Alwmark reported in 2009 that the Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived.
Furthermore, the orientation of the PDFs, e.g., ω {1013}, π {1012} and r, z {1011}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15– 20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa.
The author concluded that the presence of multiple sets of decorated PDFs in quartz grains oriented parallel to, for impact deformed materials typical crystallographic planes in the infill breccia of the Granby structure provides unambiguous evidence that the structure is impact derived. The decorated nature of the PDFs implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix originating from mature sandstone and angular grains in fragments derived from crystalline basement rocks. The rounded quartz grains display a heterogeneous distribution of shock effects and several high-index PDF orientations. The PDFs in quartz grains from the basement rock are exclusively oriented along crystallographic planes belonging to the zone ω {1013}. The difference in frequency of PDF orientations is most likely a reflection of the different behavior under shock pressure of the two different target lithologies, rather than variation in shock pressure per se.
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