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.