Textural, geochemical and mineralogical evidence for the sources of aeolian sand in central and southwestern Nebraska, U.S.A.

Sedimentary

Geology

ELSEVIER

Sedimentary Geology 101 (1996) 85-98

Textural, geochemical and mineralogical evidence for the sources of aeolian sand in central and southwestern Nebraska, U.S.A. N.R. Winspear, K. Pye Postgraduate Research Institute for Sedirnentology, P. 0. Box 227, The Unit,ersity, Whiteknights, Reading RG6 6AB, UK Received 16 November 1994; revised version accepted 27 March 1995

Abstract

Stabilised Quaternary dunefields in the central United States underwent multiple episodes of dune activity in the Holocene and may undergo extensive reactivation in response to future changes in global climate. However, the sources of aeolian sand and the history of sand supply in these dunefields are poorly understood. An investigation was therefore undertaken to identify the source(s) of aeolian sand in the largest dunefield on the High Plains, the Nebraska Sand Hills, and an adjacent area of dunes referred to here as the Dickens Dunefield. The specific hypotheses relating to sand provenance were: (1) that the Nebraska Sand Hills dune sands were derived from the underlying fluvial sands; (2) the Dickens dune sands were derived from migrating Nebraska Sand Hills dunes; and (3) the Dickens dune sands were derived from South Platte River sand. Statistical comparison of the textural and compositional characteristics of the aeolian and possible source sands suggested that the Nebraska Sand Hills were indeed derived from the underlying fluvial sediments, following an extensive episode of fluvial reworking which occurred at some time between the late Pleistocene and middle Holocene. The Dickens dune sands were probably derived from the South Platte River at some time during the Holocene.

1. Introduction

Stabilised Quaternary dunefields in the central United States were active most recently on a large scale during the Holocene (Madole, 1981a, b, 1994; Ahlbrandt et al., 1983; Holliday, 1989; Gaylord, 1990; Forman et al., 1992; Swinehart et al., 1994). It has been suggested that many of these dune fields, especially those on the southern High Plains, may experience an increase in dune activity in response to predicted increases in mean annual temperature and windiness, and a decrease in mean annual precipitation caused by possible future global climate changes (Muhs and Maat, 1993). New dunefields could also form in

response to the predicted changes in climate, and it is therefore important to understand the history of sand supply and the pathways of sand transport which supplied sand to existing dunefields. This paper summarises the results of an investigation to identify the source(s) of the aeolian sand forming the largest area of dunes on the High Plains, namely the Nebraska Sand Hills, and a nearby area of dunes referred to here as the Dickens Dunefield, after the town of Dickens, Nebraska (Fig. 1). The Nebraska Sand Hills occupy an area of ca. 50,000 km 2, and are composed primarily of transverse and crescentic megadunes in the central, northern and western regions which pass east-

0037-0738/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 3 7 - 0 7 3 8 ( 9 5 ) 0 0 0 4 9 - 6

N.R. Winspear, K. Pye /Sedimentary Geology 101 (1996)85-98

86

wards into a belt of domal dunes and southeastwards into linear dunes (Fig. 1). Studies of external dune morphology indicate that northwesterly winds were prevalent during the most recent episode of dune formation, although secondary southwesterly winds are also likely to have been important (Smith, 1965; Warren, 1976). The modern wind regime is also dominated by northwesterly winds, but with an additional component of southerly and southeasterly winds. It has been estimated that the modern wind regime would be very similar to that which formed the transverse megadunes if the southerly wind component was not present (Ahlbrandt and Fryberger, 1980). The source of the Sand Hills dune sands has previously been suggested to have been the fluvial sediments which underlie the Sand Hills (Reed, 1965; Reed and Dreeszen, 1965; Warren, 1976; Ahlbrandt et al., 1983; Swinehart, 1990), although

this relationship has not previously been rigorously tested. There is considerable disagreement regarding the time of formation of the Sand Hills dunes. The three main hypotheses are: (1) that the entire aeolian accumulation occurred during the Wisconsin glaciation, in association with strong periglacial winds (Wells, 1982); (2) accumulation occurred largely during the Wisconsin period, with limited Holocene aeolian activity (Smith, 1965; Lugn, 1968; Wright et al., 1985); and (3) accumulation occurred largely during the Holocene, especially in the middle and late Holocene (Bradbury, 1980; Ahlbrandt et al., 1983; Swinehart, 1991). Support for at least localised middle Holocene dune activity has recently been provided by unpublished optical dating evidence from the northern Sand Hills (Stokes and Swinehart, in prep.). However, this does not preclude

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102 o

Complex megaDorchan Domol dunes Dunesand FluvioI sand Johnson Loke fluvioI sond ( lute Pleistocene )

Fig. 1. The distribution of sampling sites in central and southwestern Nebraska• Sampling sites within the Sand Hills include a variety of dune morphologies. Modified from fig. 3.19 in Swinehart (1990).

