the origin and significance of boxwork clay coatings on dune sand grains from the nebraska sand...

SEDIMENTARY GEOLOGY

ELSEVIER Sedimentary Geology 94 (1995) 245-254

The origin and significance of boxwork clay coatings on dune sand grains from the Nebraska Sand Hills, USA

N.R. Winspear, K. F’ye * Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 2AB, UK

Received 26 April 1994; revised version accepted 18 July 1994

Abstract

Aeolian dune sands of Late Pleistocene to late Holocene age in the Nebraska Sand Hills are widely characterized by the development of partial clay coatings which form connected ridges and “boxwork” structures on the grain surfaces. The proportion of sand grains with clay coatings typically shows considerable variability with depth in any given section. Distinct bands of sand enriched in fines are quite common, especially just below the present, or buried, dune surfaces. The morphology, texture and mineralogical composition of clay coatings from two sites, the Merritt Reservoir megabarchan and the Dismal River Ranch section, have been investigated using a combination of SEM, X-ray microanalysis, XRD and Coulter-Multisizer analysis in order to determine their mode of formation and environmental significance. Based on the analytical results and field observations it is concluded that the boxwork clay coatings formed in the vadose zone mainly as a result of precipitation- and surface-runoff induced infiltration of clay and very fine silt from the dune surface. The main source of fines is concluded to be fine-grained allochthonous aeolian dust derived principally from the High Plains or more distal sources.

1. Introduction

Previous studies have shown that clay-rich coatings on sand grains may develop in near- surface sediments as a result of several different mechanisms, including: (1) infiltration of aeolian dust deposited by dry deposition or in precipitation (Walker, 1979; Pye and Tsoar, 1987); (2) infiltration of allochthonous fines introduced by hillwash, river floods and laterally migrating groundwater (Walker and Crone, 1974; Mole- naar, 1986; Matlack et al., 1989; Pye and Tsoar,

* Corresponding author.

1987, 1990, pp. 256-2.57); (3) in-situ weathering of feldspars, other silicates or volcanic glass shards (e.g. Walker, 1976; Walker et al., 1978; Gardner, 1981; Pye, 1983a); and (4) breakdown and re-distribution of detrital clay pellets introduced into the aeolian environment by wind or water (e.g. Bowler and Magee, 1978). Thin, sub-horizontal bands of fine material, which have been termed textural sub-soil lamellae (Dijkerman et al., 1967), are commonly observed in soil profiles developed on sandy parent materials and have been at- tributed to translocation and deposition of silt and clay where the downward-moving fines en- counter a finer-textured sand layer, or at the lower limit of water penetration (Wurman et al.,

0037-0738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00090-5

1959; Robinson and Rich, 1960; Bond, 1967; Wright and Foss, 1968). Bands of silt and clay-rich material can also form within otherwise sandy sequences when mud deposited on dune margins and in inter-dune depressions by fluvial, colluvial or lacustrine waters (e.g. Langford, 1989; Purvis, 1991) is subsequently buried by further sand deposition. The formation of early post-depositional clay coatings on sand grains, and of fine-grained interbedded laminae, is of significance because these features may significantly influence the subsequent erodibility of the sediments due to the action of wind and water, and therefore have an important control on their agricultural potential (Lutz, 1941; Tsoar and Zohar, 1985). The formation of early diagenetic clay coatings can also have a major effect on later diagenetic processes such as cementation and compaction, with important implications for reservoir properties (Mole- naar, 1986; Matlack et al., 1989).

The principal morphological characteristics of adhering clay emplaced by infiltration are de- pendent upon the environment of accumulation

of the clay relative to the water table (Crone, 1974). In the saturated zone, clays become oriented tangentially to the surfaces of the frame- work grains, and may show some concentration on the upper surfaces of grains. Clay deposited on grains in the vadose zone, which is subject to periodic wetting and drying, commonly accumu- lates as ridges where water menisci were present prior to drying. Clay ridges of this type have been described from Holocene delta-plain and point- bar environments by Matlack et al. (1989). Al- most continuous clay coatings, referred to in the soil science literature as “argillans”, which are absent only at grain contacts, develop in the vadose zone after long periods of translocation and illuviation of allochthonous or autochthonous clay (e.g. Pye, 1983a).

