Geology

The Pocatello Field Office (PFO) area can be divided into three distinct geologic provinces: the Idaho-Wyoming Thrust Belt, the Basin and Range, and the Snake River Plain.

The Idaho-Wyoming Thrust Belt generally comprises the northern and eastern half of the PFO area. It is part of the larger, Middle Rocky Mountain Province. The thrust belt is characterized by folds and thrust faults. The fold amplitudes may be small or up to several miles across. The faults may have as much as 50 to 100 miles of eastward displacement. The mountain ranges of the area reach elevations of nearly 10,000 feet. The valleys lie above 6000 feet and commonly contain gravels or basalts. The folds and faults of the region have created long, linear exposures of the Phosphoric Formation, key in the extraction of phosphate. Fossiliferous, shallow marine sediments of Cambrian through Jurassic age compose the majority of the region’s stratigraphy.

The Basin and Range physiographic province makes up the western half of the PFO area. East-west extension beginning about 17 million years ago has created a series of north trending mountain ranges. The valleys may contain thousand of feet of late Tertiary and Quaternary gravels or may contain Quaternary basalt flows. Generally, surface water in the Basin and Range flows towards evaporative basins and does not reach either the Pacific or Atlantic Oceans. Thick sequences of Paleozoic marine, sometimes fossil-rich sediments compose the majority of the region’s stratigraphy. The eastern region of the Basin and Range has significant exposures of late Proterozoic sediments and volcanics.

The Eastern Snake River Plain bounds the PFO area to the northwest. The area is characterized by volcanic terrain approximately 60-70 miles wide, running from southwestern Idaho to Yellowstone National Park. Volcanic activity started about 17 million years ago in the western portion of Idaho and migrated eastward. The rocks are commonly composed rhyolite and basalt flows. The volcanic package may be up to 10,000 feet thick and locally contains sedimentary interbeds between basalt flows. In the Pocatello area, the rhyolites generally range from 8 to 10 million years old and grade into basalts about .6 million years old. Locally, basalts may be as young as 5, 000 years old. The aquifer contained within the Snake River Plain is a major regional water source.

The PFO area’s varied geology is favorable for the occurrence of several mineral resources. Mineral resources of interest in the region include the non-energy leasable mineral phosphate; locatable minerals, such as gold, limestone, and zeolites; salable minerals, including sand, stone, gravel, and pumice; and fluid leasable minerals such as oil and gas and geothermal resources. The development of the phosphate mineral resource is of significant importance to the local economy and the national phosphorus fertilizer and chemical demand.

Pocatello Field Office | 4350 Cliffs Drive | Pocatello, ID 83204
208-478-6340 | Fax: 208-478-6376 | Office hours: 7:45am - 4:30pm,