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

the possibility of earlier episodes of dune formation. The Dickens Dunefield, which occupies an area of ca. 1200 krn 2 (Swinehart and I~ope, 1992) to the south of North Platte (Fig. 1), is characterised by stabilised domal and parabolic dunes and sand sheets, with scattered blowouts. The dunefield rests upon an erosional remnant of a fluvially dissected upland area composed primarily of late Pleistocene Peoria loess, but which also contains isolated remnants of the Bignell loess, which accumulated during the Holocene (Maat and Johnson, 1994; Pye et al., 1995). The timing of initial accumulation of the Dickens Dunefield is uncertain. It has been suggested that the Dickens dune sands were derived by southerly migration of dunes from the Nebraska Sand Hills over the

43*

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87

dried-up bed of the South Platte River during the Last-Glacial Maximum (Swinehart and l_x3ope, 1992). Alternatively, the South Platte River could have provided the source of the dune sand. The aims of the work reported in this paper were to test the hypotheses that: (1) the Nebraska Sand Hills dune sands were derived from the underlying fluvial sands; (2) the Dickens dune sands were derived from migrating Nebraska Sand Hills dunes; and (3) the Dickens dune sands were derived from South Platte River sand.

2. S a m p l i n g rationale A n a t t e m p t was m a d e to c h a r a c t e r i s e t h e varia t i o n in t e x t u r e a n d c o m p o s i t i o n o f t h e S a n d

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silty

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Dune sand

Wavy iron-slainin 9 <'--- Dune/fluvial conlact U S T L 14 (272-+40ka B.P.) Grey semi- indurated fluvialsand

--7" -? " 7 - - 7 1

Fig. 2. Stratigraphy exposed in an inspection pit dug into the north bank of the north fork of the Dismal River, approximately 0.8 km west of the Highway-97 bridge crossing. Note the absence of a well-developed weathering horizon at the contact between the dune and fluvial sand. Iron staining in the vicinity of the contact suggests that the water table existed at this level during an interval following the deposition of the basal dune sands.

88

N,R. Winspear, K. Pye /Sedimentaty Geology 101 (1996) 85-98

Hills and Dickens dune sands, and the possible source sands, by collecting samples of between 100 and 150 g from the sites shown in Fig. 1. A total of twenty samples of dune sand were collected from sites within the central, northern and southeastern parts of the sand sea, from a variety of topographic locations and stratigraphic settings within dunes including dune crests, interdune depressions and undifferentiated aeolian sand beneath the level of the interdune surfaces. Each sample specifically included a mixture of adjacent sand layers in order to provide a representation of the range of variation in textural and compositional characteristics for the immediate vicinity of the sample. Although the number of samples collected within the Sand Hills was relatively small, the sample set does not possess a large range of variability (e.g. O'mean g r a i n s i z e ~--- _+9%; O'sorting = ______13%; O r k u r t o s i s = ~___7 % ) , and it is likely that the set is broadly representative of the variation encountered in the sand sea. This conclusion is supported by the work of Ahlbrandt and Fryberger (1980), who found a similar level of textural variation between sands from adjacent layers within individual dunes as between those from widely separated dunes. Thirteen samples of fluvial sediments immediately underlying the dune sand were collected from sections along the Dismal River in the central Sand Hills. The distribution of these fluvial sediments is uncertain, although they are known to exist beneath dune sand approximately 5 km north of the Dismal River (Ahlbrandt et al., 1983). The sediments occur as lenticular fluvial sand bodies which infill shallow palaeovalleys cut into the underlying silty sand, and are composed primarily of very fine to medium sand with interbedded silt layers; peat lenses occur at or near the top of the fluvial section. It is suggested that deposition of the fluvial sediments occurred in meandering fluvial environments. The age of the sub-Sand Hills fluvial sediments has been documented by Ahlbrandt et al. (1983), who obtained radiocarbon ages from organic-rich fluvial sands and silts ranging between 8410 + 110 and 3000 + 400 14C yr B.P. from two sites along the South Fork of the Dismal River. However, a thermoluminescence (TL) age estimate of 272 + 40 ka B.P.

