The Rio Peruaçu Basin: An Impressive Multiphased Karst System
Résumé
The Rio Peruaçu basin is located on the left bank of the Rio São Francisco, in the north of the state of Minas Gerais. While its upper portion collects the waters flowing from the sandstone formations of the Urucuia, its lower part cuts into the Bambuí limestones, carving out a narrow canyon about 200 m deep for which the site is known. Over more than 17 km, the stream opens a bed that disappears underground six times. These underground sections are the remainders of an extensive and complex primitive karst network. Although some segments consist merely of simple but majestic arches, others, such as the Brejal and especially the Janelão, offer kilometers of underground galleries that can attain exceptional dimensions. For example, the Janelão cave reaches a ceiling height of 106 m and a width of 60 m. This canyon is punctuated by large and flat areas that function as reservoirs, namely the poljes of Silu and Terra Brava, and at the confluence with the Rio dos Sonhos. The system results from a complex Cenozoic evolution, including several spectacular episodes. As currently reconstituted, this evolution comprises at least three main episodes. The impressive landscapes of this region are not its only interesting feature: humans have occupied the basin since prehistoric times, as attested by about a hundred rock shelters and open-air dwelling sites that have not yet revealed all their secrets.