Studies of the Idaho black shale belt: Stratigraphy, depositional environment, and economic geology of the Permian Dollarhide Formation

Abstract

The Permian Dollarhide Formation, which consists of three informal members and is ∼2,300 m thick, is contained within the central Idaho black siliceous shale belt. The formation is a tectonostratigraphic unit within the Pioneer Mountains thrust stack. Major lithologies in the formation consist of calcareous, carbonaceous, siliceous siltite inter-bedded with fine-grained calcareous sandstone; fine-grained calcareous quartzite; and medium to dark gray, thin-bedded limestone. In the middle member, there are distinctive calcareous quartzites, and the upper member has conspicuous black (carbonaceous), pyritic argillite units. Sedimentary structures contained within the Dollarhide Formation include convolute bedding, cross-bedding, graded beds, flame structures, small-scale diapiric structures, synsedimentary folds, and load casts. It is suggested that the Dollarhide Formation correlates to the upper portion of subunit 6 and to subunit 7 of the Wood River Formation, which was deposited shoreward of the Dollarhide Formation. Petrology suggests deposition near and within an oxygen-minimum zone, rapid sedimentation at times, and, in places, turbidity-flow–emplaced beds. Mature quartz clasts and heavy minerals suggest possible derivation from a successor to the Antler highland to the east, with a sedimentary (Ordovician Kinnikinic Quartzite?) and crystalline basement as sources. Suggested depositional setting of the Dollarhide Formation is slope (lower and middle members) and basin outboard of the slope (upper member), thereby indicating transgression or subsidence during deposition. Trace-element geochemistry of the Dollarhide Formation rocks of the lower and middle members shows that they conform to the general values in black shales, as defined for Ag, Co, Cr, Mn, and Mo. The upper member, however, hosts epigenetic lead-silver-zinc veins and is locally the hanging-wall sequence of a syngenetic barite horizon.