Black Canyon of Gunnison | GeologyVirtualTrips

BLACK CANYON OF THE GUNNISON N.P.

FIELD TRIP STOP -- BLACK CANYON OF THE GUNNISON, COLORADO (PreCambrian Metamorphic and Igneous Granite including Pegmatites)

LOCATION: In west-central Colorado, about 8 miles east of Montrose near the intersection of Routes 50 and 347.

GEOLOGIC FEATURES: PreCambrian Metamorphic Gneiss, Schist and Granite; Pegmatites; Unconformities (Non-Conformity); Foliation

DESCRIPTION: The Black Canyon of the Gunnison is composed of 1.8 byo Gneisses and Schists that have been intruded by 1.4 byo granites and granite pegmatites. Mainly outside the park boundaries, the metamorphic and igneous rock are unconformably overlain by Upper Jurassic to Upper Cretaceous strata (160 to 85 myo) which are then overlain in angular unconformity by 30 myo (Oligocene) volcanics.

The greatest depth of the Canyon is 2722 ft at Warner Point while the narrowest width at the rim is 1100 ft. The Park is named for its steep cliffs which are darkened by a lack of sunlight, as low as 33 minutes per day. Painted Rock, with its Pegmatite-filled walls, is the tallest vertical cliff in Colorado at 2,250 ft.

Following deposition of Mesozoic strata, uplift commenced with the Laramide Orogeny (about 70-40 mya). This uplift was also associated with mid-Tertiary volcanic activity. Subsequent erosion, perhaps enhanced by a sporadic uplift of the Colorado Plateau within the past few million years by rivers, eventually exposed the crystalline rocks of the Canyon about two million years ago.

Photographs below were taken in April 2021 and October 2022.

STUDENT QUESTIONS:

(01) Describe a Non-Conformity. Where does it occur in the Park and how did it form chronologically?

(02) What are the distinguishing characteristics of a Pegmatite?

(03) What Geologic Process is responsible with the carving of the Canyon?

(04) What is a probable cause of the Laramide Orogeny? What Mountain System was the result of the this orogeny?

(05) CHALLENGE: Describe the geologic history of the area within and near the Black Canyon of the Gunnison. Beginning in the PreCambrian, include all episodes of Deposition, Metamorphism, Igneous Activity, mountain building, and erosion in the area.

SELETED REFERENCES:

-National Park Service, Black Canyon of the Gunnison National Park Geology. U.S. Department of the Interior (updated Oct. 22, 2019). Accessed April 25, 2021 from https://www.nps.gov/blca/learn/nature/geology.htm

-National Park Service, Black Canyon of the Gunnison National Park Geology. U.S. Department of the Interior (updated April 9, 2022). Accessed Nov. 8, 2022 from https://www.nps.gov/blca/learn/nature/geologic-story.htm

-USGS. Black Canyon of the Gunnison National Park. U.S. Department of the Interior (updated Oct. 22, 2019). Accessed April 25, 2021 from https://www.usgs.gov/science-support/osqi/yes/national-parks/black-canyon-gunnison-national-park

Cross-sectional Figure illustrates the geology in and around Black Canyon of the Gunnison N.P. (from Hanson, Wallace R. 1965. The Black Canyon of the Gunnison, USGS Bulletin 1191.)

Map Figure of the National Park shows roads and notable features (from Park Brochure)

PHOTOS:

Figure 1 - Granitic Pegmatites (1.4 byo) intruded parallel to foliation in 1.8 myo Gneiss and Schist photographed from Gunnison Point.

Figure 2 - View of the Canyon immediately south of the South Visitor Center (Tomichi Point).

Figure 3 - View from Gunnison Point near South Visitor Center showing the Gunnison River and multiple craggy spires due to weathering.

Figure 4 - At Cross Fissures View we see a long zone of large, deep joints.

Figure 5 - View of Painted Wall illustrating the numerous Granitic Pegmatites (1.4 byo) intruded into the older Metamorphic Rock. This Wall is the tallest vertical cliff in Colorado.

Figure 6 - Close-up of a Granitic Pegmatites as exposed on the stairway down to Gunnison Point. Pegmatites are largely composed of Quartz, Orthoclase Feldspar, and Biotite Mica.

Figure 7 - The Gunnison River cuts a deep and narrow canyon through the steep vertical walls of Painted Rock, a 1.7 byo gneiss punctured with Pegmatites. Vertical joints can be seen in the distance

Figure 8 - As viewed along the south road leading to the Park are Mesozoic sedimentary rocks (some with cross-bedding) that are deposited unconformably above the 1.7 byo metamorphics seen in the Park.