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The Geologic Story of Colorado's Sangre De Cristo Range

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The Geologic Story of Colorado's Sangre De Cristo Range The Geologic Story of Colorado’s Sangre de Cristo Range Circular 1349 U.S. Department of the Interior U.S. Geological Survey Cover shows a landscape carved by glaciers. Front cover, Crestone Peak on left and the three summits of Kit Carson Mountain on right. Back cover, Humboldt Peak on left and Crestone Needle on right. Photograph by the author looking south from Mt. Adams. The Geologic Story of Colorado’s Sangre de Cristo Range By David A. Lindsey A description of the rocks and landscapes of the Sangre de Cristo Range and the forces that formed them. Circular 1349 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2010 This and other USGS information products are available at http://store.usgs.gov/ U.S. Geological Survey Box 25286, Denver Federal Center Denver, CO 80225 To learn about the USGS and its information products visit http://www.usgs.gov/ 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Lindsey, D.A., 2010, The geologic story of Colorado’s Sangre de Cristo Range: U.S. Geological Survey Circular 1349, 14 p. iii Contents The Oldest Rocks ...........................................................................................................................................1 Uplifts and Unconformities ...........................................................................................................................2 A Map of the Range .......................................................................................................................................4 The Ancestral Rockies ..................................................................................................................................6 The Laramide Orogeny ..................................................................................................................................8 The Rio Grande Rift ......................................................................................................................................10 The Ice Ages .................................................................................................................................................13 References ....................................................................................................................................................14 Acknowledgments .......................................................................................................................................14 Figures 1. Precambrian rocks south of Crestone: (A) layered gneiss; (B) pink granite; (C) dark igneous rock (gabbro) with granitic veins .................................................................1 2. Uplifts and unconformities. Rocks of early Paleozoic age lap onto an old highland to the south and were partly dissolved and eroded before rocks of late Paleozoic age were deposited. .....................................................................................................................3 3. Geologic map of Colorado’s Sangre de Cristo Range .............................................................4 4. The Ancestral Rockies. Map and photographs of Crestone Conglomerate .......................6 5. At Marble Mountain, fossil mounds called bioherms in limestone of the Minturn Formation have been thrust over the younger Sangre de Cristo Formation along the Marble Mountain thrust fault ........................................................................................................7 6. A redbed under the microscope, showing iron oxide ............................................................8 7. The Laramide orogeny. Photographs of folded and faulted rocks of Pennsylvanian and Permian age and section showing structure ...................................................................9 8. The Rio Grande rift. Map and photographs of the Sangre de Cristo fault in the San Luis Valley ....................................................................................................................................10 9. The ice ages. Aspen cover a lateral moraine in Black Canyon, north of Valley View Hot Springs ..................................................................................................................................13 iv The Geologic Story of Colorado’s Sangre de Cristo Range By David A. Lindsey A There is no record of the beginning of time in the Sangre de Cristo Range*. Almost 3 billion years of Earth history are missing, but the rest is on spectacular display in this rugged mountain landscape. This is the geologic story of the Sangre de Cristo Range. The Oldest Rocks The oldest rocks are like those in other Colorado moun- tains—Precambrian gneiss from 1.8–1.7 billion years ago. B Gneiss (layered metamorphic rock formed by heat and pres- sure—fig. 1A) makes up much of the range east of the Great Sand Dunes. Once the gneiss was sedimentary or igneous rock, but it rarely retains the original texture of these rocks. Chemi- cal and isotopic composition is the only guide to the origin of gneiss. Much of the light-colored gneiss on Blanca Peak is thought to have formed by metamorphism of a large igneous intrusion (called a “pluton”). Plutons form when large balloon- shaped bodies of magma rise into the middle and upper levels of the Earth’s crust and cool. About 1.7–1.4 billion years ago, after the first episodes of metamorphism and igneous intru- sion had ended, four more plutons invaded the crust; they are exposed along the west side of the range north and south of C Crestone, and in the headwaters of Sand Creek west of Marble Mountain. Although these intrusions all look like granite (fig. 1B), their mineral composition actually varies; geologists assign specific names to them, names like monzogranite and quartz monzonite. They determine these names by identifying and estimating the amount of each mineral in the rock. Besides the large granite plutons, dikes of gabbro (a heavy, dark igne- ous rock composed of iron and magnesium minerals—fig. 1C) and pegmatite (a coarse-grained version of granite) intrude both gneiss and younger plutons. The forces of metamorphism and intrusion come from plate tectonics, the process by which continent-sized slabs of the Earth’s crust break apart by rifting and regroup by plate collision. During Precambrian time the old core of Figure 1. Precambrian rocks south of Crestone: (A) layered gneiss; the continent lay to the north, in Wyoming. Sea-floor plates (B) pink granite; (C). dark igneous rock (gabbro) with granitic veins. from the south—some with volcanic island chains—collided Picks (12 inches in length) and pen (6 inches) show scale. *The Sangre de Cristo Mountains extend from near Santa Fe, New Mexico, to Salida, Colorado. In Colorado and northernmost New Mexico, the name “Culebra Range” refers to the Sangre de Cristo Mountains south of La Veta Pass, and the name “Sangre de Cristo Range” refers to the mountains north of the pass. This document discusses the Sangre de Cristo Range north of the pass. 2 The Geologic Story of Colorado’s Sangre de Cristo Range with this old continent. Sea-floor plates usually are heavier Manitou Dolomite, Harding Sandstone, and Fremont Dolomite, than continents and are swept beneath them (a process called and the Devonian Chaffee Formation (dolomite and sandstone). “subduction”). However, some plate fragments are torn away At different places, both the Manitou and Harding lie uncon- and plastered against the continent—this is how continents formably on Precambrian rocks. This unconformity represents 1 grow. Rocks in the subducted plate melt and plumes of molten billion years of Earth history missing from the Sangre de Cristo rock rise through the newly enlarged continent, solidifying Range; all of the evidence for this history is gone. Whatever below the surface as plutons or erupting as volcanoes. Today rocks formed in this place, whether igneous or sedimentary, the results of this process of ancient continental growth are were removed by erosion before the Ordovician seas covered it. exposed as gneiss and granite in mountain uplifts like the By Ordovician time, a broad highland stood above the Ordovi- Sangre de Cristo Range. cian sea over what is now southern Colorado. The Ordovician sea slowly encroached on the uplift, producing an onlapping sequence of successively younger formations. By Mississip- pian time, when the Leadville Limestone was deposited, the sea Uplifts and Unconformities probably covered the entire highland. In fact, we find marine limestone of this age throughout the Rocky Mountains. At many places in the world, including the Sangre de Then sea level fell, and a second unconformity began Cristo Range, a great gap in time separates younger rocks from to form. Land displaced shallow seas; rocks were exposed those of Precambrian age. We recognize time gaps where rock to weathering and erosion. In the warm, humid climate, layers that should be present—because they are seen else- limestone dissolved; caves and caverns formed. The Lead- where—are missing. Geologists call these horizons of miss- ville Limestone collapsed to form a jumble of sharp rock ing rocks
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