¹C. Alwmark,²G. G. Kenny,¹S. Alwmark,³P. Minde,⁴J. Plado,⁴S. Hietala, ², M. J. Whitehouse
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14280]
¹Department of Geology, Lund University, Lund, Sweden
²Department of Geosciences, Swedish Museum of Natural History, Stockholm, Sweden
³Arctic Planetary Science Institute, Rovaniemi, Finland
⁴Department of Geology, Institute of Ecology and Earth Sciences, University of Tartu, Tartu, Estonia
⁵Geological Survey of Finland, Kuopio, Finland
Published by arrangement with John Wiley & Sons

Here we report on findings for four rock samples with melt texture found in a gravel pit within a glaciofluvial deposit near the small town of Kitkiöjärvi in northernmost Sweden. The samples are comprised of granitic clasts embedded in a brown fine-grained melt matrix. The samples all contain quartz grains and/or clasts exhibiting multiple sets of planar deformation features oriented parallel to crystallographic planes characteristic of shock metamorphism. The samples also contain Former Reidite In Granular Neoblastic (FRIGN) zircon. We therefore conclude that the investigated samples represent impact melt rock. We interpret a U-Pb concordia age of 658.9 ± 6.9 Ma (Cryogenian) derived using secondary-ion mass spectrometry analysis of shocked zircon, as the best estimate for the age of the impact event that formed the melt rocks. Zircon grains from two of the samples yield younger lower intercept ages, raising the possibility that the samples came from multiple impact events of different ages. Although we cannot exclude this possibility, we interpret the younger ages from the clast-rich melt rocks to reflect non-impact-related Pb loss events and suggest that all samples likely came from the same structure. Analysis of the glaciofluvial history of the region, along with the relatively high frequency of finds (five in total, as one similar melt rock was found in the pit in 2018), points to a short-distance glacial transportation of the samples from the southwest. Since there are no known impact structures in Sweden within that area and/or of similar age, we conclude that an old (the oldest known yet) impact structure in Sweden potentially is yet to be discovered somewhere in the vicinity of the gravel pit.

¹Roger L. Gibson,¹S’lindile S. Wela,^2,3Auriol S. P. Rae,¹Marco A. G. Andreoli
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14275]
¹School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
²Department of Earth Sciences, University of Cambridge, Cambridge, UK
³School of GeoSciences, University of Edinburgh, Edinburgh, UK
Published by arrangement with John Wiley & Sons

The 369 m deep M4 drill hole, located ~18 km NNW of the center of the 146 Ma Morokweng impact structure (MIS), intersects shocked Archean granitoid gneisses and subsidiary dolerite intrusions that are cut by faults, cataclasites and mm- to m-wide suevitic and pseudotachylitic breccia dikes. The shock features in quartz in the gneisses and breccia dikes include decorated planar deformation features (PDFs), planar fractures, feather features, and toasting. Other minerals show features that may be shock-related, such as multiple sets of planar features and alternate twin ladder structures in feldspars, kink bands in biotite, and planar features in titanite, apatite, and zircon; however, these are variably annealed and/or overprinted by hydrothermal alteration effects, and confirmation of their origin awaits further study. Universal Stage measurements of PDF sets in quartz from 12 gneissic target rocks and from lithic and mineral clasts in three suevitic and three pseudotachylitic breccia dikes reveal four dominant sets: (0001), {10⁢1¯⁢3}, {10⁢1¯⁢4} and {10⁢1¯⁢2}. Based on these observations, the average peak shock pressure in these rocks is estimated at ≤16 GPa, which supports the original proximity (within one transient cavity radius) of these rocks to the point of impact. No discernible depth-dependent shock attenuation was noted in the core. These shock levels and the elevated structural position of the rocks in the M4 core relative to the impact melt sheet intersected in drill holes closer to the center of the MIS suggest that the M4 lithologies represent part of the parautochthonous peak ring volume that subsequently experienced 1.5–2 km of post-impact erosion before it was buried beneath younger sediments. Numerical modeling using the iSALE-2D code suggests that the original Morokweng crater had a rim-to-rim diameter of between 70 and 80 km, and that the rocks in the M4 core were originally located at a depth of 7–8 km and a radial distance of 8–9 km from the point of impact.