Journal of Arid Environments (2001) 48: 475}499
doi:10.1006/jare.2000.0776, available online at http://www.idealibrary.com on
Textural attributes, mineralogy and provenance of
sand dune fields in the greater Al Ain area,
United Arab Emirates
Mohamed M. Abu-Zeid,* Ashraf R. Baghdady* & Hassan A. El-Etr-
*Department of Geology, Faculty of Science, United Arab Emirates
University, P.O. Box 17551, Al Ain, UAE
?National Authority for Remote Sensing and Space Sciences, Cairo, Egypt
(Received 28 January 2000, accepted 6 December 2000)
Sand dunes and interdune sediments around Al Ain city have markedly high
carbonate contents which increase towards Jabal Hafit mountain and the
Arabian Gulf coast. The dunes are composed predominantly of well-sorted
fine sands, consisting of unstrained quartz and carbonate grains together with
minor proportions of chert and feldspars. Scanning electron microscope
(SEM) examination reveals that quartz grains display various mechanicallyand chemically-formed surface textures. The heavy minerals are dominated by
opaques and pyroxenes with minor tourmaline, garnet, rutile, epidote, monazite, zircon, hornblende and staurolite.
Interdune sediments consist of fine and very fine, poorly- to extremely
poorly-sorted sands together with small concentrations of gravel, silt and clay.
The light sand fractions compositionally resemble those of dunes but contain,
in addition, gypsum, anhydrite, halite and celestite. Percentages of pyroxenes
are higher whereas those of zircon, tourmaline and rutile are lower than in
dunes. X-ray diffractometry reveals that the clay fractions consist solely of
palygorskite. Generally, interdune sediments are much less mature texturally
and mineralogically than dune sands; the maturity of both types of sediments
decreases toward the NE of the study area.
Sand dunes in the various districts of the greater Al Ain area are genetically
related. Also, there is a partial genetic relationship between the dunes and
interdune sediments; both are mainly multicyclic. Their major parts were
brought mainly by the dominant north-west (El Shamal) winds from older
dune fields in other localities in U.A.E., Qatar and El-Rub El-Khali. Local
contributions to the dune fields from Jabal Hafit mountain, the Oman Mountains and the calcareous coast of the Arabian Gulf were, in cases, significant.
Also, authigenesis by groundwater under highly evaporitic conditions played
a major role in the formation of interdune sediments through the genesis of the
clay and some nonclay minerals.
2001 Academic Press
Keywords: Arabian Gulf; United Arab Emirates; arid regions; sand dunes;
interdune sediments; aeolian processes; authigenesis
Introduction
The United Arab Emirates is in the eastern section of the Arabian Peninsula (Fig. 1)
and has three main landform systems; aeolian, mountain-bahada and coastal-marine
0140}1963/01/080475#25 $35.00/0
2001 Academic Press
476
M. M. ABU-ZEID ET AL.
Figure 1. Simplified geologic map of the study area showing the major landform units and
sampling sites.
(Embabi & El-Sharkawy, 1989; National Atlas of U.A.E., 1993). The aeolian system
covers about 90% of the total surface area of the country, and consists mainly of sand
dunes and intervening interdune low-lying belts. The dune types are linear, barchan,
barchanoid, transverse and star (Embabi, 1991).
The greater Al Ain area lies within a hot desert zone characterized by meagre and
variable rainfall with an annual mean of 100 mm. It is commonly localized and falling in
only a few days each year. Shade temperature is frequently above 453C. The mean
annual evaporation is 3)6 mm. Humidity ranges from 45}75%. Dessicating northwesterly winds prevail with a speed varying between 10 and 13 km h\1 (cf. Cox 1985;
Alshamsei, 1993; Garamoon, 1996).
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
477
The greater Al Ain area is subdivided into six geomorphic units; namely: mountains,
gravel plains, drainage basins, inland sabkhas, sand dunes and interdune areas
(Garamoon, op. cit.) (Fig. 1). Mountains are those of Jabal Hafit, Jabal Malaqet and
Jabal Mundassah (Fig. 2(a)). Jabal Hafit is a Tertiary anticlinal structure reaching an
elevation of about 1160m a.s.l. The rocks are composed of carbonates, marls and clays
ranging in age from Early Eocene to Miocene (Cherif & El-Deeb, 1984; Hamdan
& Bahr, 1992). Jabal Malaqet and Jabal Mundassah are parts of the Oman Mountains
and are located approximately 17 km north-east of Jabal Hafit. Their rocks are composed of serpentinized peridotites (in the core), conglomerates and carbonates of Late
Cretaceous age, overlain by marls and carbonates of Paleocene to Late Eocene age
(Warrak, 1986; and 1987; Hamdan & El-Deeb, 1990). Two gravel plains terminate the
eastern part of the greater Al Ain area fringing the Oman Mountains and Jabal Hafit.
They are loosely-cemented carbonates, sands and silts. Also, two drainage basins are
present belonging to these Jabals (Alshamsei, 1993). Inland sabkhas are developed on
the lower parts of gravel and sand plains. They are low lands occupied by evaporitic
sediments and are sites of groundwater discharge.
Sand dune fields dominate the northern, western and southern parts of the greater
Al Ain area and are responsible for many environmental problems such as desertification and air-pollution (Hindy & Baghdady, 1996). Embabi (1991) emphasized that
branched-linear and star dunes are the dominant types. Branched-linear dunes
occupy the northern and western parts of the area, whereas star dunes are present
mainly in its south-eastern part. Hunting (1979) subdivided the dune areas into:
regions of active sand transport and accumulation, regions of impeded sand transport,
dunes stabilized by vegetation, ablation hollows and flats, interdune areas and inland
sabkhas.
The objectives of this study are: (1) to recognize the various sand dune fields in the
greater Al Ain area; (2) to study the textural and mineral characteristics of the dune and
interdune sediments; and (3) to determine their provenance and sedimentary history.
