Ancient sandstone injected into Pikes Peak Granite along the Ute Pass
Steven Wade Veatch (USA)
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, earlier geologic body. 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 (Fig. 1).
The ancient dikes, composed of Cambrian Sawatch Sandstone (Fig. 2), formed long after the Pikes Peak
Granite first appeared. When this granite was subjected to intense compression during the Laramide Orogeny, extreme force was concentrated on the sandstone, resulting in it being pressurised, heated and turned to fluid. When fully fluidised, the sandstone was injected into enormous openings in Pikes Peak Granite, which were also formed by faulting pressures. Today, the Sawatch Sandstone dikes are preserved in certain areas of Teller County in Colorado.
Winding into the mountains, US Highway 24 closely follows the Ute Pass fault, which is 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 just under a 100km 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 (Fig. 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. Near Crystola, there is a 100m-thick sandstone body, forming a resistant ridge of injectite sandstone. The dike – or injectite – dips at about a 75o angle to the west and strikes parallel to the Ute Pass fault.
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. Generally, the colour of the injectites 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 fluidised sand into openings in 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.
Today the injectites remain a source of much scientific debate and, this summer (2012), another scientist has apparently probed the mysteries of this ancient sandstone embedded in Pikes Peak Granite (J Temple, personal communication). I look forward to seeing the results.
References
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.