The mineralogical composition of the fine fraction within a sand body may also provide clues to the origin of the material. Illite and mixed-layer illite-smectite are the most common authigenic clay minerals formed by weathering under warm desert and semi-arid conditions

MR = Merritt Reservoir Cample.megoDorchon

DRR = Dismal River Ranch

Fig. I. Location of sampled localities in the Nebraska Sand Hills.

N.R. Winspear, K. Pye /Sedimentary Geology 94 (1995) 245-254 247

(Walker, 1976; Walker et al., 19781, whereas kaolinite and/or gibbsite are more important authigenic produces formed by weathering under humid tropical conditions (Gardner, 1981; Pye, 1983a). Fine fractions which contain a wide range of mineralogical types often indicate an allochthonous origin (Pye and Tsoar, 1987; Pye, 1987).

The purpose of this paper is: (1) to report the occurrence and nature of clay coatings which occur quite widely in late Quaternary dune sand sequences in the Nebraska Sand Hills; and (2) to discuss the possible origin of the clay and formation and environmental significance of the clay coatings. Additional information about the composition and age structure of the aeolian sediments in central Nebraska is presented in Win- spear and Pye (1994) and Pye et al. (1995).

2. Study area

The Nebraska Sand Hills occupy an area of about 57,000 km2 in central and northern Ne- braska (Fig. 1). At present the dunes and interdune areas are mostly stabilized by grass vegetation (Warren, 1976; Swinehart, 1990). Active sand movement is restricted to 2-3% of the area of the dunefield, largely where overgrazing has led to the formation of blowouts on some dune crests (Muhs and Maat, 1993). The ultimate age of the Sand Hills is unknown, but parts of the dunefield were active at several times in the Late Pleis- tocene and Holocene (Ahlbrandt and Fryberger, 1980; Ahlbrandt et al., 1983). Optical luminescence dating results obtained by S. Stokes and J.B. Swinehart (pers. commun., 1994) suggest that the most recent phase of significant activity oc-

0.21 to.06 ko B.P.

0.43tO.IOkaB.P.

3.61’0.64ka B.FI

5.73f0.71 kaB.P.

Fig. 2. Diagram showing the relative stratigraphic position of the weakly clay-cemented clay layers sampled in inspection pits on the blowout wall of the Merritt Reservoir megabarchan. The directions and angles of dip of the foreset laminae in each inspection pit are also shown. Optical luminescence age estimates are those obtained by S. Stokes and J.B. Swinehart (pers. commun., 1994).

248 N.R. Winspear, K. Pye /Sedimentary Geology 94 (199.5) 245-254

curred only 400-450 years ago. The intervening periods of dune stability were characterized by soil formation, but in many places the palaeosols have been destroyed by later aeolian reworking. The episode of dune stability which has characterized the last few hundred years has resulted in the formation of a surface entisol which is charac- terised by poorly developed horizonation (Ruhe, 1984).

Layers of weakly indurated, clay-rich dune sand occur in a number of different settings within the Sand Hills. Some are clearly related to the modern surface soil, to palaeosol horizons or to interbedded fluvial deposits, but in a significant number of localities no such associations are evident. Ahlbrandt and Fryberger (1980) noted that, throughout the Sand Hills, the sands in the upper part of a dune exposure are commonly characterized by dark, discontinuous wavy bands which represent concentrations of fine material super- imposed on the original stratification. Similar features observed in Brazilian coastal dunes were termed “dissipation structures” by Bigarella (19751. Ahlbrandt and Fryberger (19801 applied this term to the wavy bands present in the Ne- braska Sand Hills, and suggested that the major- ity of bands were formed by concentration of infiltrated fines at the top of a seasonally frozen sand layer. These authors also noted that an immediate source of clay and fine silt is present within the overlying dune sands as clay coatings on sand grains, but did not comment on the possible origin or significance of these coatings.

