Lecture 15 - The Rocky Mountains

Outline

I. Geomorphic Provinces in the U.S.

II. The Southern Rocky Mtns.- Rocky Mtn. National Park

III. The Northern Rocky Mtns.- Glacier National Park

IV. The Middle Rocky Mtns.- Grand Teton National Park

I. Geomorphic Provinces in the U.S.

Geomorphic provinces represent large regions that share a common geologic origin and history. The Colorado Plateau is one geomorphic province where we visited several National Parks including The Grand Canyon, Zion, Bryce, Canyonlands and Arches. This area is composed of a very high plateau consisting of primarily flat-lying sedimentary rocks. It was elevated due to tectonic forces in the last 20 million years. The Cascades is a second geomorphic province that we studied and consists of primarily andesitic volcanoes that result from th esubduction of the Juan de Fuca plate beneath North America. Later in this class we will visit parks in the Sierra Nevada, Basin and Range, and Blue Ridge provinces. This class, we will visit the Rocky Mountain geomorphic province. This province is subdivided into three sections: northern Rockies which extend from Montana well into northern Canada and includes Glacier National Park, the middle Rockies which include most of the mountains in western Wyoming and contains Grand Teton National Park, and the southern Rockies which cover many mountain chains in Colorodo and New Mexico. All of the Parks that we will visit in the Rocky Mountain province consist of primarily very old Precambrian rocks that have been uplifted and the younger rocks stripped off exposing the cores of the mountains. As we will see, while igneous and metamorphic Precambrian rocks make up the mountains in Rocky Mountain and Grand Teton National Parks, Precambrian sediments make up the mountains in Glacier.

II. The Southern Rocky Mtns.- Rocky Mtn. National Park

The geology revealed in Rocky Mtn. National Park is similar in many respects to what we will see in Grand Teton National Park. Int hese parks very old Precambrian igneous and metamorphic rocks are exposed in the uplifted mountain peaks. The relationship between these rocks indicates that mountains existed over the middle and southern Rockies during Precambrian time. The mountains in Glacier are Precambrian sediments that have been little deformed or metamorphosed indicating that mountain building was not going on herein the Precambrian. All parks experienced similar events throughout the Paleozoic, Mesozoic and Cenozoic. The history of the Rocky Mountains as seen in Rocky Mtn. and Grand Teton National Park and only slightly different in Glacier National Park is as follows:

  1. Great quantities of sediments accumulated over this region in Precambrian time.
  2. The sediments deposited in the southern 2 parks were deeply buried, folded and metamorphosed in a mountain building event in Precambrian time and are now schists and gneisses.
  3. Associated with a long period of mountain building, igneous rocks intruded the metamorphosed rocks in Rocky Mtn. and Grand Teton NP between 1.2 and 1.4 billion years ago.
  4. After a period of erosion, in Paleozoic time, sea-level rose and fell laying down a thick sequence of Paleozoic, primarily marine sedimentary rocks. In middle Paleozoic time, compressional forces formed the relatively low-lying "Ancestral Rockies" which were quickly eroded away.
  5. During the Mesozoic period primarily non-marine sediments continued to accumulate. At the end of the Mesozoic, the seas advanced again lying down the last marine deposits in this region.
  6. The end of the Mesozoic-beginning of the Cenozoic brought the Laramide mountain building event or orogeny. Compressional forces developed due to subduction off of the western margin of North America. The nearly horizontal angle of the oceanic lithosphere being subducted caused the compression to extend very far into the North American continent. Rocks became folded, faulted (reverse or thrust faults in compressional regime) and uplifted, elevated temperatures at depth melted some rock to produce igneous activity near the surface (The Absaroka range near Yellowstone is a good example of volcanic activity during this period).
  7. Erosion followed and then renewed uplift and tensional forces that produced the present topography.

III. The Northern Rocky Mtns.- Glacier National Park

The major difference between the rocks exposed in Rocky Mtn. National Park and Glacier are that the mountains in Glacier are composed of Precambrian sedimentary rocks. These old sedimentary rocks make up the Belt Supergroup and range in age between 800-1600 million years old. What is amazing about these sediments is that they are so old yet remain undeformed and unmetamorphosed. The other important feature of Glacier National Park is the Lewis thrust(reverse) fault. This reverse fault developed due to compressional forces during the Laramide orogeny (latest Mesozoic through earliest Cenozoic time) and thrust the old Belt Supergroup rocks over much younger Mesozoic aged sediments. It is the Precambrian sedimentary rocks of the Belt Supergroup that make up the hanging wall block of the Lewis fault and make up the high mountains in the park.

IV. The Middle Rocky Mtns.- Grand Teton National Park

Although most of the surrounding mountains in the Middle Rocky Mountains show compressional features (folds and reverse faults)associated with the Laramide orogeny, the Teton Range owes its height to a large normal fault that uplifted it along the east bounding Teton Fault. The Tetons are on the upthrown footwall block of the fault while Jackson Hole is on the downthrown hanging wall bolck. The Teton Fault was activated about 9 million years ago when the tectonics of the region shifted from compressional to tensional. This fault is active today with the Tetons continuing to go up at a rate of millimeters per year. The rocks that make up the Teton Range are Precambrian gneisses and granites as old as 2.5 billion years! These metamorphic and igneous rocks experienced a mountain building episode in the Precambrian. Paleozoic and some Mesozoic sedimentary rocks are exposed on the edges of the Tetons where they have not been erodedaway.

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