Desert loess in Ras Al Khaimah, United Arab Emirates

Journal of Arid Environments (2000) 46: 123–135doi:10.1006/jare.2000.0663, available online at http://www.idealibrary.com on

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Desert loess in Ras Al Khaimah,United Arab Emirates

A.S. Goudie*, A.G. Parker*, P.A. Bull*, K. White- & A. Al-Farraj?

*School of Geography, University of Oxford, Mansfield Road,Oxford, OX1 3TB, U.K.

?Department of Geography, University of Reading, Whiteknights,Reading, RG6 6AB, U.K.

@Department of Geography, United Arab Emirates University, Al-Ain,P.O. Box 16722, United Arab Emirates

(Received 2 March 1999, accepted 25 May 2000)

Wind-blown silt deposits, generally termed loess, have been described froma range of desert areas. This paper describes the location, character and originof a newly discovered coarse loess deposit in Ras Al Khaimah, United ArabEmirates.

( 2000 Academic Press

Keywords: loess; deserts; United Arab Emirates

Introduction

Loess is generally regarded as a largely unconsolidated, non-stratified sediment,consisting predominantly of silt-sized materials, and deposited primarily by thewind. Very large deposits occur in China, Central Asia, the Mississippi Valley, andelsewhere.

There has long been debate concerning the existence of desert or peri-desert loess(Smalley & Vita-Finzi, 1968; Chao & Zing, 1982; Tsoar & Pye, 1987). The origin ofmany of the world’s largest loess deposits has been attributed to glacial forelandenvironments as a result of the deflation by wind of glacially ground material. However,it is also evident that deflational activity and dust storms are prevalent in arid areas, andthere are a number of sources of silty material including alluvium, inland playas, coastalsabkhas, winnowed sand seas, and rock flour produced by salt weathering. Moreover, inrecent years a number of loess deposits have been identified in arid regions including theMatmata area of Tunisia (CoudeH -Gaussen et al., 1982; Dearing et al., 1996), the Namib(BluK mel, 1982), northern Nigeria (McTainsh, 1987), eastern Afghanistan (Pias, 1971),the Potwar Plateau of Pakistan (Rendell, 1984), the Negev Desert (Yaalon & Dan1974), Syria (RoK sner, 1989), Iran (Lateef, 1988), and the Island of Bahrain (Door-nkamp et al., 1980). It has also been postulated that loess exists in mainland Arabia, bothin Yemen (Nettleton & Chadwick, 1996; Coque-Delhuille & Gentelle, 1998) and inSaudi Arabia (P. Vincent, pers. comm.).

The purpose of this paper is to record and describe the presence of a loess deposit(Fig. 1) in Ras Al Khaimah in the United Arab Emirates (Fig. 2).

140-1963/00/100123#13 $35.00/0 ( 2000 Academic Press

Figure 1. A loess ramp against the Oman Mountains.

124 A. S. GOUDIE ET AL.

Location

Ras-Al Khaimah is located on the Arabian Gulf and to the west of the Oman Mountains(Fig. 3). The mountains rise to an altitude of over 2000 m and are composed, for themost part, of limestones and dolomites of the Musandam Group (Lower Jurassic toLower Cretaceous) and of the Permo-Triassic Elphinstone Group, Ru’us Al JibalGroup, and the Ramaq Formation. A tectonic window in Wadi Haqil, to the north-eastof Ras Al Khaimah, exposes cherts, shales, limestones and volcanics of the Hawasinaseries, which are thought to be of late Campanian to Maastrichtian age. Between themountains and the Arabian Gulf are some major fans of probable Pleistocene andHolocene age. These have been laid down by rivers, such as the Wadi Naquab, Wadi AlBih and Wadi Haqil, that arise in the Oman Mountains. The largest alluvial fan is that ofthe Wadi Al Bih. Its maximum dimensions are 8)0 km from east to west and 18 km fromnorth to south. Between the fans and the coastline are a series of bars, spits and coastalmarshes (sabkhas) (Goudie et al., 2000).