As part of a wider investigation of the sedimentology and age structure of aeolian sediments in the region (Winspear, 19931, five samples of poorly indurated, clay-coated dune sand were collected from inspection pits dug on the lee (south-southeastern) side of a megabarchan exposed at Merritt Reservoir on the Snake River (Figs. 1 and 2). The consistent southerly or south- easterly orientation of foreset laminae within this dune record the dominance of northerly and northwesterly winds during multiple periods of dune reactivation. Interbedded palaeosols are not well-preserved, but sedimentological discontinu- ities and optical luminescence dating results provide evidence of episodic aeolian accumulation

and reworking during the Holocene (Fig. 2). Clay coatings are present on some sand grains throughout the sequence, but at several levels they are present on virtually all sand grains, giv- ing the sands a slightly greater than average cohe- sion. For purposes of comparison, eight similar samples of weakly indurated dune sand were also collected from inspection pits in the flank of a transverse ridge exposed along the southern fork of the Dismal River, near Dismal River Ranch (Fig. 1).

3. Analytical methods

The surface textures of the dune sand grains from each sample were initially examined with a binocular microscope and representative sub- samples taken for more detailed examination using a JEOL JXA-840 scanning electron microscope (SEMI equipped with a LINK SYSTEMS AN 10085 energy-dispersive X-ray analysis (EDXRA) system. Air-dried grains were mounted on aluminium stubs using double-sided adhesive tape and subsequently sputter-coated with a con- ducting gold layer.

The coarse silt and finer fractions were sepa- rated from splits of the Merritt Reservoir samples by ultrasonic vibration of the sand in 1% sodium hexametaphosphate solution for a period of 30 min. The suspension was then washed though a 31 pm sieve. The material passing through the sieve was subsequently analyzed using a Coulter Multisizer (capable of analyzing particles in the size range 0.7 pm-31 pm>.

Clay-size (< 2 pm> fractions were also extracted from samples of dune sands from the Merritt Reservoir and Dismal River sections after dispersion in sodium hexametaphosphate solution and ultrasonic vibration. After the mixture had been allowed to stand for 3.52 h the clay fraction remaining in suspension was extracted and deposited onto ceramic tiles by vacuum suction, producing a randomly oriented layer. X-ray diffraction (XRD) analysis using Cu Ka radiation was then performed on tiles which were subjected to the following pretreatments: (1) air drying; (2) exposure to warm ethylene glycol vapour for 12 h; and (3) heating in an oven for 1 h at 375°C.

N.R. Winspear, K. Pye/Sedimentary Geology 94 (1995) 245-254 249

Fig. 3. SEM micrograph showing clay ridges forming a superficial boxwork texture on sand grains (sample S363) from the megabarchan at Merritt Reservoir.

Fig. 4. SEM micrograph showing discrete plates of clay adhering tangentially to the surface of a quartz grain in dune sand sample S363. Ridges composed of clay oriented perpendicularly to the grain surface are also evident.

250 N.R. Winspear, K. Pye / Sedimentary Geology 94 (1995) 245-254

Fig. 5. SEM micrograph showing the outline of a former grain contact, marked by ridges of clay oriented perpendicularly to the surface of a K-feldspar grain. The feldspar grain does not show evidence of extensive chemical weathering.

4. Results

SEM examination of both the Merritt Reser- voir and Dismal River Ranch samples revealed that the surfaces of many of the dune sand grains

are characterized by ridges of alumino-silicate material which was interpreted to consist largely of clay (confirmed by XRD). In the Merritt Reservoir section the proportion of grains with clay ridges varies from < 10% to almost lOO%,

I Si

K CO Ti Fe AU

A 5.780 keV 10.9 >

Fig. 6. EDXRA spectrum of a typical clay bridge on the surface of a quartz grain (sample is gold-coated).

N.R. Winspear, K. he /Sedimentury Geology 94 (1995) 245-254 251

depending upon the position within the megabarchan sand sequence (Figs. 2 and 3). Al- though the clay occurs principally as elongate ridges oriented perpendicularly to grain surfaces, in some instances discrete platelets are oriented tangentially to the grain surface (Fig. 4). Clay is conspicuously absent at grain contacts (Fig. 5), and the ridges commonly join together to form a “boxwork” structure. Weathered grains of feldspar, other alumino-silicate minerals or volcanic glass shards, which might provide an intrin- sic source of clay, were not observed during SEM examination. Clay coatings found on detrital quartz and feldspar grains had a similar appear- ance. EDXRA revealed that the clay coatings contain appreciable quantities of Si, Al and Fe, with subsidiary quantities of K and Ca, but Na was not detected (Fig. 6).

