New geochemical evidence for the origin of North America's largest dune field, the Nebraska Sand Hills, central Great Plains, USA

Highlights

• The Nebraska Sand Hills region is the largest dune field in North America.

• The source of sediment for this large sand sea is still controversial.

• We report trace element compositions of aeolian sand and possible sources.

• The most likely source is the Tertiary Ogallala Group, upwind of the Sand Hills.

• The Ogallala landscape is dissected and sands can be eroded during droughts.

Abstract

The Nebraska Sand Hills region is the largest dune field in North America and has diverse aeolian landforms. It has been active during both the late Pleistocene and late Holocene. Despite decades of study, the source of sediment for this large sand sea is still controversial. Here we report new trace element compositions of aeolian sand that are compared to four hypothesized sediment sources, Tertiary rocks of the Arikaree Group and Ogallala Group, unconsolidated sands of Pliocene age, and Platte River system sands. All four potential sources have a mineralogy that is similar to the Nebraska Sand Hills. K/Rb, K/Ba, Sc-Th-La, Eu/Eu*, La_N/Yb_N, As/Sb, and Fe/Sc values show, however, that Pliocene sediments and sands from the Platte River system are not likely sources. The Arikaree Group could be a minor contributor, but sands from the Ogallala Group appear to have the best compositional fit to the Nebraska Sand Hills. Although past studies have proposed the Ogallala Group as an important sand source, the hypothesis has been questioned, because the unit is well cemented by calcrete in its upper part. However, examination of the landscape upwind of the Nebraska Sand Hills shows that the Ogallala Group, where it occurs at the land surface, is highly dissected in much of this region, which makes sand-sized particles available for aeolian entrainment whenever drought conditions diminish a protective vegetation cover.

Introduction

Dune fields of Quaternary age occupy large areas of the world's arid and semiarid regions, many of them in subtropical deserts and in rain-shadowed zones of the mid-latitudes (Wilson, 1973; Lancaster, 1989, Lancaster, 1995; Pye and Tsoar, 1990; Cooke et al., 1993; Lancaster, 1989, Lancaster, 1995; Livingstone and Warren, 1996; Goudie, 2002; Muhs et al., 2013; Warren, 2013; Lorenz and Zimbelman, 2014). Some of the largest dune fields, in the deserts of Asia and Africa, are unvegetated and fully active (Yang et al., 2004; Lancaster, 2007; Sun and Muhs, 2007). In contrast, dune fields of South America and North America are mostly vegetated and inactive (Zárate and Tripaldi, 2012; Muhs, 2017). In Australia, dune fields are also largely inactive, but dune field stabilization is due not only to vegetation cover, but also cementation and pedogenesis (Hesse, 2011).

Whether a dune field is active or stable, as well as what sort of dune forms evolve, requires an understanding of the controls on sand entrainment, transport, and deposition. More than seven decades ago, Hack (1941) articulated what are now considered to be the main variables in dune field activity and evolution, namely sand supply, wind strength, and degree of vegetation cover. Later studies have emphasized the importance of climatic factors, primarily moisture balance (precipitation vs. evapotranspiration) as controls on the degree of vegetation cover on sand dunes (Lancaster, 1988; Muhs and Maat, 1993; Muhs and Holliday, 1995; Wolfe, 1997; Lancaster and Helm, 2000). Still other studies have proposed wind strength as the dominant climate factor in explaining degree of dune activity (Tsoar, 2005; Tsoar et al., 2009; Yizhaq et al., 2009). All of these studies assume, however, that sand supply is adequate to form a dune field in the first place. Thus, understanding the source or sources of sand is key to understanding the origin and subsequent evolution of a dune field. In support of this concept, Kocurek and Lancaster (1999) proposed the idea of an “aeolian system sediment state,” which includes, within a dune field, the degree of aeolian activity vs. stability. These investigators emphasized the components of sediment supply and sediment availability, in addition to transport capacity of the wind. Halfen et al. (2016), in a recent review of North American dune fields of late Quaternary age, also emphasized the importance of sediment supply and sediment availability (as a result of drought episodes) as controls on dune field activity. Thus, provenance of sand is fundamental to understanding the genesis of a dune field and its potential for future activity.

Inland dune fields of Quaternary age, most of them stabilized by vegetation, are common across much of North America (see reviews in Wolfe, 2007 and Muhs, 2017). Within the USA, areas of aeolian sand are concentrated in three physiographic regions, the Basin and Range, the Colorado Plateau, and the Great Plains (Fig. 1). The largest areas of aeolian sand are those within the Great Plains province (Fig. 2). The Great Plains region is semiarid because it lies within the rain shadow of the north-south-trending ranges of the Rocky Mountains, which shield the region to the east from westerly storm tracks. Active aeolian sand in the Great Plains region is limited to the southernmost dunes in Texas and New Mexico and a few other areas where human activities, such as grazing, cultivation, or development of infrastructure have reactivated previously stabilized sand.

