Navegando por Assunto "Ictiólitos"

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Análise paleoambiental da Formação Pirabas no litoral do Maranhão, Brasil
(Universidade Federal do Pará, 2011-06-03) OLIVEIRA, Samantha Florinda Cecim Carvalho de; ROSSETTI, Dilce de Fátima; http://lattes.cnpq.br/0307721738107549
The Pirabas Formation is a geological unit known for its abundant fossil content, which includes a large number of invertebrate and vertebrate groups. This unit is exposed in the north and northeast coast of Brazil, in the States of Para, Maranhão and Piaui. The majority of the studies focusing the fossils of the Pirabas Formation emphasized initially, invertebrates. However, studies using ichthyoliths have been increasingly emphasized in the last years, which is due to their resistance to dissolution, transportation and deposition. Moreover, the small size favors their continuous recovery along different stratigraphic levels, allowing their use as an additional tool in paleoenvironmental interpretations. This work aimed to prospect ichthyoliths of the Pirabas Formation exposed along the coast of the State of Maranhão, as well as their identification and integration with facies analysis. This study area is part of the São Luís Basin, which is filled with a 4,000 m-thick sedimentary succession represented mainly by Cretaceous rocks, with a thin Cenozoic cover, the latter represented by the Pirabas and Barreiras Formations deposited mostly in the Miocene. The exposures studied occur along various cliffs between the towns of Alcantara and Guimarães. Miocene carbonates with ichthyoliths record in this location are occasional, occurring as thin layers up to 2 m thick, which are laterally and vertically intergraded with siliciclastic deposits. These strata occur as three stratigraphic units, with the second one documenting fossiliferous carbonates related to the Pirabas Formation. The cliffs studied include the ones in the localities of Canelateua, Mamuna Grande, Peru and Base. We analyzed 16 thin sections sampled from these localities, which resulted in description of four carbonate microfacies, and one of mudstone. The samples provided 30 ichthyoliths, which were photographed, identified and described under a scanning electron microscope. In addition to the ichthyoliths, the petrographic study recorded the presence of other fossils, including bryozoans, foraminifera, gastropods, bivalves, algae, and echinoids. The integration of paleontological data and microfacies is consistent with deposition in carbonate paleoenvironments predominantly with low energy, reducing conditions and subjected to the frequent introduction of siliciclastic grains. These characteristics, added to the low fossil frequency in most of the samples, corroborate previous interpretations that the deposition of these strata occurred in paralic, probably estuarine palaeoenvironments. However, the abundance of marine fossils in some samples, associated with the presence of ichthyological elements common in environments with normal salinity, shows periodic introduction of saline inflows. Therefore, we can conclude that the strata analyzed were deposited in association with an estuarine system, but representing more distal facies of this system, representative environments more exposed to marine influence.
Acesso aberto (Open Access)
Contribuição dos sedimentos e ictiólitos para a identificação do ambiente de formação do Sambaqui do Moa (Saquarema-RJ)
(Universidade Federal do Pará, 2014-08-28) MACHADO, Sauri Moreira; SILVA, José Francisco Berrêdo Reis da; http://lattes.cnpq.br/1338038101910673; COSTA, Marcondes Lima da; http://lattes.cnpq.br/1639498384851302
The Moa’s shellmound is an archaeological site located in Itaúna, Saquarema coastline from the State of Rio de Janeiro. Three moments of occupation were recognized this site: stratum 3 corresponding to the beginning of the occupation of this site basis; 2, intermediate, points to a more intense occupation with great concentration of mollusks, fish bones and human burials; and 1, the most superficial, related to the last occupation. To identify the environmental conditions of development from Moa’s shellmound, sediment samples and zooarchaeological material were collected in these three strata. The zooarchaeological material is represented by microscopic fish remains (ichthyoliths), composed by microteeth with different morphologies: canines, incisors and molars. The sediments according to XRD analyzes are composed by quartz and kaolinite, calcite, aragonite and fluorapatite. The latter is the main mineral of zooarchaeological material, while calcite and aragonite reflect the remains of shells contained there, abundant in this site. The mineralogical analyses were confirmed by chemical analysis, in which high levels of P2O5, CaO and FB (H2O, CO2), account for the fluorapatite, calcite and aragonite. Chemical modifications variations of the C and P in microteeth suggest that these experienced mineralization, an initial fossilization process, post-deposition. The data of stable isotopes 13C and 15N allowed defining the source of organic matter from Moa’s shellmound as marine/brackish, where the vegetation is represented, predominantly, for C3 plants by type of rainforest. The 87Sr/86Sr ratios in ichthyoliths confirm that the environment surrounding the shellmound as estuary. The tooth morphology allowed us to recognize five families previously not recorded for the site, as Labridae, Serranidae, Ariidae, Erythrinidae and Characidae, which confirm the estuarine environment. The age of the Moa’s shellmound by radiocarbon from the sediments showed disruption of stratum 2, causing a reversal of aged strata which can be explained by processes of formation and/or changes in the rivers changed the geological and geomorphological settings area. Another explanation could be human interference, due to the large number of burials (over 30), has upset the order of the moments of occupation from Moa’s shellmound, and possible erosion. The Moa’s shellmound is therefore installed in an area of marine-estuarine transition.
Acesso aberto (Open Access)
Ictiólitos da Formação Pirabas, mioceno do Pará, Brasil, e suas implicações paleoecológicas
(Universidade Federal do Pará, 2011-06-01) COSTA, Sue Anne Regina Ferreira da; RICHTER, Martha; http://lattes.cnpq.br/9381228195500524; ROSSETTI, Dilce de Fátima; http://lattes.cnpq.br/0307721738107549
The Pirabas Formation (Oligo-miocine), which is represented by carbonates deposits through the Brazilian Coastal Area between the States of Pará and Piauí, is recognized for its fossil richness. Among the many fossils paleoichthyofauna stands out for its abundance in outcrops, but the absence of stratigraphic and facies control of these fossil restricted their potential use in paleoenvironmental interpretations. The joint use of the technique of wet sieving for the recovery of ichthyoliths and microfacies and facies analysis of sediments taken from Pirabas Formation deposits, which were exposed in the B17 mine (Capanema-PA), allowed for the first time the use of vertebrate fossil for the elaboration of an interdisciplinary paleoenvironmental reconstruction model. The 3594 recovered ichthyoliths, along with the 5 facies associations and the 4 carbonate microfacies, led to the conclusion that these deposits of Pirabas Formation were formed in marine-marginal depositional system, with different environments genetically associated as shore face (surf zone), beach, lagoon, tidal channel and tidal delta, which would be part of an estuarine system with the influence of wave, marked by four different depositional cycles related to possible transgressiveregressive episodes. The estuarine environment was confirmed by the assembly of ichthyoliths, observing that this assembly is formed by both freshwater specimens, represented by the Characidae family, an unheard occurrence for the unit until then, as for marine specimens, for example, the sharks. Several dental types, possibly attributed to representatives of brackish waters, such as the genera Dasyatis and Sarpa, both equally registered for the first time, also strengthens the proposed environment. The distribution of the ichthyological community evidences palaeoenvironmental control, having registered their highest abundance in deposits of tidal channel. However, the influence of relative sea level resulted in the development of transgressive-regressive depositional cycles, which also exerted strong control on the stratigraphic distribution of the B17 Mine registered ichthyoliths, reinforcing the importance of interdisciplinary studies in refining palaeoecological reconstruction of this unit.