Shock metamorphism of quartz at the submarine Mjølnir impact crater, Barents Sea

• Main Authors: Sandbakken, P. T., Langenhorst, F., Dypvik, H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Meteoritics & Planetary Science Archives 2005
• Subjects: Mjølnir;Planar microstructures;Shock metamorphism;Shocked quartz;Submarine impact; Barents Sea; Hekkingen
• Online Access: https://journals.uair.arizona.edu/index.php/maps/article/view/15183

Shock metamorphosed quartz grains have been discovered in a drill core from the central peak of the Late Jurassic, marine Mjlnir structure; this finding further corroborates the impact origin of Mjlnir. The intersected strata represent the Upper Jurassic Hekkingen Formation and underlying Jurassic and Upper Triassic formations. The appearance, orientation, and origin of shock features in quartz grains and their stratigraphic distribution within the core units have been studied by optical and transmission electron microscopy. The quartz grains contain planar fractures (PFs), planar deformation features (PDFs), and mechanical Brazil twins. The formation of PFs is the predominant shock effect and is attributed to the large impedance differences between the water-rich pores and constituent minerals in target sediments. This situation may have strengthened tensional/extensional and shear movements during shock compression and decompression. The combination of various shock effects indicates possible shock pressures between 5 and at least 20 GPa for three core units with a total thickness of 86 m (from 74.00 m to 171.09 m core depth). Crater-fill material from the lower part of the core typically shows the least pressures, whereas the uppermost part of the allochthonous crater deposits displays the highest pressures. The orientations of PFs in studied quartz grains seem to become more diverse as the pressure rises from predominantly (0001) PFs to a combination of (0001), , and orientations. However, the lack of experimental data on porous sedimentary rocks does not allow us to further constrain the shock conditions on the basis of PF orientations.