Estudo de um depósito de fluxo gravitacional associado ao Lineamento de Patos em Nova Olinda, Ceará

Abstract

The city of Nova Olinda and the Araripe Basin present an mass flow deposit ever studied, therefore appropriate to a recent study of sediment transport, more specifically, gravitational transport. The tectonic context of the basin evolution with high cliffs and climate setting subject to episodic rains couid be favourable to mass-flow induced movements that once identifled, allow analysing texture, composition and sedimentary structures related to these movements. The relevance of this research is the fact that deposits fomied by high viscosity flows had never been the main focus of a detailed investigation in natural deposits, whether or not related to the evolution of Araripe basin and Nova Olinda region. Performing fades analysis, identification of the framework rock types, matrix sampiing and laboratory and statistics analyses, the present study evaluated the provenance and dynamics of high viscosity sedimentary transport of the targeted deposit. The performed analyses define the deposit as a stacking of paraconglomerates grading to coarse and muddy sandstones, with occurrence of subarkose granule clasts. The grain size analyses points to coarse particles, especialiy granule-sized gravei, very coarse, coarse and medium sand, and including low but clear mud content. The binocular microscope analysis revealed a low sphericity grade of faceted to sub-faceted grains, and a roundness from angular to subangular. The resemblance between the samples suggests a same reworking levei from the source area and low textura) maturity of the matrix. The rheology expressed by texture and sedimentary structures makes aliuvial fans, precisely it depositional lobes, the better geological model for the studied outcrop. The mylonitic gneiss texture of the grains reinforces the association to shear deformed areas, affected by the Patos Lineament. The provenance of the deposit is related to metamorphic souce areas as pointed by a gravei framework compatible with the Piancó-Alto Brígida and Alto Pajeú terrains. The origin of the deposit remains uncertain between either the Silurian of the Araripe Basin (Cariri Formation) or late Cenozoic successions. The triggering agent also remains undefined in its nature between climate (rains), tectonics (associated to the Patos Lineament), or both combined.