TONSTEINS DA FORMAÇÃO RIO BONITO NO DEPÓSITO DE CARVÃO CANDIOTA, RS

Tonsteins are the most notable and conspicuous chrono-stratigraphic markers known in sedimentary sequences. They are actual chronosurfaces due to their instantaneous formation in the geological time table scale. Considering the mineralogical composition and stratigraphic behavior of the tonsteins of the Candiota Coalfield, southernmost Brazil, it was possible for the first time to demonstrate that they were originally of fine pyroclastic-fall material, transported by wind over Permian deposition areas of the Parana Basin. The tonsteins are nowadays composed of kaolinite, originated from post sedimentary transformation of volcanic glass. The transformation of volcanic glass to kaolinite was favored by organic acids coming from coal-bearing beds.Scattered in the kaolinitic mass, accessory minerais such as euhedral zircon, beta-quartz paramorph and apatite are considered as pyroclastic minerais preserved during diagenesis. The occurrence of these minerais confirms the origin of Rio Bonito Formation tonsteins as deposits transformed from tephra. On the other hand, pyrite associated with fossil vegat al remains scattered in tonsteins are considered authigenic. The lateral continuity of beds for more than ten kilometers with small thickness variation of the tonsteins is considered as typical of distai pyroclastic-fall deposits. The occurrence of several tonstein beds interlayered with coal beds indicates several episodes of tephra sedimentation, represented by ash and dust, during a period of time corresponding to Parana Basin coal beds.