Aron K. Clymer, David M. Bice, Alessandro Montanari; Shocked quartz from the late Eocene: Impact evidence from Massignano, Italy. Geology ; 24 (6): 483–486. doi: https://doi.org/10.1130/0091-7613(1996)024<0483:SQFTLE>2.3.CO;2
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This paper reports the discovery of shocked quartz grains within a well-dated upper Eocene marlstone at the Massignano outcrop (Ancona, Italy). The shocked quartz reaches a maximum concentration of 1.6 ± 0.4 grains per cubic centimetre of bulk rock. A modest Ir anomaly is also coincident with the shocked quartz, and together they strongly support an impact origin. Moreover, this impact layer has a precise interpolated radiometric age of 35.7 ± 0.4 Ma. Although impact evidence found throughout the world indicates that at least two large impacts did occur during this time, the actual number, timing, and environmental effects of these impact events remain unclear. The late Eocene was a critical time in Earth history because of significant climatic, biotic, and environmental crises.