The Serra da Cangalha impact structure, Brazil: Geological, stratigraphic and petrographic aspects of a recently confirmed impact structure

July 22, 2017 | Autor: Alvaro Crosta | Categoria: Geology, Geophysics
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Journal of South American Earth Sciences xxx (2013) 1e15

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Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames

The Serra da Cangalha impact structure, Brazil: Geological, stratigraphic and petrographic aspects of a recently confirmed impact structure Marcos Alberto Rodrigues Vasconcelos a, *, Alvaro P. Crósta a, Wolf Uwe Reimold b, c, Ana Maria Góes d, Thomas Kenkmann e, Michael H. Poelchau e a

Institute of Geosciences, University of Campinas, Campinas, SP, Brazil Museum für Naturkunde, Leibniz-Institute for Evolution and Biodiversity Research, Invalidenstrasse 43, 10115 Berlin, Germany Humboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany d Institute of Geosciences, University of São Paulo, Brazil e Institut für Geowissenschaften, Albert-Ludwigs-Universität Freiburg, Germany b c

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Article history: Received 18 October 2012 Accepted 18 March 2013

Serra da Cangalha is a complex impact structure with an apparent diameter of 13.7 km located in essentially undisturbed sedimentary rocks of the Parnaíba basin in northeastern Brazil. The stratigraphy of the crater region includes, from bottom to top, the Longá, Poti, Piauí and Pedra de Fogo formations of Devonian to Late Permian age. The age of the impact event is constrained to 8e10 GPa for the formation of PDF in quartz are currently the only basis on which to assign approximate shock pressures experienced by crystalline or sedimentary rock. Displacement was observed along some FF and PF planes in shocked quartz. Some authors have suggested that these planes represent microfaults along planes of high shear stress (Christie et al., 1964; Gratz et al., 1988, 1992; Poelchau and Kenkmann, 2011). Christie et al. (1964) interpreted that planar fractures are initiated by plastic slip on crystallographic planes, which then propagates by brittle fracture mechanism. These authors observed the occurrence of an isotropic phase comprised of Si, Al, O along PF and they interpreted this as quenched glass formed by frictional melting. The occurrence of these phases also indicates formation at stress concentrations, and a high mobility allowing elemental movement along the planar fractures. Microfolds and faults present in the Longá rocks are indicative of pseudo-ductile and brittle regimes that occur during the formation of the central uplift. In situ fracturing of rocks of all the stratigraphic units within SdC gives the target rocks at SdC clearly different characteristics when compared to similar undeformed rocks of the Parnaíba Basin. As a result of brittle deformation, broken grains produce a false appearance of a poorly-sorted rock. Hence, although these features are not diagnostic of impact, they can be used also to characterize the degree of deformation imposed by the impact. 6. Concluding remarks We present here new detailed observations regarding the stratigraphy within the SdC structure, and new findings related to macroscopic and microscopic impact-induced deformation e including diagnostic shock features. The occurrence of diagnostic shock deformation features in quartz is restricted to the Carboniferous rocks of the Poti and Piauí formations within the innermost, prominent collar of the central uplift and, particularly, to the exposures in the central depression corresponding to the Devonian to Upper Carboniferous Longá Formation. The analysis of the shock features provided constraints that allow to estimate shock

pressures of w5e10 GPa for the rocks of the innermost collar of the structure. These constraints are based on measurements of several sets of single PDF (0001 direction) found in quartz of shatter cones from the central depression, and one set found in a drill core sample into the Longá Formation. Some quartz grains in sandstones from the southern part of the collar exhibit PF and FF, thus indicating pressures
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