Distribution, mineralogy, petrography, provenance and significance of Permian ash-carrying deposits in the Paraná Basin

摘要

The study of thin sections taken from drill core samples and outcrops in southern Brazil has demonstrated the occurrence of repeated ash fall episodes during the Permian period between 280 and 245 Ma. Scattered or concentrated altered volcanic glass shards were detected in strata of Permian sedimentary rocks of the Rio Bonito and Tatui Formations. The shard-carrying sediment is usually a silty or cherty mudstone with variable amounts of calcrete calcite. Burial and alteration of unstable glass debris led to the development of analcite and less commonly, calcite, silica minerals, zeolites or montmorillonite. Sources of explosive Permian volcanism, responsible for ample ash falls have been sought in South Africa, in the Andean Cordillera and along the Paraná Basin margin. A swarm of rhyolitic centers described in the Cordillera Frontal and in the Central Argentinian Provincia de La Pampa (Patagonia) is here proposed as the most adequate Permian source. In that area, volcanoes must have expelled ashes that traveled thousands of kilometers before settling in deltaic or shallow marine environments. Twenty-three occurrences of Permian shard-carrying sediments have been plotted in a Paraná Basin map. The number of ash-carrying sediments decreases northeastward. The authors envisage dense clouds originated in Patagonia traveling NE, depositing progressively smaller quantities of ashes but reaching Australia in pre-drift Gondwana. Shard carrying sediments must be distinguished from"tonstein", ash fall or ash flow tuffs, also recorded in the Permian Gondwana, which would indicate closer ash source area.