Sandstone dissolution landforms in the Furnas Formation, Southern Brazil

摘要

The Furnas Formation (Silurian to Devonian of the Parana Basin) consists of medium to coarse sandstones (quartzarenites) cemented by kaolinite, which reduces its porosity and makes the rock more resistant. A Mesozoic crustal upwarping called the Ponta Grossa Arch and subsequent erosion gave origin to the Devonian Scarp, which is a cuesta supported by the sandstones. The scarp serves as a boundary line for the Second Parana Plateau, a step of the en echelon relief in the State of Parana in southern Brazil. In the outcropping areas the sandstones display a ubiquitous phenomenon of superficial exfoliation by dissolution of the kaolinitic mesodiagenetic cement and reprecipitation of microcrystalline telodiagenetic kaolinite. The dissolution of kaolinite contributes to the development of small scale superficial landforms such as alveoles, runnels, dissolution basins, concavities at the base of rocky walls, polygonal joints, anastomosing tunnels and erosion columns. In the Quebra-Perna River area near the Devonian Scarp, the sandstones of the Furnas Formation show many features of significant subterranean erosion, such as 'furnas' (collapsed hollows), ponds, wet and dry depressions, sinkholes, tunnels and grikes. Such features are associated with the presence of brittle structures (faults, fractures, dikes) in two directions: the main one, NE-SW, corresponds to reactivations of the Proterozoic basement of the Parana Basin and the secondary one, NW-SE, runs parallel to the Ponta Grossa Arch axis. Furnas and other associated features frequently occur at the crossing of such structures. These findings indicate that the great features of subterranean erosion are the result of kaolinite dissolution, which intensifies along structural directions, in a region of scarped relief and steep hydraulic gradient. As a consequence, the Furnas Formation displays landforms that are characteristic of a both structural and karstic aquifer. Copyright (c) 2007 John Wiley & Sons, Ltd.