(USTL14) was obtained from fluvial sand located 30 cm beneath the basal dune sand along the North Fork of the Dismal River (Fig. 2; Pye et al., 1995). Whilst this age is unlikely to be accurate since it lies outside the commonly accepted age range of ca. 100,000 years for TL dating, it strongly suggests that these fluvial sediments are of pre-Wisconsin age, and may be much older. The apparent wide range in ages of the sub-Sand Hills fluvial sediments may be explained by a long history of fluvial reworking in this area, with the "old" sediment identified by T L dating representing a remnant of a formerly more extensive deposit. Six samples of aeolian sand from the eastern part of the Dickens Dunefield were obtained from inspection pits in blowout exposures along State Highway-83. Five samples of fluvial and aeolian sands were collected from a single outcrop about 2.1 km northeast of Johnson Lake in order to test the hypothesis that the Dickens dune sand was derived from South Platte River sand. The sequence forms the fill of a palaeovalley previously occupied by the South Platte River, and radiocarbon age estimates from palaeosols at the base and near the top of this outcrop indicate that these sediments were deposited between ca. 34 and 24 ~4C ka B.P. (May and Souders, 1988). It was not possible to collect additional samples of late Pleistocene fluvial and aeolian sands because of the general absence along the Platte River Valley of well-dated sections which are exposed beneath the blanket of late Pleistocene Peoria loess. During the late Pleistocene the North and South Platte Rivers were probably not connected, and there is evidence that the North Platte River may have occupied a palaeovalley which has since been buried by dune sands (Fig. 3; Winspear and Pye, 1995a). The possibility therefore exists that the North Platte River contributed some sand to the central and southeastern Sand Hills under prevailing northwesterly winds. However, there are no well-dated exposures of fluvial sediments in the area now covered by the southeastern Sand Hills, and it was not possible directly to test this hypothesis. As an alternative measure, it was decided to compare the textural and composi-

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996)85-98

SD

w¥

IA

KS

~

Aeoliansand

o

l

,80,m

I

" ~ Prevailing wind direction ( Late Pleistocene ~ Holocene )

Fig. 3. Major fluvial drainage systems in Nebraska during the late Pleistocene and during most of the Holocene. Following the suggested capture of the North Platte River by the South Platte River at some time during the Holocene, southeasterly dune migration covered the broad, shallow valley immediately to the northwest of the Loess Hills. IA = Iowa; MO = Missouri; KS = Kansas; CO = Colorado; WY = Wyoming; SD = South Dakota.

tional characteristics of the Sand Hills dune sand with those of the fluvial and aeolian sands exposed in the Johnson Lake outcrop, which were derived from the South Platte River. Earlier work has demonstrated that late Pleistocene loess (silt and fine sand) derived from the North Platte River is very similar in terms of texture and composition to that derived from the South Platte River (Winspear and Pye, 1995a). This evidence suggests that the source sediments in both river valleys were similar in terms of lithological composition.

3. Analytical methods Fingerprinting of the aeolian and possible source sediments was achieved by establishing the textural, geochemical and mineralogical properties of a standardised grain size fraction. This procedure was adopted primarily to enable direct comparison of the geochemical and mineralogical compositions of sands which were deposited in different sedimentary environments, and which would be expected to have significantly different textural characteristics. The upper limit of the standardised grain size fraction ( - 0 . 2 5 4') repre-

89

sents the maximum grain size encountered in the aeolian sands, and was determined by dry sieving of 30 g sub-samples of bulk samples of Sand Hills and Dickens dune sands at 0.25 4, intervals. In the case of the Dickens dune sands this value was adopted because it represents the maximum grain size encountered in two-thirds of the sand samples. The remaining one-third of the samples contained an isolated population of coarse grains in the size range - 0 . 5 0 to - 0 . 7 5 &, which constituted up to 0.4% by weight of the sand. Sub-samples of similar size (ca. 30 g) were then prepared from the possible fluvial source sediments, and were passed through a 1180 /~m mesh sieve to remove grains coarser than - 0 . 2 5 4,. These subsamples were then wet sieved to remove silt- and clay-sized material ( < 4 4,) and to facilitate comparison of the fluvial sands with the silt- and clay-poor Sand Hills and Dickens dune sands (which have minimum fines contents of 0.16_+ 0.11% and 0.67 _+ 0.81%, respectively). After drying, the sub-samples were sieved at 0.25 4' intervals, and the resulting particle size distribution data was processed using the Granny textural package (McLane, 1989). Graphical textural measures were used because these parameters are more appropriate than moment measures for characterising truncated particle size distribution data. X-ray fluorescence spectrometry (XRF) was used to determine the major element composition of the sediment samples as a relatively rapid alternative to modal mineral analysis which requires extensive point-counting of thin sections. Representative sediment sub-samples (approximately 6 g) were initially wet sieved to remove silt and clay-sized material and dry sieved to remove grains coarser than - 0 . 2 5 4' in size. The subsamples were then ground to produce a crushed powder and thoroughly mixed with lithium tetraborate using a sample to flux ratio of 1:4 (cf. Rose et al., 1962). Lithium tetraborate fusion beads were subsequently prepared using a "Claisse Fluxy" automated bead maker. The mineralogical compositions of selected - 0 . 2 5 to 4.0 4' grain size fractions were determined by point-counting of thin sections, for comparison with the XRF major oxide composi-

