Resurfaced paleodunes from the Botucatu erg amid Cretaceous Paraná volcanics

Abstract

Extensive exposure of Cretaceous paleodunes partly covered by basaltic rocks is a rare surficial structure in the geomorphology of continents. The Paraná Basin offers the opportunity in southernmost Brazil to study the relationships between the major Botucatu paleoerg-turned into the Guarani Aquifer (1.2 million km2) and the overlying, first flows from the Paraná Volcanic Province (1 million km2). We describe the Inhanduí Paleodune Field (IPDF; 15,000 km2) preserved by the first two (up to six) flows of the Paraná Volcanic Province. The field was exposed by erosion of volcanic overburden in Fronteira Oeste of southernmost Brazil. A plan view of the field is examined in the extensive grasslands and rice crops of the pampas. Observational geology from satellite images was followed by field description, gammaspectrometry, petrography and whole-rock geochemistry. Results show that bedforms include thousands of varied individual and compound paleodunes, including crescentic (barchane), linear, circle (blowout), and complex forms. Cerro do Jarau was considered an impact crater, and is here characterized as a compound paleodune. The paleoerg sand was silicified and deformed by hot water from the Guarani aquifer and the explosion of vapor in IPDF. The active dunes were flooded with tholeiitic lavas; heating by residual heat from volcanism (raised geothermal gradient) led to sand injection into overlying volcanic rocks. Silicification accompanied and followed the injection process. The Botucatu Formation includes the paleodunes and was the source bed of the sand injectites (Novo Hamburgo Complex) into the overlying volcanic group. We thus uncovered the processes associated with the flooding of a major, active erg by lavas from a large volcanic province. Hyperdry climate prevailed in the sandy Botucatu desert and continued in the rocky Serra Geral desert. The studied geological relationships are unique in the geological records; their understanding fosters the correct description of geological evolution of other terrains where some of the parameters are present.