Abstract
The 3.6 km-diameter Colonia impact crater, centred at 2352'03S and 4642'27W, lies 40 km to the south-west of the S?o Paulo city. The structure was formed on the crystalline basement rocks and displays a bowl-shaped with steeper slope near the top that decreases gently toward the centre of the crater. Over recent years were drilled two boreholes inside the crater, which reached a maximum depth of 142 m and 197 m. Geological profile suggests four different lithological associations: 1) unshocked crystalline basement rocks (197 - 140 m); 2) fractured/brecciated basement rocks (140 - 110 m); 3) polymictic allochthonous breccia deposits (110 - 40 m); and 4) post-impact deposits (40 - 0 m). Petrographic characterisation of the polymictic allochthonous breccia reveals a series of distinctive shock-metamorphic features, including, among others, planar deformation features in quartz, feldspar and mica, ballen silica, granular texture in zircon and melt-bearing impact rocks. The occurrence of melt particles and very high-pressure phase transformation in suevite breccia suggest a shock pressure regime higher than 60 GPa.
Highlights
The proposal of impact cratering as a geological process was severely criticised by some scientists at the beginning
Petrographic characterisation of the polymictic allochthonous breccia reveals a series of distinctive shock-metamorphic features, including, among others, planar deformation features in quartz, feldspar and mica, ballen silica, granular texture in zircon and melt-bearing impact rocks
The presence of a few rounded granules of sedimentary origin in the polymictic allochthonous breccia suggests that a thin sedimentary layer would have partially covered the crystalline basement before the impact
Summary
The proposal of impact cratering as a geological process was severely criticised by some scientists at the beginning. An Atlantic tropical climate prevails over the coastal region that heightens the processes of weathering and erosion In such geological settings, the terrestrial crust material is continuously being reworked and the likelihoods of preserving an impact structure are remote. The kinetic energy is transformed into high-pressure shock waves that propagate at a supersonic velocity [2]. Under these extreme physical conditions, rocks and minerals are strongly modified and a wide variety of unusual microstructures of irreversible changes is formed [20,21,22,23]. In this paper is reported, for the first time, a set of representative optical features of shock-inducted microdeformations in rock fragments and mineral clasts that were preserved in allochthonous breccia deposits from the Colônia Crater
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