Climate

Climate data are available for Ras Al Khaimah airport. The mean annual rainfall for theperiod 1975/76 to 1991/92 was 119)7 mm, but there is considerable variability from yearto year. In 1983/4 the annual rainfall was only 15 mm, while in 1981/2 it was 257)2 mm.Rainfall tends to fall in the winter months. Temperatures in the area are high. Thehottest month is July (with a monthly mean maximum temperature of 42)53C, a monthlymean temperature of 28)83C, and a monthly mean minimum temperature of 11)73C).The average relative humidity peaks in January (with a mean maximum of 89%, a meanof 70%, and a mean minimum of 22%). The wind regime is complex. Just under 52% ofsand-moving winds blow from the west and north-west, but just under 28% blow fromthe south-east.

Figure 2. The location of Ras Al Khaimah.

DESERT LOESS IN RAS AL KHAIMAH 125

The deposit

A deposit of silty and fine sand material, interspersed with angular limestone slope debrisand forming a ramp against the mountain front, was identified over a distance ofapproximately 3)0 km (Fig. 3). Its GPS location is 253 47)03@ N and 563 02)14@ E. Itattains a thickness of around 4 m. Its topographic position and morphology indicatesthat it is not an alluvial deposit. Two quarry sections were sampled, with seven samplesbeing taken from one face and eight from another (Fig. 4). Section 1 occurred on a rampwhich lies in a westward facing position between the large alluvial fan of the Wadi Bihand the limestone mountains. Section 2 occurred in a small embayment to the side of theWadi Al Bih mouth and is located on the surface of a small tributary fan.

The colour of the deposit is consistently 10 YR 7/3 (very pale brown). It has a highcarbonate content, as determined by digestion in 10% hydrochloric acid. The meanvalue for the 15 samples was 58)5% (Table 1).

The granulometry of the samples was determined by CILAS Laser Granulometer andwas performed on both a non-carbonate-free and a carbonate-free basis. The data arepresented in Table 2. All material coarser than 355 lm was removed by sieving.

The average median size of the material prior to removal of carbonate was 77)95 lm,the amount of material finer than 2 lm was 4)3%, finer than 63 lm was 36%, and finer

Figure 3. The geomorphological setting of Ras Al Khaimah and the loess deposit.


than 125 lm was 85%. The material is therefore composed dominantly of silt and finesand.

The average median size of the material after removal of carbonate fell to 49 lm. Theamount of material finer than 2 lm was 3%, finer than 63 lm was 61%, and finer than125 lm was 95%.

Although the carbonate content of the deposit is high, it is not exceptional in terms ofdesert loess. The Bahrain loess (Doornkamp et al., 1980, Table 10)2) has a combineddolomite and calcite content of 86%, while the loess of southern Spain has a carbonate contentthat varies from 35% in Tarragona to up to 70% in Granada (Brunnacker & Lozek, 1969).

The relative coarseness of the deposit is also not exceptional in terms of desert loess.The median grain size of the Matmata loess deposit in Tunisia, for example, rangesbetween 57 and 66 lm (CoudeH -Gaussen et al., 1982, Fig. 3).

Surface textures

A small amount of material from each sample that was collected was dusted ontopolished aluminium stubs prior to gold coating and viewing by a Cambridge stereoscan

Figure 4. Section logs.


equipped with EDAX for mineralogical identification. The samples were viewed atmagnifications up to 10,000 times and all grains analysed were checked by EDAX tomake sure that they were indeed quartz. Despite the very high mean carbonate contentof the samples (58)5%), most of the silt and fine, sand-sized particles were quartz; fineclay-sized and occasional grains were calcium carbonate.

Up to 100 quartz silt grains were analysed per sample, although the uniformity ofquartz type usually meant that only 30 grains were needed to achieve a true representa-tion of their character. Grains were selected in a random manner, using a nearestneighbour method (Culver et al., 1980).

Grains were analysed by identifying a range of surface textures (Bull, 1981; Higgs,1982). Given the small size of the grains, large-scale mechanical breakage phenomenawere not identified. Particular attention was given to smaller scale breakage blocks,conchoidal fractures, late grain breakage, shape and roundness, gels, euhedrals, chem-ical precipitation and solution features. Once features were identified individual grainswere categorized according to their assemblage. That is to say, individual grains wereidentified as ‘type grains’. Thus, a sample is represented, not only as a matrix of grainsurface texture percentages for a count of grains, but simply as a mixture of two or threegrain types which characterize the whole sample analysed (Table 3).

