Rocky Mountain Geology
South-Central Colorado

J.S. and S.W. Aber
Emporia State University

Table of Contents
Field setting Geologic history
Related websites References

True-color MODIS satellite image of the Rocky Mountains in Colorado and New Mexico. Image acquired Sept. 20, 2002, showing the first significant snowfall of the autumn season. Pink asterisk indicates La Veta-Cuchara vicinity in Sangre de Cristo Range. Image adapted from NASA's MODIS gallery.

View from the High Plains east of Walsenburg toward Spanish Peaks and the Culebra Range of the Sangre de Cristo Mountains. A - East Spanish Peak, B - West Spanish Peak, C - Trinchera Peak of the Culebra Range, D - Cuchara Mountain Resort, our field accommodation. Image date 3/05, © J.S. Aber.

Field Setting

The following maps depict geology and shaded relief of the Rocky Mountain vicinity in south-central Colorado. Four major geologic provinces are included, from west to east: I - San Luis Valley, II - Sangre de Cristo Mountains, III - Raton Basin, and IV Apishapa Uplift. We will take excursions in these regions for advanced tectonics; exact sites and routes will depend on autumn weather (snow) conditions. Blue numbers on the geologic map indicate some of the localities that we may visit for field observations and projects. Click on the map below for discussion of individual sites and features.

Geologic map adapted from Colorado Geologic Highway Map (1991).

Major geologic zones
Geologic Province Geologic Setting
I - San Luis Valley Part of the Rio Grande rift system
II - Sangre de Cristo Mountains Part of the Rocky Mountains
III - Raton Basin Paleogene sedimentary basin
IV - Apishapa Uplift Broad arch in the High Plains

Selected field localities
1. Huerfano Butte 2. Walensburg vicinity
3. Goemmer Butte 4. Dakota hogback at Sulfur Springs
5. Spanish Peaks and dikes 6. Cucharas Pass
7. Culebra Range and Trinchera Peak 8. Monument and North Lakes vicinity
9. Dakota hogback at Stonewall 10. La Veta Pass and Mount Maestas
11. Zapata Falls 12. Great Sand Dunes National Monument


Shaded relief map adapted from Colorado Geologic Highway Map (1991).

Selected landmark features
Landmark Geologic Setting
1. Cuchara River Tributary of Arkansas River
2. East Spanish Peak Intrusive structure in Raton Basin
3. West Spanish Peak Intrusive structure in Raton Basin
4. Trinchera Peak Culebra Range, Sangre de Cristo Mts.
5. Monument/North Lakes Foothills of Culebra Range
6. Purgatoire River Tributary of Arkansas River
7. La Veta Pass Crest of the Sangre de Cristo Mts.
8. Blanca Peak Sangre de Cristo Mountain range
9. Great Sand Dunes San Luis Valley, Rio Grande rift

Return to beginning.

Geologic history of the region

The ancient basement rocks of southern Colorado were formed during Proterozoic orogenies, mostly in the middle Proterozoic, 1.0 to 1.8 billion years ago. A great variety of granites and metamorphic rocks make up the Proterozoic crust. These rocks have been uplifted to form the cores of many ranges of the Rocky Mountains, including the Culebra Range of the Sangre de Cristo Mountains. The erosional resistance of the these crystalline rocks supports the high peaks.

The period from late Proterozoic through middle Paleozoic was a time of stable continental conditions in which various sedimentary strata were deposited in shallow seas and low-lying land environments. Limestone, dolostone, sandstone, and shale mark this interval. At times the region underwent erosion, so no rock record was preserved. Rocks of this age are not well exposed in the field geology region.

Beginning in the Pennsylvanian, a significant change took place in Colorado tectonics. A mountain range was uplifted. Known as the Ancestral Rocky Mountains, this uplift took place in the same position as the modern Rocky Mountain Front Range, which includes the Sangre de Cristo Range. Substantial uplift combined with rapid erosion to produce immense quantities of coarse clastic sediment--sand and gravel, which was deposited in basins adjacent to the mountain front. These sediments are represented today by thick redbed sequences of Pennsylvanian and Permian age, which are exposed in the foothills along the eastern margin of the Culebra Range. By the end of the Permian, the Ancestral Rocky Mountains had been eroded down to low hills and plains. Through the following Triassic and Jurassic, the region remained continental with accumulation of alluvial and aeolian sediments.