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

90 1.0'

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Sand Hills dune sands ] Dickens dune sands [ Sub-dune fluvial sands[ A Johnson Lake sands J

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Graphic mean grain size (phi units) Fig. 4. Cross-plot of mean grain size (q~) versus standard deviation for the - 0.25 to 4.0 & grain size fraction of sand samples from the Nebraska Sand Hills, Dickens Dunefield, sub-Sand Hills fluvial sands, and the Johnson Lake sands.

tional data. Sub-samples of approximately 0.5 g were obtained from each sample using a microsplitter, and were subsequently mounted in stained araldite resin, thin sectioned and stained for carbonate. A total of 300 points per slide were counted using a transmitted-light binocular microscope. The non-parametric Mann-Whitney U test was then applied to identify the possible presence of significant differences between two sample populations, using a two-tailed test at the 95% significance level. The textural and compositional characteristics of the Sand Hills dune sands were compared with those of the sub-Sand Hills fluvial sands and with the Johnson Lake sands. Similarly, the Dickens dune sands were compared with Sand Hills dune sands and with the Johnson

Lake sands. Comparisons between the sub-Sand Hills fluvial sands and the Johnson Lake sands were also undertaken to assess whether these sediments can be resolved using their textural and compositional characteristics.

4. Results

4.1. Textural analyses The variation in mean grain size and standard deviation (sorting) of individual sediment samples from the different sample populations is shown in Fig. 4. Mean values for the graphical textural parameters calculated for the sample populations

Table 1 Mean graphical textural values of the sample populations, determined by processing of 0.25 ~b interval dry sieving data Sample population

Ns

Graphic m e a n grain size ( ~ b )

Graphic standard deviation

Graphic skewness

Graphic kurtosis

Nebraska Sand Hills dune sands Dickens dune sands Sub-Sand Hills fluvial sands Johnson Lake sands

20 6 13 5

2.23 2.72 2.20 2.17

0.53 0,56 0.58 0.80

-0.06 -0.20 -0.02 -0.02

1.01 +_ 0.07 1.10 + 0.14 1.06 + 0.13 0.93 + 0.08

_+ 0.20 + 0.28 + 0.34 + 0.34

_+ 0.07 + 0.14 + 0.11 + 0.11

_+ 0.09 _+ 0.10 _+ 0.08 _+ 0.09

The data refer to the - 0.25 to 4.0 ~b grain size fraction of samples of aeolian and possible source sands. N s = n u m b e r of samples used in dry sieving analysis. ( + ) denotes the value for one population standard deviation (%).

N.R. Winspear, K_ Pye / Sedimentary Geology 101 (1996) 85-98

91

Table 2 Lowest values of the Mann-Whitney U parameter obtained during significance testing of textural data between the aeolian and possible source sands Graphic mean grain size (U) Nebraska Sand Hills dune sands versus: Sub-Sand Hills fluvial sands 115.5 Johnson Lake sands 46.0 Dickens dune sands versus: Nebraska Sand Hills dune sands Sub-Sand Hills fluvial sands Johnson Lake sands

6.0 * 16.0 * 4.5

Sub-Sand Hills fluvial sands versus: Johnson Lake sands

29.5

Graphic standard deviation (U)

Graphic skewness (U)

Graphic kurtosis (U)

fcritical

107.0 1.0 +

94.5 39.0

106.5 17.0

76.0 20.0

16.0 * 6.0 * 3.0 *

45.0 34.0 6.0

27.0 16.0 3.0

31.0

17.0

12.0

58.0 33.0 4.0

6.0 *

The data refer to the -0.25 to 4.0 ~b grain size fraction of samples of aeolian and possible source sands. Asterisk indicates that a difference exists between the sample populations at the 95% significance level.

