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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 55 (1971)

Issue: 8. (August)

First Page: 1391

Last Page: 1391

Title: Dresbachian Faunal Zones in Pilgrim Formation of South-Central Montana: ABSTRACT

Author(s): Rena Mae Bonem

Article Type: Meeting abstract


Dresbachian faunas were collected in Montana from the Pilgrim Formation on lower Beaver Creek near Helena, at Castle Rock on Mill Creek south of Livingston, and on Wolf Creek near Red Lodge.

The Aphelaspis zone is present at the top of the Pilgrim Formation only at Wolf Creek. The Crepicephalus zone is the highest zone observed at Mill Creek and the Cedaria zone is the highest at Beaver Creek. Four genera of trilobites, Aphelaspis, Blountia, Cheilocephalus, and Glaphyraspis; two brachiopod genera; and Hyolithes are recognized from the Aphelaspis zone. The few genera and numerous individuals are typical of that zone and may be interpreted as indicating cool currents moving south during the time of deposition of the Aphelaspis zone. Only species characteristic of the lowest part of the zone are recognized, though higher beds may have been deposited originally.

The most common species is Aphelaspis walcotti Resser which shows steplike growth pattern in the ratio of the preglabellar field length to the glabellar length.

The upper 34 ft of the Crepicephalus zone in the Pilgrim Formation at Wolf Creek consists of rounded limestone-pebble conglomerate, mottled, partly dolomitized oolites, and abundant glauconite in a micritic matrix. This lithology is interpreted as characteristic of carbonate tidal flats. Calcareous sandstone with fragmentary fossils and frosted quartz grains comprises the upper 2 in. of the Crepicephalus zone and the lower 7 in. of the Aphelaspis zone. The uppermost beds of the Aphelaspis zone are medium- to coarse-grained biosparite with a well-preserved fauna and indicate a lower energy environment than that in which calcarenites are deposited.

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