SOBRE A GEOLOGIA DA ÁREA BALIZADA PELAS CIDADES DE BARRA DO GARÇAS E GUIRATINGA, MATO GROSSO E JATAÍ E AMORINÓPOLIS, GOIÁS

New observations on Devonian and Carboniferous (?) (Aquidauana Group) rocks of the Southwest Goias and nearby areas of the State of Mato Grosso, Brazil, are here presented. This region constitutes the north section of the Parana Basin. The Devonian rocks have been preserved in places where they are downfaulted. Afossiliferous, coarse to medium grained, cross-stratified sandstones, interbedded with lenticular micaceous siltstones, are developed in the base of the sequence, attaining a thickness of almost 200 meters. Above them occur fossiliferous shales and siltstones interbedded with coarse to fine cross-bedded sandstones, 175 m thick Closing the Devonian occurs a sequence of fossiliferous shales, siltstones and fine sandstones without cross-bedding, 80 m thick. The Aquidauana Group is more than 1000 meters thick, mostly argillaceous sandstones with some beds of siltstones and conglomerates. The conglomerates have pebbles and cobbles of differente lithology, mostly without stratification and could be tillites. Occa sionaly there are beds of oolitic to pisolitic limestones. One of these beds is fossiliferous. One species of pelecypod, belonging to a new genera, is here described. It is the first fossil of the Aquidauana Group, so far described. Near the top of the first third part of the sequence, occur 90 meters of rithmites, made up of mudstone and siltstone laminae. The rithmites are very constant in the region and are here called Caiapo Formation. This formation is showed in the map of the fig. 2. The rocks of the Aquidauana Group are mostly reddish or brownish in the outcrops. These colors are mainly secondary inasmuch as the whitish hues predominate in cores from drillings. Brownish hues however are present in the cores made up of fine clastics and are ubiquitous in the first 90 meters which suggest that red colors were present in the source areas.