GRAND TETON N.P. – THE TETON RANGE

 

FIELD TRIP STOP – A VIEW OF A SHARPLY RISING MOUNTAIN RANGE ADJACENT TO A NORMAL FAULT.

 

LOCATION:  Grand Teton National Park is located 10 miles south of Yellowstone and North of the town of Jackson, Wyoming.

 

GEOLOGIC FEATURES: Normal Fault (The Teton Fault); Unconformity between PreCambrian crystalline rocks and Cambrian Sandstone; Glacial carving of landscape.

 

DESCRIPTION: The Teton Range is perhaps best known its sharp change in elevation, with a local relief of up to 7000 ft. above the valley floor. This rapid change in elevation (without foothills) is due primarily to motion along the 40-mile long Teton Fault.

 

The Teton Fault is a Normal Fault which dips to the east. The western upthrown block contains the Teton Range which is composed primarily of metamorphic and igneous rocks.  The downthrown block is represented by the valley of Jackson Hole.

 

Starting at about 3.0 b.y.a. sediments began to be deposited in this geographic area. Subsequently, about 2.7 b.y.a. heat and pressure transformed the sediments into metamorphic gneiss which was subsequently intruded by granite at about 2.5 b.y.a. Other intrusive igneous rocks include a few 1.3 b.y.a. vertical dikes of dark-colored diabase up to 200 ft thick. One of the most visible dikes is a 150 ft thick unit near the top of Mt. Moran.

 

Uplift and erosion of the metamorphic and igneous rocks down to sea level provided a surface for deposition of Cambrian and other Paleozoic and Mesozoic sediments. This unconformity represents a gap in geologic time of nearly 700 m.y. (At the very top of Mt. Moran, the gneiss and diabase sill are unconformably overlain by Cambrian sandstone.)

 

During the Laramide Orogeny, which ended in the Eocene (35 m.ya.), the Teton block was uplifted several thousand feet by compressive forces associated with subduction to the west. Cessation of compression was followed about 13 m.y.a. by Normal Faulting (Teton Fault) along the eastern side of the present Teton Range making this the youngest mountain range in the Rocky Mountains. Much uplift and tilting associated with fault movement has eroded much of the Paleozoic and Mesozoic  sediments and has exposed the PreCambrian crystalline rocks along the east side of the Teton Range. On the western side of the Range, Paleozoic sediments unconformably overlie the PreCambrian crystalline rocks. East of the fault, the down dropped block (Jackson Hole valley floor) is underlain by Tertiary and younger sediments.

 

Pleistocene glaciation sculpted the Tetons leaving moraines, sharp peaks and ridges, U-shaped valleys, lakes, hanging valleys and outwash plains.

STUDENT QUESTIONS:

(1) How does one determine the Hanging Wall versus the Foot Wall of adjacent fault blocks?

(2) The Laramide Orogeny ia associated with compressional forces (Thrust and Reverse Faults).  The Teton Range is a result of tensional forces (Normal Fault). How does each of the three fault types differ from the other two? (Use the terms Hanging Wall and Foot Wall in your explanation.)

(3) How does a Strike-Slip Fault differ from a Normal or Reverse Fault? Give an example of a famous Strike-Slip fault found in the U.S.

(4) Define an Unconformity. What is the largest unconformity in the Teton Range? How many years of time are missing a this unconformity? (Show how you calculated this number.)

(5) The Teton Fault is thought to have experienced up to 30,000 fee of displacement in the past 9-10 million years, What is the average rate of displacement per 1000 years?

(6) How is the movement along the fault associated with earthquakes that occur within the Park? Is movement along the fault sudden and periodic, or is it slow and continuous?

(7) CHALLENGE: What is the Farallon Plate? What is its role in the Laramide Orogeny?

(8) CHALLENGE: If the Teton Range is the result of tensional forces, then what circumstances might account for such a large uplift?

SELECTED  REFERENCES:

-Harris, A. and E. Tuttle. 1983 (3rd ed.). Geology of the National Parks. Kendall/Hunt Publishing Company. Dubuque, IO. 554 pp.

-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.

-USGS. Geology and Ecology of National Parks - Grand Tetons National Parl Photo Tour - https://www.usgs.gov/science-support/osqi/yes/national-parks/grand-tetons-national-park-geology-photo-tour. Accessed on March 26, 2020.

CROSS-SECTIONAL DIAGRAM AND PHOTOS:

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Figure 1 - Looking south from the north end of the park. Lake Jackson lies on the valley floor of Jackson Hole.

Figure 2 - From Glacier View showing the major peaks of the Teton Range. The peaks rise sharply from the floor of Jackson Hole.

Figure 3 - Mount Moran of the Teton Range.  Of particular note is the dark-colored 150 ft.-wide vertical dike of diabase at the top of the mountain. This 1.3 b.y.o. igneous rock intrudes into the 2.3 b.y.o.gneiss that composes eastern face of the Teton Range. The very top of the diabase is unconformably truncated by the horizontal Cambrian Flathead Sandstone.

Figure 4 - The Teton Range characteristically rises abruptly 5000 - 7000 ft above the floor of Jackson Hole along the Teton Fault.

Figure 5 - From left to right are found the peaks of South Teton, Middle Teton, Grand Teton, Mt. Owen, and Teewinot Mt.that rise abruptly above the floor of Jackson Hole.

Figure 6 - The major peaks of the Teton Range as viewed from Jenny Lake.

Figure 7 - Abrupt change in elevation along with hanging valleys and sharp peaks carved into 2,5 billion years old gneiss by Pleistocene glaciers.

Figure 8 - The southeast part of the park has herds of bison maintained by the National Park Service.

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   GRAND TETONS N.P.  

Figure 9 - A view of Jackson Hole valley from Signal Mountain in the eastern side of the park.

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Figure 10 - On June 23, 1925, layers of shale lubricated with excess water triggered one of the largest landslides in North America. Being 1 mile long, 2000 ft. wide and several hundred feet thick, nearly 50 million cubic yards of earth flowed down slope above the shale leaving a scar in the mountainside.