Revista Brasileira de Geociências 42(4): 785-798, dezembro de 2012Arquivo digital disponível on-line no site www.sbgeo.org.br 785 ResumoAs rochas siliciclásticas da Serra do Tepequém são correlacionadas aos Grupos Araí e Suapi, pertencentes ao Supergrupo Roraima de idade paleoproterozoica. A análise estrutural destas rochas revelou que o acamamento exibe arranjos com mergulhos preferencialmente para SE e NW, individualizados em domínios limitados por zonas de falhas oblíquas sinistrais com rejeitos normais e inversos, com direção NE-SW. Essa estruturação é formada por dobras forçadas quilométricas do tipo kink bands e chevrons. O novo arcabouço geométrico observado é característico de um ambiente de deformação de nível crustal raso a médio. A história tectônica é controlada por reativações dos planos de fraqueza preexistentes nas tramas antigas, dúcteis, do embasamento. Esse modelo diverge dos regionais prévios para a região, os quais consideram as dobras existentes como produtos de ambiente dúctil sob tectônica colisional. Os resultados evidenciam a importância da presença de estruturas antigas do embasamento, relacionado ao Escudo das Guianas, como controladoras da geometria das rochas da Serra do Tepequém, em ambiente rúptil.Palavras-chave: Serra do Tepequém; paleoproterozoico; Grupo Arai; análise estrutural; dobras kink bands. Abstract Deformation of paleoproterozoic siliciclastic rocks of the Araí Group as an example of basement fault reactivations, Serra do Tepequém, Roraima, NorthernBrazil. Serra do Tepequém region comprises paleoproterozoic siliciclastic rocks from Araí and Suapi Groups, which are part of Roraima Super-group. Field data indicate that bedding is dipping towards SE and NW in different domains limited by NE-SW sinistral oblique faults with both normal and also reverse displacements. The structural setting is formed by regional scale forced folds represented by kilometre scale kink bands and chevron folds. The proposed framework is compatible with upper-to-middle crustal level. The studied faults were controlled by reactivation of early ductile basement fabric. These findings differ from previous regional models, based on folding under ductile conditions related to colisional tectonics. Results evidence the importance of Guiana Shield early basement structures in controlling the geometry of the brittle structures, which were seen on the cover rocks of Serra do Tepequém.

Ponta Negra Beach is located on the banks of the Rio Negro, in the city of Manaus, state of Amazonas, Brazil. In 2012, a revitalization of the beach was carried out, with emphasis on the construction of an artificial beach landfill, in order to perpetuate the population's access to the beach throughout the hydrological year. This work resulted in the formation of several irregularities throughout the terrain, such as cavities, plumbing and pipes exposed on the surface, and sudden cavities between shallower and deeper parts of the bed. These problems have resulted in several fatalities since the opening of the landfill. Given these questions, this study aimed to assess the changes that occur on the beach surface, through analysis of sediments, topography and space, as a result of the construction work, for a period of two to three years, and thus, to identify the cause of these problems to understand what failures occurred in the construction of the landfill that led to the formation of these irregularities. The results showed that the grounded sediments from the artificial beach are being transported more easily by the force of the river and the rains because they are finer sediments than those of the natural beach, intensifying the erosion rates and resulting in the generation of ravines and depressions along the entire beach, being responsible for the movement of sandy mass of the ladfill and in the formation of the irregularities found. This study proves that the anthropic relationships interfered in the natural processes that act on the beach's morphology, warning the importance of serious previous studies in the execution of works with landfills and their continuous maintenance