Large and varied brachiopod faunas that may be used for rather refined age dating occur in several beds in the Middle Pennsylvanian Minturn Formation in the McCoy area, northwestern Colorado. The oldest major fauna is comparable to faunas from late Derryan beds in New Mexico and in the Midcontinent region; those higher in the section are typically early Des Moinesian in age. A number of brachiopod species are sufficiently restricted stratigraphically to be used in the McCoy area for recognition of zones as narrow as those based on fusulinids. Recognition of these brachiopod zones in other sections in the Colorado Trough is difficult, however, because adequate faunal information is generally lacking.

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