in the grain size ranges prescribed above are presented in Table 1, whilst the results of statistical comparisons between the sample populations are presented in Table 2. There are no significant textural differences between the Sand Hills dune sands and the sub-Sand Hills fluvial sands, whereas the Sand Hills dune sands are better sorted than the Johnson Lake sands. Significant differences exist between the Dickens and the Sand Hills dune sands in terms of mean grain size and skewness. In contrast, the Dickens dune sands

are texturally similar to the Johnson Lake sands, differing only in skewness. The sub-Sand Hills fluvial sands are better sorted than the Johnson Lake sands. Dry sieving of the Dickens and Sand Hills dune sands revealed the presence of grain aggregates in the < 2.25 ~b size fraction. It was not possible to estimate the abundance of these features using dry sieving because individual grains form the major portion of the sand in this size fraction. However, modal analysis using thin sec-

• • O A

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SandHills dune sands Dickensdune sands Sub-dunefluvial sands JohnsonLake sands

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.+ 6"

O

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O

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0 0

2

80

90

100

SiO2 (%) Fig. 5. Cross-plot of SiO 2 versus A I 2 0 s (%) for the - 0 . 2 5 to 4.0 (k grain size fraction of sand samples from the Nebraska Sand Hills, Dickens Dunefield, sub-Sand Hills fluvial sands, and the Johnson Lake sands.

20

6

12

7

Nebraska Sand Hills dune sands

Dickens dune sands

Sub-Sand Hills fluvial sands

Johnson Lake sands NL= 2

1.30 _+0.12

NL= 2

0.78 _+0.08

NL = 2

1.49 +0.12

NL = 3

0.74 _+0.07

LOI

1,086 _+0.033

0.948 _+0.439

1,178 _+0,041

1.002 +0,274

Na20 (%)

0.281 _+0.062

0.069 _,%0.090

0.218 -+0.074

0.024 +0.025

MgO (%)

7.779 _+0.321

6,145 -+ 1,657

8.276 -+0.461

6.302 -+0.828

AI20 3 (%)

83.541 _+ 1.058

88.612 _+3.446

83.729 -+1,402

88.058 _+ 1.870

SiO 2 (%)

2.490 _+0.067

1.910 _+0.385

2.572 _+0,117

1.849 _+0.163

K20 (%)

1.102 -+0.198

0.844 -+ 0.328

1.036 -+0.167

0,882 +0.188

CaO (%)

0,210 +0.039

0,093 + 0.054

0,224 _+0.062

0.083 _+0.023

TiO 2 (%)

0.022 _+0.004

0,010 _+0.003

0.020 -+0,003

0,008 _+0.001

MnO (%)

1,093 _+0.184

0.484 _+0,251

1.044 _+0,204

0.407 _+0.066

Fe20 3 (%)

0 * 8.0 * 13.0

30.0 25.0 1.0 *

41.0

Dickens dune sands versus: Nebraska Sand Hills dune sands Sub-Sand Hills fluvial sands Johnson Lake sands

Sub-Sand Hills fluvial sands versus: Johnson Lake sands

13.0 *

0 * 6.0 * 9.0

109.0 3.0 *

a1203 (U)

3.0 *

3.0 * 4.0 * 18.0

118.0 2.0

SiO 2 (U)

3.0 *

0 * 2.0 * 13.0

94.0 0

K20 (U)

23.0

41.0 25.0 16.0

118.0 30.0

CaO (U)

5.0 *

0 * 4.0 * 20.0

119.0 0

TiO 2 (U)

0.5 *

0 * 0 * 13.5

72.0 0

MnO (U)

4.0 *

0 * 6.0 * 17.0

98.0 0

Fe203 (U)

18.0

27.0 14.0 6.0

69,0 34,0

gcritical

The data refer to the -0.25 to 4.0 q5 grain size fraction of samples of aeolian and possible source sands. Asterisk indicates that a difference exists between the sample populations at the 95% significance level.

3.0 *

82.0 0

MgO (U)

Nebraska Sand Hills dune sands versus: Sub-Sand Hills fluvial sands 116.0 Johnson Lake sands 65.0

Na20 (U)

Table 4 Lowest values of the Mann-Whitney U parameter obtained during significance testing of geochemical data between the aeolian and possible source sands

Major oxide compositional data were determined by X R F analysis of beads. The data refer to the - 0 . 2 5 to 4.0 4~ grain size fraction of samples of aeolian and possible source sands. Nx = number of samples used in X R F analysis. N L = number of samples used in loss-on-ignition analysis. (_+) denotes the value for one population standard deviation (on).

