Bighorn Basin

The Bighorn Basin is a large northwest-trending, Laramide-age and -style sedimentary basin in northwestern Wyoming. It is bounded by mountain ranges on all but the northern end, which extends some distance into Montana. The Bighorn Mountains lie to the east, the Owl Creek Mountains lie to the south, the Absaroka Range lies to the west, and the Beartooth Mountains lie to the northwest. The southern end of Montana’s Pryor Mountains bounds the northeast edge of the basin. The Bighorn Basin is drained by the Bighorn River, named by Lewis and Clark in 1804 when they passed the mouth of the river at its confluence with the Yellowstone (in Montana) on their way back from the Pacific coast. The river leaves the basin through a spectacular canyon it has cut across and into the north end of the Bighorn Mountains; Bighorn Canyon is now partially filled by Bighorn Lake, impounded for 90 miles behind Yellowtail Dam in Montana.

Structurally, the basin is an asymmetric syncline with the deepest part on the west side; the synclinal axis extends southeastward from the Beartooth mountain front to several miles east of Cody, to a few miles east of Meeteetse, to north of Thermopolis. The basin is very deep in a few places, with structural relief in the 30,000-foot range. Structural relief is defined by the difference between the elevation of the Precambrian-Cambrian contact in the deepest part of the basin and the elevation of the contact in the highest part of the adjacent mountains. Like the Powder River Basin, the interior of the Bighorn Basin consists of a thick sequence of relatively flat-lying Eocene and Paleocene rocks surrounded by bands of Mesozoic rocks dipping more steeply into the basin. These Mesozoic rocks, along with the underlying Paleozoic rocks, are folded into anticlines and synclines in a zone between the basin interior and the surrounding mountains. Many of the large anticlines contain important oil and gas resources that are developed in well-known fields such as Elk Basin, Oregon Basin, Grass Creek, Byron/Garland, Buffalo Basin, and Badger Basin.

The western edge of the Bighorn Basin is defined primarily by the eastern extent of the Absaroka Range volcanics, but many of the rocks and structures in the basin actually extend westward under the great volcanic pile. Some geologists refer to that part of the Bighorn Basin hidden beneath the volcanics as the Absaroka Basin. This area is poorly known, as the terrain is very rugged, much of it is in roadless wilderness areas, and the western part has been affected by the Yellowstone hot spot. Hints of the complex geology in this area can be seen where rivers have eroded westward into the volcanic rocks, especially in the valleys of the Greybull and Wood rivers and North and South forks of the Shoshone River.

Many geologic structures between the surrounding mountains and the interior of the Bighorn Basin are very well exposed and have been cut through by rivers that once flowed across them. Places like Rattlesnake Mountain and Shoshone River Canyon, Clarks Fork Canyon, Dead Indian Hill and Sunlight Basin, the Red Lodge area near the Beartooth uplift in Montana, Shell and Tensleep canyons, Five Springs area on the western flank of the Bighorn Mountains, and others are ideal settings in which to study structural geology and observe Laramide-style tectonics. Heart Mountain, a lone mountain north of Cody, has been a unique and perplexing puzzle to geologists for more than a century. This feature is intimately associated with, and has implications to help unravel much of, the geologic history of the nearby Absaroka Range.

The Bighorn River is the principal drainage for the basin, flowing north out of Wind River Canyon along the east side of the basin, and exiting the north end of the basin through Bighorn Canyon. All the streams and rivers draining the western two-thirds of the Bighorn Basin have their headwaters in the Absaroka Range; they flow eastward across the basin’s interior and into the Bighorn River. These major drainages include, from south to north, Owl Creek, Cottonwood Creek, Gooseberry Creek, the Wood and Greybull rivers, the North and South Forks of the Shoshone River, the Shoshone River, and Clarks Fork of the Yellowstone.

The geomorphic history and evolution of the Bighorn Basin has been studied in detail, primarily because much of the evidence is still preserved in an extensive sequence of terraces, benches, and other alluvial features throughout the basin. For example, north of Powell is a 700-foot-high, gravel-capped surface called Polecat Bench. This bench was once the valley floor of the ancestral Shoshone River, which exited from the basin to the Yellowstone River by way of the deep (now dry) canyon called Pryor Gap (in Montana). Headward erosion by a tributary of the Bighorn River captured the ancestral river and diverted the flow into the channel now occupied by the Shoshone River between Cody and Lovell.

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