Introduction
Deflation basins in central Nebraska, sometimes called rainwater basins (e.g., Zanner et al., Reference Zanner, Kuzila and Geiss2006) because they collect overland flow seasonally, are prominent and iconic landforms (Fig. 1). Although a large number of these basins were drained for agricultural purposes, many of them still hold water in the spring and early summer, making them vital stopovers for hundreds of thousands of migratory birds. Numerous other terms have been applied to these landforms, including “depression” (Kuzila, Reference Kuzila1988), “basin” (e.g., Kuzila, Reference Kuzila1994), “rainbasin depression” (e.g., Starks, Reference Starks1984), “rainbasin” (e.g., Krueger, Reference Krueger1986), and “playa” (Johnsgard, Reference Johnsgard2012; Nebraska Game and Parks, 2025a). Likewise, multiple names have been applied to the enclosing region, such as: (1) the Loess Plains or Loess Plain (Condra, Reference Condra1906, Reference Condra1920), or Nebraska Plain (e.g., Lugn, Reference Lugn1935), or Nebraska Loess Plain (e.g., Bengston, Reference Bengston1947), which are accurate names for a large part of Nebraska south of the Platte River; (2) the Rainwater Basin (e.g., Johnsgard, Reference Johnsgard2012; Nebraska Game and Parks, 2025b), a term applied to an appropriately restrictive geographic area, but a problematic one, because that area is neither a topographic nor a structural basin; and (3) the Rainwater Basin Plains (e.g., Chapman et al., Reference Chapman, Omernik, Freeouf, Huggins, McCauley, Freeman, Steinauer, Angelo and Schlepp2001), a name that we prefer because it aptly describes the entire geomorphic region and also includes a term for its most distinguishing features.
Figure 1. Map of the Rainwater Basin Plains showing the basins (gray), major rivers and cities, and the pre-Illinoian glacial limit. The inset map shows the outline of Nebraska and the extent of Figure 1. Our study sites, including Axtell, Fairfield, Fairmont, and Kearney (K), are shown as well as the Charleston basin (CB) studied by Krueger (Reference Krueger1986) and the McMurty Marsh (MM) and Ong basin (OB) studied by Kuzila (Reference Kuzila1988, Reference Kuzila1994).
Geomorphologists have generally agreed, since at least the early 1980s, that Nebraska’s rainwater basins are products of aeolian deflation, but divergent theories have also been proposed, including extraterrestrial impact structures potentially related to the Carolina Bays (e.g., Zamora, Reference Zamora2017) and depressions referred to as “fairy circles” associated with the release of geologic hydrogen (Zgonnik, Reference Zgonnik2020). Nevertheless, the rainwater basins have striking similarities to other groups of depressions and playas on the Great Plains (see Reeves Reference Reeves1965, Reference Reeves1966; Holliday et al., Reference Holliday, Hovorka and Gustavson1996, Reference Holliday, Mayer and Fredlund2008; Bowen and Johnson, Reference Bowen and Johnson2011) for which aeolian erosional origins have also been proposed. Our study is the first to systematically study rainwater basins by providing age control that directly dates basin features, thereby linking their development to widespread climatic and environmental changes during the late Pleistocene.
Most of the rainwater basins are ovoid, with long axes aligned prominently northeast-southwest, and are partially bounded by one or more lunettes along their southern, southeastern, or eastern margins (Fig. 2). Lunettes are crescentic mounds found around deflation basins of various kinds in semiarid regions worldwide (Hills, Reference Hills1940; Lancaster, Reference Lancaster1978; Goudie and Thomas, Reference Goudie and Thomas1986; Bowen and Johnson, Reference Bowen and Johnson2011). For consistency’s sake, we use the term “basin” throughout this paper. Certainly, the rainwater basins are occasionally referred to as “playas,” because they closely resemble those characteristic landforms of the Southern Great Plains in Kansas, Colorado, Oklahoma, and Texas. Although the genetic origins of both these playas and Nebraska’s rainwater basins are similar, their geologic settings are different.
Figure 2. Plan view of western and eastern basins and lunettes studied overlying a hill shade constructed from LIDAR data. The hill shade was constructed using a 3× vertical exaggeration. The Kearney (A and B), Axtell (C and D), Fairfield (E), and Fairmont (F) study locations are shown. The locations for all cores taken in this study are shown, as well as the topographic cross sections in Figures 3 and 4 and in Supplementary Figures S5 and S6. Dashed lines indicate the basins, and the gray polygons show the lunettes.
Understanding the developmental chronology of the rainwater basins is crucial for maintaining their ecosystem services. Previous studies of these features established a rudimentary chronology through the relative dating of basin sediments relative to loess stratigraphy and a small number of radiocarbon ages of bulk organic carbon from paleosols formed in and around the basins (Krueger, Reference Krueger1986; Kuzila, Reference Kuzila1988, Reference Kuzila1994). Our study improves markedly upon prior work, because we generate a suite of new optically stimulated luminescence (OSL) dates that directly date the emplacement of lunettes that accreted contemporaneously with the progressive aeolian erosion of immediately adjacent basins.
Setting
Johnsgard (Reference Johnsgard2012) estimated the areal extent of the Rainwater Basin Plains to be 11,000 to 14,400 km2, but based on our comprehensive mapping of recognizable basins and associated lunettes, we estimate the area to be approximately 16,000 km2 (Fig. 1). The area is characterized as a low relief plain bounded on the north by the Platte River Valley, and to the east, south and west by dissected loess. The Platte River occupied the Rainwater Basin Plains beginning around 200,000 years ago, and it still flowed through its western portion as recently as ∼30,000 years ago, before attaining its present course (Swinehart et al., Reference Swinehart, Dreeszen, Richmond, Tipton, Bretz, Steece, Hallberg and Goebel1994). The soils of the Rainwater Basin Plains formed predominantly in Peoria Loess, which was mapped by Mason (Reference Mason2001) to range between 4 and 12 m in thickness in this setting. Depending on the location, the Peoria Loess overlies older loess (Gilman Canyon Formation and/or Loveland Loess) or aeolian sand that was reworked from underlying alluvium. Alluvial sediments underlie all these aeolian sediments.
