Region 3: The Rocky Mountains

The rocks of the Rocky Mountain region are the most varied in the Northwest Central, ranging from Archean gneisses—some of the oldest rocks found in the US—to Paleozoic reefs, oil shales, volcanic fields, and glacial till. This great variety of rock types is mainly a result of the Laramide and Sevier orogenies, which uplifted numerous discrete blocks of terrain along thrust faults that accommodated compressional shortening and thickening of the crust. The overlying sediments were subsequently eroded to expose deeper Precambrian rock as well as Mesozoic and Paleozoic sedimentary formations. The thrust-faulted uplift also produced adjacent basins, which subsequently accumulated sediments eroded from the surrounding mountains.

The oldest rock found so far in the Rocky Mountain region is a 3.65- to 3.8- billion-year-old granitic gneiss found in the Wind River Range. Other Archean-aged rocks, including gneisses, amphibolites, schists, and iron formations, are found throughout the uplifted ranges of Wyoming, Montana, and Idaho, including the Teton, Bighorn, Beartooth, and Wind River mountains (Figure 2.18). These rocks were formed when cratons collided between 3.6 and 3 billion years ago, producing belts of metamorphosed and deformed rock. In southwest Montana, the mountains contain excellent examples of metamorphosed sedimentary rocks with interesting occurrences of minerals (blue calcite, rubies, and more), schists, marble, quartzite, iron formations, and greenstone. At the southern end of the Wind River Mountains, a greenstone belt hosts gold deposits, and the Granite Mountains host a thick iron formation with metamorphosed sediments, greenstone, gold deposits, and good examples of komatiites.

Figure 2.18: Cathedral Peak in the Wind River Range, Wyoming, is composed of Archean-aged granitic gneiss.

Greenstones

A greenstone belt is a term used to describe a series of interlayered volcanic and sedimentary rocks that have been metamorphosed into meta-sedimentary rocks and amphibolite. The rocks are called “greenstones” due to the presence of metamorphic minerals that give the rock a greenish-grey color. Many geologists believe these belts are the result of deposition in volcanic arc environments. An unusual volcanic rock type—komatiite—is often found in Archean greenstone belts. Komatiites are mafic volcanic rocks richer in magnesium and erupted at a higher temperature than basalts. They are restricted to the Archean, when the mantle temperatures were higher at the depths where magma is generated. Komatiites often exhibit “spinifex texture,” which is an unusual crystallization-cooling texture that produces large, long crystals.crystals.

Spinifex texture in a komatiite from the Komati Formation greenstone in South Africa. Similar rocks are common in the Archean greenstones of Wyoming.

Two main groups of Proterozoic rocks record the early formation of the North American continent: the Snowy Pass Supergroup and the Belt Supergroup. The Snowy Pass Supergroup, 2.4 - 2.5 billion years old, is located in the Medicine Bow Range in southern Wyoming. These strata—thick sequences of sandstone, conglomerate, and limestone—were deposited in a continental shelf environment on the passive margin of proto-North America. The sediments were later metamorphosed by an orogenic episode accompanied by volcanic activity. Today, the Medicine Peak quartzite forms high cliffs along the ridge of the Medicine Bow Range (Figure 2.19). Metamorphosed limestones in the Snowy Range also host 2.3-billion-year-old stromatolites, or mats of colonial cyanobacteria (Figure 2.20).

Figure 2.19: Medicine Bow Peak, a ridge of 2.4-billion-year-old quartzite (metamorphosed sandstone) in the Medicine Bow Range, Wyoming.

Figure 2.20: Stromatolite in metamorphosed Proterozoic dolostone from the Nash Formation, Medicine Bow Range, Wyoming.

Stromatolites

Stromatolites are regularly banded accumulations of sediment created by the trapping and cementation of sediment grains in bacterial mats (especially photosynthetic cyanobacteria). Cyanobacteria emit a sticky substance that binds settling clay grains and creates a chemical environment that leads to the precipitation of calcium carbonate. The calcium carbonate then hardens the underlying layers of bacterial mats, while the living bacteria move upward so that they are not buried. Over time, this cycle of growth combined with sediment capture creates a rounded structure filled with banded layers. Stromatolites peaked in abundance around 1.25 billion years ago, and likely declined due to predation by grazing organisms. Today, stromatolites exist in only a few locations worldwide, such as Shark Bay, Australia. Modern stromatolites form thick layers only in stressful environments, such as very salty water, that exclude animal grazers. Even though there are still modern stromatolites, the term is often used to refer specifically to fossils. For more information, see Chapter 3: Fossils.

A stromatolite from the Green River Formation (Eocene) of southwestern Wyoming.

