Laramide mountain ranges and Yellowstone Park

The Laramide orogeny refers to a phase of mountain building that affected parts of Montana, Wyoming, Colorado, and lesser areas of nearby states (see the shaded area in the figure below).  The Laramide orogeny occurred from approximately 70 million years ago to 40 million years ago during a period when the Farallon oceanic plate was subducting rapidly beneath the western coast of the U.S.  The most unusual aspect of the Laramide orogeny is the fact that the mountain ranges created during this period were so far inland from the subduction zone, much farther than is usually the case for mountains built inland from subduction zones. Although the reason for this is still debated, independent evidence for dramatic shifts in the location of volcanic activity during the Laramide age (see the previous lecture) suggests that the Farallon plate was subducting at an extremely shallow angle beneath the western U.S. from 70-40 Ma.  This postulated flat subduction may have caused stresses at the base of the continental crust that were large enough to build mountains far inland from the trench.

Click here for map that shows names of major mountain ranges in the maps above.

The easternmost Laramide uplift is the Black Hills of western South Dakota. Immediately west of the Black Hills, the arc-shaped, isolated Big Horn Mountains are found in north-central Wyoming. The Powder River basin, which is located between the Black Hills and Big Horns, also formed during the Laramide orogeny.  It contains up to 18,000 feet of clastic sedimentary rocks (sandstones, shales, limestones), some related to the great interior seaway that covered much of the central U.S. 75 million years ago (see previous lecture), and some from sediments shed off from the adjacent Big Horn Mountains.  The Powder River basin contains enormous oil, natural gas, and coal deposits - by some estimates, this basin contains 25% of all of the coal-bed natural gas in the United States.  

The Laramide uplifts also include the frontal Rockies of Colorado, the primitive Beartooth and Absaroka Mountains, the Wind River and Uinta Mountains, the Bighorns, and numerous other small and large mountain ranges. These uplifts reach heights that exceed 13,000 feet and have classic alpine features that include mountain glaciers, U-shaped valleys, high mountain lakes, and glacially carved topography. Surprisingly, there are extensive flat areas at high altitudes in some of the Laramide-age ranges. These flat areas are relicts of  the once lower flat land surfaces that were uplifted by as much as 40,000 feet in some areas with respect to basement in the adjacent basins. The Laramide mountain ranges are amongst the most beautiful  in the United States.

The Beartooth Highway (US 212), which extends 65 miles from Red Lodge to Cooke City, Montana, deserves special mention since it is considered by many people to be the most scenic drive of any road in the lower 48 states.  Climbing to altitudes of nearly 11,000 feet, the highway goes through some of the most beautiful alpine mountainous terrain in the country.  If you are driving to Yellowstone, strongly consider taking this little-used route to the park.  Just make sure to leave enough time since it can take a few hours to drive the 65 miles and longer in the likely event that you stop to hike or snap photos.  The Beartooth and Absaroka mountains are some of the most primitive and wildly beautiful mountains in the country.  There are more than 20 peaks that extend above 12,000 feet and the high terrain is beautifully glaciated.  The Absaroka-Beartooth Wilderness is a 900,000-acre roadless area protected in its wild state.  It guards the northeastern entrance to the adjacent Yellowstone National Park.  If you are adventurous, this is a great place to hike!

The Bob - the Bob Marshall Wilderness of northwestern Montana is considered by many to be the crown jewel of America's wilderness system (excluding Alaska, which is in a category by itself).   At more than 1,500,000 acres, the Bob extends more than 40 miles from east-to-west and 100 miles from north-to-south without any roads. It includes three adjacent wilderness areas and spans a wide topographic range, from valley bottoms to 9000-foot-high peaks. As the best preserved mountain/valley ecosystem in the lower 48 states, the wilderness is reserved for hikers and horse packers, with many interior park areas requiring days to reach on foot.  