The Nebraska Sand Hills dune field, in the central part of the Great Plains, covers an area of ~57,000 km² (Fig. 2). Much of the dune field was active during the last glacial period, based on extensive drilling and optically stimulated luminescence (OSL) dating reported in a landmark study by Mason et al. (2011). Although most dunes in the Nebraska Sand Hills are presently stabilized by vegetation, there is ample evidence that sand also has been active within the late Holocene, based on radiocarbon and OSL dating (Ahlbrandt et al., 1983; Swinehart, 1990; Loope et al., 1995; Muhs et al., 1997; Stokes and Swinehart, 1997; Goble et al., 2004; Mason et al., 2004; Forman et al., 2005; Miao et al., 2007; Schmeisser et al., 2010; Schmeisser McKean et al., 2015) and even historical accounts (Muhs and Holliday, 1995). These studies have shown that this large dune field is highly sensitive to changes in moisture regime that control the degree of stabilizing vegetation.

Although there has been a significant effort to understand the timing of dune activity in the Nebraska Sand Hills, there have been few studies of the actual sources of the sand. Recently, Muhs (2017) re-examined the origin of many of the dune fields of the Great Plains and Basin and Range provinces. This investigation utilized K/Rb and K/Ba values as proxies for the compositions of K-feldspars from different source sediments. Results show that some previous concepts of dune field origins are supported by the new data, some dune sand sources not previously considered to be important are actually significant contributors, and the origin of some dune fields still remains elusive. One of the dune fields whose source sediments are still uncertain is the Nebraska Sand Hills.

In the Nebraska Sand Hills region, modern observations show that on an annual basis, the dominant sand-moving winds (computed as resultant drift directions, or RDD, using methods of Fryberger and Dean, 1979) come from the northwest (Fig. 3). Late Holocene paleowinds, based on the orientations of parabolic dunes examined on aerial photographs, indicate that the dominant sand-moving winds were also from the northwest (Fig. 3). Thus, a simple interpretation might be that past wind directions were similar to those of the present and sources of the Nebraska Sand Hills must lie to the northwest of the dune field. However, in a continental-interior, mid-latitude region such as Nebraska, there are strong seasonal contrasts in temperature, precipitation, and wind direction. During October through April, the RDD of sand-moving winds in Nebraska are dominantly from the northwest (represented in Fig. 4a by RDD for April). However, during winter months, soils and sediments of the central Great Plains are frozen and/or snow-covered for extended periods, limiting particle mobility by wind. In contrast, during the warmer months of May through September, although resultant drift potential (RDP) values are lower, meaning weaker winds, many localities in Nebraska show sand-moving winds dominantly from the south, southwest, or southeast (represented in Fig. 4b by RDD for August). Thus, although dune sand sources in areas situated to the northwest of the Nebraska Sand Hills might be important in the cooler months of the year, dune sand sources to the south, southwest, or southeast may be more important during the warmer months.

Trace element geochemistry is a powerful tool for investigating the origin of dune fields, particularly those with high quartz content, where major element geochemistry may provide only limited interpretations. A number of investigators have used trace element compositions, including the rare earth elements (REE) to determine the origin of dune fields in Mexico (Kasper-Zubillaga et al., 2007), China (Yang et al., 2007; Liu and Yang, 2018), and Africa (Garzanti et al., 2012). Here, four potential sources of aeolian sand for the Nebraska Sand Hills, identical to those studied by Muhs (2017) are considered in light of a larger suite of trace elements.

Potential source sediments for the Nebraska Sand Hills span a considerable range of geologic ages. One of these sources is unconsolidated fluvial sediments, sands from the Platte River system (North Platte, South Platte and Platte rivers), situated to the west, southwest, south, and southeast of the dune field (Fig. 5). Other potential source sediments for the Nebraska Sand Hills are of pre-Quaternary age. Pliocene sheet sands, some of which are thought to be of aeolian origin, occur beneath the Nebraska Sand Hills. These sediments, mostly unconsolidated, are as much as 28 m thick. The full geographic extent of these Pliocene sands is unknown, but a minimum distribution has been documented by Myers (1993), Swinehart et al. (1994b), and May et al. (1995), primarily beneath the south-central part of the Nebraska Sand Hills (Fig. 5). In the present study, samples were analyzed from localities where Pliocene sands are exposed either in quarries or along cut banks of streams. Two other potential dune sand sources, exposed to the northwest of the Nebraska Sand Hills, are the Miocene Ogallala Group and the Oligocene-Miocene Arikaree Group. These rocks occur at both the surface and subsurface over much of western Nebraska and adjacent parts of southern South Dakota, eastern Wyoming, and northeastern Colorado (Fig. 3, Fig. 5). Some sediments of the Ogallala Group and Arikaree Group are well cemented due to long-term diagenesis or pedogenesis, but other occurrences of these units are loose, unconsolidated sands. Sediments of the Niobrara River valley, situated to the northwest and north of the Nebraska Sand Hills, are also a potential source, but because this river drains only rocks of the Arikaree and Ogallala Groups (Burchett, 1969; Love and Christiansen, 1985), possible contributions from it can be assessed from those two sources.