Paleoambiente e isótopos de C e O da capa carbonática de Tangará da Serra (MT), margem Sul do Cráton Amazônico.

Abstract

The paleoclimatic and paleoceanographic conditions during the formation of Neoproterozoic post-glacial carbonates have been intensely debated in the last decades. During the end of the Proterozoic, these conditions influenced the biological evolution of the planet, which was punctuated with global glacial events extensive to the low latitudes. The post-glacial geological record comprises carbonate layers with diagnostic structures overlying glacial diamictites. These cap carbonate were intensely studied in cratons worldwide, using geochemical, sedimentological, petrographical and isotopical (C and O) analysis. Similar studies were carried out in newly discovered 50 m thick cap carbonate in the Southern Amazon Craton, in the regions of Tangará da Serra and Mirassol d’Oeste, 200 km away from each other and comparable to the other cap carbonate occurrences around the world. The Tangará da Serra cap carbonate includes two lithostratigraphic units belonging to the Araras Group: the Mirassol d’Oeste Formation characterized by microcrystalline dolomites overlaid by interbedded limestones and shales of the Guia Formation. The peloidal dolograinstones and pink dolomudstones/dolopackstones of the Mirassol d’Oeste display even parallel and quasi-planar lamination with low-angle truncation, often truncated by tubestone and changing vertically into symmetric megaripple bedding with wavy corrugated and internal asymmetric laminations interpreted as deposits of shallow to moderately deep platform influenced by wave. The Guia Formation comprises marls and fine grained limestones with megaripple bedding, sandy limestones with asymmetric wavy laminations interbedded with dark mudstones with hydrocarbon interpreted to have formed in moderately deep mixed platform dominated by current and wave deposits. Upwards in the Guia Formation were observed fine grained, terrigenous-rich limestones interbedded with laminated shales forming tabular layers, tens of meters long with ripple marks, crystal fans (calcite after aragonite pseudomorphs), neptunian dykes, slump structures, convolutions, faults and intraformational breccias (clasts of limestones and dolomites). These features were interpreted as related to CaCO3 supersaturated, deep water platform deposits. δ13C values are depleted (-8‰) at the contact between the Mirassol d’Oeste and Guia formations relatively to the other parts of the succession (5‰ in dolomites and limestones). The isotopic ratios at the base of the Guia Formation can be affected by dolomitization and neomorfism processes. Petrography data indicates that: 1) diagenetic alteration was stronger in limestones than in dolomites, also supported by C (-6‰ a -5‰) and O (-6‰ a -4‰) isotopic rations; 2) the good preservation of depositional features, including lamination, porosity and macropeloids is indicative of primary origin for the dolomite in the Mirasson d’Oeste Formation; 3) The source of Mg and the mechanism of dolomite precipitation were the seawater and the action of sulphatereducing bacteria; 4) hydrocarbons are scarce in the low permeability dolomitic limestones compared to the upper portions of the sequence. Shales and limestones of the Guia Formation were the source rocks of the hydrocarbons. Five successive deformational events of synsedimentary nature were recognized in the two occurrences of Neoproterozoic cap carbonate: 1) large to small-scale load cast structures in the contact zone between the dolostones and glaciogene sediments; 2) tubestones crosscutting stromatolitic lamination; 3) generation of vertical to subvertical fractures and faults and large-scale syncline and anticlines with subordinated chevron folds; 4) vertical to subvertical fractures and faults infilled with conglomerate and breccia, forming neptunian dykes enclosed by undeformed beds; and 5) formation of slump and sliding deposits in the top of the cap limestone. The event layers 1, 2 and 5 are genetically related to the depositional environment, whilst the preferred orientation of the deformational structures in the event layers 3 and 4 are compatible with regional scale tectonics that produced seismically active extensional structures where earthquakes triggered the deformation of sediments associated with fault slip and large scale mass movement. Slump deposits were formed by gravity flows along a ramp, caused by high productivity carbonate. The cap carbonate exposed in the Tangará da Serra region registers a ramp platform environment with a steep slope at the edge of the platform (distally steepened ramp) eventually disturbed by seismic shocks.