Geological field guide to the early cretaceous inhanduí hotsprings and their host botucatu paleodunes

Structures formed by geological processes that occurred during the Early Cretaceous are preserved in large areas of the Fronteira Oeste Rift in Rio Grande do Sul, Brazil. Among these areas, the Inhanduí Paleodune Field (IPDF), located in northeastern Alegrete, is part of the Botucatu paleoerg, the largest paleodune field in Supercontinent Gondwana. This area shows hundreds of circular and curvilinear dunes of silicified sandstone of variable sizes, which were covered by intraplate Paraná volcanic rocks. This field guide provides the information for a field trip from Porto Alegre, Brazil to two hills (Cerros) in the IPDF, Cerro Farrapo and Cerro Figueirinha. Important features formed by hot water, volcanic rock and sediment interaction are preserved in large outcrops in these hills, including extensive silicification and deformation of the buried dunes, injection of fluidized sand and explosive hydrothermal activity. Hydrothermal structures include preservation of subterranean conduits that emerged at the Earth’s surface to form surging hot pools, geysers and spouts. These features are preserved as bowls, rounded to polygonal cavities and prismatic and lozenge-shaped clasts and blocks. Access to the outcrops is mostly via paved roads, with 40 km in unpaved roads.