Materials and methods
Topographic sheets of various scales and geologic maps of Hunting (1979), Warrak
(1987) and Whittle & Alsharhan (1994) were used in combination with Landsat TM
and SPOT images to study the geology of the greater Al Ain area and the distribution
of the various sand dune fields. Sampling of the dune fields was carried out along
the roads: Al Ain } Al Wagan, Al Ain } Abu Dhabi, Bu Samra } Sweihan } Al Hayer,
Al Ain } Dubai, and Al Hayer } Al Madam (Fig. 1). Thirty test samples were
first collected from the crests, and lee and stoss sides of ten sand dunes in the five
districts and subjected to textural and mineral investigations. The results obtained
revealed that samples taken from different locations within a given dune have
similar distributions of size grades and mineral composition. Therefore, 66 crest
samples were collected from the dunes along the above-mentioned roads at an interval
of &5 km. The surface layer (2}3 cm) of the crest was first removed and a core
sample (30 cm long) was taken. Thirty-one samples were collected from the interdune
sediments. They were taken from a surface layer which is about 0)15 m2 and 5 cm
thick.
Preliminary examination of the samples revealed that they are composed of detrital
carbonate and noncarbonate grains. In addition, the interdune samples were found to
contain authigenic carbonates and gypsum. Determination of the carbonate content was
carried out for all the dune and interdune samples using the method of Ireland (1971).
A portion of the sample weighing 100g was treated with warm HCl (10%), washed
several times, dried and weighed. Grain-size analysis was conducted for the
same samples using the sieving and pipette procedures adopted by Folk (1966) and
478
M. M. ABU-ZEID ET AL.
Figure 2. Dune fields in the greater Al Ain area. (a) A Landsat (TM) image showing Al Ain city
(A), sand dunes (reddish to light yellow), interdune areas (I), carbonate plain (P), Jabal Hafit (H),
Jabal Malaqet (M) and Jabal Mundassah (U). (b) A SPOT image showing star dunes (S) and
interdune areas (I) along Al Ain } Al Wagan road. (c) One of the star dunes near Al Ain along the
road to Abu Dhabi. Note the effect of dune migration on the high voltage tower located close
to the road. (d) A photograph showing a part of sand dune and cultivated interdune area along Al
Ain } Abu Dhabi road. (e) A complex barchanoid dune along Bu Samra } Sweihan } Al Hayer
road. Note the huge size of the dune and its position in respect to the road. (f ) A Landsat (TM)
image closing up on the branching linear dunes (L) around Al Ain city (A). The letter (H)
designates Jabal Hafit. Note the colour variation in sand dunes from white (light), close to the
Arabian Gulf coast (U), to reddish yellow (darker) close to the Oman Mountains (M).
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
479
Galehouse (1971). Authigenic carbonates and gypsum were picked out from the
interdune samples prior to the analysis. The roundness of quartz sand grains was
examined in 29 dune and 19 interdune samples. For each sample, two slides were
prepared from the fine sand (2}3H) and very fine sand (3}4H) fractions. In each
slide, 300 grains were examined microscopically and described using the terminology
proposed by Power (1953). Surface textures of quartz sand grains were examined in 25
dune samples using the scanning electron microscope (SEM). The samples were
prepared for examination following the procedure recommended by Krinsley & Doornkamp (1973). A representative sample weighing about 5 g was boiled in HCl (10%) for
10 min, washed thoroughly with distilled water and dried. About 15}20 grains were
mounted on the speciemen holder, coated with gold and examined. The light and heavy
minerals were separated from the fine sand and very fine sand fractions of 29 dune and
19 interdune samples. Separation was conducted using bromoform and the light and
heavy fractions were washed with alcohol, dried, weighed and their percentages were
calculated (cf. Lindholm, 1987). The separated fractions were examined microscopically and the relative proportions of the various light- and heavy-mineral species were
determined by counting of at least 300 grains. X-ray diffractometry was used to
identify the nonclay and clay minerals in 18 interdune samples. Unoriented-particle
mounts were prepared from the ground bulk samples. Oriented-particle mounts were
prepared by pipetting suspensions of the separated clay fractions onto glass slides which
were left to dry. Three mounts were prepared for each sample: untreated, glycolated and
heated at 5503C for 2 h. The mounts were X-rayed using a Philips diffractometer
with a Ni-filtered Cu}Ka radiation run at 40 Kv and 30 mA potential and scanning
speed of 23 2h min\. Identification of mineral species is based on data of the American
Society for Testing and Material (ASTM) and those published by the International
Center for Diffraction Data (1995).
Results and discussion
Distribution and general characteristics
Preliminary field investigations revealed the following characteristics of the sand dune
fields in the greater Al Ain area:
(1) Al Ain } Al Wagan district is dominated by star dunes ranging in height from 10
to 49 m (Fig. 2(b)). The interdune areas near Al Wagan contain sabkha deposits.
(2) Al Ain } Abu Dhabi district has various types of sand dunes with heights between
10 and 57 m (Fig. 2(c) and 2(d)). The abundance of star dunes increases toward
Al Ain. The dunes disappear toward Abu Dhabi where they are replaced by sand
sheets or sabkha flats.
(3) Bu Samra } Sweihan } Al Hayer district has sand dunes which are 25 to 80 m high
(Fig. 2(e)). The interdune areas are quite broad.
(4) Al Ain } Dubai district has sand dunes ranging in height between 5 and 70 m;
becoming smaller toward Dubai. The interdune areas are rare and mostly
replaced by sabkha flats towards the same direction.
(5) Al Hayer } Al Madam district has sand dunes which are 3 to 15 m high; with
a general decrease in height toward Al Madam. The interdune areas are sandy,
slightly gravelly and contain no sabkha.
(6) There is evidence of considerable migration of sand dunes toward most of the
above-mentioned roads (Fig. 2(c) and 2(e)).
(7) Toward the Arabian Gulf, the dunes become smaller, change in color from
reddish yellow to white and are, in places, replaced by sand sheets and/or sabkha
deposits (Fig. 2(f )).
