Abstract

Early study of what is now called the Opohonga Limestone in west-central Utah indicated that it was part of a massive formation of Carboniferous age. Later studies identified fossils from the formation's older carbonates that were known to be Early Ordovician. A significant conodont fauna from a section on Gardison Ridge permits an even more precise definition of the formation which includes correlation with the classic North American Ibexian sequence of western Utah. While an important part of the earliest Ibexian sequence is missing on Gardison Ridge, the Opohonga Limestone there contains conodont elements representing seven zones and subzones of the type Ibexian and can be firmly correlated with conodont zones of the upper House and most of the Fillmore Formations of the Pogonip Group in the Ibex area. This correlation also confirms that the Sauk III–IV sequence boundary occurs in the lower part of the formation. The oldest conodont faunas of the Ibexian Series were not recovered from the lowest beds of the Opohonga Limestone, and it is possible that missing earliest Ibexian species in the oldest Opohonga beds may indicate that the poorly known underlying Ajax Dolomite, considered to be upper Cambrian, may actually include part of the basal Ordovician Ibexian Series.

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