The Belt Supergroup is located in northwestern Montana and adjacent Idaho, and is composed of a sequence of low-grade metamorphic sandstones, siltstones, mudstones, shales, and limestones over 10 kilometers (6 miles) thick. These rocks come in many colors— orange, yellow, rusty, red, purple, and green with white quartzite. They were deposited in a large sedimentary basin between 1.4 and 1.1 billion years ago, and they preserve many fossils as well as sedimentary structures including ripple marks, mudcracks, and raindrops. Rocks from the Belt Supergroup are best seen in Glacier National Park, Montana, where they have been exposed by an extensive system of thrust faults and folds related to the subduction of the Farallon plate beneath western North America in the late Cretaceous. The Belt Supergroup is of particular note due to its age and excellent preservation. It is extremely rare for sedimentary rocks of over a billion years in age to not have been warped, tilted, metamorphosed, or otherwise altered. The Belt Supergroup is also famous for its abundant and well-preserved stromatolites. In addition, ancient tillites (glacial deposits) found in Idaho represent major glaciation events that occurred during the Proterozoic (Figure 2.21).

Figure 2.21: Diamictite, a type of tillite from the Pocatello Formation near Pocatello, Idaho. This rock is thought to have been deposited during the “Snowball Earth” Proterozoic glacial period.

While most of the ranges and uplifts in the Rocky Mountains are cored by Archean rocks, two major areas are not. The Snake and Salt River Ranges of western Wyoming and adjacent Idaho, and the range just west of Choteau, Montana, consist of Paleozoic and Mesozoic units faulted and uplifted during the Sevier Orogeny. The ranges of central Idaho are primarily made up of the three major lobes of the Idaho Batholith, a set of late Cretaceous granitic intrusions (Figure 2.22).

Figure 2.22: Extent of the Idaho Batholith.

The Cambrian to Mississippian rocks of the Rocky Mountain region are a succession of sandstones, limestones, and shales that were deposited on the continental shelf of what was then the western shore of North America (Figure 2.23). From the Pennsylvanian through the Permian, a transition to shallow and evaporating seas deposited sandstones, mudstones, limestones, and phosphate-rich rocks. During the Triassic, a hot and arid landscape stretched across the region, as the shallow seas of the previous era retreated. This led to the deposition of continental rocks on nearshore marine environments and vast floodplains: red beds, sandstones, mudstones, and limestones. Others were deposited by aeolian processes; the Nugget Sandstone, found in parts of southwest Wyoming, exhibits cross-bedding and was deposited by wind on a Jurassic shoreline or desert. The bright red and orange colors of many Mesozoic siltstones and sandstones are caused by the presence of iron oxides (Figure 2.24). During the Cretaceous, shales, sandstones, and coals formed when the epicontinental Western Interior Seaway covered the area (see Figure 2.8).

Figure 2.23: Ridges of the Mississippian-aged Madison Limestone, exposed by thrust faulting along the Rocky Mountain Front in Montana.

Between the main ranges of the Rockies, there are a series of intermontane basins and mesas (Figure 2.25); surface rocks here are predominantly of Cretaceous and early Cenozoic age. Most of the rocks were formed when eroded sediment from the uplifted mountains was deposited by rivers into alluvial fans in lakes, basins, and swamps. These deposits eventually formed conglomerates, sandstones, mudstones, shales, evaporites, coal, and limestone. Thick blankets or wedges of Paleogene and Neogene sediments were deposited on the flanks of uplifted mountains. Paleozoic, Triassic, and Jurassic rocks crop out where they are uplifted at the margins of uplifts and ranges, but are typically buried within the basins.

The most important intermontane basins in the Rocky Mountain region are the Green River, Bighorn, Wind River, and Red Desert basins. These areas were centers for the deposition of thick layers of shale and mudstone into lakes, later forming evaporite beds as the lakes dried. The best known of these basin deposits are the sediments of the Green River Basin, which include well known fossil beds, oil shales, and large coal deposits. It is also the world’s largest source of trona, a non-marine evaporate mineral, along with related minerals including sodium bicarbonate (baking soda). Because of the basins’ isolated nature, early Cenozoic deposits are mainly basin-specific, with individual units often restricted to a particular basin or set of related basins. Some basins also contain igneous outcroppings—for example, Boars Tusk, an isolated butte within the Green River Basin, is the heavily eroded lamproite core of a 2.5-million-year-old volcano (Figure 2.26).

Figure 2.24: An outcrop of Triassic sandstone near Thermopolis, Wyoming.

Figure 2.25: Geologic basins of the Northwest Central US.

Figure 2.26: Boars Tusk, located within Sweetwater County, Wyoming, is the core of an extinct volcano.

During the Neogene, large volcanic eruptions related to the Yellowstone hot spot periodically buried parts of the region in thick layers of ash, forming tuff (Figure 2.27). Active Eocene volcanism and plutonism produced the Absaroka Volcanic Field in northwestern Wyoming, as well as smaller fields and intrusive bodies in Montana and Idaho. The Absaroka volcanics are up to 1500 meters (5000 feet) thick, and are composed of andesites, dacites, basalts, tuffs, and mudflows with minor related igneous intrusions (Figure 2.28).

Figure 2.27: Volcanic ashfall tuff from the Eocene, Green River Formation, Wyoming.

