Fácies e estratigrafia da parte superior da Formação Pedra de Fogo, Permiano da Bacia do Parnaíba, região de Filadélfia-TO

Abstract

The Permian Pedra de Fogo Formation, which belongs to the Balsas Group of the Parnaíba Basin, northern Brazil, is characterized a siliciclastic-carbonate succession with expressive chert content, rare stromatolites and fossils of animals and plants, especially silicified trunks, mainly of the Psaronius genus. In order to emprove the stratigraphic and paleoenvironmental knowledgement of the upper portion of the Pedra de Fogo Formation, outcrop-basedm facies analysis was carried out in the Filadélfia region, State of Tocantins, Southwestern Parnaíba Basin. This study allowed the definition of a 100 m thick siliciclastic succession with subordinate occurrences of carbonate and evaporites. Twenty-five sedimentary facies grouped into six facies associations (FA) were identified: FA1-Lacustrine with ephemeral river deposits; FA2- Storm wave-influenced lake deposits; FA3-Continental sabkha deposits; FA4- Central lake deposits; FA5-Dune field deposits; and AF6-Lake/oasis deposits with inunditos. These associations indicate that during Permian, an extensive lacustrine system developed with adjacent dune fields and continental sabkha, as well as with contributions from ephemeral rivers. Which when debauching into the lakes provide the formation of suspension lobes and sheet flows (FA1). Sabkha plains (FA3) were formed in the marginal portions of the lake, eventually influenced by storm waves (FA2), while the central zone was site of intense pelitic deposition (FA4). The low supply of wind-blocon sand led to the formation of restricted dune fields (FA5) with development of interdune lakes (oasis), where giant ferns, sporadically flooded by ephemeral rivers (FA6). The predominanc of smectite and the lack of kaolinite as well as the ocorrence of evaporites in the Association AF3 support the facies data that the apper part of the Pedra de Fogo Formation was laid during a hot and arid climate.