Ordovician Chitinozoa and Biostratigraphy of Brazil

Marine sedimentary rocks of Ordovician and supposed Ordovician age have been investigated for chitinozoans in five Paleozoic basins of Brazil, viz. Solimões, Amazonas, Parnaíba, Jatobá, and Paraná basins. The lithology is predominantly sandstones with subordinate siltstones and shales. In the Solimões Basin, the basal Paleozoic Benjamin Constant Formation yielded a characteristic north Gondwana assemblage of late Arenig to early Llanvirn age. Overlying beds are of Silurian age. Within or in connection to the Parnaíba and Paraná basins continental sedimentation occurred in restricted basins with molasses and acid to intermediate igneous rocks of supposed Early - Middle Ordovician age. A more widespread transgression started probably in late Caradoc and gave rise to the first Paleozoic marine sedimentation of the Amazonas (Autás-Mirim Fm), Parnaíba (Ipú Fm, possibly Jatobá Fm, Tacaratu Fm), and Paraná (Rio Ivaí Fm) basins. A diagnostic Ashgill (Rawtheyan) chitinozoan assemblage is present in the upper part of the Autás-Mirim Fm. The Ordovician-Silurian boundary lies approximately at the boundary between the Autás-Mirim Fm and the overlying Nhamundá Fm. No contemporary chitinozoan faunas are known elsewhere in Brazil.