Region 5: The Basin and Range

A tiny corner of the Basin and Range region— a huge physiographic region that extends from southeastern Oregon to west central Mexico—extends into the Rocky Mountains of southeastern Idaho. While the formation of the Basin and Range is a recent event that began only 30 million years ago, the bedrock that makes up the region’s up-thrust ranges and down-dropped basins is very old. In this tiny area of Idaho, rocks can be found from nearly all periods of the Phanerozoic. This is largely because the region’s most recent geologic activity involved crustal extension that has exposed many deeper, older layers. During the Paleogene, magma upwelling from the mantle weakened the lithosphere, lowering its density. This stimulated uplift, stretching the bedrock in an east-west direction. The crust along the Basin and Range stretched, thinned, and faulted into some 400 separate mountain blocks. Movement along the faults led to a series of elongated peaks and down-dropped valleys, also called horst and graben landscapes. In a manner similar to books toppling when a bookend is removed from a shelf, the blocks slid against each other as they filled the increased space (Figure 2.38).

Figure 2.38: Alternating basins and ranges were formed during the past 17 million years by gradual movement along faults. Arrows indicate the relative movement of rocks on either side of a fault.

Figure 2.38: Alternating basins and ranges were formed during the past 17 million years by gradual movement along faults. Arrows indicate the relative movement of rocks on either side of a fault.

Since the region’s formation, the bedrock of the basins has been covered by young deposits, including loose sediment washed down from the mountains and evaporite deposits left behind in dried-out lakes. The ranges, however, particularly the Sevier Orogenic Belt (also known as the “Overthrust Belt”), expose rocks whose ages span from Precambrian to Cenozoic. The Basin and Range’s Paleozoic rocks, a succession of sandstones, limestones, and shales, were deposited on the western shore of North America during the Cambrian to the Mississippian. This was followed during the Pennsylvanian to the Permian by a transition to shallow and evaporating seas, which deposited sandstones, mudstones, limestones, and phosphate-rich rocks. Mesozoic rocks include red beds, sandstones, mudstones, and limestones of the Dinwoody, Nugget, Twin Creek, Morrison, and Stump Formations. Good outcrops of these rocks can be seen in uplifted ranges such as the Bear and Aspen Range. These Paleozoic and Mesozoic sediments were thrusted during the Sevier Orogeny, then involved in the Basin and Range style of extension during the Paleogene. Valleys formed by this extensional faulting were filled with later Cenozoic sediments.

Younger rocks from the Cretaceous and the Cenozoic cover the valley floor, filling the region’s basins (Figure 2.39). These rocks are mainly conglomerates, sandstones, and mudstones originating from erosion of the nearby uplifts. In the case of the Idaho Basin and Range, the basin fills also include Cenozoic volcanic rocks produced by nearby volcanic activity on the Snake River Plain. Pleistocene deposits include glacial till, outwash, and glacial lake deposits. These gravels, sands, silts, and tills are mostly associated with glaciers in the adjacent Teton and Snake River Ranges of Wyoming.

Figure 2.39: Basin fill in the Basin and Range.

Figure 2.39: Basin fill in the Basin and Range.