Navegando por Assunto "Quimioestratigrafia"

Agora exibindo 1 - 2 de 2

Acesso aberto (Open Access)
Paleoambiente e quimioestratigrafia da Formação Itaituba, carbonífero da borda sul da bacia do Amazonas, região de Uruará – Pará
(Universidade Federal do Pará, 2014-10-29) CAMPOS, Amélia Carolina Pimenta Parente de; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/886783626882099; MACAMBIRA, Moacir José Buenano; http://lattes.cnpq.br/8489178778254136
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).
Acesso aberto (Open Access)
Paleoambiente e quimioestratigrafia da porção superior do Grupo Araras, neoproterozoico da faixa Paraguai Norte, estado do Mato Grosso.
(Universidade Federal do Pará, 2015-06-22) RUDNITZKI, Isaac Daniel; ADER, Magali; NOGUEIRA, Afonso César Rodrigues; http://lattes.cnpq.br/8867836268820998; 8867836268820998
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.