GEOCHEMICAL CHARACTERIZATION AND MIOSPORE BIOCHRONOSTRATIGRAPHY OF THE FRASNIAN ANOXIC EVENT IN THE PARNAIBA BASIN, NORTHEAST BRAZIL

fRadioactive shales of Frasnian age in the Parnaiba Basin present high concentrations oi organic matter. They correspond to a condensed sect ion related to the Devonian maximum marine transgression. Combined geo chemical, palynological and clay mineral data point out to a dominant algal contribution in the composition of the organic matter, as well a s to anoxic depositional settings. This radioactive shale interval cor responds 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 sou rce rocks oi Devonian age in the Palaeozoic basins oi north Brazil, an d therefore represents a target oi potential interest for hydrocarbon exploration.