Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
6431287 | Journal of South American Earth Sciences | 2016 | 16 Pages |
â¢The Carrancas Formation (base of the Bambui basin) was deposited in a rift basin.â¢A pre-Marinoan sedimentation is suggested for the Carrancas Formation.â¢Carbon isotopic chemostratigraphy imply correlation with the global Keele peak.â¢The Carrancas Formation correlates with the Tijucuçu sequence, in the Aracuaà belt.â¢Cryogenian interglacial strata was not previously known on the São Francicso craton.
The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuà 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 Jequitaà 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 δ13C values ranging from +7.1 to +9.6â°, which is a unique C isotopic composition for the lowermost Bambuà Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaà fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ13C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ13C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ13C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaà Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuà Group strata and has implications for the development of the Bambuà basin.
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