Ancient Sandstone Injected into Pikes Peak Granite is found Along Ute Pass

Figure 1. View of historic Ute Pss circa 1890s
Postcard from the S. Veatch collection.

Along the Front Range of Colorado, there are more than 200 sandstone dikes emplaced in ancient igneous (crystalline) rocks. Dikes are rock bodies that cut across another geologic body that formed first. These Front Range dikes are unique because most sandstone dikes are found in other sedimentary rocks. The Front Range dikes have been perplexing scientists since 1894, when they were first studied by the noted field geologist, Whitman Cross (1894).

The sandstone dikes are unique in the world, and are only exposed in just a few sites along Ute Pass (Figure 1). The ancient dikes, composed of Cambrian Sawatch Sandstone (Figure 2.), formed long after the Pikes Peak Granite was present. When the Pikes Peak Granite was subjected to intense compression during the Laramide orogeny, extreme force was concentrated on the sandstone, resulting in it being pressurized, heated, and fluidized. When fully fluidized, the sandstone was injected into enormous openings in Pikes Peak Granite; these openings were also formed by faulting pressures. Today, the Sawatch Sandstone dikes are preserved in certain areas of Teller County, Colorado.

Figure 2. Historical photo (1914) of Sawatch Sandstone resting on even erosion surface (nonconformity) of Pikes Peak granite: Ute Pass, near Manitou Springs, El Paso County, Colorado. 1914. Plate 6 in U.S. Geological Survey. Folio 203.

Winding into the mountains, U.S. highway 24 closely follows the Ute Pass fault, a major fault that separates the Rampart Range from the Pikes Peak massif and the rest of the Front Range. Starting southeast of Cheyenne Mountain, the Ute Pass fault can be traced for about 60 miles, and heads north along Teller Co. Route 67 beyond Woodland Park. The fault zone is relatively wide and filled with broken and fractured rocks that create the course of Fountain Creek in Ute Pass.

At least three resistant ridges made up of Sawatch Sandstone are exposed along Ute Pass: one sandstone dike (or “injectite” as they are sometimes called) is exposed in Crystola; and two injectites (Figure 3) are exposed in Woodland Park (Temple, et al., 2007). The injectites are easy to spot—instead of Pikes Peak Granite being present, the sandstone injectites are there—with Pikes Peak Granite on either side.

Figure 3. Three injectite samples. 
The one in the foreground appears to have ripple marks formed by primordial wave action. 
Photo date December, 2011 © by Steven Veatch.

Near Crystola there is a 100-meter-thick sandstone body, forming a resistant ridge of injectite sandstone. The dike—or injectite—dips at about a 75 degree angle to the west and strikes parallel to the Ute Pass fault.  There are also much smaller dikes present at this site.

These remarkable sandstone injectites can be thought of as “fault slices” of Cambrian Sawatch Sandstone “jammed” in Pikes Peak Granite during past movements of the Ute Pass fault. The injectites are made of fine- to medium-grained, well-rounded, and poorly sorted sandstone. Some samples show that they are sorted. Generally, the color of an injectite is reddish or maroon, but some of the weathered injectites have a buff discoloration on weathered surfaces that is related to the iron oxide cement present in the sandstone.

An examination of the dike rocks reveals an alignment of sand grains and granite fragments inside the injectites, which relates to the forceful injection of fluidized sand into openings into the granite caused by the pressures of faulting (Harms, 1965). Some angular fragments of Pikes Peak Granite found in the injectites result from granite being plucked off of the wall rock during movement (Figure 4).



Figure 4. View of  sandstone with highly indurated fragment of Pikes Peak Granite.
Photo date March 2, 2012.© by Steven Veatch

There is brecciated granite and brecciated sandstone formed from intense faulting (Figure 5).  The sandstone contains angular clasts. The granite breccia clasts are not as angular.

Figure 5.Angular fragments of Sawatch Sandstone created from faulting (Ute Pass Fault).
Photo date May 25, 2012 © by Steven Veatch.

Slickenside surfaces—further evidence of faulting—are common. Fluidized sandstone can be seen as veins entering into Pikes Peak Granite (Figure 6)..

Figure 6. A vein of fluidized Sawatch Sandstone has been injected into Pikes Peak Granite.
Photo date May 25, 2012. © by Steven Veatch.

Today the injectites remain a source of much scientific debate, and this summer a new scientist will probe the mysteries of this ancient sandstone embedded in Pikes Peak Granite (J. Temple, personal communication).

References cited:

Cross, W., 1894, Intrusive sandstone dikes in granite: Geological Society of America   Bulletin, Vol. 5, p. 225-230.

Harns, J., 1965, Sandstone dikes in relation to Laramide faults and stress distribution in the southern Front Range, Colorado: Geological Society of America Bulletin, Vol. 76, pp. 981-1001.

Temple, J., R. Madole, J. Kelle, and D. Martin, 2007, Geologic map of the Mt. Deception quadrangle, Teller and El Paso counties, Colorado: Colorado Geological Survey Open File Report OF-07-7.