HUERFANO BUTTE

 

Steven Wade Veatch

 

Abstract Huerfano Butte, 10 kilometers north of Walsenburg, Colorado, is a conical hill rising 100 meters above the valley bottom of the Huerfano River. The butte is a hypabyssal plug of biotite alkali gabbro intruded by a dike of biotite monzonite and a smaller dike of alkali lamprophyre. Radiometric dates of the biotite gabbro (25.2 - 26 Ma) and the monzonite (26 Ma) are Oligocene, synchronous with the alkaline intrusive rocks of the Spanish Peaks 50 kilometers to the southwest. Remote sensing images and aerial photos provide a synoptic view of the intrusive bodies.

 

 

Introduction and Historical Background  Ecology  Geology  Chronology  Igneous Petrology of Huerfano Butte  Remote Sensing

 

Introduction and Historical Background

   El Huerfano – the orphan – is a stark and solitary rock outcrop that stands silently in the foreground of the Spanish Peaks. El Huerfano, named in the late 1700s by a Spanish trader, had been for centuries a landmark that guided earlier Indians and Spaniards traveling through this region. Today El Huerfano is known as Huerfano Butte, and is located east of mile marker 60 on I-25 (10 kilometers north of Walsenburg, Colorado, see figure 1). Surrounding areas have yielded a multitude of relics, from Indian artifacts to various parts of Spanish armor.

    This area became part of Mexico after 1821, and migrating Mexicans soon built small agricultural communities along the Huerfano River and nearby streams. The Mexican government granted extensive tracks of land to encourage settlement. The Vigil-St. Vrain grant, covering over four million acres, was established here. John C. Fremont’s last western expedition, a railroad survey, passed Huerfano Butte on its way to California in 1853. Huerfano Butte and the surrounding grasslands continued to attract many settlers including a Frenchman by the name of Beaubois who, in 1858, started a ranch and built a fort near Huerfano Butte. A band of hunters and teamsters who had worked for Bent, St. Vrain, and Company later set up a stage stop known as Butte Valley, in 1862. These men – with sympathies toward the South – harbored Confederate soldiers, guerrilla gangs, bandits, and stagecoach robbers until the First Colorado Cavalry ran them out of the area.

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Figure 1 (Map courtesy of Dr. Brian Penn)

Ecology

 

    A diverse fauna, characteristic of the semi-desert habitat, thrives in the Huerfano Butte area. Many species of mammals, including the mule deer, coyote, cottontail rabbits, black-tailed jackrabbits, raccoons, and ground squirrels live here. A variety of birds can be seen in the region, such as several species of raptors, pinion and scrub jays, and western meadowlarks.

 

    The grasslands are composed of blue grama, western wheat grass, and several other common grasses. The dominant plants associated with the grasslands are the yucca, cholla, cactus, rabbit brush, and four-winged salt bush.

 

   

 

 

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Geology

    The geology varies from simple strata of recent deposits of alluvium to the emplacement of the conical Huerfano Butte that rises 100 meters above the landscape. Viewed from a distance, Huerfano Butte appears to be volcanic neck, however there is no evidence that magmas ever vented to the surface. Huerfano Butte is a hypabyssal plug – a minor intrusion of a vertical mass of igneous rock that has crystallized under conditions between plutonic (intrusive) and volcanic (extrusive). Pierre Shale, which extends 80 meters from Huerfano Butte, has been metamorphosed to argillite. The plug is associated with the emplacement of the Spanish Peak intrusives, which were synchronous with the start of the Rio Grande Rift in Southern Colorado during the late Oligocene and the early Miocene.

    The Spanish Peaks rise above the far western edge of the Great Plains 50 kilometers to the southwest of Huerfano Butte. Many intrusive features, including stocks, dikes, sills, laccoliths, plugs, and injections dominate the landscape. East Spanish Peak (4,152 meters) and West Spanish Peak (3,873 meters) are the largest stocks. There are two sets of radial dike systems in the region: a swarm of over 500 radial dikes is concentrated just west of West Spanish Peak and a second radial dike system is centered around Silver Mountain, 27 kilometers NNW of West Spanish Peak (Johnson, 1961).

Chronology

    The landforms seen in the Huerfano Butte area are related to the Spanish Peak intrusives – emplaced during a period of continuous intrusive activity from about 26.6 to 21.4 million years ago (Penn, 1998, pers. comm.). Huerfano Butte represented the earliest alkali-gabbro and lamprophyre phase of intrusion in the Spanish Peaks area. The next phase of intrusives included (calcium-alkaline) West Spanish Peak and East Spanish Peak and all the radial dikes (mostly monzonitic to syenitic porphyries). The last phase of intrusive rocks (potassium and strontium rich) are found in the northeast portion of the Spanish Peaks region (Penn, 1998, pers. comm.).