The basins are complex geomorphic features. Although many of the basins have only one associated lunette, some basins show evidence for multiple formative periods, including basins that are superposed onto one another and in some cases smaller basins that are found within larger basins. The lunettes are also complex and appear to show multiple periods of formation in some instances, and while some basins do not have a distinct lunette, others have multiple lunettes. Most lunettes lie on the southeastern margins of basins, but several examples lie on the northeastern margins of particular basins. Krueger (Reference Krueger1986) and Kuzila (Reference Kuzila1988, Reference Kuzila1994) interpreted the stratigraphy within and around rainwater basins at three sites. Both of these authors illustrated Peoria Loess as a continuous deposit draping both the surrounding plain and floors of basins. Although Kuzila (Reference Kuzila1994, fig. 4) depicted the Gilman Canyon Formation with a similar draping geometry, interpretive cross sections in Krueger (Reference Krueger1986, fig. III-7) imply that both the Gilman Canyon Formation and Loveland Loess are partially to completely eroded under the basin. Neither of these authors depicted significant amounts of subaqueously deposited basin-filling sediments. Thus, lunettes appear to be the primary sedimentary record directly related to the development of the rainwater basins.
The Rainwater Basin Plains, which we outline in Figure 1, are within the High Plains in the Great Plains Physiographic Province (Fenneman and Johnson, Reference Fenneman and Johnson1946). The mean annual temperature of the area ranges from 16.7°C to 17.2°C, and the mean annual precipitation ranges from 740 mm in the east to 660 mm in the western Rainwater Basin Plains (U.S. Climate Data website, n.d.). The native vegetation is upland tallgrass prairie in the east and loess mixed-grass prairie in the west (Kaul and Rolfsmeier, Reference Kaul and Rolfsmeier1993); however, most of the area is now under irrigated row crops.
Krueger (Reference Krueger1986) and Kuzila (Reference Kuzila1988, Reference Kuzila1994) demonstrated that the basins formed by aeolian deflation and that lunettes formed by the accumulation of deflated sediment immediately downwind. A similar mechanism has been proposed for features in Kansas (Bowen and Johnson, Reference Bowen and Johnson2011) and Texas (Holliday et al., Reference Holliday, Hovorka and Gustavson1996, Reference Holliday, Mayer and Fredlund2008). Krueger (Reference Krueger1986) also attributed the shapes of the larger playas to northwesterly winds driving erosive lake waves that gradually elongated the basins perpendicular to that primary wind direction. The same kind of mechanism has been demonstrated in several studies of oriented lakes in the Arctic (Livingstone, Reference Livingstone1954; Carson and Hussey, Reference Carson and Hussey1962) and of playas in Australia (Killigrew and Gilkes, Reference Killigrew and Gilkes1974) and Texas (Reeves, Reference Reeves1966). Strong northerly to westerly winds can also be inferred from different aspects of aeolian deposits in Nebraska. Dune morphologies in the Sand Hills also suggest such a wind direction, as do sedimentary structures in the Holocene (Schmeisser et al., Reference Schmeisser, Loope and Mason2010) and Pleistocene (Mason et al., Reference Mason, Swinehart, Hanson, Loope, Goble, Miao and Schmeisser2011) dunes. Likewise, there is a marked regional thinning of the Peoria Loess southeastward, and presumably downwind, from the Sand Hills (Mason, Reference Mason2001). Alternative models for the formation of the rainwater basins have been proposed, such as hydrogen seeps or extraterrestrial impacts (e.g., Zamora, Reference Zamora2017; Zhang et al., Reference Zhang, Perkovich, Li, Weihermann and Curmmett2025), but these models are not supported by stratigraphic analysis and geochronological data, and they account for neither the associated landforms nor the Quaternary history of landscape evolution in the enclosing region.
Previous studies identified that rainwater basins and the associated lunettes were buried by Peoria Loess (Krueger, Reference Krueger1986; Kuzila, Reference Kuzila1988, Reference Kuzila1994) and concluded that the basins were formed either during or before the deposition of the Gilman Canyon Formation (GCF), a distinct loess deposit that predates the Peoria Loess. The paleosol within the GCF also formed in sands beneath lunettes in several of our coreholes. The GCF has been dated to ∼38 to 26 ka in the central Great Plains (Johnson et al., Reference Johnson, Willey, Mason and May2007; Mason et al., Reference Mason, Joeckel and Bettis2007) and the Peoria Loess between 25 and 14 ka (Miao et al., Reference Miao, Mason, Swinehart, Loope, Hanson, Goble and Xiaodong2007; Mason et al., Reference Mason, Miao, Hanson, Johnson, Jacobs and Goble2008, Reference Mason, Swinehart, Hanson, Loope, Goble, Miao and Schmeisser2011; Muhs et al., Reference Muhs, Bettis, Aleinikoff, McGeehin, Beann, Skipp, Marshall, Roberts, Johnson and Benton2008, Reference Muhs, Bettis, Roberts, Harlan, Paces and Reynolds2013).
