¹Jörg Fritz, ²Ansgar Greshake, ²Vera A. Fernandes
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12845]
¹Saalbau Weltraum Projekt, Heppenheim, Germany
²Museum für Naturkunde Berlin, Berlin, Germany
Published by arrangement with John Wiley & Sons

The current shock classification scheme of meteorites assigns shock levels of S1 (unshocked) to S6 (very strongly shocked) using shock effects in rock-forming minerals such as olivine and plagioclase. The S6 stage (55–90 GPa; 850–1750 °C) relies solely on localized effects in or near melt zones, the recrystallization of olivine, or the presence of mafic high-pressure phases such as ringwoodite. However, high whole rock temperatures and the presence of high-pressure phases that are unstable at those temperatures and pressures of zero GPa (e.g., ringwoodite) are two criteria that exclude each other. Each type of high-pressure phase provides a minimum shock pressure during elevated pressure conditions to allow the formation of this phase, and a maximum temperature of the whole rock after decompression to allow the preservation of this phase. Rocks classified as S6 are characterized not by the presence but by the absence of those thermally unstable high-pressure phases. High-pressure phases in or attached to shock melt zones form mainly during shock pressure decline. This is because shocked rocks (<60 GPa) experience a shock wave with a broad isobaric pressure plateau only during low velocity (<4.5 km s⁻¹) impacts, which rarely occur on small planetary bodies; e.g., the Moon and asteroids. The mineralogy of shock melt zones provides information on the shape and temporal duration of the shock wave but no information on the general maximum shock pressure in the whole rock.

 ¹J. Ormö, ²A. T. Nielsen, ³C. Alwmark
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12816]
¹Centro de Astrobiologia (INTA-CSIC), Torrejon de Ardoz, Spain
²Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
³Department of Geology, Lund University, Lund, Sweden
Published by arrangement with John Wiley & Sons

The ≤27 m thick Vakkejokk Breccia is intercalated in autochthon Lower Cambrian along the Caledonian front north of Lake Torneträsk, Lapland, Sweden. The spectacular breccia is here interpreted as a proximal ejecta layer associated with an impact crater, probably ~2–3 km in size, located below Caledonian overthrusts immediately north of the main breccia section. The impact would have taken place in a shallow-marine environment ~520 Ma ago. The breccia comprises i) a strongly disturbed lower polymict subunit with occasional, in themselves brecciated, crystalline mega-clasts locally exceeding 50 m surrounded by contorted sediments; ii) a middle, commonly normally graded, crystalline-rich, polymict subunit, in turn locally overlain by iii) a thin fine-grained quartz sandstone-<30 cm thick. The upper sandstone is sporadically either overlain, or replaced, by a conglomerate. In progressively more distal parts of the ejecta layer, the lower subunit is better described as only slightly disturbed strata. The lower subunit is suggested to have formed by ejecta bombardment of the strata surrounding the impact crater, even causing some net outwards mobilization of the sediments. The middle subunit and the uppermost quartz sandstone are considered resurge deposits. The top conglomerate may be caused by subsequent wave reworking and slumping of material from the elevated rim. Quartz grains showing planar deformation features are present in the graded polymict subunit and the upper sandstone, that is, the inferred resurge deposits.