Estudo geológico, geofísico e morfológico da estrutura circular de São Miguel do Tapuio, Piauí - Brasil

• Autor(a) principal: Martins, Jackson Alves
• Data de Publicação: 2016
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Ceará (UFC)
• Texto Completo: http://www.repositorio.ufc.br/handle/riufc/59993
• topic: Bacia do Parnaíba; Astroblema,; Sensoriamento remoto; Geofísica; Geocronologia.

The São Miguel do Tapuio Circular Structure (SMTCS) is a remarkable geomorphological feature with 20 km diameter recognized in Mesopaleozoic sandstones of Parnaíba Basin (Piauí State, Brazil). Studies concerning the origin of the SMTCS are divided between an endogenous origin and other caused by a meteoritic impact. In this study, we performed an extensive bibliographic research, analysis of the previous cartographic studies, field geology and laboratory works (remote sensing, geophysics, topography, petrography and geochronology). The remote sensing data were used to delineate the superficial morphological configuration of the structure, its diameter and compartments, as well as the ring pattern of drainage and the multi-directional pattern of the relief. The information of gravimetric and topographic surveys shows the shallow nature of SMTCS with ~ 1.4 km depth, there is a strong correlation with the theoretical model of the complex type impact structures, there are no regional anomalies associated with a non-outcropping igneous body and the positive Bouguer anomaly observed in one of the profiles is consistent with the observed gravity anomalies in large craters. The petrographic studies did not allow us any identification of typical structural or textural features of a clear impact origin. However, it was found features that could vaguely be regarded as shatter cones and also indicative of micro-deformational features of meteoritic impact (Planar Fractures (PFs), Feather Features (FFs) and Healed Fractures (HFs)), possibly associated with an inferior position to the strongest deformation zone of the structure. The structure core sandstones thermal metamorphism characterizes intense recrystallization and elimination of all previous matrix. They have thrust listric faults with steep dip converging to the structure central point and suggest to be penetrative in shallow subsurface character, highlighting the remarkable correlation with the 2D Euler deconvolution products. The geochronological results from U-Pb not allowed to find the ages of possible isotopic imbalances that could possibly be related to SMTCS (< 400 Ma), but marked the crystallization ages of the zircons and their source areas, with Paleoproterozoic influence (~2200-1800Ma), Upper Mesoproterozoic (~1200-900 Ma) and Neoproterozoic (~800-550 Ma). Finally, it was concluded that although it was not found unequivocal petrographic-mineralogical features of impact processes, all the evidence and characteristics obtained in this research suggests an exogenous source for SMTCS. The current surface may represent the lower portion of an ancient crater, in this related to an astrobleme.