Estratigrafia e geoquímica orgânica da formação Longá, neodevoniano/eocarbonífero da Bacia do Parnaíba, região de Pedro Afonso-To

Abstract

The Late Devonian is marked by considerable paleoclimatic and paleogeographic changes related to a widespread colonization of the continent by vascular plants, exponential increase of organic production and development of background anoxia in the epeiric seas, typical of that period. In addition, during Famennian age the Gondwana continent was in high latitudes and glacial pulses occurred in South America. A postglacial phase in Late Famennian was responsible for the occurrence of a marine transgression and beginning of the Long Formation sedimentation until the Tournaisian. Devonian-Carboniferous boundary is marked by deposition of black shales in various parts of the world. The goal of this work is a paleoenvironmental reconstruction and a geochemical characterization of the postglacial organic matter deposits of Long Formation from southwest Parnaíba Basin. A lithofaciological analysis was accomplished of Longá deposits plus petrographic and X-ray diffraction. The quantification of Total Organic Carbon (TOC) plus Rock-Eval pyrolysis and biomarkers was the geochemical analysis of organic matter within Longá Shales. From the sedimentological analysis of the deposits five lithofacies was identified: conglomerate lag (Gmm), laminated shales (Fl), coarse sandstone with megaripple bedding (Sm), fine to medium hummocky cross-stratified sandstones (Sh) and wavy-linsen shale interbedding with fine sandstone (Fwl). The lithofacies were interpreted as belonging to a single facies association, representing offshore to shoreface deposits. The lower Longá Formation boundary is well delimited by a transgressive lag produced by waves that separate it from the non-marine deposits of the Cabeças Formation. The formation is characterized by the predominance of thick layers of the Fl and Fwl facies with episodic events of storms and occurrence of debris rain as result of remaining icebergs melting. In Fwl facies occur typical elements of the Cruziana Ichnofacies characterized by predominance of horizontal trace fossils. These characteristics suggest a stressful environment caused by changes in salinity during the input of ice melting water within the platform. The organic geochemistry analyzes showed that Longá shales had a low TOC content, less than 1%, values of free hydrocarbons (S1) below the detection limit of the Rock-Eval 6 equipment, and very low (0.06 to 0.23) hydrocarbons generation potential (S2). The maximum pyrolysis temperature (Tmax) as well as the thermal maturation parameters calculated on the biomarkers show that the Longá shales are immature. The values of pristane/n-C17, phytane/n-C18, Hydrogen Index (IH) and Oxygen Index (IO) suggest kerogen type III and IV, formed from conical land leaves, needles and plant waxes as are support by total sulfur (TS) <0.2 wt%, terrigenous/aquatic ratio (TAR), C29-sterol, hopane/sterane and Methylphenanthrene (MPs). This organic matter was deposited in an epeiric sea where prevailed shallow water, oxidizing conditions, normal salinity, and low temperatures.