Paleoambiente e quimioestratigrafia da porção superior do Grupo Araras, neoproterozoico da faixa Paraguai Norte, estado do Mato Grosso.

Abstract

In the central portion of the South-America Platform occurs the carbonate succession of the Araras Group, that register an extensive carbonate platforms of the Neoproterozoic, developed along a passive margin at S-SW portion of the Amazon Craton. These sedimentary basins were submitted to a compressional collisional event at Ediacaran-Cambrian transition, resulting in the origin of the Paraguay Belt. While the lower Araras Group are recognized as a post-glacial record of the Marinoan event (~635 Ma), the final paleoceanographic and sedimentary events that led the end of carbonate deposition are discussed, for the first time, in this thesis. The upper Araras Group, represented by Serra do Quilombo and Nobres formations, are the last stage of Araras carbonate platform sedimentation. These units are covered by siliciclastic deposits of Alto Paraguay Group, Middle Ediacaran, which the basal deposits are related to the glacial Gaskiers event (~580 Ma), reported only on the eastern portion of the Belt. Stratigraphy, sedimentology and carbon isotope studies were performed on carbonate rocks exposition in Cáceres and Nobres regions, respectively the western and eastern portion of the belt, Mato Grosso state, Brazil. The Serra do Quilombo and Nobres formations are a unique carbonate ramp in a homoclinal system composed by: i) deep ramp with massive thin dolostone of offshore. ii) Middle ramp with massive thin dolostone, intraclastic and sandy dolostone with hummocky and swaley cross stratification, planar and wave lamination, oncolitic dolostone with wavy bedding and massive oolitic dolostone, from shoreface, oolitic bar complex, and foreshore/shallow subtidal zones; and iii) inner ramp comprising in massive thin dolostone, intraclastic dolostone and dolomitic sandstone with megaripple bedding, microbialites, chert, evaporite molds, laminated sandstone mudstone, deposited in tidal flats. These deposits were developed during highstand conditions after the Marinoan transgressive event , and preceding the implantation of siliciclastic platform from Alto Paraguay Group during the Ediacaran-Cambrian transition. The palinomorphs assemblages from Nobres Formation are represented by spheroidal forms, such as Leiospharidia, rare filamentous and acanthomorphous acritarchs represented mostly by Tanarium correlated to biozone ECAP (Ediacaran Complex Acantomorph Palynoflora) that suggest an age between 600-550 Ma. The High frequency stratigraphy appoint to orbital force as the principal mechanism responsible to accommodation space generation and peritidal cycles composition, and secondarily influenced by tectonic. The patters of carbon isotopes from carbonate (δ13Ccarb), organic matter (δ13Corg) and the coupling of carbon isotope composition (Δ13Ccarb-org = δ13Ccarb - δ13Corg) associated with low TOC suggests an oxidant ocean in equilibrium with atmosphere, with minimal redox variation on sediment/water interface, that includes: i) anoxic layer with δ13Ccarb around ~0‰ and δ13Corg <-28‰, restrict to the distal zone of deep ramp with early organic diagenesis conduct by chemosynthetic organism; ii) oxic layer with δ13Ccarb around ~0‰ and δ13Corg between -28 a -25‰ in open sea system of deep and middle ramp, with primary photosynthetic activities; iii) oxic layer with low pCO2 associates to positives signal of δ13Ccarb (+3‰) and δ13Corg (>-25‰) in shallow middle ramp associated with high ration of inorganic carbonate precipitation; and iv) oxic layer restrict features by negative δ13Ccarb (-2‰) and δ13Corg between -31 to -25‰, exclusive to inner ramp, related to primary photosynthetic production and effective organic early diagenesis drive by sea level changes. These data define the upper Araras Group as the record of a carbonate ramp with restrict coastal environments, on an oxidant ocean, tropical zones with warm waters, high evaporation rate and photosynthetic biomass, during the Middle Ediacaran. The pre-glacial influence indicated by carbon cycles anomalies or glacioeustasy they were not documented to the upper Araras Group. The final sedimentation stage of Araras carbonate platform was influenced by initial continental uplift source areas responsible for the siliciclastic input on carbonate coastal zone, preceding a progradation of continental-coast systems from Alto Paraguay Group at Middle Ediacaran.