Nx

Sample populations

Table 3 Mean oxide compositions and loss on ignition (LOI) data (weight%) of the sample populations

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N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

93

and the sub-Sand Hills fluvial sands could not be distinguished in terms of geochemical composition, whereas significant differences exist between the Sand Hills dune sands and the Johnson Lake sands in terms of MgO, A1203, SiO 2, K 2 0 , CaO, TiO2, MnO and Fe203 contents. The Dickens dune sands differ only in terms of N a 2 0 content from the Johnson Lake sands, whereas significant differences exist between the Dickens and the Sand Hills dune sands in terms of MgO, A1203, SiO2, K 2 0 , TiO2, MnO and Fe203 contents. The sub-Sand Hills fluvial sands and the Johnson Lake sands differ in terms of MgO, A1203, SiO2, K 2 0 , TiO2, MnO and F e : O 3.

tions showed that the proportion of grain aggregates in the Dickens dune sands is about 0.5 _+ 0.6%. Analysis of several samples of Sand Hills dune sands using scanning electron microscopy (SEM) indicated that individual grain aggregates are cemented by clay which forms a boxwork-type structure between grains (Winspear and Pye, 1995b). The grain aggregates in the dune sands are interpreted to have formed by cementation of sand grains following infiltration of allochthonous dust in rainwater. A proportion of the aggregates are likely to be preserved even after dune reactivation, since these features were still present throughout a dune in the northern Nebraska Sand Hills which experienced multiple episodes of aeolian activity during the Holocene (cf. Winspear and Pye, 1995b).

4.3. Mineralogical (thin section) analyses An example cross-plot showing the two main constituent minerals (quartz and plagioclase feldspar) of the sample populations is shown in Fig. 6, whilst the mean mineralogical compositions of the sample populations are presented in Table 5. It was necessary to normalise the composition of the Johnson Lake sand using a scaling factor of 1.04, to exclude the relatively high content (3.6%) of grain aggregates which were identified by point-counting. This is because the pres-

4.2. Geochemical analyses The range of variation in the SiO 2 and AI203 content of the sample populations is shown in Fig. 5, and accompanying values describing the mean major oxide composition and loss on ignition data are presented in Table 3. The results of comparisons between the sample populations are presented in Table 4. The Sand Hills dune sand

16"

• 14"

Sand Hi~s dune sands

•

Dickensdune sands

O

Sub-dune fluvialsands Johnson Lake sands

12&

0

10•

•

A 0

0

0

0

60

70

80

Quartz content (%)

Fig. 6. Cross-plot of quartz versus plagioclase feldspar (%) for the -0.25 to 4.0 ~b grain size fraction of sand samples from the Nebraska Sand Hills, Dickens Dunefield, sub-Sand Hills fluvial sands, and the Johnson Lake sands.

94

N.R. Winspear, K. Pye /Sedimentary Geology 101 (1996)85-98

ence of these features will lead to error in estimating the major mineral composition of the sediment, and is assumed to be reasonable since thin section and SEM studies showed that an essentially random distribution of mineral types are involved in the aggregates. The average mineralogical composition of each of the four sample populations corresponds to that of a feldspathic litharenite according to the classification of Folk (1968). Comparisons involving point-counted mineralogical data were restricted to minerals present in both populations in estimated proportions > 2%. This value was adopted since a large degree of statistical uncertainty relative to the estimated value is associated with minerals present in estimated quantities of < 2%. The 95% confidence limit for a mineral estimated to be present in an abundance of 2% is about 1.5% (see fig. 1 in Van der Plas and Tobi, 1965), indicating that the true mineral proportion may range from 0.5 to 3.5%. The results of comparisons between sample populations are described in Table 6. The Sand Hills dune sands are similar to the sub-Sand Hills fluvial sands in mineralogical composition, whereas the Sand Hills dune sands and the Johnson Lake sands differ in quartz content. The Sand Hills and Dickens dune sands are similar in mineralogical composition, and the Dickens dune sands contain a different proportion of plagioclase feldspar compared with the Johnson Lake sands. The sub-Sand Hills fluvial sands and the Johnson Lake sands are similar in mineralogical composition.

