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Open-File Report 2006-1250

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A USGS Zonal Table for the Upper Cretaceous Middle Cenomanian−Maastrichtian of the Western Interior of the United States Based on Ammonites, Inoceramids, and Radiometric Ages

By William A.Cobban, Ireneusz Walaszczyk, John D. Obradovich, and Kevin C. McKinney

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From the Introduction

This provisional table is based mainly on the molluscan fossil record of the central and northern parts of the Western Interior of the United States. Some of the ammonite zones are known in Europe, such as Watinoceras devonense, Collignoniceras woollgari, Prionocyclus germari, Scaphites hippocrepis, Didymoceras stevensoni, and Didymoceras cheyennense, whereas more than one-half of the inoceramid zones are known also in Europe. A few of the ammonite zones are known from only a few localities, but the diagnostic species may occur in abundance. Among these are the zones of Acanthoceras granerosense, A. bellense, Dunveganoceras problematicum, Burroceras clydense, Watinoceras devonense, Collignoniceras praecox, and Scaphites mariasensis. All fossils listed in the table are in the national collection housed in Building 810 at the Federal Center, Denver, Colo.

Version 1.0

Posted November 2006

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