A variety of microspherules is found in clay layers of the Upper Cretaceous and lower Tertiary sediments at Gubbio, Italy. Of particular interest are spherules containing K-feldspar or K-feldspar and glauconite that were found in 12 of 15 clay layers examined. Their ages range from the Turonian to the Paleocene, a span of 22 m.y. They are most abundant in the clay layer at the K/T transition, but are clearly not unique to this horizon. Their origin does not seem to be related to a single extraterrestrial impact at K/T time, nor to a few impacts over a 1–2-m.y. interval. In fact, they contain no unique features that would restrict their origin to an impact event. We conclude that the microspherules are diagenetic in origin. The conditions that led to a concentration of microspherules at the K/T horizon are unique only in terms of the magnitude of their effects.

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