on Bebedouro Formation in order to provide additional information regarding depositional environment and paleogeography. In addition, a comparative study of provenance between diamictites from Bebedouro Formation and surrounding basement rocks age allowed evaluation of recent glacial models for ice cover dynamics, whether they were spanned latitude or localized altitude glaciers.
Facies analysis allowed the recognition of two facies associations: (1) rain out deposits including massive and pebbly sandstone and massive diamictite, interpreted as cohesive sub-aqueous debris flows, and (2) massive to cross-bedded pebbly sandstone grading to very thick mudstone and ritmite with outsized clasts interpreted as derived from decantation processes and scarce turbidity current events in an interglacial marine setting.
Stratigraphic correlation among the three studied sites points to a predominance of rain out deposits in the northern portion where 8 m thick mudstone are overlain by 9 m of diamictite (the maximum thickness identified in the region). The identification of a thick, over 50 m mudstone succession overlying turbidite deposits suggests that Una-Utinga and Irec depositional environments were susceptible to turbidity currents and ice-rafting probably within a deep marine basin.
Provenance results from macroscopic lithotype counting at each depositional site indicates that deposits of the Una-Utinga are oligomitic and were likely controlled by local source areas whereas at the Salitre deposits are polymictic and show source area variability. These interpretations and the evidence of more than one diamictite level suggest more than one glacier advance and retreat event, indicating a particular Neoproterozoic climatic and sedimentological dynamics in the studied region.
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