Earth Surface Processes Team - Central Region

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

Origin of the Great Dunes

Speculation about the origin of the Great Sand Dunes has appeared intermittently in various publications since 1873. These publications have focused primarily on the source of the sand. Related questions such as when and under what conditions sand was mobilized have not received much attention. Hypotheses proposed for the source of the dune sand fall into four categories:

  1. The dunes are a remnant of the Miocene-Pliocene Santa Fe Formation (the principal basin-fill sediment in the Rio Grande rift).
  2. The sand was derived from the late Pliocene-early Pleistocene Alamosa Formation, which is at or near the surface over much of the San Luis Valley.
  3. The sand was derived from late Pleistocene flood-plain deposits of the Rio Grande.
  4. The sand was derived from dry lakebeds on the valley floor intermittently over a time span of unknown duration, but which probably included much of the Pleistocene.
A lack of information about the age and origin of most surficial deposits in the San Luis Valley has hindered the testing of any hypothesis. Consequently, the surficial geologic map of the Great Sand Dunes area is providing new perspectives on the age and origin of the Great Sand Dunes.

geologic time chart

*After Fullerton and others (2003).
Figure 1. Geologic Time Chart showing divisions of the late Cenezoic Era.

Chronology of Thought about Great Sand Dunes Origin

Hayden (1873) was the first to comment on the origin and age of the Great Sand Dunes, although his contribution was a single sentence in an otherwise lengthy report. He believed the Great Sand Dunes are a remnant of the Santa Fe Formation.

Endlich (1877) was in complete disagreement with Hayden (1873) when he wrote (p. 143): "These dunes seem to be of comparatively recent date, geologically speaking, and belong to the Post-Glacial age." ("Post-Glacial" inferred to mean post-Pinedale in today's terminology.) Endlich was the first to recognize that much of the sand in the Great Dunes was of volcanic origin and that it was derived from sediment that originated in the San Juan Mountains. He also recognized that sand was transported by winds that blew primarily from the southwest.

Siebenthal (1910) made a comprehensive study of San Luis Valley geology, but did not make a geologic map. Siebenthal, like Hayden (1873), believed that the Great Dunes are a remnant of the Miocene Santa Fe Formation that had been "broken down by the winds".

Between 1910 and 1967 several short papers were published (Wegemann, 1939; Merk, 1960; and Burford, 1960, 1961) that either echoed or blended ideas expressed by Hayden, Endlich, and Siebenthal. Wegemann (1939) acknowledged that the essential facts in his paper were from Siebenthal (1910), and, although he did not cite Endlich's (1877) work, he blended some of Endlich's ideas with those of Siebenthal (1910). However, Wegemann differed markedly from Siebenthal in attributing part of the sand in the Great Dunes to deflation of the Alamosa Formation, a formation that Siebenthal had named and clearly did not consider to be the source of the Great Dunes. Burford (1960, 1961) also believed that the Alamosa Formation was a source of at least some of the sand in the Great Dunes. Although Endlich (1877) was unequivocal about the Great Dunes being postglacial, he did identify a specific valley-floor source, such as the Alamosa Formation or any of the younger (post-Alamosa Formation) surficial deposits described by Siebenthal (1910).

Johnson (1967) introduced new ideas about the origin and age of the Great Sand Dunes, which have been widely accepted for nearly 40 years. Johnson believed that the Great Sand Dunes were derived from alluvium deposited by the Rio Grande in late Pleistocene time. He wrote (p. C181-C183): "During late Pleistocene time the Rio Grande flowed directly eastward from the San Juan Mountains across the San Luis Valley to the vicinity of San Luis Lake where the river made a 90° bend and flowed directly south through the San Luis Hills. Since that time the Rio Grande has gradually moved southwestward away from this sharp bend until it now occupies a gently curved channel between the east flank of the San Juans and the north end of the San Luis Hills. The area between the oldest and youngest channels now consists of shallow crescent-shaped swales that are abandoned oxbow lakes and low serpentine-shaped mounds of loose sand and silt that are ancient natural levees of the Rio Grande." ... "This area of loose sand and silt was the ready, immediate source of abundant material for the prevailing southwesterlies to build parabolic dunes to the northeast." ..."Newly formed parabolic dunes migrated downwind until they reached nearly to the mountain front. There, wind turbulence and eddying were so violent that the sand began to pile up..."