Igneous Petrology of Huerfano Butte

    The Butte is composed of a plug of biotite alkali-gabbro. Gabbro is a coarse-grained mafic igneous rock composed of ferromagnesian minerals and calcium rich plagioclase feldspar. The alkali-gabbro, which is the intrusive equivalent of basalt, has been dated between 25.2 and 26 million years ago. (Penn, 1995). Well-developed jointing in the fine-grained alkali-gabbro is exhibited on south side of the plug. Hexagonal columns form as the result of the contracting and cooling of magma, and the jointing reveals the shallow intrusive nature of the alkali-gabbro.

    The alkali-gabbro plug is intruded by two east-west trending dikes. The larger felsic dike, about 10 meters thick, is composed of monzonite that cuts through the middle of the plug and extends through the notch and over the top of the Butte. The notch has formed because the monzonite weathers more quickly than the alkali-gabbro. The monzonite intruded the gabbro plug about 25.2 million years ago (Penn, 1995). Monzonite (see figure 2), a lighter color than the gabbro, contains approximately equal amounts of alkali feldspar and plagioclase, and little or no quartz. Augite is the main mafic mineral.

    The smaller mafic dike, visible only from the east side, is an alkali-lamprophyre. Lamprophyres are a complex group of dark alkaline igneous rocks that share mineralogical similarities with some kimberlites. Dark minerals occur as phenocrysts and in the lamprophyre groundmass. A white material, caliche (calcium carbonate), is deposited nearby.

   Figure 2

 

Thin-section of monzonite porphyry from central dike of Huerfano Butte

Photo courtesy of Dr. Brian Penn

Remote Sensing

    Remote sensing reveals through photos and images the stocks, dikes, plugs, and other intrusive features that dominate the landscape of the study area. A satellite image, unlike a photograph, is a rendition of a target’s features depicted through the use of spectral reflectance and stored digitally. These reflectance values are stored in a form suitable for input into a computer. Software and hardware designed to analyze these images provides the ability to see a pictorial representation of remotely sensed targets.

    Landsat 5 images, taken at an altitude of 705 kilometers, provides a synoptic view of the Spanish Peaks area. The Landsat 5 multispectral sensors are designed to operate in several different bands of the electromagnetic spectrum. Electromagnetic radiation produced by the sun is the source of all signals collected by most remote sensing instruments. Landsat 5 used a second generation scanner, the Thematic Mapper, a nonphotographic imaging system that utilizes an oscillating mirror and seven arrays of detectors that sense electromagnetic radiation in seven different bands. These sensors record the light levels of the Earth’s reflected energy (visible and near-infrared) and transmit digital image data to the ground in four to seven wavelength bands (groups) for each Landsat scene. On the ground the digital data are converted to images similar in appearance to aircraft acquired photographs. The Thematic Mapper has improved spectral separation, ground resolution, geometric fidelity, and radiometric accuracy for computer image processing (Baylin and Gaisford, 1993).

    The Landsat 5 image in figure 3, a color-infrared composite of multispectral data, shows the Spanish Peaks area and a well-exposed system of radial dikes. Vegetation in this image is bright red, and rivers are highlighted in red due to the vegetation growing along their banks. The stocks of West and East Spanish Peak are located left of center. The Landsat 5 image shows that the dikes are focused on West Spanish Peak. The two peaks appear white because there is no vegetation present above the tree line (3,350 meters or 11,000 feet). The other white objects are clouds. The change from light red to dark red around the peaks is due to the change from scrub oak and pinion trees to pine and fur trees. Deciduous leaves generally are more reflective than evergreens and their leaves tend to be brighter and richer in hue than the needles from the pine and fur trees. (Short, 1998). This image also depicts the Cretaceous Dakota Hogback near Stonewall. Silver Mountain, a small stock, shows a set of dikes radiating outward. A number of stock ponds and reservoirs are shown in figure 3.

    The National Aerial Photography Program (NAPP), an interagency Federal program that is coordinated by the USGS, produces aerial photos. These photos, taken from airplanes flying at 6,100 meters (20,000 feet) on about a 5-year cycle, cover the coterminous United States. Each 9-by-9 inch photo covers an area just over 5 miles on a side. Aerial photos, either color-infrared and black-and-white, are the most important and commonly used kinds of remotely sensed images. Figure 5 and 6 are NAPP photos.

