A Pennsylvanian saline-alkaline lake in Gondwana mid-latitude: Evidence from the Piauí Formation chert deposits, Parnaíba Basin, Brazil

Abstract

The Pangaea assemblage resulted in significant tectonic and eustatic changes, but also in a well-known desertification triggered by the increasing continentalization. While the northern hemisphere basins have been thoroughly studied, several recent studies on southern hemisphere basins bring new insight on how these events affected the Gondwana hinterlands. In this study, sedimentological and geochemical analyses of chert deposits from the Pennsylvanian Piauí Formation of the Parnaíba Basin are examined as new evidence for the onset of continentalization and aridification in the northwestern Gondwana mid-latitudes. The chert deposits correspond to three facies (stromatolites, ooidal arenites and breccias) that together suggest deposition in a shallow water environment and biomarkers analysis of stromatolites indicated predominance of organic matter produced by cyanobacteria in an environment characterized by high salinity and elevated pH. Such evidence suggests that the Piauí Formation cherts deposited in a shallow, saline-alkaline lacustrine environment during a period marked by increasing arid climate conditions.