The long-standing chronological interpretation of basins and lunettes in the eastern part of the region was based on regional loess stratigraphy and a limited number of bulk soil radiocarbon ages (Krueger, Reference Krueger1986; Kuzila, Reference Kuzila1988, Reference Kuzila1994). Kuzila (Reference Kuzila1988, Reference Kuzila1994) reported five radiocarbon ages collected from the tops of paleosols underlying the Peoria Loess. One of these was from beneath a lunette, two were from the terrain surrounding a basin, and the depositional environment of the other two samples was not specified. We calibrated these ages using Calib 8 (Reimer et al., Reference Reimer, Austin, Bard, Bayliss, Blackwell, Bronk Ramsey and Martin Butzin2020), and the resulting ages using 1σ errors range from 30.1 to 25.3 cal ka BP. Krueger (Reference Krueger1986) collected three radiocarbon ages from three paleosols underlying the Peoria Loess at one basin. One of these was from the edge of a lunette and the other two were from the area surrounding a basin. These ages range from 33.3 to 25.0 cal ka BP. One of these ages was collected from within the fill of a basin, and the age and stratigraphy suggest that the basin was excavated before ∼27.2–26.5 cal ka BP. Although several of these ages were from samples of the basin fill or the lunette, collectively they suggest both were formed before ∼33 to 25 ka.
Methods
Using either a split-spoon drill rig or a direct-push Geoprobe, we collected cores at two sites in the eastern Rainwater Basin Plains and two in the western part thereof (Fig. 1, Table 1). All of these cores were taken from lunettes on the southern or southeastern margins of seven basins, whether single cores or multiple closely spaced cores along transects of individual lunettes. In the west we collected 3 cores (K 1–2, K 6) in lunettes along two basins at the Kearney site (Fig. 2A and B) and 12 cores (A 1–12) in lunettes along two basins at the Axtell site (Fig. 2C and D). We collected two cores (Fd 10–11) along a basin at the Fairfield site (Fig. 2E) and four cores (Ft 8–11) in one transect across the lunette of a basin at the Fairmont site (Fig. 2F). We also collected two additional cores (Fd 8–9) in loess-mantled dunes at the Fairfield site (Fig. 2E). Our Fairfield site is approximately 15 km to the southeast of the McMurtry Marsh site and 18 km northwest of the Ong site of Kuzila (Reference Kuzila1988, Reference Kuzila1994; Fig. 1). Our Fairmont site is approximately 18 km to the southeast of the Charleston site studied by Krueger (Reference Krueger1986); furthermore, it is our only site located east of the pre-Illinoian glacial limit (Fig. 1).
Table 1. Elevations, depths and locations of Nebraska Rainwater Basin cores.
We described sediments for particle size, soil properties, and redoximorphic features according to Schoeneberger et al. (Reference Schoeneberger, Wysocki, Benham and Broderson2002) and described bedding characteristics in each of our cores. We distinguished lunette and basin sediments from alluvial sediments by using a combination of color, grain size, sedimentary structures, and geomorphic and stratigraphic relationships. Particle-size analysis was performed on sediment samples collected from four cores from four lunettes. These samples were treated with 10 mL of 50 g/L sodium hexametaphosphate to disperse clays and then agitated for at least 10 hours before analysis. Samples were sonicated for 60 s before analysis in a Malvern 3000 laser particle-size analyzer, and particle-size results for selected cores are shown in Supplementary Figure S1.
Optically stimulated luminescence (OSL) dating
We selected 22 OSL samples from lunettes at six basins at sites in both the eastern and western parts of the Rainwater Basin Plains. These samples were collected from sand-dominated lunette sediments at depths of 6.1 to 14.9 m beneath the present land surface. Four additional samples were collected, including two from loess-mantled dunes on the alluvial plain adjacent to the Fairfield basin and two from the underlying alluvium. OSL ages and dose-rate data are shown in Table 2.
Table 2. Equivalent dose, dose-rate data, and optically stimulated luminescence (OSL) age estimates for Rainwater Basin samples.
a Assumes 50% variability in estimated moisture content.
b Central Age Model (Galbraith et al., Reference Galbraith, Roberts, Laslett, Yoshida and Olley1999).
c Accepted disks/all disks.
d Overdispersion.
OSL samples were analyzed at the Landscape Evolution and Luminescence Geochronology Laboratory at the University of Nebraska–Lincoln. Samples were dated using quartz OSL and prepared by sieving to isolate 90–150 µm grains followed by treatments in hydrochloric acid to remove carbonates, flotation in 2.7 g/cm3 sodium polytungstate to remove heavy minerals, and hydrofluoric acid to remove feldspars and to etch the outer portion of the quartz grains. We used 5 mm masks for the aeolian samples and 2 mm masks for the alluvial samples. All samples were analyzed using the single-aliquot regenerative protocol (Murray and Wintle, Reference Murray and Wintle2000) using a preheat temperature of 220°C as determined through a preheat plateau test. We rejected individual aliquots if they had recycling ratios that were > ±10% or if they had signals above background levels when exposed to IR diodes. Final D e values were based on a minimum of 29 accepted aliquots (Table 2) and were calculated using the Central Age Model (CAM) (Galbraith et al., Reference Galbraith, Roberts, Laslett, Yoshida and Olley1999) using the LDAC program (Liang and Forman, Reference Liang and Forman2019). Representative dose recovery results, shine-down, and growth curves, as well as kernel density plots, are included in the Supplementary Data (Supplementary Figs. S2 and S3). The environmental dose-rate values were estimated using concentrations of K, U, and Th measured using a high-resolution gamma spectrometer. We applied 10% moisture content for each sample and estimated the contribution of gamma radiation on the sample’s burial depth using equations from Prescott and Hutton (Reference Prescott and Hutton1994). Dose rates were calculated using the LDAC program (Liang and Forman, Reference Liang and Forman2019) and equations from Aitken (Reference Aitken1998) and Guérin et al. (Reference Guérin, Mercier and Adamiec2011). All of our OSL samples were collected from >1 m below soils or paleosols to avoid issues with bioturbation (Hanson et al., Reference Hanson, Mason, Jacobs and Young2015) and the ages are reported in years before 2025 CE.