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5. Discussion

Important differences exist between the subSand Hills fluvial sands and the Johnson Lake sands in terms of textural and geochemical parameters. However, differences between these sediments cannot be resolved using mineralogical criteria, which probably reflects the lack of sensitivity of point-counting to compositional variation compared with XRF analyses. Conclusions regarding the derivation of the aeolian sands from

4.1

U

z

._= E

,.~

.=

O 09

[-

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ii

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

one of these possible sources were therefore based primarily on geochemical and textural criteria. Whilst the use of graphical textural measures to differentiate between inland dune sand and source sand has previously been questioned (e.g. Moiola and Weiser, 1968), recent work using a standardised grain size fraction has demonstrated that the Algodones dune sands of southeastern California can be distinguished from the lake shoreline source sands because aeolian sorting processes have operated across a distance of 5-25 km between the sand source and the dunes (Winspear and Pye, 1995c). The Algodones study also showed that it is possible to identify slight but significant geochemical differences between source sands and dune sands which have undergone a considerable distance of transport, including longshore and aeolian transport (Winspear and Pye, 1995c). However, it is difficult to differentiate, using textural and geochemical evidence, between the sub-Sand Hills fluvial sands and the directly overlying Sand Hills dune sands, which implies that the dune sands have been reworked in situ from the underlying fluvial sands with only limited transportation. Considering that aeolian transportation appears able to produce only subtle differences in geochemical composition between aeolian sands and their source sands, it is unlikely that the major differences in geochemistry which exist between the Sand Hills and

95

Johnson Lake sands developed purely as a result of aeolian transportation. The North Platte River can therefore be rejected as a source of Sand Hills dune sands. The apparent textural similarity of the Sand Hills dune and underlying fluvial sands implies that the dune sands have been transported a distance considerably less than that experienced by the Algodones dune sands ( < 25 km). The fluvial sands must therefore have been present beneath most of the area now covered by the Sand Hills, although the limits of their distribution are uncertain. The textural and compositional similarity between the dune sands, which were collected from widely separated sites within the central and southeastern Sand Hills, and the fluvial sands, which were collected from a small area of about 2 km 2 in the central Sand Hills, demonstrates that the -0.25 to 4.0 ~b grain size fraction of the fluvial sands was relatively uniform in texture and composition across the area of deposition. Some differentiation in terms of texture and composition is likely to have occurred between the first formed Sand Hills dune sands and the precursor fluvial sands through the influence of aeolian transport processes, including saltation, reptation and creep. However, the distinction may have been blurred by alternate fluvial and aeolian reworking of the sand bodies. Radiocarbon and

Table 6 Lowest values of the Mann-Whitney U parameter obtained during significance testing of point-count data between the aeolian and possible source sands

(u)

Quartz

(u)

M/qtz

K-fsp

(u)

(u)

Plag fsp

Chert

Uc, itic~l

Nebraska Sand Hills dune sands versus: Sub-Sand Hills fluvial sands Johnson Lake sands

13.0 2.0 *

12.5 5.5

NC NC

9.5 11.0

14.0 12.0

3.0 2.0

Dickens dune sands versus: Nebraska Sand Hills dune sands Sub-Sand Hills fluvial sands Johnson Lake sands

4.0 5.0 10.0

14.0 10.0 6.0

NC 10.0 9.0

3.5 0 * 0 *

15.[) 11.0 9.(1

3.0 2.0 1.0

Sub-Sand Hills fluvial sands versus: Johnson Lake sands

4.0

9.0

9.0

6.0

9.5

1.0

(u)

The data refer to the - 0.25 to 4.0 q5 grain size fraction of samples of aeolian and possible source sands. Asterisk indicates that a difference exists between the sample populations at the 95% significance level. NC = no comparison because one or both of the sample populations contained < 2% of a particular mineral.

