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Publication Authorized by the Director, U. S. Geological Survey, Washington, D. C.

Paleomagnetic and potassium-argon studies support geologic evidence that the lower member of the Bandelier Tuff was deposited 1.4 m.y. ago. The upper member erupted about 1.0 m.y. ago and was followed by caldera collapse which formed the 12- to 14-mile diameter Valles Caldera. Postcaldera activity which resulted in the eruption of rhyolite domes and pyroclastic material, has occurred at about 0.9, 0.7, 0.5, and 0.4 m.y. ago, with later undated eruptions that were estimated at about 0.1 m.y. ago.

These data from the Valles Caldera are the basis for the previously published age revision of the Brunhes-Matuyama geomagnetic polarity epoch boundary from 1.0 to 0.7 m.y. ago, and they were used to define the Jaramillo normal polarity event at about 0.9 m.y. ago (Doell and Dalrymple, 1966).

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