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Usually karst morphology is described in carbonate rocks such as limestone, dolomites, and marble due to the high solubilization of carbonate mineral (especially calcite) in acid water environment. Similar morphology has been described in silicate rocks domains (Young, 1988; Wray, 1997) also called as pseudokarst (Summerfield, 1991; Jennings, 1983). This work discusses the interactions between geological structures and quartzite dissolution that favor the origin and development of karst features in the middle Preto river basin, on the border of Rio de Janeiro and Minas Gerais states. This basin is located in a stable tectonic region and presents highly weathered rocks. A sequence of neo-proterozoic metassedimentary rocks is related to the Andrelndia group (Ribeiro et al., 1990; Heilbron et al. 1995) composed by banded biotite-gneiss and quartzite and folded at least by three deformation phases. Gneiss is rich in biotite, quartz, feldspar and garnet, with well marked metamorphic foliation and grain size between 2 and 5 mm. Detailed geological mapping (scale 1:10,000) shows two lithotypes in the quartzite layers: (i) very pure quartzite, more than 95% of quartz, crystals between 3 and 8 mm, homogeneous, poor foliation and sacaroidal texture; (ii) impure foliated quartzite, where quartz is associated with feldspar and sometimes muscovite, grain size from 1 to 3 mm. The map also shows an extensive recumbent folding and sub-horizontal boundaries. Both lithologic and structural conditions in association to vertical and sub-vertical joints favor water infiltration especially in the hillslope summit and exfiltration in the sideslopes and valley bottoms. This percolation cause rock dissolution, particularly in the pure and impure quartzite boundaries, giving origin to caves on sideslopes and sinkholes along the river channels. Cave development can lead to doline formation, shafts and towers, most of which are guided by joint trends. Analysis in cave groundwater and river flow carried out by mass spectrometry methods (ICP-MS, 810 Varian) focusing Si, Al, Ca, Fe(total), Mg, K, and, Na have shown values (mg.L-1) respectively between: 0.24 to 0.80; 0.04 to 3.80; 0.18 to 0.72; 0.09 to 0.50; 0.02 to 0.57; 0.08 to 1.01; and 0.05 to 1.74. One observe differences between groundwater and river flows concentration; but data is not yet well documented for a better explanation. Field observations point out that these karst morphologies associate both mechanical and chemical processes: mineral dissolution (quartz included) cause decrease of the rock cohesion and favor mechanical erosion by fallen rock blocs and grain transport by water flows.
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