The analytical results for the 15 samples collected (Table 3) reinforce the overallconcept that quartz-rock silt fragments range in shape from blocky to platey; and from

Table 1. Carbonate content of loess and of alluvial deposits

Sample No. Carbonate content %

Site 1 1 57)342 56)593 58)474 55)295 56)346 55)797 52)87

Site 2 8 59)669 60)23

10 67)5511 59)6112 62)4613 62)8014 52)0415 60)54X 58)50

Alluvium 16 64)5717 59)7118 53)86X 59)38


spherical to bladed, reflecting the provenance of the quartz material rather more than thetransportational histories of the grains (Bull, 1978).

Inevitably, in such a salt-rich environment, post-depositional modification effectsoccur but they are limited to chemical edge-rounding and, more spectacularly, tocomplete grain breakage (Fig. 5) caused by salt growth within quartz grain cavities (seeGoudie et al., 1979).

The two sections sampled (Fig. 6) were analysed for quartz grain surface textureassemblage and type, summarized in Table 3. Site 1, comprising seven samples,exhibited only two quartz grain types. Type A (Table 3 and Fig. 7) characterizes grainsof sub-angular, high-relief form with blocky breakage and normally heavy post-depo-sitional precipitation of quartz gels and cryptocrystalline coatings of the type stimulatedby highly saline environments (Magee et al., 1998a, 1998b).

Type A grains exhibit no mechanical edge abrasion and no subaqueous transportationmodification by river or sea. They imply near-distance, low energy or gravity-feddeposition with subsequent post-depositional saline-induced chemical modification.Samples 1–5 (Site 1, Fig. 5) exhibit a preponderance of this grain type.

Type B grains (Fig. 8) are typified by well-rounded/sub-rounded, low relief grainswith angular point indentors and upturned plates, modified by post-depositional, salt-induced gel precipitation and solution. This type is composed of classically mature,wind-blown, often desert-derived (or at least long-distance aeolian transported) grains(Krinsley & Doornkamp, 1973). Associated with this grain type are adhering particles offlat and shard-shaped quartz silt and clay of relatively fresh appearance (Fig. 9). Thesegrains appear to have suffered no depositional modification, and can be bestexplained as fragments of quartz grains broken by salt weathering (Goudie et al., 1979).They are either in situ or have been wind-winnowed over short distances at low enoughenergy levels to avoid transportational modification of the grain surfaces.

Table 2. Loess granulometry (carbonate free) on (355 km (a) and (b) loessgranulometry (including carbonate) on (355 km

Median(a) Sample size lm (2 lm (63 lm (125 lm

Site 1 1 28)68 4)5 77)6 99)12 43)67 3)6 64)9 97)93 29)88 4)0 73)7 98)84 38)30 3)9 69)1 98)35 38)08 3)2 68)9 97)96 46)50 2)7 62)0 95)27 34)90 3)4 76)3 99)0

Site 2 8 44)12 2)6 61)9 93)39 66)26 2)7 62)0 95)2

10 63)22 2)5 49)8 92)311 46)50 2)7 62)0 95)212 73)69 1)9 38)8 88)213 65)67 2)2 47)4 89)614 66)66 2)1 46)6 87)815 55)07 2)4 55)7 91)7X 49)41 2)96 61)11 94)63

Median(b) Sample size lm (2 lm (63 lm (125 lm

Site 1 1 60)34 6)6 52)8 97)62 59)19 7)7 54)2 98)23 66)71 5)7 44)6 96)54 62)50 5)5 50)5 97)05 68)29 4)1 42)7 94)66 65)34 4)0 46)8 96)37 61)33 4)8 51)7 97)4

Site 2 8 89)53 3)2 25)3 75)59 81)91 4)2 28)3 85)1

10 86)95 3)3 23)5 82)611 78)59 5)3 32)0 87)212 94)41 2)7 21)0 72)313 94)30 2)9 24)3 69)814 103)59 2)5 20)1 63)515 96)28 2)3 21)9 69)2X 77)95 4)32 35)98 85)32


Whilst the fragmented quartz particles can be found in appreciable numbers through-out Section 1 there are significantly large amounts of this material (Table 3) in samples5, 6 and 7. (Such a distribution through the section could also be accounted for bydown-section migration and winnowing of quartz fines).