480
M. M. ABU-ZEID ET AL.
Textural attributes
Sand dunes
Grain size. The dune sands in all districts are almost entirely composed of sand-sized
grains (carbonate and non-carbonate), whereas mud is present in trace concentrations
(averages 0)1}0)4%) (Table 1). Histograms representing their mechanical composition
display unimodal distributions in which the modal class falls into the fine and, rarely, the
very fine and medium sand grades. Cumulative curves were constructed (Fig. 3) and
used to calculate the statistical grain-size parameters applying the equations of Folk
& Ward (1957). The results obtained (Table 1) reveal that the average mean size (Mz)
values (2)50}2)71H) represent fine sand. Sands of Al Hayer } Al Madam are slightly
coarser than those of the other districts, whereas sands of Al Ain } Abu Dhabi are the
finest. The standard deviation (p') values average 0)34}0)46H indicating well-sorting;
sands of Al Ain } Al Wagan are the best sorted. The average skewness values
(0)03}0)13) correspond to near-symmetrical to fine-skewed distribution curves. Kurtosis values average 1)03}1)12 and indicate that sands of Bu Samra } Sweihan } Al Hayer
have mainly leptokurtic curves whereas those of the other districts have mesokurtic
curves. Generally, the statistical grain-size parameters of the studied dunes are similar to
those reported by Ahlbrandt (1979), Al Saoud (1986), Yehia & Hashem (1989) and
Alsharhan et al. (1995) for sand dunes in the Arabian Peninsula, Qatar and other parts of
U.A.E.
Grain roundness. The fine sand fractions of the dune sands in all districts are dominated by the subrounded (average 35)0}40)7%) and rounded (average 28)9}39)9%)
grains (Table 2). However, their proportions decrease remarkably in the very fine sand
fractions whereas those of subangular (average 14)8}27)7%) and angular (average
4)5}16)6%) grains increase.
The roundness classes were grouped into two clans: the rounded clan (well-rounded
to subrounded) and the angular clan (subangular to very angular). Table 2 shows that
the fine sand and very fine sand fractions in all districts are dominated by the rounded
clan (average 76)7}91)4% and 57)4}66)1%; respectively). Its proportions display only
slight variations among the various districts with a general decrease towards the NE of
the study area.
Surface textures of grains. SEM examination of quartz sand grains in dune sands
revealed that they variably display mechanically- and chemically-formed surface textures (Figs 4 and 5). Mechanical features include upturned plates, shrinkage cracks,
Table 1. Averaged carbonate contents, mechanical compositions and statistical grain-size parameters of
the dune sands
District
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan
} Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Number Carbonate Mechanical
of
%
composition Statistical size parameters
samples
Sand Mud Mz
p
'
%
%
(H) (H) SKI KG
13
17
7
17)4
33)5
36)0
99)9
99)7
99)7
0)1
0)3
0)3
2)56
2)71
2)62
0)34
0)43
0)46
0)03 1)04
0)12 1)11
0)07 1)03
15
14
38)1
32)7
99)6
99)7
0)4
0)3
2)60
2)50
0)45
0)41
0)11 1)06
0)13 1)12
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
481
Figure 3. Cumulative curves of the dune sands.
meandering ridges, V-shaped pits, stepped fracture planes and dish-shaped depressions.
Upturned plates (Fig. 4(a)) are a series of thin parallel plates, either continuous or
discontinuous, and commonly oriented at some angle to the grain surface. They are
observed on quartz sand grains in Al Ain } Abu Dhabi, Al Ain } Dubai and Al Hayer
} Al Madam districts. They appear irregular and discontinuous as a result of the
presence of pits and grooves (formed by chemical dissolution) and silica precipitates.
Curved and irregular shrinkage cracks (Figs 4(a, b)) are displayed by sand grains in Al
Ain } Dubai, Bu Samra } Sweihan } Al Hayer and Al Hayer } Al Madam districts.
Meandering ridges (Fig. 4(c)) are defined as the intersections of slightly concoidal
breakage patterns and graded arcs. They are shown by quartz grains of Al Ain } Al
Wagan dunes. V-shaped pits (Fig. 4(d)) are characterized by somewhat irregular sides
and central unevenly depressed areas. They are displayed by grains from Al Ain } Al
Wagan and, especially, Al Ain } Abu Dhabi dunes. Stepped fracture planes (Fig. 4(e))
are recorded in quartz grains from Al Ain } Dubai and Al Ain } Abu Dhabi dunes.
Dish-shaped depressions (Fig. 4(f )) are single or multiple rounded concavities acquiring different sizes. They are displayed by quartz grains especially from Al Ain } Abu
Dhabi, Al Ain } Dubai and Al Ain } Al Wagan dunes.
Chemical surface features include those produced both by dissolution and precipitation of silica on grain surfaces. Dissolution features include triangular grooves, linear
grooves and deep haloes. Triangular grooves (Fig. 5(a)) are displayed by quartz grains
482
Table 2. Averaged percentages of the various roundness classes of sand grains in the dune sands
District
Fine sand (2}3 H)
Classes %
W.R.
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra }
Sweihan Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
WR: Well-rounded;
R.
Very fine sand (3}4 H)
Clans %
Classes %
V.A. R.C. A.C. W.R.
R.
S.R. S.A.
Both fractions
Clans %
S.R. S.A.
A.
7
6
4
13)6 37)1 40)7 7)7
9)8 39.1 37)5 12)5
10)7 39)9 35)5 11)2
0)7
0)8
1)7
0)2
0)3
1)0
91)4 8)6
86)4 13)6
86)1 13)9
9)2
6)2
7)2
23)4 33)6 26)5 4)5
19)9 31)3 26)8 10)9
19)5 34)3 27)7 7)4
2)9
4)9
3)9
66)1 33)9
57)4 42)6
61)0 39)0
78)7
71)9
73)6
21)3
28)1
26)4
7
5
12)2 31)3 35)0 19)6
11)6 28)9 36)2 21)4
1)4
1)4
0)5
0)5
78)6 21)4
76)7 23)3
6)7
2)7
14)8 39)5 22)4 11)0
11)2 48)5 14)8 16)6
5)6
6)2
61)0 39)0
62)3 37)7
69)8
69)5
30)2
30)5
R: Rounded; SR: Subrounded; SA: Subangular; A: Angular; VA: Very angular;
A.