Results
Kearney site
We collected three cores from ridges along two basins at the Kearney site (Hanson et al., Reference Hanson, Larsen, Raymond and Howard2016). The westerly basin measures 2.8 km × 1.3 km (Fig. 2A), and the easterly one is 2.2 km × 1.3 km (Fig. 2B). Both basins are drained by small creeks that dissected them long after they formed. The top of the lunette along the more westerly basin is 15.2 m above the basin floor and 8.4 m above the surrounding loess-mantled alluvial plain (Fig. 3). Cores K 1 and K 2 are 10.7 and 19.8 m deep, respectively, and were collected from the lunette on the more westerly basin (Fig. 2A). Neither of these cores penetrated below the surface of the adjacent basin. The two cores exhibit similar stratigraphy, with ∼5.5 m of Peoria Loess that overlies a weakly developed paleosol formed atop packages of fine to medium sands that are locally laminated or cross-stratified (Supplementary Fig. S4). The sand units are 4.5 to 6 m in thickness, and we attained three OSL ages from them. Two samples from depths of 8.6 and 11.6 m in K 1 date to 20.5 ± 1.2 ka (UNL-4223) and 35.3 ± 2.0 ka (UNL-4224), respectively. An OSL sample from sand collected from K 2 at 8.8 m in depth was dated to 26.3 ± 1.4 ka (UNL-4225).
Figure 3. Topographic cross sections and cores showing general stratigraphy in the western Kearney basin (top; Figure 2A) and the Fairmont site (bottom; Figure 2F). Cores show Peoria Loess (black) over lunette sand (gray). Dashed line represents level of the highest closed topographic contour encircling the basin.
Core K 6 was taken from near the top of the lunette on the Kearney east basin, and it penetrated to a depth of 19.8 m (Fig. 2B). The top of this lunette is 19.6 m above the adjacent basin floor and 13.4 m above the surrounding loess-mantled alluvial plain (Supplementary Fig. S5). The bottom of core K 6 is at approximately the lowest elevation of the adjacent basin. Core K 6 has 7.4 m of Peoria Loess that overlies a silty paleosol formed on the top of an ∼12-m-thick package of locally laminated, very fine to medium sand (Supplementary Fig. S4). Two OSL samples were collected from these sands. The sample collected from 8.8 m in depth, or approximately 1.2 m below the paleosol described in this core, dates to 26.3 ± 2.9 ka (UNL-4230) and the one from 14.9 m depth dates to 24.3 ± 1.2 ka (UNL-4231).
Axtell site
The two Axtell basin sites lie in the western Rainwater Basin Plains (Fig. 1). We collected cores in three transects of two lunettes (Hanson et al., Reference Hanson, Larsen and Howard2018) (Fig. 2C and D). The more westerly of the two basins measures approximately 2 km × 4.5 km (Fig. 2D) and the easterly one is 1.4 km × 2.5 km (Fig. 2C). We drilled two transects of four cores (A 1–4 and A 5–8) at each at the more easterly of the Axtel East sites (Figs. 2C and 3). Cores A 1–4 are located on a 145-m-long transect from near the top of the lunette toward the basin. The lunette at this site is 12.5 m above the basin and 9 m above the surrounding loess-mantled alluvial plain. The four cores ranged between 14 and 21 m in depth, and the deepest penetrated to ∼17 m below the basin floor (Fig. 4). The cores show that between 4.6 and 8.5 m of Peoria Loess overlies 9–15 m of locally laminated, fine to medium sand that underlies both the lunette and at least a portion of the basin (Fig. 5). The lower 2 to 2.5 m of Peoria Loess here is laminated. Three OSL samples were collected from the sandy sediments within the lunette. In A 1, samples collected from 9.1 and 9.8 m depth dated to 29.9 ± 1.6 ka (UNL-4405) and 29.6 ± 1.7 ka (UNL-4406), respectively. A sample collected from 6.1 m depth in A 3 dated to 20.0 ± 1.0 ka (UNL-4407).
Figure 4. Topographic cross sections and cores showing general stratigraphy in the two eastern Axtell basin sites (Figure 2C). Cores show Peoria Loess (black) over lunette sand (gray) and alluvium (dark gray). Dashed line represents level of the highest closed topographic contour encircling the basin.
Figure 5. Logs showing stratigraphy, soils, and optically stimulated luminescence (OSL) ages (in years ago) for cores Axtell 1–4 (Figure 2C).
Cores A 5 through A 8 were drilled along a 269 m transect ∼1100 m east of the transect described earlier (Fig. 2C). They extend from the crest of the lunette into the basin. The lunette is ∼13 m above the basin floor and 10 m above the surrounding loess-mantled alluvial plain (Fig. 4). Each of these cores is ∼15 m long, and the deepest penetrated to ∼12.5 m below the current basin floor, and Peoria Loess ranged from 3.7 to 8.2 m in thickness. Underneath the Peoria Loess are 7 to 11 m of very fine to medium sand with local lamination (Fig. 6). OSL samples collected from the sands at 6.7 m depth in A 5 and 7.6 m in A 6 dated to 30.4 ± 1.8 ka (UNL-4408) and 30.0 ± 1.7 ka (UNL-4409), respectively.
