Argilominerais da Forma？？o Codó (Aptiano Superior), Bacia de Grajaú, nordeste do Brasil

this work combines facies, stratigraphy, x-ray diffraction, sem and petrographic analyses in order to investigate argillaceous rocks of the codó formation exposed in the codó and grajaú areas, northeastern brazil (fig. 1). the main goal is to characterize the clay mineral assemblage and record its variability along the depositional cycles in the attempt to discuss its origin and test its applicability as paleoclimatic and paleoenvironmental proxy indicators. the codó formation records the upper aptian deposition of the grajaú basin, a tectonically subsiding structure originated during the early stages of rifting that culminated with the opening of the south atlantic ocean along the equatorial brazilian margin (aranha et al., 1990). this basin has been interpreted as a unique structure consisting of an intracontinental semi-graben, which was formed by combination of pure shear stress and strike-slip deformation (góes and rossetti, 2001). the sedimentary successions of this basin include three depositional sequences, designated as s1, s2 and s3 (rossetti, 2001; fig. 2), with the first including the codó formation studied herein. the codó formation records a depositional system dominated by closed and hypersaline lakes and sabkhasalt pan complex (paz, 2000; paz & rossetti, 2001; paz & rossetti, 2005a). in the codó area, these deposits encompass a 25 m-thick prograding lacustrine succession arranged as shallowing-upward cycles that are attributed to central, transitional, and marginal lake facies associations. central lake deposits include mostly evaporites and bituminous black shales. transitional lake deposits consist of green to grey shales interbedded with limestones (lime-mudstone, laminated to massive peloidal wackestone to packstone and sparstone) formed in more oxygenated waters relative to the central lake deposits. marginal lake deposits include massive and indurated pelite with brownish-red colours, fenestral calcarenite of calcite grains, ostracodal wackesto