Clans %
V.A. R.C. A.C. R.C.
RC: Rounded clan; AC: Angular clan.
A.C.
M. M. ABU-ZEID ET AL.
No. of
samples
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
483
Figure 4. Quartz sand grains showing mechanically-formed surface features. (a) well-developed
upturned plates (U) and shrinkage cracks (S); (b) shrinkage cracks (S); (c) meandering ridges
(M) modified by chemical dissolution; (d) v-shaped pits (P); (e) stepped fractures (F); (f )
dish-shaped depressions (D).
484
M. M. ABU-ZEID ET AL.
Figure 5. Quartz sand grains showing chemically-formed surface features. (a) triangular grooves
(T); (b) nearly straight and markedly wide linear grooves (L); (c) slightly curved linear grooves
(G); (d) a deep halo (H); (e) silica overgrowths (S) partially filling pre-existing fractures;
(f ) smoothening by silica precipitation.
District
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan }
Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Number Carbonate
of
%
samples
Mechanical composition %
Statistical grain size parameters
Gravel
Sand
Silt
Clay
Mz(H)
r (H)
'
SK
'
K
%
10
5
6
29)8
43)0
46)4
1)1
7)2
14)8
83)0
71)7
69)6
11)1
15)6
9)8
4)8
5)5
5)8
3)22
2)62
1)80
1)35
3)01
4)92
0)40
0)04
!0)41
2)28
2)79
6)69
4
6
40)9
30)5
9)7
5)8
75)3
82)0
8)6
8)5
6)4
3)7
1)70
2)32
4)36
2)48
0)02
0)05
3)18
3)68
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
Table 3. Averaged carbonate contents, mechanical compositions and statistical grain-size parameters of the interdune sediments
485
486
M. M. ABU-ZEID ET AL.
of Al Ain } Abu Dhabi dunes. Linear grooves are observed on surfaces of the grains
constituting Al Ain } Al Wagan, Al Ain } Abu Dhabi and Al Hayer } Al Madam dunes.
They are straight (Fig. 5(b)) or curved (Fig. 5(c)) and, in some cases, branched. Deep
and elongate haloes are recorded on quartz grains from Al Ain } Abu Dhabi dunes.
Precipitation features, on the other hand, are represented by silica overgrowths, silica
globules and smoothness by silica. Silica overgrowths (Fig. 5(e)) are displayed by grains
from Al Ain } Abu Dhabi dunes partially filling the mechanically-formed fractures.
Silica globules are found on grains from Bu Samra } Sweihan } Al Hayer and Al Hayer
- Al Madam dunes. They are commonly associated with mesh-like precipitation of silica
and may destroy the mechanically-formed upturned plates. Smoothing by silica is
commonly displayed by grains of the studied dunes which show also subsequent
development of deep haloes (Fig. 5(f )).
Interdune sediments
Grain size. The interdune sediments are composed of sand-sized grains (average
69)6}83)0%) together with much smaller proportions of gravels (average 1)1}14)8%),
silt (average 8)5}15)6%) and clay (average 3)7}6)4%) (Table 3). Generally, the proportions of gravels are highest in the central part of the study area and decrease toward the
SW and NE to the advantage of those of sand. Percentages of silt and clay remain almost
unchanged.
Figure 6. Plots of the interdune sediments on the ternary diagram proposed by Folk (1954). Al
Ain–Al Wagan ( ); Al Ain}Abu Dhabi ( ); Bu Samra } Sweihan } Al Hayer ( ); Al Ain } Dubai
( ); Al Hayer } Al Madam ( ); Average values ( , , , , ).
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
487
In the textural nomenclature scheme of Folk (1954) the interdune sediments are
classified as sand, muddy sand, slightly gravelly sand, gravelly sand, slightly gravelly
muddy sand, gravelly muddy sand and sandy gravel (Fig. 6). The averaged mechanical
composition of Al Ain } Al Wagan sediments lies in the field of ‘slightly gravelly muddy
sand’, whereas those of the other districts fall into the ‘gravelly muddy sand’ field.
Histograms representing the grain-size distributions in the interdune sediments are
uni-, bi-, or polymodal. In the unimodal distributions, the modal class commonly lies in
the fine sand grade. For the bi- and polymodal distributions, the primary mode falls into
the fine sand or the very fine sand grade, whereas the scondary mode commonly lies in
the gravel size grade. Cumulative curves (Fig. 7) were used for the calculation of the
statistical grain-size parameters (Table 3). Mz values (average 1)70}3)22H) correspond to very fine, fine or medium sand. Al Ain } Al Wagan sediments are the finest
whereas those of Al Ain } Dubai are the coarsest. Standard deviation values average
1)35}4)92H and indicate poor- to extremely poor-sorting; sediments of Al Ain } Al
Wagan are relatively better-sorted than those of the other districts. Values of SKI
(average!0)41 to 0)40) indicate that the distribution curves are near-symmetrical and,
Figure 7. Cumulative curves of the interdune sediments.
488
M. M. ABU-ZEID ET AL.
rarely, strongly fine}or strongly coarse}skewed. KG values average 2)28}6)69 indicating
that they are very to extremely leptokurtic.
Grain roundness. Table 4 shows that the grain-roundness characteristics of the interdune sediments are remarkably similar to those of sand dunes, except for the presence of
higher proportions of angular grains. Based on averages of both sand fractions, the
percentages of the rounded (78)9}83)3%) and angular (16)7}21)1%) clans show only
slight variations among the various districts.
