Contribuição ao estudo da mineralogia de sedimentos na plataforma continental dos Estados do Ceará, Piauí e Maranhão através da microscopia analítica SEM/EDS

Abstract

The knowledge of the morphology and chemical composition of the minerals allows inferring about the predominant rocks of the local geology, the distance travelled by mineral debris from the parent rock, transportation processes, and can indicate the paleoclimatic condition of the sediment deposition environment. The study region is located between the latitudes of 2°30’0’’S and 3°0’0’’ S and longitudes of 42°30’0’’ W and 40°0’0’’W, on the Northeastern Equatorial Brazilian continental shelf NEB, along the coasts of the states of Ceará, Piauí and Maranhão. Marine sediments were evaluated by Scanning Electron Microscopy associated with Energy Dispersive Spectroscopy (SEM/EDS). Energy dispersive spectra and chemical maps were obtained with electron beam ranging from 500 pA and 1.0 nA , acquisition time 90s, working distance of 8.5 mm, detection angle of 35°, voltage of 25 keV, using a XVP microscope ZEISS SEM EVO 40. The analytical Microscopy (SEM/EDS) allowed identify the morphometry and the chemical composition of siliciclastic minerals quartz, zircon and kyanite; oxides, ilmenite and rutile; monazite formed by O, P, La, Ce, Nd and Th. Different fases of carbonatic minerals were identified: calcite, aragonite and magnesian calcite. Halite, gypsum and framboids of pyrites completed of the mineralogy observed in this region. Generalizations about the predominance of biogenic carbonate sedimentation to the (CE- PI- MA) coast should be revised in the light of the existence of authigenic carbonates observed in this study. Halides and sulfates suggest that the marine waters of this region were exposed to intense evaporation process due to semi-arid climatic condition of the Northeastern Brazil. Sandy plains and fields submerged dunes covered with algae species Halophila decipiens, were identified in the western portion of the study area. The spatial distribution of siliciclastic and carbonate minerals in sediments showed areas of Ceará coast where increased river inflow occurred, and clearly allow the delineation of the transition zone between siliciclastic carbonate and sectors of the Northeast Brazilian Equatorial continental shelf.