A switch to marine environments took place in the Cretaceous as shallow seas transgressed over the mid-continent region. These marine transgressions resulted from local subsidence of the crust combined with global rises in sealevel. In Colorado, marine sandstone, shale, and chalk accumulated to considerable thickness during the Cretaceous. These strata are well exposed within the Apishapa Uplift and around the margins of the Raton Basin, where more resistant strata form escarpments and hogbacks.

The Larimide orogeny began in latest Cretaceous time and continued through the early Paleogene. This orogeny formed the fundamental structures of the modern Rocky Mountains. Mountain ranges were uplifted as tilted crustal blocks bounded by thrust and reverse faults. Proterozoic crust was thrust over Paleozoic and younger strata. Major thrust faults mark the eastern edge of the uplifts, as in the Culebra Range of the Sangre de Cristo Mountains. Uplift of the mountain ranges culminated in the Eocene in the central Rocky Mountains (Fan et al. 2014).

Uplift was accompanied by subsidence of marginal basins, such as the Raton Basin, which were filled by great thicknesses of clastic sediment. More than a kilometer of Paleogene sediment is preserved in the Raton Basin in vicinity of Spanish Peaks, for example. Larimide structural deformation was essentially complete in the southern Rocky Mountains by the end of the Eocene.

The mid-Cenozoic witnessed a change from crustal compression to crustal extension, as the Rio Grande rift system began to open up west of the Sangre de Cristo Mountains. This rift propagated northward from New Mexico into south-central Colorado during the Oligocene and Miocene. Widespread magma intrusions and volcanic eruptions took place in Colorado, New Mexico, and western Texas in connection with rifting. The Raton Basin was a focus for igneous activity within the field-geology region. Thick Tertiary sediments of the basin were intruded at Spanish Peaks, Goemmer Butte, Mount Maestas, Silver Mountain and White Peaks, and great dike systems were formed in connection with several of these intrusions. Most of this igneous activity took place between 27 and 21 million years ago in latest Oligocene and early Miocene times (Penn and Lindsey 1996).

Tectonic activity gradually diminished during the Neogene and Quaternary. A few volcanic centers continued to erupt in New Mexico, and the Rio Grande rift zone became relatively stable. Beginning in the Pliocene, the mid-continent region underwent a dramatic rise. Crustal uplift of the entire southern Rocky Mountains and Colorado Plateau regions exceeded one mile (1.6 km) in vertical movement. Rivers entrenched deep canyons, such as the Royal Gorge of the Arkansas River west of Canon City, and massive erosion of the landscape took place. Soft sedimentary strata were washed away leaving more resistant rocks to form the plateaus, buttes, peaks, and ridges of the modern landscape. The history of erosional downcutting is revealed by prominent terraces and pediments within the Cuchara drainage basin.

Aerial view looking northward along the crest of the Sangre de Cristo Mountains, Colorado. Present elevation of the mountains is a consequence of late Cenozoic crustal uplift and deep stream erosion. Image date 2/02, courtesy of J. Zupancic ©.
Aerial view looking northward over a well-formed hogback in the Colorado Plateau. Tilted sedimentary strata are revealed by erosional patterns. The hogback consists of a resistant formation within a softer sequence of rocks. Image date 2/02, courtesy of J. Zupancic ©.

The most recent geological event of note was the Ice Age during the Pleistocene Epoch, 1 million to 10,000 years ago. The high peaks of the Sangre de Cristo Mountains supported numerous small glaciers. These glaciers carved a classic assemblage of alpine landforms, including cirques, horns, aretes, and cols. Lower in the glaciated valleys, various kinds of till and stratified sediments accumulated to form moraines. Small kettle lakes occupy lateral and end moraine complexes. Most of the glacial deposits and landforms date from the last glacial phase, known in the Rocky Mountains as the Pinedale Stage. The glaciated terrain is among the most picturesque in the high alpine environment today. Alpine glacial landforms are depicted in the following topographic maps obtained from TopoZone.

Overview of Culebra Range

Details of Trinchera Peak

Details of Hells Canyon

Stream and river downcutting increased during the middle and late Pleistocene. According to Dethier (2001), the Culebra Range and adjacent High Plains experienced stream incision rates of 10-15 cm per 1000 years during the past 600,000 years. This adds up to 6-9 m (20-30 feet) of stream downcutting, which is thought to be a consequence of increased precipitation and runoff from the southern Rocky Mountains rather than a result of crustal uplift.

Return to beginning.

Related websites

References

Return to beginning.

All text and images © J.S. Aber (2014).