The wet desert system of the Upper Jurassic of the Parnaiba Basin, in the region between Formosa da Serra Negra and Montes Altos, Maranhão State, Brazil

Abstract

The Mesozoic was marked by significant geological changes, resulting of the Gonduanide Orogeny, which propitiated the implantation of desert systems concomitantly with expressive magmatic events. In Brazil, the record of these events is found in the central portion of the Parnaíba Basin, represented by jurassic siliciclastic deposits of Corda Formation, that overlie the basaltic flow of the Mosquito formation. Outcrop based stratigraphic and facies analysis, carried out in the Formosa da Serra Negra regions and Montes Altos, State of Maranhão, allowed the paleoenvironmental and paleoclimatic reconstitution of this part of Parnaiba Basin during the Jurassic. We identified fifteen sedimentary facies grouped into three facies associations (FA), representative of a wet desert system: 1) braided stream channel deposits (FA1), characterized by conglomerates and fine to coarse-grained sandstone, with grains and pebbles of volcanic rocks, even parallel and trough cross stratifications; 2) sandy sheets (FA2), comprising fine to coarse-grained sandstone and subordinate mudstones with even parallel lamination, climbingripple cross lamination, adhesion ripples and gutter casts; 3) dune field (FA3), constituting fine to medium-grained sandstones with rounded grains and planar and tangential cross bedding. Ephemeral fluvial flow (FA1) adjacent to the dune field (FA3), frequently reworked the upper portion of the volcanic and sporadically migrate into the sandy sheet setting. The Jurassic desert of Corda Formation was wetter and smaller than to the Perm-Triassic ergs, preceding the Cretaceous fluvial to coastal systems of the Parnaíba Basin.