Spanish Peaks
Dikes and Sills

James S. Aber
Emporia State University

Table of Contents
Spanish Peaks Dikes
Age & composition Tectonic setting

Spanish Peaks

Spanish Peaks consist of "twin mountains" located at the westernmost edge of the High Plains in south-central Colorado. The mountains were formed by magma intrusion of the surrounding sedimentary rocks during the late Oligocene and Miocene epochs (Penn and Lindsey 1996). Magma intruded along two central stocks within the Cuchara Formation (Eocene). These injections split the surrounding crust, and magma flowed into these fractures to form dikes. Magma was also injected along bedding planes and formation boundaries within the sedimentary sequence to form sills.

East Spanish Peak (left) stands 12,683 feet high, and West Spanish Peak (right) is nearly a thousand feet higher at 13,626 feet in elevation. Seen here from Goemmer Butte. Two major dikes are visible--dike of Profile Rock (right center) and the Great Wall (to left). These dikes radiate from West Spanish Peak. Image date 6/99; © J.S. Aber.
View of West Spanish Peak, as seen from Goemmer Butte, with the dike of Profile Rock (center) in line with the peak. The lower portion of the dike has been eroded away by the Cuchara River, which cuts across the near end of the dike. Part of the Great Wall dike can be seen to left. Image date 6/99; © J.S. Aber.
Closeup view of West Spanish Peak seen from Cordova Pass. Notice the nearly horizonal strata which consist of metamorphosed beds of Cuchara Formation. These resistant rocks make up the bulk of Spanish Peaks. Image date 6/03; © J.S. Aber.

Astronaut photograph of Spanish Peaks.

Dikes of the Raton Basin

Most of the intrusions took place along the axis of the Raton Basin, perhaps because of crustal fracturing of basement rocks along the basin trough. Besides Spanish Peaks, several other significant intrusions are present along the axis and western portion of Raton Basin, including Silver Mountain, Mount Maestas, and White Peaks. According to Penn and Lindsey (1996), three distinct groups of dikes exist in the Raton Basin of southern Colorado.

  1. Radial dikes -- These dikes diverge from a central point like the spokes of a wheel. Two sets of radial dikes are associated with West Spanish Peak and Silver Mountain. More than 500 dikes are known in the West Spanish Peak group, although none has a direct contact with the stock of West Spanish Peak.

  2. Subparallel dikes -- Dikes with approximate east-west trend (80-260°) are present throughout the Raton Basin, especially south of Spanish Peaks. These are among the longest dikes, up to 27½ km (17 miles) in length.

  3. Independent dikes -- Randomly oriented dikes within the region.

Profile Rock, so called because it appears to be the profile of a person lying down (top of head to left). This dike is described as rhyolite in composition (Penn and Lindsey 1996). The adjacent country rock has been baked almost black by contact metamorphism. Image date 6/99; © J.S. Aber.
Dike of Devils Stairsteps next to Colorado highway 12. This spectacular dike is well exposed next to the highway. The dike rock is described as granite or granodiorite porphyry (Penn and Lindsey 1996). Image date 6/99; © J.S. Aber.
A small dike (dark rock) has attached remnants of the Cuchara Formation (light rock) on its side. The Cuchara sandstone displays cross bedding and other sedimentary structures. Image date 6/99; © J.S. Aber.
Dike exposed at Cordova Pass. Cobbles and boulders of the Cordova Pass Conglomerate member of Cuchara Formation (Eocene) are metamorphosed on the right wall of the dike. Trinchera Peak of the Culebra Range is center mountain in background. Image date 6/03; © J.S. Aber.

Age and composition of intrusions

Spanish Peaks, associated intrusions, and dikes were emplaced during the latest Oligocene and Miocene. Much debate has surrounded the ages and sequence of intrusions in the past. High-precision 40Ar/39Ar radiometric dating has provided a detailed chronology for the igneous rocks. Phases 1-6 of the following sequence formed between 27 and 21 million years ago. The last phase (7) was only 14 million years ago (Penn and Lindsey 1996).

  1. Silver Mountain, Black Hills, and Little Black Hills.
  2. Early phase of subparallel dikes.
  3. West Spanish Peak monzonite.
  4. East Spanish Peak granodiorite porphyry (inner stock) and granite porphyry (outer stock).
  5. West Spanish Peak syenite porphyry radial dikes.
  6. White Peaks microgranite.
  7. Late phase subparallel dikes north and northwest of East Spanish Peak.

Igneous rocks of the region display considerable range in compositions, which may be described in general as either felsic or mafic. Felsic refers to rocks rich in feldspars and silica; mafic indicates rocks that are rich in magnesium and iron and with lower silica content. Some rocks have compositions intermediate between felsic and mafic. The most common felsic rock compositions are monzonite and syenite porphyries. These rock types form the stocks of major intrusions and most of the dikes near to Spanish Peaks. Mafic rocks, such as basalt, are found farther away from Spanish Peaks.

See igneous rock classification.

Tectonic setting and present relief

Syenite is a rather unusual rock composition, similar to granite, but with little or no quartz. Syenite is composed essentially of potassium and sodium feldspars; it cooled from a felsic magma in which there was not enough silica to form quartz. Syenite indicates a particular kind of tectonic environment associated with the early phase of continental rifting. The intrusions in the Raton Basin are related to opening of the Rio Grande rift west of the Sangre de Cristo Mountains. This rift system failed to continue opening after the Miocene; it is similar to other early or failed rift systems such as the Mississippi embayment or East African rift system.

Magma injection at Spanish Peaks and associated intrusions took place entirely in subsurface sedimentary strata. Spanish Peaks are not volcanoes. The present cone-like appearance of Spanish Peaks is due entirely to post-Miocene crustal uplift and erosion. Intrusive rock and adjacent contact metamorphic rock are more resistant to erosion than are poorly consolidated sedimentary strata of the Raton Basin. From the top of West Spanish Peak (4153 m) to the Cuchara River near La Veta (2150 m) is more than two km of vertical relief. This figure should be regarded as a minimum value for the amount of stream downcutting in the vicinity.

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GO 547/ES 747 © J.S. Aber (2008).