Basaltos del Paraná (Formación Alto Paraná)

Along the Paraná River, much of eastern Paraguay’s bedrock is tholeiitic basalt of the Alto Paraná Formation, equivalent to Brazil’s Serra Geral Group. It is part of the Paraná–Etendeka magmatic province, one of the planet’s largest flood-basalt provinces, whose main phase has been dated to 135–133 million years ago, before the South Atlantic Ocean opened.

These basalts overlie the sandstones of the Paraná Basin and act as a low-permeability “lid” on the Guaraní Aquifer. When they weather, they give the iron-rich red soils of Alto Paraná, Canindeyú and Itapúa (tierra colorada).