Figure 2.28: An andesite dike intrudes through a volcanic debris flow conglomerate exposed by a roadcut in the Absaroka Range, Wyoming.

Pleistocene glaciation produced glacial till and outwash materials in the region’s mountains and basins. Alpine glaciers, rather than continental ice sheets, carved cirques and deposited moraines in mountain valleys.

Greater Yellowstone Area

Yellowstone National Park and its surrounding area are the latest and current manifestation of the Yellowstone hot spot, whose trail from Oregon to Wyoming produced the Snake River Plain in Idaho. The most recent caldera eruption associated with this hot spot occurred 640,000 years ago, and more recent minor eruptive activity produced rhyolitic domes and basalt flows. The Yellowstone area is rich with volcanic features, such as calderas, resurgent domes, lava flows, and hydrothermal explosion craters. Yellowstone also has the world’s greatest number of geysers, along with hot springs, fumaroles, and mudpots. Earthquakes are common here, and many are related to faults connected with the movement of magma, groundwater, thermal expansion, or contraction of the ground.

Due to the Yellowstone area’s history of volcanism, rocks in the park are primarily volcanic. Major explosive caldera eruptions in the Yellowstone area occurred 2.1, 1.3, and 0.63 million years ago, with multiple minor eruptions occurring between the major caldera-forming eruptions. The volume of material ejected during these major eruptions has led to Yellowstone’s classification as a supervolcano (Figure 2.29). A wide range of volcanic rock types and textures are present in the park, including basalts, rhyolites, obsidian (volcanic glass), agglomerates (volcanic flows that picked up cobbles or fragments of other volcanic rocks), and ashflows. In some areas, volcanic ashflows are mixed with sedimentary conglomerates, sandstones, and mudstones from stream and mudflow deposits.

The rocks formed during Yellowstone’s past eruptive events can be seen in exposures throughout the park. The Huckleberry Ridge ash bed, laid down during the caldera explosion of 2.1 million years ago, is exposed in the walls of Golden Gate Canyon (Figure 2.30). An eruption around 590,000 years ago produced the Canyon Rhyolite flow, which can be seen in the Grand Canyon of the Yellowstone River on the eastern side of the park (Figure 2.31). Here, the rhyolites in the canyon walls have been altered by oxidation and acidic groundwater, resulting in striking yellows, pinks, and lavenders. Obsidian Cliff formed from a rhyolite lava flow that occurred 180,000 years ago (Figure 2.32); it contains abundant obsidian and was an important source of tool-making material for prehistoric peoples in the area.

Figure 2.29: The extent of the three most recent ashfalls from Yellowstone supervolcano eruptions, as compared to the eruption of Mt. St. Helens in 1980.

Figure 2.30: The pink- and yellow-hued rocks exposed in Golden Gate Canyon are composed of Huckleberry Ridge Tuff, formed from an ashfall after a Yellowstone supervolcano eruption 2.1 million years ago.

Figure 2.31: Brightly colored rhyolites are exposed in the Grand Canyon of the Yellowstone.

Figure 2.32: Thick veins of obsidian run through the rhyolite of Obsidian Cliff. Note the sunglasses used for scale.

The thermal features of Yellowstone are driven by heat from the cooling magma body beneath the caldera. Groundwater circulates through the hot rock, rises in hot springs, or erupts as geysers if there is a pressure buildup within the water. Fumaroles are openings that emit gaseous steam. Mudpots occur where hot water mixes with clay that has weathered from volcanic rock; the thick mud bubbles and spatters as it boils or releases gas. The hydrothermal solutions that circulate within Yellowstone’s geysers and hot springs dissolve minerals from the bedrock, precipitating them out at the surface to form intricate structures made of silica or travertine (Figure 2.33).

Figure 2.33: The travertine terraces of Mammoth Hot Springs in Yellowstone National Park precipitated over thousands of years as hot water from the spring cooled and deposited calcium carbonate. Over two tons of carbonate minerals in solution flow through the hot springs every day.

During the Quaternary, the Cordilleran Ice Sheet covered part of the Rocky Mountains, and a small ice sheet covered Yellowstone. This glaciation event left behind glacial till, creating moraines and outwash deposits from sediments deposited by meltwater. Geyser and hot spring activity continued beneath the ice cover. The combination of ice, meltwater ponds and lakes, and underground heat sources caused hydrothermal explosions—these are not volcanic eruptions, but rather occur when water contained in near-surface rock at superheated temperatures flashes to steam and violently disrupts the confining rock. In the case of the Pocket Basin, located in the western part of the park, an ice-dammed lake existed over a heat source, and a hydrothermal explosion was triggered by an abrupt decrease in confining pressure when the dam failed and the lake drained. This event created a crater-like basin, in which steam works its way through silica-bearing volcanic silt to create mudpots (Figure 2.34).

Figure 2.34: Mudpots in the Pocket Basin, Yellowstone National Park. The mud is composed of hot water mixed with volcanic clay.