Mineralogy
Sand dunes
The dune sands have high contents of carbonates (17)4}38)1%); the least calcareous are
those of Al Ain } Al Wagan district (Table 1). In this latter district, the carbonate
contents increase toward Al Ain city. Similarly, in Al Ain } Abu Dhabi and Al Ain
} Dubai districts, the carbonate content of the dune sands increases toward Abu Dhabi
and Dubai, respectively.
The light sand fraction. The light sand fractions of the dune sands consist of quartz
and carbonate minerals together with minor proportions of chert and feldspars
(Table 5). Percentages of quartz (average 53)0}74)4%) show a marked increase at the
SW of the study area at the expense of those of carbonates (average 18)2}40)4%); the
latter increase toward the NE. The proportions of chert (average 5)1}7)3%) and
feldspars (average 0)3}1)1%) remain almost unchanged throughout the study area.
Single quartz grains with uniform extinction markedly dominate both the fine sand
and very fine sand fractions in dunes of all districts (Table 6). Their proportions are
highest at Al Ain } Al Wagan but generally decrease toward the NE of the study area to
the advantage of those of single grains displaying slightly undulose extinction and
composite grains with strongly undulose extinction. The percentages of other quartz
types fluctuate among the various districts.
The carbonate grains consist of micritic calcite and, less commonly, sparitic dolomite.
They are partially or completely coated with iron oxides. Chert fragments are very
angular to subrounded. Feldspars are represented by plagioclase and microcline grains
which are occasionally altered.
The heavy sand fraction. Table 7 shows that the total averaged values of heavy index
for both the fine sand and very fine sand fractions (1)6}3)6) show a slight gradual
increase towards the NE of the study area. The recorded heavy-mineral assemblage
consists of (in order of decreasing abundance): opaques, pyroxenes, tourmaline, garnet,
rutile, epidote, monazite, zircon, hornblende and staurolite. Opaques dominate the
heavy fractions (average 29)3}33)3%). Therefore, in order to facilitate comparison
between different samples, their percentages were omitted and the proportions of
non-opaque minerals were recalculated to 100%.
Based on the averaged percentages of both sand fractions, pyroxenes are the most
dominant non-opaque minerals in dune sands of all districts (averages 39)6}60)3%).
Tourmaline comes next in abundance to pyroxenes and exists in markedly lower
concentrations (averages 15)1}25)7%). The proportions of pyroxenes increase whereas
those of tourmaline, zircon and rutile (ZTR index"maturity index; Hubert, 1962)
decrease toward the NE of the study area. Other non-opaque minerals are present in
minor or trace concentrations which fluctuate among the various districts.
Pyroxenes are represented by enstatite, hypersthene and, less commonly, diopside
and augite. The grains are often pitted and show cockscomb-like terminations. Tourmaline grains are well-rounded, have prismatic or discoidal shapes and may contain
District
No. of
samples
Fine sand (2}3 H)
Classes %
WR
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan } Al
Hayer
Al Ain } Dubai
Al Hayer } Al Madam
WR: Well-rounded;
R
SR
Very fine sand (3}4 H)
Clans %
Classes %
SA
A
VA
RC
AC
WR
5
5
28)3 34)3 29)7 6)9
10)3 35)5 41)7 12)5
0)8
0)0
0)0
0)0
92)3 7.7
87)5 12)5
3
3
3
7)6 32)5 46)7 11)7
10)7 31)9 50)3 7)1
12)4 41)2 41)9 4)9
1)5
0)0
0)0
0)0
0)0
0)0
86)8 13)2
92)8 7)2
95)1 4)9
R
Clans %
Clans %
SA
A
VA
RC
AC
RC
AC
7)8
8)0
27)1 39)5 22)5
24)8 37)6 21)6
2)2
6)2
0)9
1)8
74)4 25)6
70)4 29)6
83)3
78)9
16)7
21)1
5)8
4)7
5)3
21)5 45)5 19)7
24)9 37)7 24)7
23)4 40)7 20)1
5)8
6)6
6)5
1)7
1)4
4)0
72)8 27)2
67)3 32)7
69)3 30)7
79)8
80)0
82)2
20)2
20)0
17)8
R: Rounded; SR: Subrounded; SA: Subangular; A: Angular; VA: Very angular;
SR
Both fractions
RC: Rounded clan; AC: Angular clan.
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
Table 4. Averaged percentages of the various roundness classes of sand grains in the interdune sediments
489
490
M. M. ABU-ZEID ET AL.
Table 5. Averaged composition of the light sand fractions of the dune sands
District
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra - Sweihan }
Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Number of
samples
Quartz
%
Carbonates
%
Chert
%
Feldspars
%
7
6
4
74)4
62)2
60)8
18)2
31)1
33)8
6)3
6)2
5)1
1)1
0.5
0)3
7
5
53)0
59)0
40)4
33)1
6)2
7)3
0)4
0)6
Total average for the fine sand and very fine sand fractions.
unidentifiable inclusions. The grains of garnet are angular to rounded and occasionally
show opaque inclusions and rugged surfaces suggestive of extensive etching. Rutile is
represented by the yellowish and reddish brown varieties constituting elongate and
rounded grains. Epidote and monazite grains are rounded and, in cases, contain
inclusions. Zircon grains are subrounded to rounded, have euhedral prismatic shapes
and contain abundant inclusions. Hornblende and staurolite grains are subrounded and
may have inclusions of opaques.
The heavy-mineral assemblage recorded in sand dunes of the greater Al Ain area is
closely similar to that reported by Embabi & Ashour (1985) for the dunes in Qatar, by
Yehia & Hashem (1989) for the south-eastern dune belt in Qatar and by Ahmed et al.
(1995) for the dunes in the eastern province of Abu Dhabi Emirate.
Interdune sediments
The bulk sediments. The interdune sediments are richer in carbonates (average
29)8}46)4%) than the adjacent sand dunes (Table 3). The carbonate contents of the
sediments in Al Ain } Al Wagan, Al Ain } Abu Dhabi and Al Ain } Dubai districts
increase toward Al Ain, Abu Dhabi and Dubai, respectively. These trends are similar to
those recorded in sand dunes of these districts.