96

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

optical luminescence dating have demonstrated that aeolian and fluvial activity occurred at several times during the late Holocene in the central and northern Sand Hills (cf. Ahlbrandt et al., 1983; Stokes and Swinehart, in prep.). Apparently climatically controlled intercalation of dune and fluvio-lacustrine sediments has also been reported in the western Sand Hills (Loope and Swinehart, 1992). Radiometric age estimates suggest that the initial accumulation of Sand Hills dune sands occurred some time between the late Pleistocene and the middle Holocene. The proximity of late Pleistocene fluvial sediments to the overlying dune sands along the north fork of the Dismal River, without an intervening weathering horizon, suggests either that the dune sands were derived shortly after the fluvial sand was deposited, or that a considerable proportion of sub-Sand Hills fluvial sediment of intermediate age has been removed by erosion. The second possibility appears more likely, given the long history of fluvial activity in this area. Several significant differences in geochemistry and texture exist between the Dickens dune sands and the Sand Hills dune sands. In particular, the geochemical data suggest that the Dickens dune sands are enriched in K-feldspar, which is supported by point-count data, and heavy mineral grains. Given these characteristics, the Sand Hills dune sands can be rejected as a possible source of the Dickens dune sands. However, the Dickens dune sands are very similar in texture and geochemical composition to Johnson Lake sands, which suggests that the South Platte River could have provided an important source of sand to the dunefield during the late Pleistocene or Holocene. The emplacement of the initial Dickens dune sands can be more closely constrained by considering the history of erosion of the loess uplands located to the south of the Platte River. An episode of landscape stability and pedogenesis between about 10.5 and 9 ka B.P., which allowed the formation of the Brady Soil in the loess deposits of southern and central Nebraska (Johnson and May, 1992; Maat and Johnson, 1994; Pye et al., 1995), was followed by fluvial dissection of the loess uplands. Stereographic analysis of aerial

photographs (NASA-ERTS E-1529/30 series) of the Dickens Dunefield revealed that lineations are visible in the surface of the Dickens dune sand. The orientation of these features (N N E SSW, N -S and NNW-SSE) appears to be similar to that displayed by topographic ridges formed by fluvial erosion in the surrounding loess upland area, which suggests that the dune sands may have been emplaced at some time later than 9 ka B.P. However, the preferential preservation of a loess plateau beneath the Dickens dune sands also suggests that erosion ceased at an earlier time in this area than in the surrounding loess uplands. The most likely reason for this is that a covering of dune sand led to a significant reduction in overland flow during intense rainstorms, due to the greater infiltration capacity of the sands compared with loess. The available evidence therefore implies that the Dickens Dunefield originally formed at some time during the Holocene, probably during a period of decreased rainfall which reduced the density of vegetation cover and probably led both to increased gullying and aeolian activity. The likelihood of a Holocene age for the original Dickens dune sands implies that the textural and compositional characteristics of sands carried by the South Platte River in the size range -0.25 to 4.0 ~b did not change between the late Pleistocene and the Holocene. This is surprising because a major change in the processes of sediment production occurred in the Rocky Mountain Front Range with the end of the Pinedale glaciation between 12 and 11.5 ]4C ka B.P. (cf. Benedict, 1975; Madole, 1976), but does not appear to be reflected in the textural and compositional characteristics of sediments derived from this area during the Holocene. As a hypothesis which requires further testing, it is tentatively proposed that sediments carried by the South Platte River during the Holocene were derived by fluvial reworking of sediments stored within the Colorado Front Range since the end of the Pinedale glaciation. These sediments could have provided the source of the Dickens dune sands and the Bignell loess which is present across parts of Nebraska and Kansas (cf. Pye et al., 1995). The modern Dickens dunes are likely to have

N.R. Winspear, K. Pye / Sedimentary Geology 101 (1996) 85-98

been reworked from the initial aeolian sands during multiple episodes of dune formation which occurred in northeastern Colorado during the middle and late Holocene (cf. Forman et al., 1992; Madole, 1994).

6. Conclusions On the basis of textural and geochemical evidence, it is concluded that the Nebraska Sand Hills were formed principally by aeolian reworking of the underlying deposits of fluvial sand. Stratigraphic and chronological evidence suggests that an extensive history of fluvial reworking predated the initial formation of the Sand Hills, which probably occurred in the late Pleistocene or early Holocene. On the basis of geochemical evidence, the Nebraska Sand Hills are unlikely to have provided a source for the Dickens dune sands, and the South Platte River appears to be a much more likely sand source. Although the possibility that formation of the Dickens Dunefield began during the late Pleistocene cannot be entirely discounted, the available evidence suggests that the dune sand is most likely to have been first emplaced at some time during the Holocene.

Acknowledgements This work forms part of PhD research undertaken by N.R. Winspear at the University of Reading between 1990 and 1993, and was funded by the Reading University Research Endowment Fund and a Leverhulme Trust Fellowship to K. Pye. The authors would like to thank Drs. J. Swinehart and D. Loope for their help and cooperation, and Ms. Mary Helen Marland for granting access to the Dismal River Ranch sections. Drafting was undertaken by A. Cross. Reading University PRIS Contribution No. 399.

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97

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