Cuenca del Chaco (llanura de sedimentos)

The Paraguayan Chaco, which covers about 60% of the territory, is a modern foreland basin that extends into Argentina and Bolivia and borders the Andean thrust front. Its surface is an alluvial basin: fine sands and clays of continental origin, of Paleogene, Neogene and Quaternary age, rather than marine sediments. At depth there are four sub-basins (Pirizal, Pilar, Carandaity and Curupaity).

That is why there is almost no bedrock in sight and few caves: a caving website describes the Chaco as a plain of loose sediments, with very little rain across much of it and with large rivers flowing down from the Andes. It is one of the largest actively filling continental sedimentary basins on Earth.