X-ray diffraction analysis of the bulk interdune sediments revealed the presence
of quartz, anorthite, calcite, aragonite, dolomite, gypsum, anhydrite, halite and celestite.
Quartz is the most common mineral followed by calcite and gypsum. The latter mineral
is the most dominant in sediments of Al Ain } Abu Dhabi district which contain also an
association of aragonite, anhydrite, halite and celestite.
The light sand fraction. Table 8 shows that the light sand fractions of the interdune
sediments are similar in composition to those of the adjacent sand dunes being consisting of quartz and carbonates, together with small proportions of chert and traces of
feldspars. However, they contain remarkably higher proportions of carbonates (average
30)4}47)5%) which decrease toward the NE and SW of the study area to the advantage
of those of quartz. Percentages of chert and feldspars remain almost unchanged.
Like in sand dunes, single quartz grains exhibiting uniform extinction dominate the
fine sand and very fine sand fractions of interdune sediments (Table 9). Their percentages increase remarkably toward the NE of the study area at the expense of those of
single grains with strongly undulose extinction. Proportions of other quartz types are
slightly higher than those in sand dunes.
The heavy sand fraction. The heavy-mineral composition of the interdune sediments is
qualitatively similar to that of sand dunes (Table 10). However, the proportions of
District
Number
of
samples
*Empirical classification of quartz types (Folk, 1974)
Fine sand (2}3 H)
Very fine sand (3}4 H)
Both fractions
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
(I) (II) (III) (IV) (V) (VI) (I) (II) (III) (IV) (V) (VI) (I) (II) (III) (IV) (V) (VI)
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan }
Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Type
Type
Type
Type
Type
Type
7
6
4
84)3 11)6
73)9 17)8
79)7 13)9
1)8
5)1
3)3
0)4
1)1
1)5
0)0
0)2
0)3
1)9
1)9
1)3
60)6 20)0 13)2
52)9 27)3 14)2
57)1 26)0 11)4
0)5
0)7
0)5
0)3
0)3
0)5
5)4
4)6
4)5
72)4 15)8
63)4 22)5
68)4 19)9
7)5
9)6
7)4
0)4
0)9
1)0
0)2
0)3
0)4
3)7
3)3
2)9
7
5
77)3 16)2
64)4 25)6
4)0
5)9
0)6
1)0
0)3
0)4
1)6
2)7
61)0 20)7 9)2
52)8 23)0 15)6
0)0
0)0
0)0
0)8
9)1
7)8
69)1 18)4 6)6
58)6 24)3 10)8
0)3
0)5
0)2
0)6
5)4
5)2
(I): Single grains with uniform extinction.
(II): Single grains with slightly undulose extinction.
(III): Single grains with strongly undulose extinction.
(IV): Semicomposite grains.
(V): Composite grains with slightly undulose extinction.
(VI): Composite grains with strongly undulose extinction.
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
Table 6. Averaged percentages of the various types of quartz grains in the dune sands
491
492
Table 7. Averaged frequency distribution of heavy minerals in the dune sands
District
Heavy
index
Opaques
%
F
F
VF
VF
Pyroxenes Tourmaline
%
%
F
VF
F
VF
Garnet
%
F
VF
Rutile
%
F
VF
7
TA
6
TA
4
TA
0)4 2)7
1)6
0)7 3)6
2)1
0)5 4)1
2)3
31)0 33)6 39)5 39)8 14)3 24)4 10)8 12)9 11)3 5)3
32)3
39)6
19)4
11)8
8)3
27)5 31)0 42)0 40)2 25)2 26)1 7)0 10)6 6)0 5)9
29)3
41)1
25)7
8)8
5)9
30)6 29)6 46)8 39)3 18)8 26)7 8)1 11)2 8)1 7)2
30)1
43)1
22)7
9)7
7)6
7
TA
5
TA
2)0 5)2
3)6
1)6 5)4
3)5
33)7 32)8 60)0 46)0 11)1 24)6
33)3
53)0
17.9
27)4 32)9 67)8 52)9 11)0 19)2
30)1
60)3
15)1
F: Fine sand fraction;
5)4 10)7
8)1
3)9 11)1
7)5
7)8 5)2
6)5
3)9 4)5
4)2
Epidote
%
F
VF
Monazite
%
F
VF
Zircon
%
F
VF
Hornblende Staurolite
%
%
F
VF
F
VF
ZTR
index
F
VF
10)6 4)8
7)7
8)7 6)1
7)4
8)5 7)3
7)9
5)3 8)4
6)9
3)7 4)5
4)1
2)6 2)8
2)7
4)1
2)1
3)1
3)3 2)8
3)1
2)8 2)0
2)4
2)2
1)5
1)8
3)2 2)0
2)6
2)2 2)1
2)7
1)9
0)8
1)4
0)9 1)8
1)3
2)1 1)4
1)7
29)7 31)7
30)7
34)4 34)7
34)6
29)6 35)9
32)8
6)3 4)3
5)3
3)4 3)9
3)8
3)3 3)7
3)5
3)5 4)3
3)9
2)1
3)2
0)8
21)1 32)0
25)5
16)6 25)2
20)9
2)2
2)1
1)7 1)7
1)7
2)5
2)8
3)6 2)0
2)8
0)8
0)8
0)4 0)9
0)7
VF: Very fine sand fraction; TA: Total average for both fractions; ZTR Index: Sum of zircon, tourmaline and rutile percentages.
M. M. ABU-ZEID ET AL.
Al Ain }
Al Wagan
Al Ain }
Abu Dhabi
Bu Samra}
SweihanAl Hayer
Al Ain}
Dubai
Al Hayer}
Al Madam
No. of
Samples
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
493
Table 8. Averaged composition of the light sand fractions of the interdune sediments
District
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan }
Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Number of
samples
Quartz
%
Carbonates
%
Chert
%
Feldspars
%
5
5
3
58)4
47)8
44)2
30)4
43)0
47)5
10)3
8)4
7)1
0)9
0)8
1)2
3
3
50)6
60)7
41)4
31)1
7)2
7)3
0)8
0)9
Total average for the fine sand and very fine sand fractions.
pyroxenes are higher whereas those of opaques and other non-opaque minerals are
lower than in sand dunes. Values of heavy index and the total averaged percentages of
opaques and pyroxenes increase toward the NE of the study area; trends which are
similar to those recorded in sand dunes. It is worthy to mention that the grains of a given
light or heavy mineral component have the same nature in sand dunes and interdune
sediments and also among the various districts.