Cores A 9–12 are on a 274-m-long transect on the more westerly of the Axtell basins (Figs. 1 and 2D). The top of the lunette here is 12 m above the basin floor and 10 m above the surrounding loess-mantled alluvial plain (Supplementary Fig. S5). The four cores ranged from 11.9 to 18.9 m depth, and the deepest penetrated ∼13 m below the current basin floor. Peoria Loess thickness ranged from 5 to 10 m overlying 6.7 to 12.5 m of locally laminated very fine and fine sand (Fig. 7). Core A 11 encountered a silt unit that overlies laminated fine sand. These sediments are interpreted as alluvium underneath the lunette, and the top of these sediments is ∼10.2 m below the basin floor (Supplementary Fig. S5). At A 9 and A 10, OSL samples were collected from the sand-dominated sediments at depths of 7.0 and 11.3 m, yielding ages of 27.2 ± 1.5 ka (UNL-4410) and 30.5 ± 1.7 ka (UNL-4411), respectively.
Fairfield site
The Fairfield site is located along a basin in the south-central portion of the Rainwater Basin Plains (Fig. 1). The basin measures 4.1 km × 1.3 km, and it is surrounded by several coalesced basins and lunettes (Fig. 2E). Cores Fd 10 and Fd 11, which were both 21.4 m in depth, were collected from the top of the lunette on the same basin, and both penetrated 10 to 13 m below the adjacent basin floor (Young et al., Reference Young, Hanson, Howard and Kuzila2015) (Supplementary Fig. S6). The lunette sits approximately 9 m above the basin and 6–7 m above the surrounding loess-mantled alluvial plain. Core Fd 10 contained 3.2 m of Peoria Loess overlying approximately 7 m of laminated very fine and fine sand and 2.2 m of silt and clay (Supplementary Fig. S7). These sediments overlie approximately 9 m of fine to coarse sand with an interbedded 2.2-m-thick silt bed. One OSL age collected from near the base of the upper sand-dominated sediments yielded an age of 25.4 ± 1.4 ka (UNL-4053) (Supplementary Fig. S7). This sample was taken from ∼5.8 m below the basal contact of the Peoria Loess.
Core Fd 11 core was collected approximately 630 m to the northeast of Fd 10 (Fig. 2E). It contains 3.1 m of Peoria Loess atop ∼12 m of laminated fine and very fine sand (Supplementary Figs. S5 and S6). These sediments overlie 6.4 m of laminated silt and clay. Two OSL samples were collected from the laminated fine and very fine sand. The sample from 7.3 m in depth yielded an age of 28.1 ± 1.4 ka (UNL-4054), and the sample from 13.9 m depth was dated to 31.5 ± 1.7 ka (UNL-4055). The upper sample was collected from ∼4.5 m below the basal contact of the Peoria Loess, and the lower sample was taken from 1 m above the underlying laminated silt and clay.
Two cores were collected from the loess-mantled alluvial plain adjacent to the Fairfield basin (Fig. 2E). Core Fd 8 showed 1.9 m of Peoria Loess overlying a thin GCF soil formed in silt and very fine sand that overlies 12.2 m of locally laminated aeolian sand and 2.9 m of fine to medium alluvial sands (Supplementary Fig. S7). An OSL sample collected from 7.5 m in depth in the fine sand beneath the loess yielded an age of 54.8 ± 2.9 ka (UNL-4049), while a sample from 16.6 m depth in the underlying alluvial sand had saturated luminescence traps and yielded an age of >82.8 ka (UNL-4050). Core Fd 9 showed 3.6 m of Peoria Loess overlying 12.2 m of locally laminated fine sand we interpret as aeolian, and 11.2 m a package of fine sand, silt and sand and gravel that we interpret as alluvium (Supplementary Fig. S7). An OSL sample from 6.4 m depth in the aeolian sand yielded an age of 47.1 ± 2.7 ka (UNL-4051), and the sample from the alluvium was saturated and yielded a minimum age estimate of >78.6 ka (UNL-4052). The aeolian sands in both of these cores are noticeably redder in color, with 7.5 YR hues in the Munsell color chart compared with other sands encountered in the study.
Fairmont site
The basin at Fairmont is our easternmost study site and the only one that lies east of the pre-Illinoian glacial limit (Fig. 1). It measures 1.7 km × 0.9 km (Fig. 2F). Four cores, ranging from 15.0 to 18.3 m in depth, were collected along a 139 m transect on the (Hanson et al., Reference Hanson, Young, Larsen, Howard and Dillon2017) lunette (Fig. 3). The cores ranged from 15.0 to 18.3 m deep and penetrated to depths of over 10 m below the basin’s surface. The top of the lunette is 8 m above the basin floor and ∼6.5 m above the surrounding loess-mantled alluvial plain. Core Ft 11, which is closest to the basin, contains 4.2 m of Peoria Loess overlying ∼12.3 m of very fine to fine sand with some silt-dominated beds (Fig. 8). These latter sediments are locally laminated, and they overlie laminated silt and clay. Two OSL ages were collected from the sand-dominated sediments at depths of 7.8 and 13.6 m below the land surface (Fig. 8). The uppermost sediment dates to 39.0 ± 2.3 ka (UNL-4325) and the lower dates to 51.1 ± 2.9 ka (UNL-4326).
Core Ft 10 lies beneath the apex of the lunette and contains 4.1 m of Peoria Loess overlying a paleosol formed in silt-dominated sediment. This silt-dominated sediment overlies 8.3 m of laminated and locally cross-stratified fine to medium sand. This sand, in turn, is underlain by laminated silt containing a paleosol. Below this laminated silt is a 4.5-m-thick unit of laminated sand and clay. OSL samples collected from 7.8 m and 12.6 m in depth yielded ages of 38.2 ± 2.0 ka (UNL-4323) and 33.1 ± 1.8 ka (UNL-4324), respectively. The lower OSL sample lies approximately 0.2 m above the underlying buried soil formed in laminated silt.
