The Chaco region that makes up the northwestern half (~60%) of Paraguay is a modern foreland basin[7] that extends into Argentina and Bolivia where it borders the Andean thrust front. Superficially the Chaco Basin is an alluvial basin composed of land-derived (in contrast to marine sediments) material, mostly fine sand and clays of Paleogene, Neogene and Quaternary age.[2] On deeper levels the Paraguayan Chaco is made up by four sub-basins, the Pirizal, Pilar, Carandaity and Curupaity basins.