Four bentonite beds near the Cenomanian-Turonian Stage boundary (ca. 90 Ma) can be traced through much of the western interior of the United States. Isopach maps constructed for these bentonite beds indicate that volcanic source areas were positioned both northwest and southwest of the central U.S. western interior basin and also indicate both easterly and southeasterly paleowind directions. Volcanism of this age has not been reported previously in the southern source area.

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