Carste em rochas siliciclásticas na Chapada Diamantina: geoespeleologia da gruta do Canal da Fumaça, Vila de Igatu, Andaraí (BA).

Abstract

The development of karst reliefs in siliciclastic rocks is a fact already recognized and studied by science in recent decades. In this sense, the Chapada Diamantina region, in the center of the State of Bahia, which is notorious for the presence of extensive cave systems installed in carbonate rocks, is currently also gaining visibility for siliciclastic karst terrain, especially in the Sincorá Ridge. This unit of mountainous relief, which reaches 1,700 m, is supported by metasandstones and metaconglomerates of the Tombador Formation (Chapada Diamantina Group, Espinhaço Supergroup), of Mesoproterozoic age. The highlights include the Lapão Cave, in the municipality of Lençóis, with 1.6 km of extension and the Castelo Cave, in the Pati Valley, municipality of Mucugê, with 383 m. Both are touristic sites within the Chapada Diamantina National Park and, therefore, have greater visibility in academia and the public. In turn, the Village of Igatu, in the municipality of Andaraí, is home to valuable karst systems, with complex networks of underground conduits and ruiniform relief, still poorly understood. In the area of the Coisa Boa River Basin, one of the tributaries of the Paraguaçu River, 12 caves are recognized, some of which already described in previous works and registered in speleological databases. In this work, the characterization of one of the karst systems, the Canal da Fumaça Cave, was carried out through speleological cartography, structural analyzes by remote sensing and field measurement, descriptions of rock samples through optical and scanning electronic microscopes, in addition to determination of chemical (FRX) and mineralogical (DRX) composition. It was observed that NNE-SSW, NNW-SSE, and ENE-WSE structures, associated with the sinistral transpressive strike that structured the Sincorá Ridge, exert an important influence on the opening of the conduits and the formation of the ruiniform relief, acting as preferential weathering fronts. The petrographic assessment suggests that the chemical attack occurs mainly on the phyllosilicates (pyrophyllite and kaolinite) that make up the matrix of metasandstone in the area, being more soluble in acidic pH. Secondarily, quartz grains are also affected. This process, called phantomization, reduces the cohesion between the grains and then piping processes remove them mechanically. On the other hand, a profound impact of the diamond mining activity, carried out in the 19th and 20th centuries, was also identified. Modifications such as excavations and detonations of conduits, construction of walls and pillars, significantly altered the interior of the cave, even helping to increase its extension and volume. With this in mind, an evolution model was proposed for the Canal da Fumaça Cave, considering karst geological processes and also anthropogenic changes.