The Carrancas Formation, Bambui Group: A record of pre-Marinoan sedimentation on the southern Sao Francisco craton, Brazil

The Carrancas Formation outcrops in east-central Brazil on the southern margin of the Sao Francisco craton where it comprises the base of the late Neoproterozoic Bambui Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitai Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have delta C-13 values ranging from +7.1 to +9.6 parts per thousand, which is a unique C isotopic composition for the lowermost Bambui Group but similar to values found in the Tijucucu sequence, a pre-glacial unit in the Aracuai fold belt on the eastern margin of the Sao Francisco craton. The strati graphic position below basal Ediacaran cap carbonates and the highly positive delta C-13 values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high delta C-13 values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative delta C-13 excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitai Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of Sao Francicso craton. Documentation of Cryogenian interglacial strata on the Sao Francisco craton reinforces recent revisions to the age of Bambui Group strata and has implications for the development of the Bambui basin. (C) 2016 Elsevier Ltd. All rights reserved.