Abstract

Lower Permian through Lower Triassic strata unconformably overlain by Cenozoic sediments and volcanics, and comprising approximately 3265 m (10,712 ft) of section, have been recognized in the main body of the Cassia Mountains, southern Idaho. The oldest three units, which compose the bulk of this study, are unnamed, but are herein referred to as the Wahlstrom Hollow sequence (Sakmarian), Third Fork sequence, and Badger Gulch sequence (Artinskian). They are overlain by the Park City Group.

The Wahlstrom Hollow sequence consists of approximately 500 m (1,640 ft) of fine-grained rock whose constituents are lime mud, quartz silt, sponge spicules, and other fine-grained bioclastic material. It accumulated in relatively deep water via turbidity currents and hemipelagic sedimentation. Metastable bed conditions resulted in slumping and other soft-sediment deformation.

A gradually increasing influx of silt-sized siliciclastics into the area throughout Wahlstrom Hollow sedimentation, culminated with the deposition of approximately 514 m (1,686 ft) of dominantly calcareous, siliciclastic silts of the Third Fork sequence. Early in the depositional history of this sequence, the basin floor was scoured by storm generated currents, but most of the unit was probably deposited below wave base in less than 50 m (164 ft) of water. Deposition of silts in the upper part of the unit was briefly interrupted by thin, dominantly hemipelagic black lime muds.

When water depths exceeded 150 m (492 ft) silt deposition slowed, and approximately 300 m (984 ft) of uniformly bedded, silty (quartz), black lime muds of the Badger Gulch sequence accumulated mostly through hemipelagic sedimentation, although weak bottom currents were also active.

Deposition of the basal Park City Group, which consists of fossiliferous limestones and less abundant siliciclastic sandstones, marked a return to shallower (within reach of storm base) well oxygenated environments.

The three sequences were deposited within a N-S seaway which was bounded by remnants of the Humboldt Belt to the west and the continental shelf to the east. In this seaway, dysaerobic conditions during Wahlstrom Hollow deposition greatly limited a benthic marine fauna; whereas, higher oxygen levels accompanied Third Fork sedimentation, and permitted the establishment of a relatively diverse deposit-feeding fauna. During Badger Gulch deposition, the seaway was divided by the E-W trending Oquirrh-Uinta Uplift. North of the uplift, in the Cassia Basin, anaerobic bottom waters excluded a benthic marine fauna.