The clay fraction. X-ray diffractometry of the clay fractions of interdune sediments revealed that they consist solely of palygorskite (Fig. 8).
In general, the textural and mineral characteristics of the dune sands indicate a high
maturity whereas interdune sediments are much less mature. The maturity of both types
of sediments differes among the various districts but shows a general decrease
toward the NE of the study area. This is indicated by the increase in heavy index and the
proportions of opaques, pyroxenes and composite quartz grains exhibiting strongly
undulose extinction.
Provenance
The marked textural and mineralogic similarities among sand dunes in the various
districts of the greater Al Ain area indicate that they are genetically-related; i.e. derived
from the same sources and have the same sedimentary history. Also, the similarities in
several petrographic aspects between the dune sands and interdune sediments strongly
suggest that they are at least partially genetically-related. Both types of sediments
(especially the dune sands) are mainly multicyclic as evidenced by their markedly high
textural and mineral maturity (cf. Carozzi, 1993). It is believed that their major parts
were brought by the dominant north-west (El-Shamal) winds from dune fields in other
localities in the U.A.E., Qatar and El-Rub El-Khali. This explains the close similarities in
the textural and heavy-mineral compositions between these dunes and those in the
greater Al Ain area.
Although the major parts of the studied dune fields were derived from distant older
dunes, however, contributions from local sources were in several cases significant. The
Oman Mountains which lie to the NE of the study area and are made up of igneous
(mafic and ultramafic) and metamorphic rocks provided materials to the dune fields
especially those located close to the mountains. This explains the decrease in maturity of
the dune and interdune sediments towards the NE of the study area. The red colouration
displayed by the dunes near the Oman Mountains is mostly attributed to the alteration
494
Table 9. Averaged percentages of the various types of quartz grains in the interdune sediments
District
Number
of
samples
*Empirical classification of quartz types (Folk, 1974)
Fine sand (2}3 H)
Very fine sand (3}4 H)
Both fractions
Al Ain } Al Wagan
Al Ain } Abu Dhabi
Bu Samra } Sweihan }
Al Hayer
Al Ain } Dubai
Al Hayer } Al Madam
Type
Type
Type
Type
Type
Type
5
5
58)3 20)6 14)6
63)7 15)4 14)7
2)3
2)4
2)2
1)9
2)0
1)9
60)5 16)4 14)9
60)0 17)3 13)9
1)4
1)2
1)2
2)1
5)6
5)5
59)4 18)5 14)7
61)8 16)4 14)3
1)9
1)8
1)7
2)0
3)8
3)7
3
3
3
71)1 12)3 11)6
71)5 15)0 9)1
73)6 12)8 10)7
1)9
2)0
0)6
1)4
0)7
0)7
1)7
1)7
1)6
62)0 18)4 7)3
63)7 12)7 11)6
69)6 15)3 6)3
1)1
2)2
1)7
2)5
0)7
0)7
8)7
9)1
6)4
66)5 15)4 9)5
67)6 13)8 10)4
71)6 14)1 8)5
1)5
2)1
1)1
1)9
0)7
0)7
5)2
5)4
4)0
(I): Single grains with uniform extinction.
(II): Single grains with slightly undulose extinction.
(III): Single grains with strongly undulose extinction.
(IV): Semicomposite grains.
(V): Composite grains with slightly undulose extinction.
(VI): Composite grains with strongly undulose extinction.
M. M. ABU-ZEID ET AL.
Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type Type
(I) (II) (III) (IV) (V) (VI) (I) (II) (III) (IV) (V) (VI) (I) (II) (III) (IV) (V) (VI)
District
Al Ain }
Al Wagan
Al Ain}
Abu Dhabi
Bu Samra}
Sweihan }
Al Hayer
Al Ain}
Dubai
Al Hayer}
Al Madam
No. of
Samples
Heavy
index
Opaques
%
F
F
VF
VF
Pyroxenes Tourmaline
%
%
F
VF
F
VF
Garnet
%
F
VF
Rutile
%
F
VF
Epidote
%
F
VF
Monazite
%
F
VF
Zircon
%
F
VF
Hornblende Staurolite
%
%
F
VF
F
VF
ZTR
index
F
VF
5
T.A.
5
T.A.
3
T.A.
0)9 3)7
2)3
2)9 4)5
3)7
0)9 6)9
3)9
27)8 24)9 64)0 39)0 12)6 27)7
26)3
51)7
20)2
23)3 25)4 55)5 46)8 10)6 24)3
24)3
51)1
17)4
27)7 28)8 60)3 38)9 12)6 29)4
28)2
49)6
21)0
3)8 10)8
7)3
6)2 7)7
6)9
3)1 10)2
6)7
4)0 6)7
5)4
6)6 6)7
5)0
8)1 6)0
7)1
3)8 10)8
5)4
4)4 5)6
6)9
5)8 3)9
4)6
2)2 4)0
3)1
5)5 3)9
4)7
2)1 4)4
3)2
4)4
3)1
3)7
7)7 2)0
4)8
2)4 1)9
2)1
2)4
2)8
2)6
2)3 2)3
2)3
5)8 3)9
4)8
0)4
0)8
0)6
1)2 0)7
0)9
0)4 1)4
0)9
23)2 33)5
28)3
25)0 33)0
29)0
23)1 38)2
30)6
3
T.A.
3
T.A.
