Physiographic Regions/Regional Context
The Bighorn Basin is an intermontane basin in the Middle Rocky Mountain Foreland geologic province. It is an asymmetric, northwest‐trending topographical and structural basin with an elliptical shape, bounded on the northeast by the Pryor Mountains, on the east by the Big Horn Mountains, on the south by the Owl Creek, Bridger and Washakie Ranges, on the northwest by the Beartooth Mountains, and open to the north into Montana. The basin is also bounded on the west by volcanic rocks of the Absaroka Mountains which were erupted and deposited atop older Laramide uplifts. The north end of the Bighorn Basin is considered to terminate structurally along a low‐lying folded and faulted zone known as the Nye‐Bowler lineament in Montana (Thomas 1965).
The topography of the Planning Area varies from rolling plains, flat mesas, and badlands to alluvial valleys, benches, foothills, and mountains (BLM 1993). Many pronounced anticlinal and synclinal folds, some of which have considerable structural relief (Pierce and Andrews 1941) occupy the foothills or “flank” areas of the basin. Riparian corridors, badlands, and benches/upland topography dominate the central basin. The paragraphs that follow further address geologic structure in the Bighorn Basin. See the Solid Mineral Occurrence and Development Potential Report for further discussion on the geology of the Bighorn Basin (BLM 2009c).
Stratigraphy and Economic Geology
Figure 3-15 provides a generalized stratigraphic and lithologic section for the Bighorn Basin; Map 76 displays a geologic map of the Planning Area.
Stratigraphically, rocks of all the geologic periods, with the exception of the Silurian Period, are represented in the numerous formations found in the Planning Area. The sedimentary rocks were deposited during the repeated advances and retreats of ancient seas and epicontinental seaways (such as the Sundance Seaway and the Cretaceous Seaway) that covered the Planning Area, and during other terrestrial depositional environments, including, fluvial, aeolian, and lacustrine.
Sedimentary rocks in the Planning Area range in age from Cambrian to Holocene, have an aggregate thickness of more than 33,000 feet, and overlie Precambrian metamorphic and granitic basement rock. Within the Bighorn Basin, younger sedimentary formations tend to be exposed toward the center of the basin, while progressively older formations crop out generally toward the eastern, southern, and western edges of the basin. Sedimentary rocks are folded and faulted as a result of uplifts of the mountains that rim the basin.
The geology of the basin is conducive to the accumulation of hydrocarbons (also known as fossil fuels) given the presence of sedimentary formations that act as source rocks, reservoir rocks, and impermeable caps to some of the reservoir rocks. See the Reasonable Foreseeable Development Scenario (RFD) for Oil and Gas for a discussion on oil and gas development potential in the Planning Area (BLM 2009e).
Some formations contain coal seams of varying thicknesses and grades. Other formations contain the remains of ancient volcanic ash deposits that were chemically altered into beds of montmorillonite and beidelite clay known as bentonite. Some formations are a source of dimension stone (material quarried as block or slabs that also meets certain size and shape specifications) and construction stone. There are thick sand and gravel deposits along rivers and streams throughout the basin.
Historical and Structural Geology
The Bighorn Basin formed as a result of the Laramide Orogeny, a compressional mountain-building and basin‐forming event, which took place from late Cretaceous time (about 80 million years ago [MYA]) to middle Eocene time (about 45 MYA) (Downs 1952). Large blocks of Precambrian‐age rock were displaced upward, generally along reverse or ramp faults of varying dips (Fanshawe 1971), with resultant folding and faulting of the overlying sedimentary rock layers. During this time, the Big Horn, Owl Creek, Pryor, Beartooth, and Washakie Ranges were uplifted, as were numerous smaller anticlinal structures along the inner flank or margin of the basin. The central portion of the Bighorn Basin was relatively undeformed during the Laramide Orogeny, and received sediment eroded from surrounding uplifts.
Approximately 10 to 12 MYA, a period of broad regional uplift and extension (epeirogeny) began in Miocene time that has continued into the present (Fanshawe 1971). This broad general uplift triggered increased erosional activities, leading to excavation of deep canyons, (i.e., Cottonwood Canyon, Wind River Canyon, Clarks Fork Canyon, Sheep Mountain Canyon, and Devil’s Canyon), and removal of thousands of feet of basin sediment via large rivers and their tributaries. Streams such as the Shoshone River, the Bighorn River, Porcupine Creek, and Cottonwood Creek were rejuvenated during this time of uplift, and began to incise deep canyons into the underlying Paleozoic shales, limestones, and dolomites.
During the Pleistocene Epoch (approximately 2 MYA to 15,000 years ago), the mountain uplifts experienced several episodes of alpine glaciation. Alpine glaciation is responsible for numerous U‐shaped glacial valleys, glacial lakes, terminal and lateral moraines, and other glacially derived landforms seen today along the Beartooth front, in the Absaroka Mountains, and in the Big Horn Mountains.
Current Geological Conditions
Currently, the Bighorn Basin is generally experiencing an erosional phase, with deposition of sediment occurring locally in rivers, streams, and lakes, and reservoirs. Erosion of sediment by rivers, streams, wind, gravity, and ice far exceeds sediment deposition in the basin. The consolidated rocks and unconsolidated sediments in the Planning Area are constantly affected by the forces of weathering and erosion. Rocks weather through mechanical processes, chemical processes, or both. Water, wind, ice, and gravity are the principal weathering agents. The mild acidity of rain or snow causes chemical erosion and tends to dissolve carbonate rocks. Water reacts with the calcium carbonate in limestone to form carbonic acid, which dissolves limestone even more aggressively than water alone.
Water weathers rocks by infiltrating pore spaces or fractures in rock, freezing, and then thawing, thereby acting to wedge the rock apart. Water flowing downslope transports sediment of various sizes down gradient. Pebbles and cobbles in channel and terrace deposits along streams and rivers in the Planning Area reveal their various sources in the varying lithologies seen in the deposits.
Bentonite, gypsum, and sand and gravel mining alter the existing geologic resources in the Planning Area, because these activities remove commercial quantities of minerals from the geologic formations. Other surface disturbances change the condition of existing geological resources by disturbing or loosening soil or rock at the surface.
The degree and direction of change to geology due to the weathering process would be imperceptible over the life of a land use plan. Typically, mining activities would tend to change the character of the surface over the short term, but over the long run, disturbed areas would be reclaimed and returned to the extent possible to the preexisting slope and vegetative cover.