Great Basin Dunes and Winds: What Might They Tell Us About the Geologic Record?

Dune fields have long been recognized as important geomorphic features of continents, both on the modern Earth and in the geologic record. While dunes can exhibit a variety of morphologies (McKee, 1979), they generally form in response to prevailing wind directions at wind magnitudes greater than 6 m/s. Dunes tend to form where there is (1) low precipitation, (2) an abundant sediment source and (3) winds capable of transporting the sediment. Dunes are predominantly composed of sand-sized sediment. The cross-bedding in sand dunes is used to infer the direction of wind that formed the dunes (Figure 1).

The world's largest modern dune fields occur between 10 and 30 degrees latitude and are more common on the western side of continents. For this reason, the presence of dune deposits in the geologic record has been extensively used to make paleoclimate reconstructions. By measuring the orientation of cross-bedding in dunes we know the mean direction the wind was blowing from. But is this always true?

The Great Basin of the western United States is a closed basin (i.e., an area of internal drainage) located between the Sierra Nevada and Rocky Mountains characterized by north-south trending mountain ranges and intervening valleys. The Great Basin occupies most of the state of Nevada, western Utah, and small portions of Idaho, Oregon, and California. The Great Basin contains a number of dune fields with different characteristics (Figure 2).

The Great Basin spans 30-40^o N latitude and thus is somewhat further north than is considered typical of modern large dune fields. Westerlies (i.e, those winds that blow from the west) constitute the prevailing, zonal wind pattern at these latitudes.

The Last Glacial Maximum (~21,000 years ago) saw the development of many pluvial lakes (formed during a period of elevated precipitation) in the Great Basin that subsequently dried up during the Holocene (roughly the past 10,000 years). These lake sediments provide an abundant sediment source for dunes. Located in the rain shadow of the Sierra Nevada Mountains, the modern Great Basin receives some of the lowest rainfall totals in the lower 48 United States. Thus, conditions considered favorable for formation of dunes are favorable in the Great Basin. Despite this, dunes in the Great Basin are small and have received relatively little attention from the scientific community. Even so, these relatively small dunes have something important to say about Great Basin climate as well as providing a cautionary note about using dune orientation to understand the winds of ancient climates.

The first step in understanding Great Basin dunes is examining the long term record of wind directions and intensities. Fortunately this has been helped greatly by digital archiving of wind records from the past 50-60 years. Approximately 500,000 hourly wind records were examined at 12 different places in the Great Basin. The direction that the wind was blowing from was placed in 16 different categories (north, north-northeast, northeast, east-northeast, and so on) (Figure 3). The 16 different directions were then combined to form a single direction that the wind was blowing toward. These wind records display certain trends (Figure 4) over the Great Basin. Winds capable of moving sand are generally from the west and relatively weak in the western Great Basin. In the eastern Great Basin, winds are much stronger and are from the southwest or the south which is not what one would expect at this latitude of predominantly westerly winds.

How do we explain these trends? The answers come largely from work done by the Department of Atmospheric Sciences at the University of Utah. It turns out that when low pressure troughs approach the Sierra Nevada Mountains, a certain amount of the energy in associated storm systems is funneled around the southern end of the Sierras. Upon emerging in the Great Basin, this energy moves northward, parallel to the Wasatch Plateau and Mountains (Shafer and Steenburgh, 2008) (Figure 5). This explains the weak westerly winds in the western Great Basin and the strong southerly winds in the eastern Great Basin.

What does this have to do with interpreting the record of sand dunes in the geologic record? The orientation of dune features has been used to infer wind direction (Figures 1 and 2) and in the case of the Great Basin, this appears justified. What is unique about Great Basin dunes however, is that the winds and dunes in the eastern portion of the basin do not conform to the westerly zonal wind pattern that might be expected. As such, the Great Basin provides a cautionary note for using the orientation of dunes to interpret wind directions in the geologic record.

• Jewell, P. W., and K. Nicoll, 2011, Wind regimes and aeolian transport in the Great Basin, U.S.A.: Geomorphology, v. 129, p. 1-13.
• McKee, E. D., 1979, Introduction to a study of global sand seas. U. S. Geological Survey Professional Paper 1052-F, 1-19.
• Shafer, J. C., and W. J. Steenburgh, 2008. Climatology of strong intermountain cold fronts. Monthly Weather Review, 136, 784-807.