The Neoproterozoic successions of the São Francisco Craton and their glaciogenic units in the Bambui/Una and in the Vaza Barris/Miaba groups
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Misi, Aroldo۱; Azmy, Karem۲; Sial, Alcides N.۳; Guimarمes, José T.۴; Oliveira, Felipe B. M.۱|
|Holding Date||10 September 2008|
The Neoproterozoic carbonate and siliciclasic successions of the São Francisco Craton were deposited in epicontinental basins that covered the cratonic area, and in passive-margin basins, which were later intensely deformed creating fold belts. The epicontinental basins include three mega-sequences: glaciogenic, carbonate platform and molasse, which likely correlate with the Vaza Barris/Miaba groups on the northeastern border of the São Francisco Craton (Sergipano Fold Belt). At least two transgressive-regressive sea-level cycles occurred during the evolution of the carbonate mega-sequence, which lies unconformably above iron-cemented glacio-marine diamictites of probable Sturtian age. Carbon, oxygen, strontium and sulfur isotope trends from analyses of well-preserved samples, together with lithostratigraphic observations provide reasonable correlations of these successions, but the precise ages of the sedimentation is still a matter of debate. They were probably deposited during extensional events related to the fragmentation of the Rodinia supercontinent, starting around 950 Ma and extending until approximately 600 Ma. These events occurred diachronously with the closure of the Pan-African-Brasiliano rift. The same cyclicity has been observed on the craton as well as in the fold belt, suggesting that the sedimentation spread continuously across the craton margins. In all studied successions there is at least one horizon of glacial diamictite. The glaciogenic mega sequence is comprised of siliciclastic rocks at the base, including conglomerate, metagraywacke, diamictite, pelite and quartzite. These lithofacies are dominantly glaciogenic, glacio-marine, glacio-continental or locally eolian in origin. Striated pavements, dropstones and other evidence of glacial origin for most of these lithofacies have been described previously for the Jequitaí and Bebedouro formations (Bambui and Una Groups). These successions unconformably overly the Paleoproterozoic/Archean basement complex or the Mesoproterozoic Espinhaço Supergroup. In the Sergipano Fold Belt, the Palestina Formation is believed to be a glaciogenic sequence. It is formed by diamictite, phyllite and lenses of iron-cemented quartzite unconformably overlying the Simão Dias Group. Diamictite clasts are heterogeneous in size, shape, and composition, including gneiss, granite, quartz, quartzite, phyllite, and metacarbonate, which are supported by a quartz-sericite matrix. In a lower stratigraphic position, the Juetê and Ribeirópolis equivalent formations overly crystalline basement (gneisses) and are composed of diamictite and arkosic sandstone, and are believed to correlate with the Jequitaí and Bebedouro formations in the cratonic area. In all the studied successions negative δC anomalies have been observed in the carbonates above the diamictites, providing likely similarities to Neoproterozoic counterparts worldwide.