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Science 25 May 1984:
Vol. 224. no. 4651, pp. 867 - 869
DOI: 10.1126/science.224.4651.867
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Articles

Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary

B. F. BOHOR 1, E. E. FOORD 1, P. J. MODRESKI 1, and D. M. TRIPLEHORN 1

1 U.S. Geological Survey, Box 25046, Denver, Colorado 80225

A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth. The shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile.

Submitted on January 24, 1984
Accepted on February 16, 1984


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