Abstract
Fascinating circular hydrothermal bowls 0.5 m in diameter occur in paleodunes of the 1.2 million km2 Botucatu paleoerg in the Fronteira Oeste of Brazil, Rio Grande do Sul, and were exposed upon erosion of presumed 2,000 m of basalt and rhyodacite cover of the Paraná Volcanic Province (1 million km2) since the Cretaceous (135 Ma). The tops of the paleodunes display low-temperature (150-50°C) hydrothermal structures comparable to those in basalts and rhyodacites. The source beds of sand injectites display evidence of sequential fault-valve action. The bowls represent a new structure in continental paleodunes. They present circular rims sealed by quartz and a flat horizontal bottom with a central sealed vent. The inclined internal rims of the bowls exhibit a radial set of linear fractures filled with quartz, forming prominent, sharp, positive structures. Their origin is ascribed to the work of water vapor originated in the heated freshwater aquifer. The concave ceiling was lifted a few millimeters along circular fractures long enough for quartz to be deposited. Erosional exhumation of the buried Botucatu hyperdry sand sea has revealed the effects, including the new structures described here, of hydrothermalism associated with a major igneous province upon a giant paleoerg turned-aquifer.
Highlights
The interplay of several processes resulted in the formation of a large variety of structures at the interface of heated freshwater (150°C) from the Guarani Aquifer with the host Botucatu paleoerg (1.2 million km2) and the overlying Paraná Volcanic Province (1.0 million km2)
The basalts and rhyodacites formed a large igneous province in Gondwana, extending from southeastern South America (Brazil, Uruguay, Argentina, Paraguay) to western Africa (Namibia, Congo, Angola).The heating of the underlying aquifer resulted in sand injection into many lava flows of the province culminating in effusion on top of flows in a continued hyperdry desert (Botucatu desert — Amarante et al 2019, Serra Geral desert — Hartmann and Cerva-Alves 2021)
In this paper we examine the bowls discovered at the top of the paleodunes present amid the effusive Catalán andesite (Hartmann et al 2010)
Summary
The interplay of several processes resulted in the formation of a large variety of structures at the interface of heated freshwater (150°C) from the Guarani Aquifer with the host Botucatu paleoerg (1.2 million km2) and the overlying Paraná Volcanic Province (1.0 million km). The basalts and rhyodacites formed a large igneous province in Gondwana, extending from southeastern South America (Brazil, Uruguay, Argentina, Paraguay) to western Africa (Namibia, Congo, Angola).The heating of the underlying aquifer resulted in sand injection into many lava flows of the province culminating in effusion on top of flows in a continued hyperdry desert (Botucatu desert — Amarante et al 2019, Serra Geral desert — Hartmann and Cerva-Alves 2021). The bowls occur in paleodunes of the Botucatu Formation (Scherer 2000, Bertolini et al 2020) from the Cretaceous (150-135 Ma; Hartmann and Cerva-Alves 2021), formed in a continental environment. Field work allowed characterization of the bowls as a result of water vapor venting through silicified sandstone, likely below 0.5 m of andesite and some sandstone
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