Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

Abstract

ABSTRACT:The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ 13 C (c. -4‰), and a large oscillation of the δ 18 O (-6 to -15‰

Sedimentary provenance in the southern sector of the São Francisco Basin, SE Brazil

“…The basal unit of the Bambuí Group is the Sete Lagoas Formation (Fig. 2), which is mostly composed of carbonate rocks with very negative δ 13 C values at the base (Alvarenga et al 2007, Kuchenbecker et al 2016, Paula-Santos et al 2017. This geological record is in agreement with the Snowball hypothesis (Hoffman et al 1998, Hoffman & Schrag 2002) that postulates at least three periods of extreme climatic shifts during the Cryogenian and Ediacaran Figure 1.…”

“…The basal carbonate rocks of the Sete Lagoas Formation display very negative δ 13 C values, which are similar to other Neoproterozoic cap carbonates (Alvarenga et al 2007, Kuchenbecker et al 2016. Babinski et al (2007) obtained a Pb-Pb isochron age of 740 ± 22 Ma in these carbonates, and correlated the glaciation to the Sturtian global event.…”

“…Either an unconformity yet to be found separates the cap carbonates from the rest of the Bambuí Group, or the lowermost part of the Sete Lagoas Formation records the entire Ediacaran period. Based on a stratigraphic and chemostratigraphic analysis, Kuchenbecker et al (2016) concluded that the existence of an unconformity is unlikely, and proposed that the basal diamictites may be Late Ediacaran in age.…”

Facies analysis, sequence stratigraphy and chemostratigraphy of the Sete Lagoas Formation (Bambui Group), northern Minas Gerais State, Brazil: evidence of a cap carbonate deposited on the Januária basement high

“…Such deposits at the base of the Sete Lagoas Formation have been interpreted as cap carbonates (Misi 2001;Martins and Lemos 2007;Lima 2011;Misi et al 2011;Caxito et al 2012;Paula-Santos et al 2015;Kuchenbecker et al 2016). The Sete Lagoas cap carbonates have been related to the Sturtian glaciation (~740 Ma; Babinski et al 2007;Vieira et al 2007), Marinoan glaciation (~635 Ma, Caxito et al 2012), and Marinonan or younger glaciation (Lima 2011).…”

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

“…Furthermore, the different preparation methods for Sr isotopes may also contribute to this problem (see Melezhik et al 2001). Kuchenbecker et al (2016) have mentioned that the Bambuí Basin may have been restricted, but did not mention occurrence of evaporite or extensive black shale deposits as further evidence. The occurrence of cosmopolitan fossils, such as Cloudina, militates against this restricted scenario.…”