Madole and Romig (2002) presented an alternative to Johnson's (1967) hypothesis. They proposed that the Great Sand Dunes were derived from dry lakebeds on the floor of the closed basin. Furthermore, they suggested that the Great Dunes are the product of multiple episodes of sand transport that were controlled primarily by climatically driven fluctuations of water table, which occurred intermittently over a time span of unknown duration, but that may have included much of the Pleistocene. During times of greater effective moisture, water table rose and shallow lakes formed on the basin floor. During megadroughts, water table fell, exposing sandy lake-floor sediment to wind erosion. During times of more effective moisture, the sand supply on the basin floor was replenished by inflow from streams originating in the surrounding mountains.


Conclusions

  1. The Great Sand Dunes are but a small part of a system of windblown sand that blankets the east edge of the San Luis Valley over a north-south distance of nearly 75 km (47 mi).
  2. The shape, orientation, and location of the wind-deposited sand indicate that it came from the closed basin in the San Luis Valley, rather than from the late Pleistocene flood plain of the Rio Grande as previously believed.
  3. Wind-deposited sand in this area is the product of multiple episodes of transport. Sand of at least four ages, ranging from latest Pleistocene to latest Holocene, have been identified at the surface. In most places, Holocene deposits bury Pleistocene deposits.
  4. Pleistocene and Holocene terrace deposits flank stream channels in the northern part of the area.
  5. Most landforms and surficial deposits in the Great Sand Dunes area are related to climatically driven fluctuations of water-table level.
  6. During megadroughts, water table fell and exposed areas of sandy sediment in playas, dry stream channels, and desiccated marshes to wind erosion. These areas became primary sources of new generations of dune and sheet sand. At the same time, drought reduced vegetation on deposits of older eolian sand allowing wind to remobilize parts of them.
  7. During wetter times, water table rose and marshes and shallow lakes formed, chiefly along the western edge of the sand system. Rising water table caused streams like Little Spring and Big Spring Creeks to aggrade their valley floors.


Rainbow at Great Sand Dunes National Park
An afternoon thunderstorm in June sweeps eastward across the Sangre de Cristo Mountains generating this rainbow as it passes.


Contact

Richard F. Madole


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References

Burford, A.E., 1960, Geology of the Medano Peak area, Sangre de Cristo Mountains, Colorado [Ph.D. dissertation]: Ann Arbor, University of Michigan, 201 p.

Burford, A.E., 1961, Petrology of the Great Sand Dunes, Colorado: West Virginia Academy of Science, Proceedings, v. 33, p. 87-90.

Endlich, F.M., 1877, Geological report on the Southeastern District, in Hayden, F.V., Report of progress of the exploration for the year 1875: U.S. Geological and Geographical Survey of the Territories, Ninth Annual Report, p. 103-236.

Fullerton, D.S., Bush, C.A., and Pennell, J.N., 2003, Map of surficial deposits and materials in the eastern and central United States (east of 102° west longitude): U.S. Geological Survey Geologic Investigations Series Map I-2789, scale 1:2,500,00.

Hayden, F.V., 1873, Geological report for 1869, Embracing Colorado and New Mexico: U.S. Geological Survey of the Territories, Third Annual Report, 261 p.

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

Madole, R.F., and Romig, J.H., 2002, The Great Sand Dunes (GSD) of south-central Colorado: Origin and implications: Geological Society of America Abstracts with Programs, v. 34, no. 6, p. 275.

Merk, G.P., 1960, Great Sand Dunes of Colorado, in Weimer, R.J., and Haun, J.D., eds., Guide to the Geology of Colorado: Geological Society of America, Rocky Mountain Association of Geologists, and Colorado Scientific Society, p. 127-129.

Siebenthal, C.E., 1910, Geology and water resources of the San Luis Valley, Colorado: U.S. Geological Survey Water-Supply Paper 240, 128 p.

Wegemann, C.H., 1939, The Great Sand Dunes of Colorado: The Mines Magazine, v. 29, no. 11, p. 556-558.


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