Basal Ordovician faunas from the Williston Basin, Montana

Three cores from the Deadwood Formation in the Williston basin of Montana have furnished trilobites and brachiopods referable to basal Ordovician Zones A, B, and possibly C of R. J. Ross, Jr. (1951) and L. F. Hintze (1952). The fossils occur in dark green micaceous, pyritic shales interbedded with a few glauconitic limestone pebble conglomerates. Zone A faunules are characterized by long-ranging graptolites and brachiopods with trilobites of Symphysurina species and Euloma cordilleri n. sp. Zone B faunules, in over 100 ft of section, contain many of the trilobites Clelandia, Bellefontia, and Xenostegium, and of Lingullela; a new problematica, Huishea flexa, is rare. A few specimens of Apheoorthis and Symphosurina appear 25 ft higher, tentatively Zone C.

This record provided courtesy of AGI/GeoRef.

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