Paleoambiente e quimioestratigrafia da Formação Itaituba, carbonífero da borda sul da bacia do Amazonas, região de Uruará – Pará

Abstract

The Amazon Basin, characterized as an intracratonic basin of about 400,000 km2, has a Phanerozoic sedimentary record of four second- order sequences related to groups Trombetas; Urupadi and Curuá; Tapajós and Javari. The Itaituba Formation, the goal of this work, is part of the Tapajós Group, which represents the last transgressive -regressive cycle of this Paleozoic basin. The Itaituba Formation presents thick packages of subtidal limestones intercalated with thicker evaporite deposits toward the top of the formation, with shales, siltstones and sandstones that represent deposits transgressive – regressive deposits of moderate energy in shallow marine infra-tidal and subtidal environment. The formation is composed of the richest strata of marine fossils in the Amazon Basin, such as the conodonts, foraminifera, corals, bryozoans, crinoids, trilobites, ostracods, gastropods, brachiopods, bivalve, escolecodonts and fish fragments. In this work, it was studied samples from a drill core (FURO 5) obtained in Uruará area, east-central state of Pará. The main objectives of this study are to obtain the depositional age of Itaituba Formation by comparison with the century 87 Sr/86Sr curve and its chemostratigraphic characterization based on major and trace elements contentes, and C and O isotopes, as well as the facies description. The stratigraphic profile of the drill core is characterized by an intercalation between carbonatic facies rich in bioclasts , stylolites and quartz druses and dolomitic facies . At the base of the profile, terrigenous facies characterized by reddish silt with carbonate clasts were recorded. Microfaciologically the following lithotipes were identified: wackstone, packstone, and more rarely dolomudstone, mudstone and grainstone. Seven microfacies were defined: Bioclastic Mudstone (Mcb), Bioclastic Wackstone (Wb), Bioclastic Wackstone with stylolites (Wbe), Bioclastic Packstone (Pb), Bioclastic Packstone with Peloids (Pbp), Bioclastic Grainstone with Peloids (Gbp) and Fine Dolomite (Dl). Among the bioclasts are: brachiopods, echinoderms, foraminifera, pelecipods, bryozoans, gastropods and ostracods. As not skeletal components there are: quartz, clay minerals, feldspars, oóides and intraclasts. The matrix is micritic and there are three types of cement: "fringe", tile and sintaxial overgrowth. Much of the profile was affected by secondary processes such as dolomitization, dissolution and compaction. The results of geochemical analyses were performed on 46 samples collected at each 50 cm, approximately. The data confirm that the profile is predominantly calcitic with small variations in the Mg content, however, dolomitised levels are present. The high values of Si in some samples indicate the presence of terrigenous minerals, in addition to the druses and fractures filled with quartz. The Sr contents of the samples are satisfactory for the isotopic analysis, and varies from 30 to 293 ppm, some of the smaller values being related dolomitic rocks. Stable isotope studies were performed on 76 samples collected at intervals of 30 cm. The values obtained for 13C and 18O range from 1.602 to 5.422 ‰ and - 8.734 to 0.804 ‰, respectively. These values are in accord with those obtained in previous studies for the Itaituba Formation carbonates, pointing a marine environment with the typical isotopic signature of Carboniferous rocks. (13C ranging from 2 to 6 ‰, and 18O ranging from - 3 to 7‰). Therefore, the study carbonates have primary isotopic composition, except for some samples, which have their values affected by diagenetic processes, like the dolomitization. The depositional age was obtained from the leaching of two shells of brachiopods, which positioned the rocks from the Itaituba Formation in the Upper Pennsylvanian with two ages intervals, between 296 and 303 Ma (Virgiliano - Missourian), and between 293 and 307 Ma (Virgiliano - Desmoinesiano), and Lower Pennsylvanian aged between 313 and 318 Ma (Morrowan).