Abstract

An Upper Devonian (Upper Frasnian), coral-dominated carbonate buildup occurs in the upper part of the “dark dolomite member” of the Jefferson Formation in east-central Idaho. The buildup formed above a thick sequence of slope to deep ramp deposits and therefore reflects regional shallowing of deep water environments. The buildup also formed in an important position for defining the Frasnian platform-to-basin transition from southwestern Montana to east-central Idaho. However, detailed regional lithofacies relationships for the platform-to-basin transition are still unclear.

The dominance of corals with fasciculate growth morphologies suggests that the buildup formed in relatively deep water, with moderate sedimentation rates and turbid water conditions. Comparison with other Frasnian carbonate buildups suggests that coral-dominated buildups probably are characteristic of deeper water settings and along carbonate ramps. The faunal succession in the Jefferson buildup probably records regional fluctuations in relative sea level, however, crustal movements, global extinction events, or influx of platform derived carbonate sediment also may have affected the buildup sequence.