Thematic Mapper satellite image is a standard false-color composite (TM bands 4, 3, 2

in red, green, blue) of the Spanish Peaks area. Image courtesy of Dr. Brian Penn.

(Figure 3)

 

    Figure 3 is a color infrared Landsat 5 Thematic Mapper image (acquired on 7/30/92 at 10:30 a.m.) of the Spanish Peaks area. The color red is associated with live vegetation. Very intense shades of red indicate dense vegetation that is growing quite vigorously. The change from light red to dark red around the peaks is due to the change from scrub oak and pinion trees to pine and fur trees. Deciduous leaves generally are more reflective than evergreens and their leaves tend to be brighter and richer in hue than the needles from the pine and fur trees. Rivers are highlighted in red due to the vegetation growing along their banks. The stocks of West and East Spanish Peak are located left of center. The southern end of the Wet Mountains is visible at the top of the image. Clouds are the white areas in the Sangre de Cristo Mountains and on the southwest slope of the West Spanish Peak. Huerfano Butte is 10 km from Walsenburg and is south of the Huerfano River. North is at the top of the image. 
 

Satellite Image (Figure 4)

 

    Figure 4 is a color infrared Landsat 5 Thematic Mapper image (acquired on 7/30/92 at 10:30 a.m. ) of Walsenburg, Colorado located at the intersection of I-25 and Colorado 10. Lathrop State Park is visible (far left of center – Martin Lake, Horseshoe Lake, and a golf course can be seen). Three long northeast trending ridges are visible in this image. From northwest to southeast they are Pictou Dike, Walsen Dike, and Unfug Ridge. Unfug Ridge is part of the earliest phase of intrusion in the Spanish Peaks region. The Cucharas River flows northeastward between the Walsen Dike and the Unfug Ridge. North is at the top of the image.

    The color infrared photo of Huerfano Butte in figure 5 (just right of center and south of Huerfano River) was taken at 6,100 meters (20,000 feet) on 8/14/88. The intense shade of red in the alfalfa fields indicates dense vegetation growing quite vigorously. Mowed alfalfa fields, where vegetation density and vigor decrease, display different tones of red – lighter pink colors. Note where plant density and activity becomes too low the faint red coloring is overcome by the stronger colors representing the soil in which the plants have been growing. The ground area appears in shades of white, blue, or green depending on the soil type and moisture content. Bare soils are white, blue, and green. The moister the soil, the darker the color. Dry, sandy land appears white in color. I-25 runs through the middle of the photo and is dark blue. The dry and sandy gravel roads have less intense colors than the asphalt of I-25. In this image these gravel roads appear white. Road 104 runs in front of Orlando Reservoir Number 2 in the right corner of the photo. The northwest edge of Orlando Reservoir is white, which reflects the high levels of built-up sand. With the increase of sediment deposits, the reservoir turns to lighter blue tones. Pristine water is black. Shallow water reflects the material in its bottom. North is at the top of the photo.

This black and white image of Huerfano Butte in figure 6 (just right of center and south of Huerfano River) was taken at 6,100 meters (20,000 feet) on 6/28/93. North is at the top of the photo.

Acknowledgments

I thank Dr. Brian Penn, Department of Geological Sciences, University of Texas at El Paso, with dating and general interpretation and Shelly Veatch for field assistance. This work is part of an earth science course requirement (ES771 Remote Sensing) completed at Emporia State University.

References:

Avery, T. E., and Berlin, G. L., 1992, Fundamentals of remote sensing and airphoto interpretation. Prentice Hall, New Jersey, p 140.

Baylin, F. and Gaisford, B (contributing editors), 1993, International satellite directory, 8th edition. Design Publishers. Sonoma, California, Paginated in sections.

Johnson, R.B., 1961, Patterns and origin of radial dike swarms associated with West Spanish Peak and Dike Mountain, south-central Colorado. Geological Society of America Bulletin, v. 72, p. 579-590.

Penn, B.S., 1995, What’s the scoop on Huerfano Butte? In EOS Transactions of the American Geophysical Union, Spring Meetings, Baltimore, MD, vol. 76, no. 17, April 25, 1995, published as a supplement to EOX, p. S311-S312.

Penn, B.S., pers. comm.: Dept. of Geological Sciences, University of Texas at El Paso, El Paso TX 79968, June 16, 1998, (letter).

Short, N.M., 1998. The remote sensing tutorial. NASA

Last update: March 16, 1999