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This project consists of two parts: one involves making a surficial geologic map of the nation's newest National Park, and the other involves providing geochronologic and paleoenvironmental information for the "Great Sand Dunes eolian system anthropological program," a multiple-agency effort administered by the National Park Service. Thus, the primary goals of this study are to determine the origin and age of a complex array of surficial deposits and to reconstruct the recent prehistory of the San Luis Valley in the vicinity of the Great Sand Dunes.
Most surficial deposits in the Great Sand Dunes area are linked to climate change and fluctuations of water-table level. Understanding these deposits and the interplay between climate change and geomorphic processes is important to management of both natural and cultural resources. Knowing the frequency and magnitude of past fluctuations of water-table level is especially important in the San Luis Valley. The Great Sand Dunes, which include the tallest dunes in North America (several taller than 200 m; 656 ft), are flanked on the west by the largest wetland in Colorado (Matthews and others, 2003). The closed basin in the San Luis Valley, from which the sand in the Great Dunes was derived, contains a vast quantity of ground water. Management of this water is important to all ecosystems in the area, which besides those of the Great Sand Dunes National Park and Preserve, include five wildlife (primarily waterfowl) refuges. In addition, groundwater in the closed basin is of strategic importance in administering the Rio Grande Compact, which assures availability of Rio Grande water to states downstream from Colorado.
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Figure 1. Map showing the location of the San Luis Valley and place names mentioned in the text. |
Wind-deposited sand blankets the east side of the San Luis Valley, the largest intermontane basin in Colorado, over a north-south distance of about 75 km (47 mi) (fig. 1). From the Great Dunes north, sand was transported primarily by winds blowing N 65°E; thus, sand lapped high onto alluvial fans and adjacent foot slopes of the Sangre de Cristo Mountains. Subsequently, much of this sand was reworked by water and deposited as alluvium on fans and footslopes, and also along stream channels and flood plains for several kilometers west of the mountain front. South of the Great Dunes, wind-deposited sand, driven primarily by southwesterly winds blowing on average N 37°E, drifted sand northeastward toward the Great Dunes rather than onto the footslopes of the Sangre de Cristo Mountains. The eolian sand area (fig. 1) parallels the axis of a relatively broad, shallow, north-northwest to south-southeast trending trough on the floor of the San Luis Valley. The trough extends southeastward from near the confluence of the San Luis and Saguache Creeks to a topographic and hydrologic divide just north of the Rio Grande. There is no outflow of surface water from the trough, nor is there an integrated drainage system within it, although a maze of relict channels indicate that streams have flowed through the area in the past. Most eolian sand is on the east (leeward) side of the trough, but some also is present on the west side. Eolian sand is the dominant surficial material in the area, but alluvium and marsh, pond, and lake deposits also are present. They cover about one-third of the area, primarily near the axis of the trough.
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Figure 2. The area of the Great Dunes is shown here on a mosaic of digital orthophoto quarter-quadrangles. The Great Dunes are defined as those that are more than 10 m (33 ft) high, contiguous, presently active, have steep, well-defined slip faces (steeply sloping surface on lee sides of dunes), and contain laminations that dip more steeply than 20°. Outlying areas of active dunes that have steep, well-defined slip faces, but which are less than 10 m (33 ft) high, are not considered to be part of the Great Dunes. (Click on image for larger view) |
The Great Dunes cover about 72 km2 (28 mi2) and rise as high as 210 m (690 ft) above adjacent terrain (fig. 2). Although the Great Dunes occupy less than 10 percent of the sand-covered area, they contain more than half of the sand in the system because elsewhere most wind-deposited sand is less than 7 m (23 ft) thick. The Great Dunes formed on the windward side of a pronounced saddle on the crest of the Sangre de Cristo Mountains. The saddle is coincident with a change in the trend of the range that forms an embayment in the mountain front (fig. 1). North of the Great Dunes, the range trends north-northwest, whereas to the south, the trend is south-southwest. Several high peaks, including Kit Carson Mountain (4317 m; 14,165 ft) and Crestone Peak (4357 m; 14,294 ft), extend north of the Great Dunes (fig. 3), and Blanca Peak (4372 m; 14,345 ft) and four other peaks ranging in altitude from 4163 to 4280 m (13,655-14,038 ft), form the range crest south of the Great Dunes. Between the high peaks north and south of the Great Dunes, the summit lowers by as much as 1300 m (4264 ft). Prevailing winds in the San Luis Valley are southwesterly and westerly. Variations in topography, sand supply, and range front geometry have caused wind to pile sand higher in the area of the Great Dunes than anywhere else along the east edge of the closed basin.
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Continue to Eolian Stratigraphy | Non-Eolian Stratigraphy | Origin and Conclusions
Matthews, Vincent, KellerLynn, Katie, and Fox, Betty, eds., 2003, Messages in stone Colorado's colorful geology: Colorado Geological Survey SP 52, 157 p.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/dunes/
Page Contact Information: ESP Web Team
Page Last Modified: Wed 15-Mar-2006 14:38:42 MST
