Abstract

The Early Ordovician was a time of fluctuating sea level. At least two events interpreted to be low sea-level stands are recognized in northern Utah. The earlier hiatus correlates with the Hirsutodontus hirsutus, Fryxellodontus inornatus, and Clavohamulus elongatus subzones of the Cordylodus proavus conodont zone. This hiatus is represented by a disconformity between the St. Charles Formation and Garden City Limestone in the southern Lakeside Mountains and at Stansbury Island, north-central Utah. The second hiatus correlates with the Hirsutodontus simplex and Clavohamulus hintzei subzones of the Cordylodus proavus zone and conodont Fauna B in the southern Lakeside Mountains, at Stansbury Island and the Bear River Range in northern Utah and southeastern Idaho. It is expressed by a disconformity between the St. Charles and Garden City formations in the Bear River Range and by a disconformity in the lower 15 meters of the Garden City Limestone in the southern Lakeside Mountains.

Mixed carbonate and siliciclastic sediments accumulated on a shallow-water, miogeoclinal shelf in western Utah and eastern Nevada and in deep-water basinal environments in central Nevada during the time of subaerial exposure in northern Utah and southeastern Idaho. The hiatuses are either absent or of durations shorter than one conodont subzone in western Utah and Nevada. The hiatuses and related dispersal of siliciclastic sediments across the Cordilleran miogeocline are attributed to eustatic sea-level changes, as opposed to local tectonic activity, because the events have been detected worldwide in separate paleotectonic settings.

As now known, the stratigraphic sections in northern Utah with the longest Early Ordovician hiatuses seem restricted to the Paris and Tintic Valley thrust sheets that are now located north, west and south of the Uinta-Cortez arch. These relations suggest that Precambrian and Lower Cambrian crystalline and siliciclastic rocks were probably exposed along the axis of the Uinta-Cortez arch and served as sources for the Early Ordovician siliciclastic sediments during low sea-level stands. However, until the sedimentology has been studied in detail, cratonic sources east of the Sevier orogenic belt or the Lemhi arch in east-central Idaho cannot be ruled out as possible contributing source areas.