The < 31 pm fractions extracted from the five samples of Merritt reservoir dune sand were found to have a mean grain size of 0.97 & 0.04 pm, a dispersion value of 0.87 f 0.04 vrn, and a skewness of 1.08 _t 0.12. The average content of very fine silt plus fine silt (2-15.6 pm) in the samples was 2.70 f 0.89%. Very little material coarser than 15.6 pm was found to be present (ca. O.OOl%), with the average maximum particle size being 16.21 + 3.4 pm.

XRD analyses of < 2 pm fractions from 13 samples from the two sampled sections showed that the principal crystalline components present in the clay coatings are smectite (72 f 3%), illite plus mixed-layer illite-smectite (22 + 3%) and kaolinite-chlorite (6 f 1%). These findings are consistent with those previously reported by Ahlbrandt and Fryberger (1980, pp. 14-15).

5. Discussion

The morphology of the clay ridges suggests that the clay accumulated post-depositionally in the vadose zone as a result of drying of clay-rich suspensions present as menisci around grain contacts. The dominance of smectite and illite- smectite in the clay fraction is consistent with clay authigenesis under semi-arid to sub-humid cli-

matic conditions, but the absence of obviously weathered feldspars or other detrital grains within the sands makes it unlikely that much of the clay has formed by in-situ post-depositional weathering of silicate minerals. The low proportion of quartz silt in the fine fraction also indicates that detrital quartz grains have not been significantly affected by post-depositional weathering and grain fragmentation (cf. Pye, 1983b), and suggests that addition of allochthonous quartz silt formed by aeolian abrasion (cf. Whalley et al., 1987) within the Sand Hills has not been important.

The topographic setting and vertical distribution of the dune sands in which the clay coatings occur suggests that infiltration of fines by hillwash or river floods is unlikely. It is also unlikely that the dispersed clay coatings owe their origin to breakdown of clay pellets introduced into the dunefield by fluvial processes or aeolian saltation (cf. Bowler, 1973; Bowler and Magee, 1978; Loch, 1991). Petrographic examination of thin sections of the dune sand revealed that some clay pellets of possible pedogenic or biogenic origin are present (Winspear and Pye, 19951, but these rarely show evidence of extensive breakdown. Although shallow lake basins are numerous in the western part of the dunefield (Loope and Swinehart, 19921, there is so far no evidence (e.g. in the form of marginal clay or gypsum lunettes) that these were ever sites of extensive saline mudflats which could have acted as major sources of clay pellets.

Previous work in the northern Negev Desert of Israel has shown that deposition of aeolian dust onto the surfaces of stabilised sand dunes and subsequent infiltration by rainwater results in the progressive build-up of fines in the near-surface sand layers (Tsoar and Moller, 1986; Pye and Tsoar, 1987). When the dust is derived from a proximal source, a wide range of particle sizes and mineralogies is typically present in the near- surface sand layers, although only finer particles (composed principally of clay minerals) are translocated to greater depths. By contrast, aeolian dust deposited in distal areas, several hun- dreds of kilometres from the dust source, is commonly dominated by clay-size material (Pye, 1987; Pye and Tsoar, 1987). It is therefore possible that much of the clay present as coatings on the

