Navegando por Assunto "Pleistoceno"

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Análise tafonômica de Eremotherim laurillardi (Lund, 1842) dos depósitos pleistocenos, município de Itaituba, Pará
(Universidade Federal do Pará, 2008-08-18) FERREIRA, Denys José Xavier; RAMOS, Maria Inês Feijó; http://lattes.cnpq.br/4546620118003936
The present work deals with the taphonomic study of Eremotherium laurillardi (Lund, 1842), a ground sloth found within a Pleistocene depositional site in Itaituba town, State of Pará. The samples comprise approximately eight hundred skeletal fragments and complete pieces of this specie deposited in the Paleontological Collection of Museu Paraense Emílio Goeldi. Taphonomy is the post mortem history of fossils, the study of processes that influence preservation of potential fossils. It consists of two major aspects: biostratinomy, the study of processes affecting organism remains prior to burial and fossil diagenesis affecting potential fossils after burial. The biostratinomic study showed that the skeletal remains had a loose packing and were poorly sorted, indicating deposition in situ. Moreover, the biostratinomic study revealed that the taxonomic composition of the skeletal fragments is monotypical and monospecific related to catastrophic death (non-selective) by abrupt burial which occurred before necrolysis. Traces of abrasion and reworking during transport of the skeletal remains are insignificant and/or non-existent. Bioerosion facies have not been identified too. The non-preservation of soft parts shows that the necrolysis occurred in an aerobic environment. However, the partial presence of pyrite in the foramina and channels of ribs and teeth analyzed point to localized reducing micro environment. The skeletal remains show their morphological structures, both external and internal, well preserved suggesting that they were not exposed to the exogenous cycle, due to the quick burial. The fossildiagenetic study, conducted on samples of ribs, teeth and vertebrae, using both optical and scanning electron microscopy, revealed that both the bone and dental structures, known as Havers` channels and dentinal tubules, respectively, remained well – preserved. Additionally EDS Analysis (Energy Dispersive Spectroscopy) carried out on these components showed a nearly unaltered chemical composition relating to Ca, P, Mg, K and Na contents.
Acesso aberto (Open Access)
Sedimentação coluvial pleistocênica na região de Presidente Figueiredo, nordeste do estado do Amazonas
(2009-06) SARGES, Roseane Ribeiro; NOGUEIRA, Afonso César Rodrigues; RICCOMINI, Claudio
This paper describe Upper Pleistocene colluvial deposits in the Presidente Figueiredo region, northestern State of Amazonas, Brazil. Outcrop-based sedimentologic and stratigraphic study, integrated with geomorphologic analysis and optically stimulated luminescence dating, allowed to characterize the architecture and lithofacies of these sedimentary successions and provided information about the denudation history and landscape changes in the central Amazonian region during the Pleistocene. The colluvial deposits consist of sands and mainly matrix-suppported gravels with massive beddding, locally with inverse grading, suggestive of deposition by gravitaty and torrential flows. Two types of colluvial deposits were identified: Colluvial deposits 1, dated at 57.000±5.000 years BP, constituted of gravels and sands with fragments of pelite, lateritic crusts and ferruginous sandstone, overlying Early Paleozoic rocks; and Colluvial deposits 2, yielding 22.100±2.600 years BP, consisting of gravels with fragments of semi-flint kaolin and lateritic crust, overlying Cretaceous kaolinic silciclastic deposits of the Alter do Chão Formation. The types of fragments indicate as sources Phanerozoic weathered rocks and lateritic bauxitic and ferruginous laterites which were removed during denudation of plateau. The two colluvial events described here seem to confirm that the main phases of geomorphogenesis are correlated with periods of forest retreat related to the two dry climatic events recorded during the Upper Pleistocene in Amazonia.