Associated minerals found through Section 1 (Table 3) include mica (often well-rounded by both chemical and mechanical modification) orthoclase feldspars, chert,gypsum, calcite and dolomite. No discernible down-section pattern emerges from their

Table 3. Quartz silt and sand grain characteristics from sections 1 and sections 2. Type A grains are near-source, sub-angular, high relief withblock breakage and post-depositional precipitation. Type B grains are mature aeolian grains. Type C grains are well-rounded, high relief

sub-aqueous grains

Type A Type B Type CSection 1 No. of grains No. of % of No. of % of No. of % of Other minerals associatedSample No. Analysed grains Total grains Total grains Total (sand-sized)

1 76 48 63 28 37 0 02 50 36 72 14 28 0 0 Mica, chert3 87 50 57 37 43 0 0 Feldspar, chert4 80 80 100 0 0 0 0 Gypsum, calcite, chert5 78 60 77 18 23 0 0 Mica, chert6 125 33 26 92 74 0 0 Chert, feldspar, gypsum7 129 50 39 79 61 0 0 Chert, feldspar, gypsum

Section 2sample No.8 90 70 78 20 22 0 0 Silica cement coating

to grains9 55 39 71 fines* 0 16 29

10 114 70 61 17 15 27 2411 66 0 0 0 0 66 100 Feldspar, rutile, tourmaline,

garnet12 58 0 0 0 0 58 10013 61 0 0 fines* 0 61 100 Garnet14 70 0 0 fines* 0 70 10015 76 0 0 fines* 0 76 100 Garnet

*Fines represent very fine silt and clay quartz plates and shards.

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Figure 5. Type A grain from Section 2 sharing grain breakage associated with Sample 9 highsaline conditions (Fig. 4).


occurrence but it does approximate to the rock types identified in the nearer regions ofthe Oman Mountains.

Section 2 (Fig. 4) comprised eight samples which were analysed for their quartz grainsurface texture assemblages and grain type. The grains from these samples commonlycontained Type A grains (as noted in Section 1) in the upper three layers sampled(Table 3, samples 8, 9 and 10), but these were absent in the lower samples (Table 3,11–15). Type B grains were represented only as silt fragments and shards rather than asthe well-travelled aeolian grains found in Section 1 and were found in most samples(implying sand-sized, grain fragmentation in response to salt weathering, rather thanloess-like rock flour).

Figure 6. A model of loess formation.

Figure 7. Type A grains from Sample 4 Section 1 (Fig. 4). Sub-angular/sub-rounded quartzgrains with high relief, blocky breakage, and quartz with salt precipitation.


In this section (Table 3), however, there was also a third type of grain. This TypeC grain was characterized as a well-rounded to sub-rounded texture of relatively highrelief, showing late grain breakage and mechanical surface texture grain modificationcaused by point indentors, indicative of sub-aqueous transportation. Present also in thesamples, where Type C grains predominate, are relatively fresh feldspars, rutile, tourmaline,

Figure 8. Type B grains from Section 1 Sample 6 (Fig. 4) showing well-rounded quartz withshallow dish-shaped concavities and surface frosting typical of an upturned plate assemblage.

Figure 9. Quartz shard, fresh in appearance, from Section 1, Sample 6 (Fig. 4).


garnet, metamorphic quartz and grains which contain fragments of cement on theirgrain surfaces. Such assemblages can indicate discrete source rocks rather than amelange of sediment provenances (Bull, 1981).

Clearly the palaeohistories of the two sections are very different. Section 1 con-tains near-source, angular quartz, mixed with long-distance, mature aeolian grains thatexhibit classic surface texture assemblages associated with wind-blown sands and silts(Krinsley & Doornkamp, 1973). For comparison, sands taken from Awafi in the sandsea to the south of Ras Al Khaimah were analysed in order to compare their grain surfacetextures with Type B grains found in Section 1. Photomicrographs (Fig. 8 and 10) showthe similarity of both quartz types which may well suggest a provenance for this TypeB material found in Section 1.