Core Ft 9 contains 5.2 m of Peoria Loess overlying a paleosol formed within the underlying 8.9 m of very fine to fine sand. These sands are locally laminated, and in two locations they preserve large root or crack fills (Fig. 8). These sediments overlie ∼3.5 m of deposits, including a thin very fine sand deposit and a thicker silt and clay deposit. Two OSL samples collected at depths of 6.6 and 10.8 m within the sandy sediment yielded ages of 37.3 ± 1.9 ka (UNL-4321) and 38.6 ± 2.0 ka (UNL-4322), respectively. The upper of these two was collected within 1.5 m of the Peoria Loess contact.
Core Ft 8 contains 5.5 m of Peoria Loess that overlies a buried soil formed in ∼5.9 m of locally laminated very fine and fine sands. Crack fills are present between 7.1 and 7.2 m depth. These sediments overlie a buried soil formed in silt and clay and, at depth, fine sand that overlies deeper silt and clay. One OSL age collected from 10.0 m depth was dated to 33.8 ± 2.0 ka (UNL-4320).
Discussion
The cores reveal several similarities in lunette stratigraphy. First, each lunette is mantled with Peoria Loess that varies in thickness from 3.7 to 10.2 m. The thickness of the Peoria Loess varies within each study area, but the loess is typically thinnest at the crest of the lunette (Figs. 3 and 4, Supplementary Fig. S5). We suspect the thinner loess cover results from sediment loss from the upper potions of the lunette due to slopewash. These trends in overall Peoria Loess thickness, and not the accumulation of thicker soils in the lower landscape positions adjacent to the lunettes, indicates that slopewash was occurring during Peoria Loess deposition. This finding shows that the terrain-mantling Peoria Loess reduced the relief on the present landscape relative to the pre-Peoria one (cf. Krueger, Reference Krueger1986; Kuzila, Reference Kuzila1994).
Second, most of our cores encountered neither buried soils nor the GCF beneath the Peoria Loess, and none of them encountered the loess of the Loveland Formation. In contrast, Krueger (Reference Krueger1986) and Kuzila (Reference Kuzila1988, Reference Kuzila1994) described the GCF beneath the Peoria Loess in the majority of their cores. In our study there were paleosols beneath the Peoria Loess in cores at Kearney (Supplementary Fig. S4) and Fairmont (Fig. 8). These paleosols are particularly sandy, and they do not resemble typical GCF sediments (e.g., Johnson et al., Reference Johnson, Willey, Mason and May2007). The cores collected that contain buried soils are found at both easterly and westerly sites and in both relatively young and old rims. This suggests that preservation potential of buried soils within lunettes differs markedly both geographically and temporally in the area.
Third, sand underlies the Peoria Loess in all our cores. Furthermore, this sand is closest to the present land surface directly beneath the highest point in the lunettes in each of our transects, and the sand lies above the level of the surrounding loess-covered Rainwater Basin Plains. This relationship, along with their lunette morphology and proximity to the basin, strongly suggests an aeolian origin for these deposits. The upper contacts of these sands lie above the level of the adjacent basin floor in all but one of our transects (Axtell cores A 1–4; Fig. 4). In some cases, the lower extent of the aeolian sand is clearly defined, as the sands overlie a paleosol or a deposit that is clearly alluvial, but in other cases, we cannot definitively determine the lower extent of lunette sediments. We used color and grain size to distinguish between the two and suspect that any misinterpretation would at most be on the order of 2–3 m.
Fourth, we encountered alluvium beneath the lunettes in a limited number of cores, which we distinguished by the presence of either stiff, clayey silt, gravel, or both. Krueger (Reference Krueger1986) observed that alluvium appears to lie approximately parallel to the ground surface surrounding the basins. Our minimum age estimates from alluvium adjacent to the Fairfield site (>77 ka) agree with estimates from Swinehart et al.’s (Reference Swinehart, Dreeszen, Richmond, Tipton, Bretz, Steece, Hallberg and Goebel1994) interpretive location of the Platte River in this area during the middle Pleistocene.
Finally, aeolian sand sheet and low-relief dunes are present beneath the Peoria Loess around our study sites. Aeolian sand is present atop the buried alluvial deposits in some of our cores (Fd 8 and 9 in Supplementary Fig. S7) and it may locally underlie the lunettes and basin fills. This stratigraphic relationship suggests that basins may have been formed by the deflation of either aeolian sediments or alluvial deposits, and in some cases both. We suggest that basins formed in places where sand was exposed at the ancient land surface. Furthermore, we presume that wherever surficial sands were more widespread, larger basins with oriented long axes formed. Cores Fd 8 and 9 suggest that relatively thick deposits of aeolian sand (∼10–12 m in thickness) are present beneath at least some portions of the Rainwater Basin Plains (Supplementary Fig. S7).
We produced 22 ages from lunettes in the Rainwater Basin Plains, and all 22 samples were from what we interpret to be aeolian sediments. Like other samples collected from aeolian deposits in Nebraska (Mason et al., Reference Mason, Swinehart, Goble and Loope2004, Reference Mason, Swinehart, Hanson, Loope, Goble, Miao and Schmeisser2011; Miao et al., Reference Miao, Mason, Swinehart, Loope, Hanson, Goble and Xiaodong2007; Hanson et al., Reference Hanson, Joeckel, Young and Horn2009; Puta et al., Reference Puta, Hanson and Young2013; Buckland et al., Reference Buckland, Bailey and Thomas2018, Reference Buckland, Thomas and Bailey2019; Mahoney et al., Reference Mahoney, Mandel, Hanson and Fritz2025), our new OSL age estimates from the rainwater basins are for the most part internally and stratigraphically consistent, and they are generally supported by relative age control. The overdispersion of the samples is low, ranging from 9.1 to 26.3 and averaging 16.0. These values are compatible with the interpretation of an aeolian depositional environment. Several relatively modest age inversions are present in our cores (A1, Fig. 5; Ft 10, Fig. 8; K 6, Supplementary Fig. S4), but in two of these cases, they fall within 1σ errors of each other, and in the other case, they are within 2σ errors. We suspect that variability in estimated water content of these sediments contributes to these apparent age inversions.
