Estratigrafia e paleoambiente da capa carbonática neoproterozóica, sul do cráton amazônico, região de Tangará da Serra (MT)

Abstract

The Neoproterozoic is marked by major climatic changes that interfere with the Crucially in biological evolution and paleoceanográfica of our planet, and mainly characterized by periods of low overall glacial that achieved latitudes. This dramatic period in the history of the planet is recorded in layers carbonate termed carbonate layers that overlap directly diamictic glaciers. In this work one Neoproterozoic cap carbonate 20 m thick was Tangara described in Limestone mine, Tangara da Serra, Mato Grosso. That sequence comprises the Mirassol d West formations (dolomite) and Guide (limestone) belonging to the lower group macaws. The cover is composed of dolomitic dolograinstones peloidais pink with inverse grading, a parallel lamination and truncation of low angle, in addition to discontinuous layers of fibrous crystals calcite (gypsum pseudomorphs second?), interpreted as records of a shallow to moderately deep platform with hypersalinity events. The cover Limestone consists of rolled and massive, siltstone rich in iron oxide, and lime thin with wavy bedding megamarcas interpreted as deposits moderately deep mixed platform dominated by waves. Limestone with fine corrugated laminating / ripple marks and fans crystals (pseudomorphs second aragonite), intercalated with shales were interpreted as deposits deep platform and supersaturated in CaCO3. With calcareous structures Slip including convoluted laminations and syn-sedimentary faults characterized slope deposits, while Neptunian dykes, filled with limestone breccias, and deformed layers between non deformed layers suggest seismic activity. Three stratigraphic surfaces divide the carbonate succession studied: S1 separates the covers dolomitica and limestone and is interpreted as transgressive surface, while surfaces S2 and S3 within the limestone cover are considered limits facies. Facies deformed occur throughout the sequence, separated by intervals without strain, and were split into three packages (A, B and C). Packages A and C exhibit ductile-brittle structures like folds, faults and bedding convoluted, while the B package contains structures formed in brittle regime as failures and fractures. Analysis of C and O isotopes showed negative values ​​similar to found in other carbonate covers the world. The data 13C isotope values ​​between -4 and -6 in the case dolomite , limestone while in the case 13C the values ​​reaches to -7 without co- variance of the 18 O isotope indicates change by meteoric fluids or by dolomitization . The sequence described in Tangara Sierra expands the occurrence of carbonate layers in South America and corroborates the interpretation of an extensive carbonate platform post- glaciation Puga , related to Marinoano event , this part of the Amazon Craton.