The Paraná Basin is a large sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande do Sul states. The remainder area is distributed in eastern Paraguay, northeastern Argentina and northern Uruguay. The shape of the depression is elliptical and covers an area of about, of which 110,000 km2 are in Paraguay. The basin developed during the Paleozoic and the Mesozoic with a sedimentary record comprising rocks from the Ordovician right up to the Cretaceous, thus spanning the time interval between 460 and 66 million years. The maximum thickness of the infill reaches 7,000 m in its central area and is composed of sedimentary and igneous rocks. The Paraná Basin is a typical intra-cratonic flexural basin, although during the Paleozoic it was a gulf that opened to the southwest. The basin genesis is related to the convergence between the former Gondwanasupercontinent and the oceanic crust of the former Panthalassa ocean. The basin formed, at least during the Paleozoic orogenesis of the Gondwanides, a foreland basin. The Paraná River, that forms Paraguay's eastern boundary, flows along the central axis of the Paraná Basin and drains the modern basin.
The Chaco region that makes up the northwestern half of Paraguay is a modern foreland basin that extends into Argentina and Bolivia where it borders the Andeanthrust front. Superficially the Chaco Basin is an alluvial basin composed of land-derived material, mostly fine sand and clays of Paleogene, Neogene and Quaternary age. On deeper levels the Paraguayan Chaco is made up by four sub-basins, the Pirizal, Pilar, Carandaity and Curupaity basins. Carandaity and Curupaity in the northwest are chieftly composed of Paleozoic sediments. Pirizal Basin is made up mostly of Late Cretaceous and younger sediments. The Pilar Basin is located mainly on Presidente Hayes Department and borders the Paraná Basin across Paraguay River to the east, geologically the basins are separated by the Asunción Anticline that roughly follows the path of Paraguay River.
In a strip along Paraná River tholeiitic basalts belonging to the Paraná Traps makes up most of the bedrock and overlies earlier sediments of the Paraná Basin. These tholeiites were erupted in the Early Cretaceous in association with the initial opening of the South Atlantic. The Cretaceous volcanism did also created minor outcrops of potassic alkaline volcanics in a graben structure developed between Asunción and Villarrica and in Amambay Department. Cretaceous volcanism did also left small units of sodic alkaline rocks in Misiones Department in southern Paraguay.