Flat, platey quartz particles are also present in Section 1 and can be interpreted aseither mechanical abrasion debris [desert loess (Whalley et al., 1982) or salt-fracturedquartz debris (Goudie et al., 1979)].

Section 2 contains, in contrast, near-source angular quartz only in its upper layers butthis is superseded in abundance by rounded quartz, mechanically modified in sub-aqueous environments. Interspersed within these quartz types are the fine, platey andshard-like quartz fragments.

Sources

The relatively coarse median size of the deposit both before and after carbonate removalsuggests that, if it were carried in aeolian suspension, it did not travel far. It isconsiderably coarser than most true loess deposits recorded in the literature.

The high carbonate content of the material could be derived either from the coastalzone or from the surface of the Wadi Al Bih fan (Fig. 5). The alluvial silts, being derivedfrom the limestones and dolomites of the Oman Mountains, are rich in carbonate, andsalt weathering of limestone clasts on the fan surface could also provide carbonate-rich‘rock flour’ for deflation. Three samples of alluvial silt collected in the area had a mean

Figure 10. Quartz sand from Awafi in the sea to the south of Ras Al Khaimah. Note comparisonwith Fig. 8.


carbonate content of 59%, which is almost exactly the same as the mean carbonatecontent of the loess ramp material (58)5%).

Buildings at the salty distal end of the fan, bordering on the sabkha, suffer fromsevere salt attack, and rock blocks placed on the surface near Daya for 17 monthsshowed a rapid rate of breakdown. Eleven samples of York Stone (a fine-grainedsandstone) lost on average 15% of their weight, while 11 samples of Bath Stone (anoolitic limestone) lost on average 10% of their weight. This could account for some ofthe observed textures under the scanning electron microscope. It is therefore likely thatthe dominant sediment-moving winds (which came from the west and north-west)deposited the material against the westward facing mountain front after crossing thealluvial fan.

Conclusion

This paper provides the first description of a small deposit of sandy loess from theeastern end of the Arabian Desert and thus adds to a growing catalogue of desert loessdeposits from the Middle East and other arid zones. The deposit is relatively coarse incomparison with most classic examples of loess, and also has a high carbonate content.Scanning electron microscopy indicates that salt weathering may have played a role inproviding a source for the loess, much of which appears to have been derived as a resultof deflation from a large alluvial fan surface and a neighbouring coastal plain. Its highcarbonate content and its coarseness result from the fact that it is of essentially localderivation.

We are grateful to the National Museum of Ras Al Khaimah, the University of Al-Ain, The BritishCouncil, and the Faculty Board of Anthropology and Geography (University of Oxford) for theirsupport.


References

BluK mel, W.D. (1982). Calcretes in Namibia and SE Spain. Relations to substratum, soil formationand geomorphic factors. Catena Supplement, 1: 67–82.

Brunnacker, K. & Lozek, V. (1969). LoK ss-Vorkommen in SuK dostspanien. Zeitschift fu( r Geomor-phologie NF, 13: 297–316.

Bull, P.A., (1978). Observations on small sedimentary quartz particles analysed by SEM. In:Johari, O. (Ed.), Scanning Electron Microscopy, Vol. 1, pp. 821–828. Illinois: SEM Inc.

Bull, P.A., (1981). ‘Environmental reconstruction by electron microscopy’ Progress in PhysicalGeography, 6: 368–397.

Chao, S.C. & Zing, Y. (1982). Origin and development of the Shamo and the Gobi of China.Striae, 17: 79–91.

Coque-Delhuille, B.C. & Gentelle, P.P. (1998). Aeolian dust and superficial formations in the aridpart of Yemen. In Alsharhan, A., Glennie, K.W., Whittle, G.L., & Kendall, C.G. St. C. (Eds),Quaternary Deserts and Climate Change. Balkema: Rotterdam, 199–208.

