Estudo sumário de algumas formações sedimentares do Distrito Federal


  • Alfredo José Porto Domingues


Distrito Federal - Brasil, Rio de Janeiro, Geologia Estrutural


            In this paper, the author, Prof. Alfredo Porto Domingues makes a study of some sedimentary formations of the Federal District.

            He studied the regions of Matoso hill, an elevation to the south of Ilha do Governador, the village of Pedra da Guaratiba and another hill between Triagem and Vieira Fazenda, along the tracks of the Central Railroad of Brasil.

            He presents, initially, an historical study of the sedimentary formations, adding his own historical studies to this part.

            In continuing, he gives an idea of the outcrops, describing the sandy clay material which contains quartz pebbles and limestone blocks; these deposits were found on Matoso hill.

            At the base of the formation, inter stratified with the clay beds, he found a particular bed composed of recent shells mixed to pebbles.

            The author makes, then, a description of the material found on the terrace at Pedra da Guaratiba, and on the hill between Triagem and Vieira Fazenda.

            The author makes, furthermore, a study of the pebbles found in the three formations, comparing these with other pebbles originator of other regions.

            The method of study is based in the comparison of histograms (graphics) which are obtained by measuring the smaller radius and the length of the pebbles. By dividing the double of the smaller curvature (roundness) radius by the length of the pebbles, the curvature index is obtained.

            The histogram is made by is made by placing on the abscissa the curvature indexes, in such an order that each division corresponds to 50% and, on the ordinate, the percentage of pebbles.

            The author makes, then, a succinct characterization of marine and fluvial pebbles studying, at the same time, the histograms.

            Finally, he arrives to his conclusions stating that the pebbles found on Ilha do Governador and Pedra da Guaratiba are marine, and fluvial those found at the hill between Triagem and Vieira Fazenda.