Graben

	TOURING COLORADO GEOLOGY
	Huerfano Butte	Back to Roadcuts
1		Driving along Interstate 25 looking to the east stands a lone "orphan" structure rising nearly 70 m above the surrounding plain. Although it looks like a volcano (a cinder cone), it is in reality a dike, an igneous intrusion that formed below the earth's surface and was subsequently exposed by erosion. Latitude: 37º 45' 13.8" Longitude: 104º 49' 34.6" Elevation: 6166' (1879m) at top Huerfano is Spanish for "orphan". The Huerfano River is named after the butte, Huerfano County is named after the river. In Colorado "Huerfano" is pronounced "WAR-fuh-no" as opposed to "WEAR-fuh-no" in proper Spanish.
2		From the south side, the butte looks pretty uniform in color (very dark gray), but looking from the west in the above photo, you can see a distinct color difference (and a notch in the top) which delineates the monzonite dike that cuts through the plug in an east-west direction. The dike is about 10 m in thickness and represents one of two dikes that cut the butte, the other an alkali-lamprophyre is discussed below.
3 - 4	A topographic map of the area above (from Terraserver) An aerial photo of Huerfano Butte on the right (from Terraserver). The east-west striking dikes can be seen cutting through the "lower" third of the butte.
5 - 6		The rock in photos 5 & 6 is classified as a olivine biotite alkali gabbro. Minerals present in the rock are olivine, biotite, plagioclase and potassium feldspar, orthopyroxene, titanaugite, apatite, and fluorite. The gabbro weathers a very dark gray color as seen in the above photo. The biotite gabbro has been dated using 40Ar/39Ar at 25.2 ± 0.8 Ma.
7 - 8		Views on the way up
9 - 10	Views from the top
11		U.S. Coast & Geodetic Survey Triangulation Station Huerfano Butte 1949 embedded in the rock at the summit elevation 6166'
12 - 13	Jointing of the gabbro is extremely pronounced throughout
14		The hypabyssal plug is cut by two dikes, one of monzonite and the other an alkali-lamprophyre (camptonite), the latter seen in the photo to the left. The monzonite dike has been dated at 25.2 ± 0.18 Ma. The monzonite's mineralogy consists of biotite, phlogopite, aegerine-augite, titanaugite, plagioclase, potassium feldspar, carbonate, apatite, sphene, analcite, and pseudoleucite. The lamprophyre has been described as a camptonite which according to the Glossary of Geology is defined as "...a lamprophyre in which plagioclase is more abundant than alkali feldspar, and total feldspar exceeds feldspathoids." The two dikes do not intersect so it is difficult to determine which was intruded first. Chemically they are so similar that they probably were probably intruded about the same time. The white material is caliche.
15		For a distance of about 80 m around the plug, the Pierre shale, in which the intrusion was emplaced, has been metamorphosed into an argillite. The term argillite, however, is probably not the best term for this material as it is clearly fissile. According to the Glossary of Geology, argillite is defined as a slightly metamorphosed mudstone or shale, but without its fissility or distinctive cleavage of slate.
	References: Foutz, Dell R., Geology of Colorado Illustrated: Jackson, Julia, 1997, Glossary of Geology: American Geological Institute. http://terraserver.homeadvisor.msn.com http://www.spanishpeakscolorado.com http://www.kmitch.com/Pueblo/origins.html http://www.geocities.com/CapitolHill/Lobby/3162/Colorado/placenames.htm Photos by John M. Ghist