fRadioactive shales of Frasnian age in the Parnaiba Basin present high concentrations oi organic matter. They correspond to a condensed section related to the Devonian maximum marine transgression. Combined geochemical, palynological and clay mineral data point out to a dominant algal contribution in the composition of the organic matter, as well as to anoxic depositional settings. This radioactive shale interval corresponds to the onset oi a long-lasting, global anoxic event which was to be intensified in the Late Frasnian, and thus can be regarded as a marker for chronostratigraphic correlations. it includes the main source rocks oi Devonian age in the Palaeozoic basins oi north Brazil, and therefore represents a target oi potential interest for hydrocarbon exploration.