Our ages from aeolian sands beneath rainwater basin rims range from ∼51 to 20 ka, with the vast majority falling within the latter half of Marine Isotope Stage (MIS) 3 (57–29 ka) and the very beginning of MIS 2 (29–14 ka). We cannot definitively determine when basin formation began, but 91% of our ages suggest that it ended by 25 ka, that is, just before the onset of Peoria Loess deposition in the enclosing region. Only two of our ages, out of 22 total, are younger than 25 ka (Table 2). The average ages from the study sites are: Axtell sites = 28.2 ka (n = 7), Kearney sites = 26.5 ka (n = 5), and Fairfield sites = 28.3 ka (n = 3). These samples were collected between 1 and 11 m below the base of the Peoria Loess. The ages from the Fairmont basin average 38.7 ka (n = 7) and are much older than those collected from the other sites. Cores collected from the Fairmont basin contained the GCF, unlike the other sites. These different ages suggest that at the least basins stabilized at different time periods in the late Pleistocene and presumably due to local conditions that would have favored stability of the basins and lunettes.
Our OSL chronology generally agrees with the ages reported from rainwater basins in previous studies. Krueger (Reference Krueger1986) generated three 14C ages on bulk material from soils buried by Peoria Loess in the Charleston basin located ∼18 km northwest of the Fairmont site (Fig. 1). Stratigraphic cross sections show the lunette at the Charleston basin formed during or shortly before the deposition of the GCF. The 14C ages collected from the top of the GCF on the top of a lunette date to approximately 25.6 to 25 cal ka BP, suggesting the lunettes formed before that date. Two other samples from the middle and lower parts of the GCF were dated between 33 and 27.7 cal ka BP. These latter ages were not within lunettes but from sediments found adjacent to the Charleston basin. These ages suggest that the basins may have formed before ∼33 ka. Kuzila (Reference Kuzila1994) produced radiocarbon ages from bulk soil material within the buried GCF in and around the Ong basin, 18 km southeast of our Fairfield site (Fig. 1). Like Krueger’s (Reference Krueger1986) stratigraphic cross sections, those produced for the Ong basin show that lunettes formed either during or before deposition of the GCF. One age from the Ong basin indicates that the lunette formed before 31.0 to 30.0 cal ka BP. Two other ages from sedimentary fill around the Ong basin indicate the GCF was deposited at 27.2 to 26.5 cal ka BP. One sample from the adjacent loess-mantled alluvial plain indicates the GCF was deposited between 28.9 and 28.8 cal ka BP. These ages suggest the basin formed before 31 to 26.5 cal ka BP. Collectively, the ages reported by Krueger (Reference Krueger1986) and Kuzila (Reference Kuzila1994) suggest that the basins and lunettes were formed before ∼33 to 25 ka, a hypothesis that accords with the chronology from the nearby basins described in the present study.
Paleoclimate studies provide evidence of climatic fluctuations in the central Great Plains during MIS 3 (Johnson et al., Reference Johnson, Willey, Mason and May2007; Layzell et al., Reference Layzell, Andrzejewski, Mandel and Hanson2024). Widespread loess deposition—albeit at rates of accumulation lower than those recorded during MIS 2—occurred throughout the upper Midwest and the central Great Plains during MIS 3. In the central Great Plains, the GCF is relatively thin (<4 m), dates to ∼42 to 25 ka, and records several periods of incremental loess deposition and pedogenesis (Johnson et al., Reference Johnson, Willey, Mason and May2007; Muhs et al., Reference Muhs, Bettis, Aleinikoff, McGeehin, Beann, Skipp, Marshall, Roberts, Johnson and Benton2008). There was greater effective moisture early in MIS 3, followed by increased aridity toward the end of that period. For example, sites in eastern Nebraska and western Iowa yield evidence for marshes and wetlands between ∼50 and 37 ka, but there is also evidence for aridification from 37 to 29 ka (Baker et al., Reference Baker, Bettis, Madel, Dorale and Fredlund2009). Similarly, marshes were present in central Kansas from ∼34 to 29 ka, but they were infilled during drier climates after 30 ka (Fredlund, Reference Fredlund1995). Furthermore, dunes on terraces in the Kansas River Valley in northeastern Kansas had stabilized between ∼40 and 28 ka (Johnson et al., Reference Johnson, Halfen, Spencer, Hanson, Young and Mason2019), suggesting that sediment was available to winds at this time, presumably from reduced vegetation densities due to aridity. Dune activity is also suggested by the emplacement of a dune dam in the Niobrara River Valley in north-central Nebraska around 45 ka and the subsequent formation of a lake (Jacobs et al., Reference Jacobs, Fritz and Swinehart2007). The lake persisted for ∼10 ka as the climate fluctuated between dry and wet conditions. In central Kansas, relatively warm temperatures and effective precipitation lower than present resulted in playa formation during MIS 3 (Bowen and Johnson, Reference Bowen and Johnson2011). Ages from the Fairfield 8 and 9 cores in the present study suggest that aeolian sand accumulated around 54 to 47 ka (Supplementary Fig. 7).
