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

Abstract

Sedimentary rocks of the Sete Lagoas Formation, exposed in the left margin of the São Francisco river (Minas Gerais State, Brazil), were deposited on the Januária-Itacarambi basement high. They show both lateral and vertical rock stacking along continuous outcrops, allowing us to carry out detailed facies analysis and sequence stratigraphy studies. Our studies also integrate data from geological mapping, macro and microscopic petrography and high-resolution C and O isotope analysis. Eight facies and four facies associations make up a sequence composed by a transgressive tract in the base, and a high stand tract in the upper portion, separated by a maximum flooding surface. The high stand tract shows a progradation stacking from the basement high apex towards progressively deeper basement areas. This stratigraphic framework, associated with others stratigraphic and isotopic features, indicates that the now exposed Januária basement high also represents a paleo-high during the sedimentation event. Aragonite pseudomorphs and dolomites coupled with δ 13 C values of -5‰ characterize the basal carbonate of the transgressive tract as a cap carbonate. Records of the Cloudina fossil, recognized in the high stand tract, indicate a Late Ediacaran age for the upper portion of the studied stratigraphic sequence. KEYWORDS

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

“…Some authors point to a possible unconformity between the basal cap carbonates of the Bambuí Group and the rest of the unit (Martins & Lemos 2007, Zalán & RomeiroSilva 2007. However, sedimentary and isotopic studies in complete borehole and field sections do not support a sedimentation gap of such magnitude at this stratigraphic level (Kuchenbecker et al 2016, Perrella Jr. et al 2017. Therefore, we argue that refinements between the subsidence and the filling histories of the São Francisco Basin are required.…”

“…Unless a major unconformity between the cap carbonates and the rest of the deposits of the São Francisco Basin is assumed, the glaciogenic units are also likely to be Late Ediacaran in age (Kuchenbecker et al 2016). Although some authors point to a sedimentation hiatus at such a stratigraphic position, because of seismic, sedimentary and isotopic features (Martins & Lemos 2007, Zalán & Romeiro-Silva 2007, studies on complete borehole (Kuchenbecker et al 2016) and field (Perrella Jr. et al 2017) sections refute that it represents a major regional unconformity. Therefore, a significant time gap at the base of the Sete Lagoas Formation is yet to be described.…”

“…Assuming any of the Cryogenian ages for the glacial deposits, the Late Ediacaran age for the base of the Bambuí Group implies that either the first 100 m of the Sete Lagoas Formation records 85 or 200 Myr of deposition or an unconformity yet to be found exists between the CI-1 and CI-2 carbonates. The first option is highly unlikely, as such durations are not compatible with a second order or a third order sequence , Perrella Jr. et al 2017 subdivision of the lower Sete Lagoas Formation (Vail et al 1991). Here, we examine the second option and its implications.…”

“…Although some works have discussed a possible unconformity at the base of the Sete Lagoas Formation (Martins & Lemos 2007, Zalán & Romeiro-Silva 2007, stratigraphic (Kuchenbecker et al 2016), sequence stratigraphic , Perrella Jr. et al 2017 and isotope chemostratigraphic data (Paula-Santos et al 2017, Perrella Jr. et al 2017 argue against the existence of an unconformity at the base of the Bambuí Group, and suggest a continuous deposition through the cap carbonates to the Cloudina bearing strata.…”

“…The C isotope data of the carbonates from the Sapé profile of Perrella Jr. et al (2017) is especially important, as it shows that the recoveries of the δ 13 C values from very negative to 0‰ are not sharp when the geological record is not condensed or incomplete. Additionally, Paula-Santos et al (2017) suggested that the initial increase in 87 Sr/ 86 Sr isotope ratios from 0.7074 to 0.7082 in the cap carbonate ) is not a peak, but rather, they stabilize close to 0.7085 in the CI-2.…”