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Surficial Geology and Geomorphology of the Great Sand Dunes Area, South-Central Colorado

Eolian Sand Stratigraphy

Wind-deposited sand is at the surface in about two-thirds of the study area. The shape, orientation, and location of wind-deposited sand indicate that it came primarily from the floor of the closed basin, rather than from the late Pleistocene flood plain of the Rio Grande as some have suggested (Johnson, 1967). Large volumes of eolian sand were deposited episodically from Pleistocene to latest Holocene time. Most of the sand at the surface is Holocene and can be divided into three units (fig. 1) on the basis of (1) topographic expression of dunes, (2) differences in degree of soil development, and (3) stratigraphic relations.

Figure 1. Distinguishing characteristics of eolian sand units in the Great Sand Dunes area. Plus symbols represent the stratgraphic position of a zone of secondary CaCO3 in unit Qes2. This zone of calcareous sand is an important stratigraphic marker throughout the Great Sand Dunes area.

Active sand Sharply defined dune morphology No soil Sand is noncalcareous (in other words, does not contain CaCO3) Age: Latest Holocene Stable and quasistable sand Distinct dune morphology Very weak soil to no soil Sand is noncalcareous, except adjacent to playas Age: Late Holocene Stable sand Subdued dune morphology Weak soil; partly eroded in many places Sand is noncalcareous except in upper 0.5-1.5 m (1.6-5 ft) Age: middle Holocene to earliest late Holocene Buried in most places Age: late Pleistocene

The youngest Holocene unit (Qes4) consists of sand that has been active in historic time. It is at the surface in about 10% of the area, chiefly in the Great Dunes, but small areas also are present in the sand sheet bordering the Great Dunes (fig. 2). Most of the unit probably has been active since first described by Zebulon Pike in January 1807 (Carter, 1978), and may have been active for a long time prior to 1807.

Figure 2. Landsat image of the Great Sand Dunes area. Numbers denote eolian sand units Qes2, Qes3, Qes4.

The middle Holocene unit (Qes3) was deposited episodically between about 2600 and 300 years ago, but most of it is probably younger than 1300 years. The middle unit is at the surface in about 70% of the area covered by eolian sand. It forms (1) fields of parabolic dunes north, northwest, and southwest of the Great Dunes, (2) lunette dunes in southern part of the area, and (3) compound parabolic dunes northeast of San Luis Lake (figs. 4, 5, 6, and 7). Unit Qes3 has little or no soil development and is noncalcareous, except in lunette and coppice dunes adjacent to playas.

Figure 4. Vertical airphoto shows an area south of Medano Creek where unit Qes3 is widespread. Lineations trending from lower left to upper right are the arms of simple hairpin-shaped parabolic dunes, a type that in this area is common only in unit Qes3. Like most streams flowing into the basin from adjacent mountains, Medano Creek (wet and dry channel floor, top of photo) is a losing stream. Even during peak flows, it does not extend beyond the area shown here before being lost entirely to infiltration and evaporation. Arrows denote location of a paleochannel of Medano Creek that predates deposition of parabolic dunes of Qes3.

Figure 5. Vertical airphoto shows the spatial relations of Holocene eolian sand units southwest of the Great Sand Dunes (see fig. 3 for photo location). The contrasting colors of units Qes2 and Qes3 are due to differences in sand composition and vegetation cover. Unit Qes3 is covered primarily by grass, forbs, and widely scattered rabbitbrush, whereas Qes2 supports abundant shrubs, predominantly halophytes, such as greasewood and saltbush.

Figure 6. Vertical airphoto of late Holocene playas and lunette dunes of Qes3 south of Lane 6 (see fig. 3 for location). X marks the location where charcoal was collected from playa-floor sediment 1 m (3.3 ft) below the base of a high (6 m; 20 ft), sharp-crested lunette dune. The age of the charcoal, which predates formation of the overlying dune, is 1050 ± 70 14C years B.P. (Before Present). The 2-sigma calibrated-age range of the charcoal is A.D. 870-1160.

Figure 7. San Luis Lake occupies a deflation basin (see fig. 3 for location). It became a source of wind-deposited sand when water table fell and the lake dried up. During Holocene time, a small analog of the Great Dunes developed here in the form of a system of compound parabolic dunes that migrated 2.3 km (about 1.4 mi) northeast from San Luis Lake. The sand came from the lake basin, but only after water table had lowered to the point that most or all the lake basin was dry. Wet or damp sand resists wind erosion. Radiocarbon and OSL ages indicate that the parabolic dunes formed during the past 1000 years (De Lanois, 1993; Madole, unpub data 2001-2004).

The oldest Holocene unit (Qes2) includes sand that was deposited at different times during the middle Holocene (8,000-4,000 years ago) and early part of the late Holocene (4,000 years ago to present). It is at the surface in about 10% of the area, primarily the west-central and southern parts. Unlike the middle unit, the oldest unit has a very weakly developed soil in some places and everywhere contains secondary CaCO₃ in the upper 0.5-1.5 m (1.6-5 ft) (fig. 11).

Figure 8. Ledge-forming secondary CaCO3 in the uppermost part of Qes2 is exposed in a borrow pit (see fig. 3 for location). Calcium carbonate bonds the sand grains and makes Qes2 more resistant to erosion than other eolian sand units, which are noncalcareous in most places. The carbonate-enriched sediment forms a ledge because it became case hardened (dry surface is harder than the more moist interior) after being exposed in the borrow pit. The sand below the carbonate-enriched ledge is noncalcareous. The zone of CaCO3 accumulation parallels the land surface across topographic highs and lows, and it is present on dune summits at levels too high to have been reached by the capillary fringe that rises from the water table. The OSL (optically stimulated luminescence) age is of noncalcareous sand just below carbonate-rich ledge (B.P., abbreviation of Before Present).

Exposures of Pleistocene eolian sand are scarce, and are limited primarily to stream banks, blowouts, and excavations (figs. 9, 10, and 11). Little is known about the extent and properties of this unit. In the southern and westernmost parts of the Great Sand Dunes area, Pleistocene alluvium and lacustrine sediment underlie Holocene wind-deposited sand at shallow depths (1.5-6 m; 5-20 ft).

Figure 9. Peaty sediment about 1 m higher than Big Spring Creek just west of the Great Dunes (see fig. 3 for location) marks the level of the valley floor that existed near the beginning of Holocene time.

Figure 10. Diagram of stratigraphy and numerical ages at Tallest Bend, Big Spring Creek, just west of the Great Dunes (see fig. 3 for location).

Figure 11. Cohesive pond or marsh sediment (low ledge at end of leader) is exposed on the floor of a blowout in unit Qes3 just south of Lane 6. The sediment provided a radiocarbon age of 9,510 ± 210 years B.P. (Before Present). Pleistocene alluvium and pond or marsh sediment, such as shown here, underlie wind-deposited sand at shallow depths in the southern and western parts of the Great Sand Dunes area.

Project Home Page | Non-Eolian Stratigraphy | Origin and Conclusions

References

Carter, C.J., 1978, Pike in Colorado: Old Army Press.

De Lanois, J.L., 1993, Climatic change during the late Holocene from a south central Colorado lake [M.S. thesis]: Tucson, University of Arizona, 50 p.

Johnson, R.B., 1967, The Great Sand Dunes of southern Colorado: U.S. Geological Survey Professional Paper 575-C, p. C177-C183.

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