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Shock metamorphic features in quartz grains from the Saarijärvi and Söderfjärden impact structures, Finland
Author(s) -
Öhman Teemu,
Preeden Ulla
Publication year - 2013
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12112
Subject(s) - geology , impact crater , breccia , impact structure , sedimentary rock , quartz , metamorphic rock , lithology , shock (circulatory) , geochemistry , dike , paleontology , astrobiology , medicine , physics
Shock metamorphic features at the Saarijärvi ( D > 2 km) and Söderfjärden ( D = 6.5 km) structures in Finland have so far only been studied tentatively, although both are considered to be proven impact structures. This work presents the first detailed universal stage study of planar deformation features ( PDF s), feather feature lamellae ( FFL ), and planar fractures ( PF s) in quartz grains from a polymict impact breccia dike from Söderfjärden, and from sedimentary crater‐fill rocks from Saarijärvi. Planar microstructures, particularly PDF s, are very rare and poorly developed or preserved in Saarijärvi, whereas in Söderfjärden they are much more common and well defined. Miller–Bravais indices of the planar microstructures in both Saarijärvi and Söderfjärden are indicative of relatively low‐shock pressure but high shear conditions, only compatible with an impact origin for these structures. Although a Proterozoic age for Saarijärvi cannot be ruled out, the observations of shock features throughout the sedimentary crater‐fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a Lower Cambrian (or younger) impact age.