GLACIER N.P. – OVERTHRUSTING IN THE ROCKY MOUNTANS

 

FIELD TRIP STOP – THE BEST VIEW OF PRECAMBRIAN SEDIMENTS IN THE WORLD SHOVED ATOP CRETACEOUS ROCK BY THE LEWIS OVERTHRUST.

 

LOCATION:  Glacier National Park is found in northernmost Montana where it forms part of the Glacier-Waterton International Peace Park.

 

GEOLOGIC FEATURES: Thick PreCambrian sedimentary strata; Stromatolites; Overthrusting; Sill; Pillow Lava; Mudcracks; Salt casts; Ripple Marks; Cirques; Hanging Valleys; Aretes; Moraines.

 

DESCRIPTION: The geology of Glacier National Park is particularly noteworthy since it represents the best-preserved thick stack of PreCambrian sedimentary rocks in the world, and displays a huge thrust fault where these PreCambrian sediments have been shoved laterally for nearly 50 miles over younger Cretaceous rock.

 

In the western part of the North American craton deposition of sediments began in a shallow basin about 1.6 billion years ago. Deposition continued periodically through the Paleozoic and into the Cretaceous Period. In the late Cretaceous and early Eocene, the Laramide Orogeny caused a massive thrust fault which moved these PreCambrian and younger sediments eastward over Cretaceous rocks for a distance approximating 50 miles. This fault is called the Lewis Overthrust and its trace extended in a north-south direction for hundreds of miles.

 

Most of the rocks in Glacier National Park belong to the Belt Supergroup, a series of Precambrian rocks nearly 14,000 feet thick and ranging in age from 1.6 billion to 800 million years old. Exceptions are a thin lava flow and igneous intrusion (1.1 byo) found within a part of the sedimentary stack. Any Paleozoic and younger rock found above the overthrust have been eroded away within the Park boundary (though they can be found stratigraphically below the Cretaceous found beneath the overthrust).

 

Sedimentary rocks of the Belt Supergroup generally represent deposition in very shallow water as evidenced by ripple marks, mud cracks, salt crystal impressions, raindrop impressions, and up to six species of algal stromatolites. Strata are generally red and green shaly argillites, and gray to buff limestone. Stromatolites are the only fossils. Proterozoic sedimentary rock formations, arranged stratigraphically, are the:

 

     Kintla (>800 ft; Red shaly argillite).

       Shepard (600 ft; Buff limestone),

         Siyeh (Helena) (3500 ft; Gray Limestone;

           Grinnell (2500 ft; red shaly argillite),

             Appekunny (3500 ft; Greenish shaly argillite);

               Altyn (2300 ft; Light Gray Limestone), Stromatolitic; Greatest cliff-maker.

 

Proterozoic Igneous rocks include the Purcell Basalt (50-275 feet; a pillow lava found above the Siyeh, and a Diabase intrusion (100 feet) found within the upper Siyeh.

 

Pillow lavas are evidence of lava erupted under water. The Diabase is a coarser grained version of the basalt due to its slower cooling while surrounded above and below by preexisting sedimentary strata (i.e., it is a Sill). Areas directly above and below the igneous intrusion are whitish in color, having been bleached by hot fluids from the magma.

 

Pleistocene glaciation is responsible for the Park’s many Cirques, Aretes, Moraines, and Hanging Valleys.

 

 

STUDENT QUESTIONS:

(1) What are Stromatolites? How do Stromatolites indicate the presence of shallow water? Which Text-Figure(s) show Stromatolites?

(2) Why is it common for thrust plane to be found in shales?

(3) Within a few vertical inches, Precambrian rocks can be found lying over Cretaceous sediments. How much of a time gap (in millions of years) is represented by the contact between the Altyn Formation and Cretaceous rock? Which Text-Figure shows the Overthrust?

(4) How do mudcracks represent environments that are alternately wet and dry?

(5) What is the Proterozoic?

(6) Define: Cirque, Hanging Valley, Arete, and include how each is formed and represent sculpting by glacial action. Name one Text-Figure associated with each of the above.

(7) CHALLENGE: Why are Pillow Lavas named as such? What do they represent? How are they take their characteristic shape?

(8) Why are the Grinnell and Kintla Formations bright red in color? What does that indicate about past environmental conditions?

 

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SELECTED  REFERENCES:

-Alt, D.D. and D.W. Hyndman. 1973. Rocks, Ice and Water: The Geology of Waterton-Glacier Park. Mountain Press Publishing Company, Missoula, Montana.

-Dyson, J. L. 1957. The Geologic Story of Glacier National Park. Glacier Natural History Association, Special Bulletin No. 3. Accessed on March 29, 2020: https://www.nps.gov/parkhistory/online_books/glac/3/index.htm

-Harris, A. and E. Tuttle. 1983 (3rd ed).Geology of the National Parks. Kendall/Hunt Publishing Co., 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

-NPS. Glacier National Park - Geologic Formations. Accessed on Mar 28, 2020 (Updated May 24,2017): https://www.nps.gov/glac/learn/nature/geologicformations.htm

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CROSS-SECTIONAL DIAGRAM AND PHOTOS: