General aspects of continental margin sediments of the Ceará State, northeast Brazil, from mineralogy and principal component analysis
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Abstract
Granulometry, mineralogy, portable X-ray fluorescence (XRFp), portable X-ray diffractometry (XRDp), and principal component analysis (PCA) have been made in samples of two deep cores of the continental margin of the Ceará State, northeast Brazil, aiming to detail the general aspects of deep sedimentation, including mechanical dispersion of continental minerals, environmental chemistry, and clay identification. The samples comprised biogenic oozes, clays, coarse silt, and fine sand to a lesser extent. The granulometric composition suggests slow and calm sediment accumulation over time. XRDp analysis pointed out that pelagic clays consist of chlorite, kaolinite, nontronite, and vermiculite. High oxidation levels are suggested for the two core samples, represented by the light brown color 5YR 6/4 of the sediments. Traces of minerals existing at the continental shelf were found, although in the core SIS-454 at 160 to 164 cm depth, there is a total absence of both heavy and light continental minerals. In core SIS-515, the total absence of heavy and light minerals was found at the depth of 87 to 92 cm. The levels with absence of continental minerals are probably associated with the absence of mechanical dispersion due to transgression and regression events in the Ceará continental shelf. The high oxidation levels can probably be linked to shallow waters caused by sea level changes, or to large period of sedimentation in the slope. From the statistical analysis, it is suggested that element contents are related to each other according to the provenance setting, whereas the pelagic sediment has provenance from marine water, resurgence currents, wind transport, cosmic dust, and from slow transportation, originated from the continent. It was deduced that sedimentation in the area was affected by at least two events with absence of mechanical dispersion of continental sediments, associated with geochemical characteristics represented by antagonism between elements of terrigenous and biogenic origin. These events are possibly associated with small-scale regressions occurred during the Flandrian Transgression in the Ceará Basin.
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