The wilderness was established in recognition of Bob Marshall, a legendary hiker and U.S. Forest Service researcher who died in 1939 at the age of only 39.  Despite frequent poor health, Mr. Marshall frequently hiked 30-40 miles per day in mountainous areas and once climbed 14 Adirondack peaks within a single 19-hour period.  Native Americans who accompanied him were reportedly unable to keep up.  An avid outdoor enthusiast and founder of the Wilderness Society, Mr. Marshall explored many of the wildest areas of the United States and Alaska. He advocated protecting wild areas long before wildland preservation was an important national issue.

 
The Chinese Wall.  Photo Courtesy of the Bob Marshall Wilderness Ranch (www.wildernessranch.org)

Much of the mountain-building in the Bob occurred during the Laramide orogeny, when rocks west of the park were thrust tens of miles eastward along the Lewis Overthrust to form mountain ranges now within the park boundaries. A 20-mile-long, 1000-foot-high vertical cliff called the Chinese Wall marks where the Lewis Overthrust reaches the surface in the park and is one of the most popular hiking destinations for its spectacular view points.  

 

 

 

The image links are meant to sample additional images of Laramide features. Please examine each of them and read the accompanying text.

(all photos and most text below are from the "Geology by Lightplane" aerial photo Web site of Professor Lou Maher.)

Black Hills, South Dakota

#1 - Devil's Tower, Wyoming - a Laramide-era intrusive volcanic feature

#2 - Devil's Tower with Missouri Buttes, more Laramide-era volcanics, in left background

Laramide-period volcanic features, South Dakota

#1 - Big Horn Mountains, WY- Piney Creek Thrust. View to northwest along strike of eastward-thrust Paleozoic formations. Precambrian crystalline rocks lie at the left. Cambrian sediments crop out in the grassy slopes. The Bighorn dolomite (Ordovician) and Madison limestone (Mississippian) form the ridge in the central part of the photo. Tertiary erosion debris--i.e. Moncrief member of Wasatch fm--is at the right. (9Apr66)

#2 - Big Horn Mountains, WY View to the south at the north side of the Piney Thrust sheet, Bighorn Mountains, WY; On the right (west) horizon is the Precambrian crystalline mountain core. The adjacent swale is underlain by Cambrian formations. The ridge at the center is composed of the Ordovician Bighorn dolomite and Mississippian Madison fm. The peak at the left is composed of the Tertiary Moncrief gravels that were eroded from the thrust plate. (22Jul56)

#3 - Big Horn Mtns, WY. Western edge where the mountains drop into the Big Horn Basin. See how the layers of sedimentary rocks are draped over and eroded away from the steep slopes along the edge of mountain range. This indicates that the range was uplifted after the sediments were deposited.

Big Horn Basin - Sheep Mountain anticline between Greybull and Lovell, WY. View to southeast. Anticline plunges to northwest. At left is northwest-plunging syncline. Note that Bighorn River has been superimposed across this structure. See Lovell-Greybull Area, Big Horn Co., WY, Department of Agriculture, Commodity Stabilization Service, 1961: Air photo BBN-3BB-66. (15Apr66 )The region between Lovell and Greybull, Wyoming, is in the Bighorn Basin, a structural basin formed during the Laramide Orogeny of the early Tertiary. The Bighorn and Pryor Mountains make up the eastern and northern margins, the Owl Creek Mountains establish the southern margin, and the Absaroka Range and the Yellowstone Plateau form the western limit of the basin. The Bighorn Basin, like the other intermountain basins of Wyoming and Colorado, was filled with the debris eroded from the surrounding uplifts even as folding continued. During the later part of the Tertiary the rivers were able to remove much of the basin fill, exposing the older bedrock. The modern topographic basin is situated in the rain shadow downwind of the Yellowstone Plateau, and few trees grow unless they can find water along the Bighorn River and its tributaries. The rocks are very well-exposed, and make a good area for air photo interpretation. They range from Mississippian to Cretaceous in age. Quaternary alluvium occurs along the major valleys.

Sheep Mountain - View to southwest. Bighorn River superimposed across the fold. The river valley that cuts through the anticline implies that the river predated this feature and eroded down through the feature as it was slowly uplifted. See Lovell-Greybull Area, Big Horn Co., WY, Department of Agriculture, Commodity Stabilization Service, 1961: Air photo BBN-3BB-110. (15Apr66)

Sheep Mountain - Red Chugwater fm (Triassic) and white Gypsum Spring fm (Jurassic) at south end of Sheep Mountain, Bighorn Basin, WY. View to northeast. Bighorn Mountains in background

#1 - Wind River Mountains, WY. View to west looking upstream over the Wind River to the Wind River Mountains. The deep notch on the mountain flank is the valley of Bull Lake. (21Jul65). What formed the deep valley emanating from the lake in the direct center of the photo?

#2 - Wind River Mountains, WY. Dinwoody lakes

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The Yellowstone Hotspot and Snake River Plain

The  Snake River plain of southern Idaho and western Montana is an arcuate-shaped 400-mile-long by 60-mile-wide belt of volcanic rocks erupted by a series of volcanos that formed above the Yellowstone hotspot over the past 17 million years.  Individual volcanos, now inactive, show up in the figures below as bumps in the beige-colored Snake River plain.  The "plain" is so named because it is a relatively flat feature that cuts across the high mountain ranges to the north and south of the plain.  Volcanic rocks produced by the volcanos in the plain buried the mountains that once extended continuously across the plain.  The ages of the volcanic rocks along the plain become progressively younger toward the northeast, with the youngest volcanics occurring in Yellowstone Park, which is positioned above the present hotspot.  Slow southwestward movement at a rate of about 0.7 inches per year of the North American plate over the hotspot, which is approximately stationary in the mantle beneath the plate, caused this line of volcanos to form progressively through time.  Please study the material following the topographic charts to gain a more in depth understanding of this unusual feature (the Snake River plain and Yellowstone).

The Snake River plain topography

Figure courtesy of the U.S.G.S.






Topography in the vicinity of Yellowstone National Park.
Red outline shows the location of the caldera that formed during a
massive eruption 630,000 years ago.  Hot springs and geysers are
evidence for the continued existence of a magma chamber beneath the
caldera.  The caldera is located within the boundaries of the
park.
Figure courtesy of the U.S.G.S.

Also see aerial photos below of specific features inside the park.

Extracted from Smith and Braile (1994) "The Yellowstone hotspot", JVGR v. 61, p.121-187

Topography and volcanic ages associated with the Yellowstone hotspot and Snake River plain. Note progression of volcanic ages ranging from 16.1 Ma at left edge of maps to 0.6 Ma at the present location of the Yellowstone hotspot in Yellowstone National Park. The two plots below show how the ages of the volcanic rocks and the age at which faulting along the Snake Rivera plain both increase with distance from the present Yellowstone hotspot. The steady increase in time is a measure of the rate at which the North American plate has moved over this mantle hotspot over the past 15 million years.

Extracted from Smith and Braile (1994) "The Yellowstone hotspot", JVGR, v. 61, p. 121-187

The next two figures show the reconstructed Yellowstone/Snake River hotspot track over the past 15 Ma. The upper rightmost panel shows the present-day geography and geologic provinces. The panel to its right reconstructs these to their configurations at 5 Ma. The panels beneath show reconstructions at 10 Ma and 15Ma from right to left. Moving forward in time from 15 Ma, the North American plate moves to the west, thereby adding to the length of the Snake River plain/Yellowstone hotspot track.

Extracted from Smith and Braile (1994) "The Yellowstone hotspot", JVGR v. 61, p.121-187

The final figure shows a conceptual model, based on a variety of scientific data, for the Yellowstone hotspot at depth. The upper panel shows how elevation about sea level increases along the Snake River plain until it reaches its highest point at the present location of the Yellowstone hotspot. Here, the high temperatures make the crust more buoyant and thus make it "float" higher on the underlying mantle than the older and cooler volcanic rocks to the west along the Snake River plain. In the lower panel, the diagram emphasizes how the temperature "plume" associated with the hotspot is smeared out beneath the Snake River plain as a consequence of the westward motion of the North American plate over the plume.