CoudeH -Gaussen, G., Mosser, C., Rognon, P. & Tourenq, J. (1982). Une accumulation de loessdu PleH istocene supeH rieur dans la Sud-Tunisien: la coupe de TeH chine. Bulletin Socie& te& Ge&ologiquede France, 24: 283–292.

Culver, S.J., Williams, H.R. & Bull, P.A. (1980). Late Precambrian glacial deposits from theRokehole Fold Belt, Sierra Leone. Palaeogeography, Palaeoclimatology, Palaeoecology, 30: 65–81.

Dearing, J., Livingstone, I. & Zhon, L.P. (1996). A late Quaternary magnetic record of Tunisianloess and its climatic significance. Geophysical Research Letters, 23: 189–192.

Doornkamp, J.C., Brunsden, D. & Jones, D.K.C. (Eds), (1980). Geology, Geomorphology andPedology of Bahrain. Norwich: Geo Abstracts. 443 pp.

Goudie, A.S., Cooke, R.U. & Doornkamp, J.C. (1979). ‘The formation of silt from quartz dunesand by salt weathering processes in deserts’. Journal of Arid Environments, 2: 105–112.

Goudie A.S., Parker A.G. & Al-Farraj A. (2000). Coastal change in Ras Al Khaimah (UnitedArab Emirates): a cartographic analysis. Geographical Journal, 166: 14–25.

Higgs, R. (1982). ‘Quartz-grain surface features of Mesozoico-Cenozoic sands from the Labradorand Western Greenland continental margins’. Journal of Sedimentary Petrology, 49: 2, 599–610.

Krinsley, D.H. & Doornkamp, J.C. (1973). Atlas of Quartz Sand Surface Textures. Cambridge:Cambridge University Press. 91pp.

Lateef, A.S.A. (1988). Distribution, provenance, age and palaeoclimatic record of the loess inCentral North Iran. In Eden, D.N., & Furkert, R.J. (Eds), Loess. Rotterdam: Balkema. pp.93–101.

Magee, A.W., Bull, P.A. & Goudie, A.S. (1988a). Chemical textures on quartz grains: anexperimental approach using salts. Earth Surface Processes and Landforms, 13: 665–676.

Magee, A.W., Bull, P.A. & Goudie, A.S. (1988b). Before rock decay: Chemical weathering ofconstituent grains by salts. In Marinos, P. & Koukis, G. (Eds), Engineering Geology of AncientWorks, Monuments and Historical Sites. Balkema, Rotterdam: 779–786.

McTainsh, G. (1987). Desert loess in northern Nigeria. Zeitschift fu( r Geomorphologie, NF 31:145–165.

Nettleton, W.D. & Chadwick, O.A. (1996). Late Quaternary, redeposited loess-soil developmen-tal sequences, South Yeman. Geoderma, 70: 21–36.

Pias, J. (1971). Les loess en Afghanistan Oriental et leurs peH dogeneses successives au QuaternairereH cent. Comples Rendus Academie de Sciences, Paris, 272 (D): 1602–1605.

Rendell, H.M. (1984). New perspectives on the Pleistocene and Holocene sequences of thePotwar plateau and adjacent areas of Northern Pakistan. In K.J. Miller (Ed.), The InternationalKarakoram Project, Cambridge: Cambridge University Press, Cambridge, Vol. 1, 389–398.

RoK sner, U. (1989). ‘‘LoK ss’’ am Rande der Wustensteppe? Erdkunde, 43 (4): 233–242.Smalley, I.J. & Vita-Finz, C. (1968). The formation of fine particles in sandy deserts and the

nature of ‘desert’ loess. Journal of Sedimentary Petrology, 38: 766–774.Tsoar, H. & Pye, K. (1987). Dust transport and the question of desert loess formation. Sedimentol-

ogy, 34: 139–153.Whalley, W.B., Marshall, J.R. & Smith, B.J. (1982). ‘Origin of desert loess from experimental

observation’ Nature, 300: 433–435.Yaalon, D. & Dan, J. (1974). Accumulation and distribution of loess-derived deposits in the

semi-desert and desert fringe areas of Israel. Zeitschift fu( r Geomorphologie, Supplementband, 20:91–105.

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Desert loess in Ras Al Khaimah, United Arab Emirates