252 N. R. Winspear, K. Pye /Sedimentary Geology 94 (1995) 245-254

Nebraska Sand Hills dune sand grains owes its origin to infiltrated aeolian dust. Its fine-grained nature, indicated by Coulter-Multisizer analysis of samples from the Merritt Reservoir section, could indicate derivation from distal sources or simply the fact that only the deeper sand layers of the former dune accumulations have been preserved at the sites sampled. The climate of the central United States is currently dominated by westerly atmospheric circulation, resulting in fre- quent incursions of Pacific air masses which have been dried by passage over the Rocky Mountains (Baker et al., 1992). Most of the precipitation in this region, which is responsible for supporting the vegetation cover of the Nebraska Sand Hills, is derived from maritime air masses which move northwards from the Gulf of Mexico during summer (Wilhite and Hubbard, 1990). During periods of dune stability, allochthonous dust deposited in the Sand Hills region could have been derived from a combination of sources in Montana, North and South Dakota, the southern prairie provinces of Canada (cf. van Heuklon, 1977), and the southern High Plains. However, a contribution of dust from proximal sources cannot be ruled out, since deposits of dated Holocene loess (Bignell Loess) up to several metres thick occur in several parts of Nebraska and adjoining states (Maat and Johnson, 1994; Pye et al., 1995). Smectite and illite-smectite are the dominant minerals present in the clay fractions of both Holocene and Late Pleistocene (Peoria) loess in these areas (Win- spear and Pye, 1995).

The depth to which dust is infiltrated within dune sand is governed partly by the amount, frequency and intensity of rainfall and partly by the grain size distribution and pore structure of the sand. In the northern Negev, where the mean annual rainfall of approximately loo-150 mm is concentrated in a small number of extreme events, the maximum wetting depth in dune sand does not normally exceed 100 cm (Tsoar and Zohar, 1985). In a more humid climate the greater frequency of rainstorms is likely to have a cumula- tive effect, causing penetration of clay to greater depths. In the Nebraska Sand Hills, where present mean annual rainfall increases from ca. 420 mm per annum in the west to ca. 570 mm per

annum in the east, with significant precipitation both in the summer (as thunderstorms) and in the winter (much as snow), the depth of clay penetration could be several metres.

Optical dating results (S. Stokes and J.B. Swinehart, pers. commun., 1994) suggest that formation of the Merritt Reservoir megabarchan involved multiple periods of aeolian reactivation, deflation and sedimentation in the Holocene. An alternating sequence of palaeosols and interbedded sands is not exposed, but the more-indurated clay-coated sand layers may represent near- surface horizons which formed by infiltration of largely allochthonous clay during periods of dune stability. The existence of weakly clay-cemented sands in all five inspection pits on the lee side of the dune indicates that the bulk of the sands were not reworked after clay infiltration.

6. Conclusions

On the basis of field observations and laboratory analytical data, it is concluded that the clay adhering to dune sand grains in the Merritt Reservoir and Dismal River sections accumulated as a result of infiltration of allochthonous clay which was introduced to the sand surface, mainly during periods of dune stability. The very fine grain size of the infiltrated material is consistent with the dust being derived mainly from distal sources, although the upper parts of the former dunes are not preserved and the original pres- ence of coarser infiltrated dust derived from proximal sources cannot be ruled out. Infiltration of the dust into the sand would have been accom- plished by rainwater, seasonal snowmelt and lo- cally by drainage of surface runoff following periods when the infiltration capacity of the sands was temporarily exceeded. The formation of distinct sub-surface bands enriched in fines could be related to the position of former wetting fronts, in some instances related to seasonally frozen sand layers as suggested by Fryberger and Ahlbrandt (1980), but in other cases related to textural inhomogeneities within the parent sand which gave rise to variations in porosity and per- meability. We suggest that the term “infiltration

N.R. Winspear, R Pye /Sedimentary Geology 94 (1995) 245-254 253

structures” is a more appropriate generic term for these features than “dissipation structures”.

Preservation of the boxwork clay texture on the dune sand grains indicates that extensive reworking of the dune sands in the Merritt Reser- voir megabarchan has not occurred since infiltration of the clay. The absence of palaeosols in the sequence suggests that some surface reactivation and erosion has occurred, but the underlying clay-coated sands have been largely preserved, perhaps partly because of their semi-cohesive character and relative resistance to aeolian deflation.

Acknowledgements

This work forms part of the PhD research undertaken by N.R. Winspear at the University of Reading. Financial support was provided by a grant from the University of Reading Research Endowment Fund and a Leverhulme Trust Fel- lowship awarded to K. Pye. S. Stokes and J.B. Swinehart provided valuable cooperation during sampling at Merritt Reservoir and gave permis- sion to cite unpublished optical dating results. Useful comments on an earlier version of the manuscript were provided by D.B. Loope and D.H. Krinsley. Drafting was undertaken by A. Cross. University of Reading PRIS Contribution No. 350.

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