Shocked Quartz at the Triassic-Jurassic Boundary in Italy

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Science  24 Jan 1992:
Vol. 255, Issue 5043, pp. 443-446
DOI: 10.1126/science.255.5043.443


Quartz grains that appear to have been shock-metamorphosed occur within three closely spaced shale beds from the uppermost Triassic ("Rhaetian") Calcare a Rhaetavicula in the Northern Apennines of Italy. The upper shale coincides with the abrupt termination of the distinctive, uppermost Triassic Rhaetavicula fauna and is overlain by the Hettangian (Lower Jurassic) Calcare Massiccio; no extinctions appear to be associated with the two lower layers, which occur 1.2 and 2.4 meters below the boundary shale. Approximately 5 to 10% of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes (basal, ω, and π) most commonly observed in shocked quartz. Textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the Triassic.