Unique record through Late Proterozoic snowball earth and hothouse aftermath stages in the core of South America

ASTINI, RICARDO A.; DE LA PUENTE, G. SUSANA; ZÜLLIGER, GUSTAVO

Córdoba

Simposio; IX Reunión Argentina de Sedimentología; 2002

Centro de Investigaciones Geoquímicas y de Procesos de la Superficie (CIGES), Universidad Nacional de Córdoba

It is increasingly recognized that the Late Neoproterozoic was one of the most remarkable periods in Earth history due to extreme climate variations driven by the interplay of tectonics, hydrosphere and biosphere. A continuous sedimentary record of latest Proterozoic sedimentation is exposed in the area of Corumbá (State of Mato Grosso do Sul), southwestern Brazil. This region is wellknown for the banded iron formation (BIF) occurrences of Urucum which stretch the borderline into Bolivia (Mutúm). Although previous regional work has highlighted the importance of the iron formation and its association with glacial subaqueous diamictites, an appropriate explanation for the origin of this gigantic and extremely iron-rich deposit with interbedded tills and the above carbonate sequences can be satisfactorily explained and tied with similar ones in Gondwana.             Two major units ?the Jacadigo and the Corumbá Groups- separated by an erosive unconformity represent a major climatic change (contrasted icehouse to greenhouse sedimentation) in apparent rift settings. Each of the groups has a basal conglomeratic unit, in the first case with glacial influence, and in the second devoid of involved glacial processes. Both seem to be pre-final assembly of Gondwana. However due to its peripheral location, the Brasiliano Orogeny has only mildly affected the sedimentary succession with broad folds and local cleavage. 640-610 Ma non-metamorphosed basic dykes intrud the Meso-Proterozoic basement and predate the Jacadigo Gp. Ediacara-like fauna (Vendian), acritarchs and Cloudina have all been described for the upper Araras Limestones (150 m) and from the Tamengo shales (>100 m) at the top of the Corumbá Gp. (450 m). Hence, the glacial interval that deposited the diamictites and dropstones through the Urucum (350 m), Corrego das Pedras (100 m) and Banda Alta (350 m) formations must correlate with the Varanger/Marinoan glaciations elsewhere. The Band Alta BIF is a chemical deposit of sedimentary origin with several different iron facies and no apparent volcanic relationship. Tabular stratofabric, absence of diagnostic proximal-distal facies trends and massive interbedded rainout tills can be taken as suggestive of a rather wide basin with flat bottom, where sedimentation took place under relatively deep water and direct glacial influence. No indications of subaerial exposure or influence of storm or wave reworking were observed, what might indicate either a deep-water environment or a highly restricted depocenter (graben?). Seasonal scale overturning and mixing of water masses in a cold stratified water body is suggested as a possible origin. Stages of high CO2 pressure could have lowered pH leading to a greater efficiency in iron leaching, transportation and concentration during weathering of continental rocks. Sea water with a lower pH would have been a major reservoir of Fe+2 in solution. Like in modern high-latitude or cold-water lakes or in restricted fjords overturning and sudden oxygenation may have induced periodic iron precipitation. However, rhythmic bedding could possibly be of tidal origin.             The Corumba limestones characterize a tectonically stable setting with warm stable climatic conditions and good light penetration as to be suitable for prolific carbonate production. Facies associations allow interpreting a variety of shallow-marine to deep-ramp environments, respectively represented by oolitic grainstones and packstones, small mushroom-like mud mounds and black lime-shales with minor graded calcarenites with rich sulphur content. Facies associations are comparable to low-latitude bahamitic type settings. Large order cyclicity is recorded as it is typical of passive margin settings. These rather unusual low-latitude ?Bahamian-like carbonates? together with the record of Ediacara-like and post-Ediacara shelly faunas may represent a distinct metazoan radiation that appeared in the aftermath of what was apparently the last snowball glaciation.