Middle Ordovician bentonites in the central and southeastern United States have been correlated on the basis of rare-earth element (REE) concentrations of primary apatite phenocrysts. Ratios of light to heavy REE, such as La/Tb or Ce/Yb, produce the best separation of the four studied bentonites. The Deicke bentonite has been correlated over a distance of more than 1000 km, from southern Minnesota to central Tennessee, and the Millbrig bentonite has been correlated over 750 km, from central Minnesota to central Missouri.

The successful discrimination of closely spaced bentonites over a long distance demonstrates that apatite chemistry is a powerful correlation tool on a continental scale.

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