We propose that fluctuations in effective moisture conditions suggested by previous studies favored the development of deflation basins that episodically held water. This statement is supported by the formation of basins during MIS 3 in Nebraska (Krueger, Reference Krueger1986; Kuzila, Reference Kuzila1988, Reference Kuzila1994), as well as playas in Kansas (Bowen and Johnson, Reference Bowen and Johnson2011). As a rule, modern deflation basins formed under semiarid climates (Goudie and Thomas, Reference Goudie and Thomas1986). The formation of the rainwater basins required that a combination of aridity, to promote deflation, and adequate effective moisture, to create conditions amenable to episodic wave erosion, were present during MIS 3. Deflation occurred chiefly during drier periods when vegetation densities were relatively low and exposed sandy alluvium and/or aeolian sands were easily deflated by predominantly northwesterly winds. The initial deflation basin depressions were likely to have been small and round, and then subsequently expanded and elongated perpendicular to the primary wind direction as water filled them intermittently and wave erosion expanded them as proposed by Krueger (Reference Krueger1986) and Kuzila (Reference Kuzila1988, Reference Kuzila1994). The depth of deflation may have been determined by the position of the water table (McKenna-Neuman and Nickling, Reference McKenna-Neuman and Nickling1989; Seppälä, Reference Seppälä1995), and perhaps the presence of deflation-resistant alluvial clays or gravels. Nevertheless, we have no means of ascertaining the position of the ancient water table, and we lack sufficient subsurface control to map subsurface sediments across the region.
Our results suggest that climatic conditions during MIS 3 resulted in some combination of relatively arid and humid climates that resulted in deflation of sediments to form the basins and sufficient effective moisture to at least seasonally fill the basins with water, allowing wave activity to expand them. The bulk of our ages suggest that these types of conditions may have been present from ∼39 to 25 ka, when most of the lunettes described in the present study formed. In contrast, playas on the Southern High Plains formed predominantly during MIS 2 (Holliday et al., Reference Holliday, Mayer and Fredlund2008), yet there is no evidence for a coeval phase of deflation-basin development on the central Great Plains. We propose that drier conditions and the burial of formerly mobile sands by Peoria Loess during MIS 2 stabilized both basins and lunettes in our study area. In contrast, sand remained mobile and basins continued to form in the southern High Plains, where rates of loess deposition were significantly lower.
Conclusions
New OSL ages produced in this study generally agree with previous results from studies of deflation basins in Nebraska. We propose a developmental sequence of events pertaining to the evolution of the Rainwater Basin Plains (Fig. 9). The entire region is underlain by braidplain sediments of the ancient Platte River (Fig. 9A). An aeolian sand sheet developed in the region by at least ∼50 ka (Fig. 9B). Our OSL chronology indicates that aeolian deflation of the rainwater basins occurred between 51 and 20 ka, but most of these ages show that basin-marginal lunettes were forming between ∼45 and 25 ka, largely during MIS 3 and in early MIS 2 (Fig. 9C). The rainwater basins were mantled by Peoria Loess between ∼25 and 14 ka, which subdued local and regional topography, but did not completely fill the basins (Fig. 9D). Our results generally agree with prior estimates of the age of alluvium underlying the Rainwater Basin Plains, regional loess stratigraphy, and previous bulk soil carbon radiocarbon ages.
Figure 9. Middle to late Pleistocene development of Rainwater Basin Plains, as interpreted from the results of this study. Dominant northwesterly winds are indicated. (A) Development of middle Pleistocene braidplain of Platte River and deposition of fluvial sediments that now underlie aeolian sediments. (B) Aeolian mobilization and development of sand sheet with local low-relief dunes. Age estimate is derived from dune sediments in our Fd 8 and 9 cores (Table 2, Supplementary Figure S7). (C) Widespread development of deflation basins in sandsheet and braidplain sediments during Marine Isotope Stage (MIS) 3 and early MIS 2. Lunettes are deposited immediately downwind of deflation basins, which episodically fill with water (as shown) and undergo wave erosion, leading to basin elongation perpendicular to the primary wind direction. (D) Mantling of entire landscape by Peoria Loess between ∼25 and 14 ka. Basins remain and continue to collect water seasonally, but relief is subdued by loess.
We emphasize that the rainwater basins formed primarily during MIS 3, a time of variable climate during which loess accumulation rates on the central Great Plains were comparatively low. Although aeolian deflation occurred during MIS 3 due to aridity, intermittent wetter periods favored the development of ephemeral lakes in the basins and the progressive wave erosion of basin margins. Thus, both wind and waves in standing water formed and gradually remodeled the basins. Indeed, several of the basins show evidence for multiple phases of deflation and lunette accretion. We did not examine any of the smaller basins nestled within larger ones, so we cannot address the complexities of multigenerational basins. Future work should address the development of multigenerational basins and examine more basins across the entire Rainwater Basin Plains to identify any larger-scale geographic or temporal trends. Additionally, the identification of any subaqueously deposited basin sediments and their analysis in the context of loess deposition would be valuable.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/qua.2025.10040.
Acknowledgments
We thank the CSD drilling program led by Matt Marxsen, as well as several undergraduate and graduate students who assisted with field and laboratory work that supported this project. This project was funded in part through the U.S. Geological Survey STATEMAP Cooperative Geologic Mapping Program for mapping the Axtell East (G17AC00373), Fairfield (G18AC00300), Fairmont (G16AC00298), and Kearney SW (G15AC00515) 7.5-minute USGS quadrangles. Some fieldwork, including test hole drilling, was conducted on the U.S. Fish and Wildlife Service’s County Line Marsh, Gleason, Massie, Meadowlark, Moger, and Prairie Dog Marsh Wildlife Protection Areas. We thank the land managers for access to those properties. We thank Mark Sweeney and an anonymous reviewer, as well as senior editor Nicholas Lancaster and guest editor Shannon Mahan for comments that improved this article.
Competing interests
The authors declare that they have no conflicts of interest.
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