Depósitos de rampa carbonática ediacarana do Grupo Corumbá, região de Corumbá, Mato Grosso do Sul

Abstract

Depositional events which occurred after Late Cryogenian global glaciations (850 - 635 Ma) were marked by the implantation of extensive carbonate platforms and ramps developed in several cratons worldwide. In this period, one of the bioevolutive innovations was the appearance of calcified skeleton animals, like the genus Cloudina (~548 Ma). In South America, one of the best records of Ediacaran carbonate deposits with these macrofossil is the Corumbá Group, exposed in the southern Amazon craton. These deposits were deformed by Brazilian/Pan-African tectonics which led to the establishment of the Paraguay belt in the Neoproterozoic-Cambrian transition. Outcrop-based facies and stratigraphic analysis, assisted with C, O and N isotope data of the Bocaina and Tamengo formations, belonging to the upper Corumbá Group, were carried out in the Corumbá and Porto Morrinhos regions, State of Mato Grosso do Sul, Brazil, and allowed the reconstitution of the Ediacaran carbonate ramp and the habitat of Cloudina. The Bocaina Formation consists of small-scale shallowing upward cycles grouped into two facies associations: 1) intertidal plain, consisting of well sorted and rounded fine-grained quartz-sandstones cemented by dolomite and fine dolomite with stratiform and columnar stromatolites, fenestral/bird´s eyes porosity and mud cracks; and 2) subtidal shoals, formed mainly by intraclastic dolomites (intraclasts of dolomicrite and peloidal dolomite) and subordinated fine-grained sandstones and shales. Deformation structures occur in both associations and are related to liquefaction and fluidization processes, possibly triggered by seismicity. The Tamengo Formation consists of the associations: 1) shoreface with oolithic bars, composed of intraclastic and oolithic packstones, lime mudstone/ bituminous shale rythmithes with Cloudina; and 2) storm-influenced offshore, consisting of crystalline limestone with rare terrigenous grains, massive bedding, hummocky/swaley cross stratification, low-angle cross lamination and Cloudina shell fragments. Mudrocks and shales separate discrete tempestite beds. Cloudina lived in protected environments in the shoreface zone, periodically reworked by storms that accumulated coquinas in the offshore zone. The high carbon concentration (TOC up to 0.41%), the enriched values of δ13C (1.5 ‰ to 5.4‰) and δ15N (between 3.5 and 4.5‰) found in the carbonate rocks with Cloudina, indicate high organic productivity and oxygen concentration for the Terminal Neoproterozoic, similar to those of the current atmosphere/ocean interface.