1)2 5)7
3)4
1)6 8)6
5)1
30)5 27)3 61)8 46)9
28)9
54)4
27)7 30)6 70)5 51)8
29)2
61)2
3)9 8)1
6)0
2)1 7)0
4)6
8)5 7)0
7)8
4)0 4)7
4)3
6)6 7)2
6)9
4)0 7)2
5)6
2)6 3)9
3)4
1)8 6)0
3)9
3)5
3)7
0)9
20)3 30)7
25)5
14)8 25)9
20)4
F: Fine sand fraction;
8)3 22)5
15)4
8)7 18)1
13)4
1)2
2)3
2)1 3)0
2)5
2)2
2)9
6)6 1)7
4)2
1)0
0)9
0)2 0)5
0)3
VF: Very fine sand fraction; TA: Total average for both fractions; ZTR Index"Sum of zircon, tourmaline and rutile percentages.
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
Table 10. Averaged frequency distribution of heavy minerals in the interdune sediments
495
496
M. M. ABU-ZEID ET AL.
Figure 8. Selected X-ray diffractograms of the clay fractions of the interdune sediments.
(U: untreated, G: glycolated, H: heated).
effect of meteoric water on the predominating pyroxenes resulting in the liberation
of iron which, upon oxidation, acts as a brownish red pigment. This is evidenced by:
(1) the marked dominance of altered pyroxene grains which are pitted and have
cockscomb-like terminations; (2) the increase in intensity of the red coloration towards
the Oman Mountains consisting of rocks rich in pyroxenes; and (3) the lack of evidence
of soil-paleosol development in the dunes which may cause their red coloration. The
possibility that the dune red coloration has been inherited from the older source dunes
with paleosols displayed is ruled out. Such paleosols were not reported by previous
workers in the source dunes in other parts of U.A.E., Qatar and the Arabian peninsula
(cf. Ahlbrandt, 1979; Al Saoud,1986; Yehia & Hashem, 1989; Alsharhan et al., 1995).
Jabal Hafit mountain played an important role in the enrichment of the nearby dune
fields in carbonate detritus. This explains the marked increase in their percentages in the
dune fields which lie in the vicinity of Al Ain city. Contributions from the calcareous
coast of the Arabian Gulf were also significant especially for the nearby dune fields. It is
believed that some of the sand components brought to the area by El-Shamal winds were
laid down in the shallow zones of the Arabian Gulf. Active marine currents resulted in
their accumulation on beaches from which they were recycled and contributed to the
dunes and interdune sediments. This is evidenced by: (1) the presence of markedly high
proportions of carbonate detritus in these sediments and their increase toward Abu
Dhabi and Dubai which are located on the Arabian Gulf coast; (2) the increase in
proportions of angular grains towards the Gulf; (3) the presence of quartz sand grains
with stepped fracture planes which characterize coastal deposits, and also those having
V-shaped pits which commonly originate in marine environments before transportation
of the grains to aeolian environments (cf. Margolis & Krinsley, 1974); (4) the presence
of grains with triangular and linear grooves and deep elongated haloes suggesting their
partial dissolution in a high-energy water environment before transportation by wind
from the coastal area (cf. Krinsley & Doornkamp, 1973; Margolis & Krinsley, 1974);
and (5) the appearance of the mineral association aragonite, anhydrite, halite and
celestite in the interdune sediments near Abu Dhabi.
TEXTURE, MINERALOGY AND PROVENANCE OF DUNE FIELDS
497
Although the major parts of the interdune sediments are detrital, however, authigenesis by groundwater under highly evaporitic conditions played a significant role in their
formation. Evidence for this authigenesis includes: (1) their remarkably lower textural
and mineralogic maturity as compared to the adjacent sand dunes; (2) the dissimilarity
between trends of variation in carbonate contents of the sediments in various districts;
(3) the presence of authigenic evaporitic minerals especially in the central part of the
study area which is known to have the shallowest groundwater levels and, hence, active
authigenesis; (4) the presence of silica precipitates (overgrowths and globules) on
surfaces of some quartz grains; and (5) the existence of palygorskite as a sole constituent
of the clay fractions of the sediments. A detrital origin for this clay mineral is precluded
since it is extremely unstable during weathering and transportation (Chahi et al., 1993).
Also, the possibility of genesis by transformation from a clay precursor is ruled out since
there is no such precursor in the studied sediments. Hence, it is most likely that
palygorskite was authigenically neoformed by direct precipitation from Mg-rich solutions and/or by alteration of Mg-rich rock fragments derived from the ultramafic and
mafic igneous rocks which constitute the Oman Mountains. This neoformation was
early diagenetic and enhanced by the hot and arid climate and flat topography of the
study area (cf. Velde, 1992; Chauchan, 1996).
Summary
Sand dunes in the greater Al Ain area are of different types and vary markedly in
height and colour. They show evidence of migration towards certain main roads. Also,
interdune sediments may vary in nature and extension.
The dunes and, especially, interdune sediments have markedly high carbonate
contents.
The dunes in the various districts have similar textural and mineral characteristics.
They are much more mature than the associated interdune sediments. This maturity
decreases towards the NE of the study area.
Generally, the grain-size characteristics and heavy-mineral composition of the studied
dunes resemble those previously reported for dunes in other localities in U.A.E., Qatar
and the Arabian Peninsula.
The dunes in the greater Al Ain area are genetically-related. Also, there is a genetic
relationship between them and interdune sediments. Both types of sediments are
multicyclic. Their major parts were brought by the dominant north-west (El Shamal)
winds from older dune fields in other localities in the U.A.E., Qatar, and El-Rub
El-Khali.
Local contributions to the dune fields from Jabal Hafit mountain, the Oman Mountains and the calcareous coast of the Arabian Gulf were, in cases, significant and
reflected on their composition and color.
Authigenesis by groundwater under highly-evaporitic conditions played a major role
in the formation of interdune sediments. It resulted in the genesis of sulphate and halide
minerals together with the sole clay mineral palygorskite.
The authors wish to express their appreciation and gratitude to the United Arab Emirates
University (UAEU) for providing various facilities during the progress of this research work.
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