The Bebedouro Formation, Una Group, Bahia (Brazil)
Published:January 01, 2011
J. T. Guimarães, A. Misi, A. J. Pedreira, J. M. L. Dominguez, 2011. "The Bebedouro Formation, Una Group, Bahia (Brazil)", The Geological Record of Neoproterozoic Glaciations, Emmanuelle Arnaud, Galen P. Halverson, Graham Shields-Zhou
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The Bebedouro Formation (Fm.) is a Neoproterozoic glaciogenic succession at the base of the Una Group on the São Francisco Craton. The glaciogenic sequence is composed of diamictites, pelites and sandstones with a variety of lithofacies that are grouped into four associations: (i) ice-contact, (ii) pro-glacial, (iii) ice-rafted and (iv) aeolian (extraglacial). Thus, the Bebedouro Fm. is interpreted to have been deposited in a shelf marine environment where glacio-proximal sedimentation dominated. It variably overlies Palaeoproterozoic/Archaean basement and Mesoproterozoic metasedimentary rocks of the Chapada Diamantina Group.
The overlying carbonate succession (Salitre Fm.) is informally subdivided into five mappable units. Unit C, at the base, is composed of red argillaceous dolostone with typical negative δ13C signatures averaging –5.1‰ (VPDB). Unit B consists of grey laminated limestones that grade upward into grey dolostone with tepee structures of Unit B1; both have δ13C values around 0‰. The overlying Unit A consists of interbeds of greyish marl, shales and siltstones grading upward into massive black organic-rich limestone with oolitic and pisolitic beds of Unit A1. Positive δ13C signatures averaging +8.5‰ VPDB characterize the black limestones of Unit A1. 87Sr/86Sr ratios of well-preserved samples of these carbonate units show values of 0.70745 and 0.70765. These units can be correlated litho- and chemostratigraphically with formations of the Bambuí Group (São Francisco Basin) to the west.
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The Geological Record of Neoproterozoic Glaciations
In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harland’s (1981) Earth’s pre-Pleistocene Glacial Record. This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.