Contrast of Opinions Concerning the Formation of the Black Canyon - Continued

Continued from Previous Post

Photo Credit: Nefratiri Weeks

At first, pyroclastic flows directed the course of the river, but over time, its course became set. When it came to the hard Precambrian rocks, it could not change course and began to cut through the hard rocks until it carved the canyon to its current form. This is where geologist Shawn La Bounty disagrees with the USGS explanation.

Shawn explains that the entire region was under an immense amount of strain from tectonic activity and this strain caused the Black Canyon Anticline. He explains, “I do not disagree that this area was blanketed with ash (in fact many times), and this ash may have directed the river in places, especially from Upper Blue Mesa to Curecanti creek, but the placement of the Black Canyon on the top of the Gunnison Uplift (Black Canyon Anticline) was no "Lucky Event"."

"It is widely known that Stress Fractures are caused by the strain hardening of anticlines.” “There are thousands … of scientific journals that explain the common occurrence of this fracturing. Thus the more plausible explanation of the depth and scope of the canyon is that the fractures of the canyon were already in place and the river was simply captured …”

Photo Credit: Nefratiri Weeks

References:

La Bounty, Shawn. Personal Interview. November 18 and 22, 2010.

Black Canyon of the Gunnison National Park & Curecanti National Recreation Area Geologic Resource Evaluation Report. Jan. 2005. National Park Service US Dept of the Interior. http://www.nature.nps.gov/geology/inventory/publications/reports/blca_cure_gre_rpt_view.pdf. Retrieved October 18, 2010.

Johnson, A. The faulting and folding of strain-hardening sedimentary rocks. Tectonophysics, Volume 62, Issues 3-4, 20 February 1980, Pages 251-278.

Contrast of Opinions Concerning the Formation of the Black Canyon

Scientists postulate the complex physical processes coupled with immense amounts of time that work together to form phenomena like the Black Canyon of the Gunnison, and often disagree. I’m going to contrast the explanation given on the USGS website with the explanation of local Geologist, Shawn La Bounty.

Black Canyon of the Gunnison
Photo Credit: Nefratiri Weeks

The USGS explanation states that about 2,000 million years ago (mya), the entire area was covered with an ancient ocean.

Gunnison River
Photo Credit: Nefratiri Weeks

In the late Cretaceous Period, the Laramide orogeny took place – tectonic plates, “jockeying for position” caused the beginning of the building of the Rocky Mountains (Thornberry-Ehrlich, 2005, p. 20). Also during this time, a local event took place, called the Gunnison Uplift, which raised the anticline where the Black Canyon is located.

About 25 mya, the Tertiary period brought volcanic eruptions from surrounding mountain ranges – the West Elks and San Juans.

This volcanic activity resulted in pyroclastic flows that covered the area with rock and ash. Coupled with snowfall, this helped form the headwaters of the Gunnison River.

Around 2-3 mya, another “broad uplift initiated a period of active erosion” which is when the Canyon was formed (ibid., p. 20). The Gunnison River began flowing across the plateau of the soft ash sediment.

Continue Reading Next Post

References:

Thronberry-Ehrlich, T. 2005. Black Canyon of the Gunnison National Park & Curecanti National Recreation Area Geologic Resource Evaluation Report. Natural Resource Report NPS/NRPC/GRD/NRR--2005/001. National Park Service, Denver, Colorado. http://www.nature.nps.gov/geology/inventory/publications/reports/blca_cure_gre_rpt_view.pdf. Retrieved October 18, 2010.

USGS: Geological Survey Bulletin 1191. Black Canyon of the Gunnison Today and Yesterday: Rock Formations - Why the Black Canyon Crosses the Gunnison Uplift. http://www.nps.gov/history/history/online_books/geology/publications/bul/1191/sec3.htm. Retrieved November 29, 2010.

Building Earth’s Landscapes: The Battle Begins...

One of the most magnificent aspects of The Black Canyon of the Gunnison is its exposure of Precambrian rocks dating over 1.7 billion years old -- one of the best in the world. These ancient rocks, also called basement rocks, are composed of gneiss, schist, and granite, as well as diorites, pegmatites, and gabbros (Thornberry-Ehrlich, 2005).

Alternating bands of gneiss and schist
Photo Credit:  NPS.gov
http://www.nps.gov/blca/naturescience/precambrian.htm

These metamorphosed sedimentary and igneous rocks underwent intense heat and pressure, which folded them within the earth -- creating an alternating pattern of gneiss and schist.

In addition, ribbons of intrusive pegmatite are interspersed, which formed as hot magma forced its way into the rock and cooled very slowly, forming large crystalline structures. (NPS.gov).

Large bands of Pegmatite Ribbons on the Painted Wall of the Black Canyon
Photo Credit: Colorado Geology Photojournals
http://www.cliffshade.com/colorado/

During the Precambrian, the area was covered in an ancient ocean. Over immense amounts of time, it underwent many geologic changes, including volcanic activity, the collision of massive landforms, compression followed by a shift toward extension, and the eventual Gunnison uplift and resulting erosion. The headwaters of the Gunnison eventually cut through the soft volcanic deposits, helping to carve the Canyon.

Photo Credit: Nefratiri Weeks

A mysterious feature of the Canyon is that the entire Paleozoic era is missing from the rock record shown by Precambrian rocks overlain with Mesozoic rocks (NPS.gov).

References:

Thronberry-Ehrlich, T. 2005. Black Canyon of the Gunnison NAtional Park & Curecanti National Recreation Area Geologic REsource Evaluation Report. Natural Resource Report NPS/NRPC/GRD/NRR--2005/001. National PArk SErvice, Denver, Colorado. http://www.nature.nps.gov/geology/inventory/publications/reports/blca_cure_gre_rpt_view.pdf. Retrieved October 18, 2010.

NPS.gov. Natural Features and Ecosystems, Geologic Story: Precambrian. http://www.nps.gov/blca/naturescience/precambrian.htm.Retrieved November 16, 2010.

USGS: Geological Survey Bulletin 1191. Black Canyon of the Gunnison Today and Yesterday: Rock Formations - Their Attributes and Geologic Setting. http://www.nps.gov/history/history/online_books/geology/publications/bul/1191/sec4f.htm Retrieved November 16, 2010.