Sedimentary and paleoenvironmental evolution of the Sete Lagoas Formation (Bambuí Group, northern of Minas Gerais State) in the context of the terminal Ediacaran sedimentary basins

Author(s)

Juliana Okubo

• Document type: Doctoral Thesis
• Press: Rio Claro. 2019-10-07.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Geociências e Ciências Exatas. Rio Claro
• Defense date: 2019-09-19
• Advisor: Lucas Veríssimo Warren

Abstract

Major climatic, tectonic, biogeochemical and evolutionary changes occurred during Late Neoproterozoic are recorded in several sedimentary successions around the world. One of these successions is the Sete Lagoas Formation, the basal unit of the Bambuí Group, present both in the southern and central part of São Francisco Craton. Because of excellent exposure and and great stratigraphic continuity of this unit, this study focused on the the sedimentary and paleoenvironmental evolution of the Sete Lagoas Formation near Januária area– MG in the central São Francisco Craton, based on a sedimentological, stratigraphic and geochemical approaches. Five facies associations were defined in the studied area: cap dolostone (FA1), peritidal carbonates (FA2), seismic-influenced carbonate ramp (FA3), oolitic belt (FA4) and tidal flat dominated by microbialites (FA5). Due to their importance in the rock record, two distinct intervals were studied in detail: the seafloor precipitates and the flat-pebble breccia from the lower and middle part of the unit, respecively. The seafloor precipitates are represented by carbonate fans (aragonite pseudomorphs), barite and authigenic apatitic cements. The latter was described here for the first time in a cap carbonate context, and this mineralogical association is interpreted as the result of iron reduction of sediments in conjunction with high alkalinity of the seawater. Since barite fans have been used to infer the Marinoan age of this cap carbonate, it is important to test whether its origin relates to early diagenetic or hydrothermal processes. The FA1 was succeeded by the deposition of peritidal carbonates (FA2), marked by the presence of thrombolites and laminated microbialites, which provided environmental conditions for the proliferation of the Cloudina and Corumbella metazoans. These shallow-water deposits are overlaid by intraformational breccias, interpreted as flat-pebble breccia, formed by seismic events (FA3). Clast measurements of the paleocurrent data from FA4 indicate a NE-SW coastline with ocean opening for SE. Extensive ooid shoals (FA4) succeeded by tidal flats dominated by microbialites (FA5), represent the last deposition cycle of the Sete Lagoas Formation, preceding the drowning of the carbonate platform represented by pelitic rocks from the Serra de Santa Helena Formation. The Sete Lagoas carbonate platform deposits are arranged into three depositional sequences with thickening towards E-NE, possibly related to an increase of subsidence rate in this part of the basin. Throughout the entire carbonate succession, C and O isotopic values show a large negative excursion in the base, increasing to positive values upward. The 34SCAS isotopic values covary with 13Ccarb, indicating that pyrite and organic carbon were oxidized and buried at the same rate. Measured carbonate-associated sulfate (CAS) concentrations show an oxygenation peak in the lower part of the unit, followed by anoxic conditions, similar to previous estimates of sulfate reservoir in other late Ediacaran carbonate units. (AU)

• FAPESP's process: 15/07391-0 - Sedimentary evolution of the Sete Lagoas formation (Bambuí group, northern Minas Gerais State) in the context of the upper Ediacaran sedimentary basins
• Grantee: Juliana Okubo
• Support Opportunities: Scholarships in Brazil - Doctorate