Ian West,

Romsey, Hampshire
and: School of Ocean and Earth Science ,
National Oceanography Centre, Southampton
Southampton University,
Website hosted by courtesy of Information Systems Services, Southampton University

Home and Contents | | Sabkhas, Salt Lakes and Other Desert Environments - Introduction | Select Bibliography on Sabkhas, Salt Lakes and Other Desert Environments | Permo-Triassic Desert Redbeds of Budleigh Salterton, Devon, UK

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1. GENERAL INTRODUCTION

This is an introduction to sedimentological features of interest in the arid environment of Qatar, Arabian Gulf. Sabkhas, evaporites, salt lakes, sand dunes and other desert environments. It is in progress, and from time to time more illustrations and more detailed descriptive text will be added. This is being reorganised from a general sabkha webpage and will be in improved condition later.

SABKHAS - Introduction

Sabkha is an arabic name for a salt-flat that has come into general use in sedimentology following classic research in the United Arab Emirates of the Arabian Gulf in the 1960s and later. They are flat and very saline areas of sand or silt lying just above the water-table and often containing soft nodules and enterolithic veins of gypsum or anhydrite. A thin crust of halite and gypsum may be present in some parts. Many ancient evaporites show sedimentary feature of sabkhas, such as gypsum nodules.

A supratidal part of a large coastal sabkha at Umm Said in Qatar. This particular area is the remains of a lagoon indirectly filled with siliciclastic sand of aeolian origin, originating from some large barchan sand dunes. The flatness is controlled by the content of capillary moisture from the water-table, which is only about half a metre (one and a half feet) down, keeping the sand damp and firm and preventing it from being blown away. Any higher dryer sand can be moved away by deflation. This particular sabkha has much granular gypsum in addition to the sand but is firm enough for a four-wheel drive vehicle where the brine is not at the surface (although it is sensible to keep only to well-used car routes and to have a mobile phone and use GPS for location).

A lower part of the same sabkha where a thin halite crust is developed. The halite is empheral and easily dissolved by a rare flood of rain. The gypsum in the sand beneath is less soluble and remains. Here the water-table is almost at the surface and the sand beneath is soft with gypsum and some clay. The salt crust can sometimes support cars until they break through and rapidly rust away in the brine. The wheels and windows will eventually be the only fossil remains of this vehicle. One hopes that the driver was able to walk away on the salt crust to the solid desert beyond.

INTRODUCTION:

Safety

With regard to safety on sabhas and other areas of geological interest, consult the local authorities, university etc. for guidance and permission where necessary.

There is, of course, a risk of sunburn, heat-stroke and dehydration when working in deserts in summer conditions. Even in cooler times of the year there can be problems with heat and sun in the middle of the day. Much water should be taken, and arrangements made in case of problems with vehicles used for transport. Mobile phones and GSP satellite location systems are necessary.

There is considerable risk on sabkhas of sinking into the saline sand or mud. On foot it is usually fairly obvious where there is a hazard, but the geologist should be very cautious. With vehicles the hazard of becoming stuck is greater. This is because there is often a hard salt crust which is easy to walk on. However, the weight of vehicle may cause it to break and the vehicle will become stuck and may be a write-off.

Snakes are not usually a problem on sabkhas because they need dry burrows. They are more likely to encountered on sand dunes or other drier areas.

If there is any chance of rainfall be very careful to avoid being caught in a low area by flash floods.

INTRODUCTION:

Setting

Qatar is introduced here because it is an interesting place for the study of sabkha and other desert environments. It has been studied by sedimentologists in the past, although not as much so as has the classic United Arab Emirates coast (i.e. Abu Dhabi region).

The Peninsula is interesting in that it is a structural dome with contrasting surface environments between north and south. Very large barchan sand dunes are developed in the south but are absent in the north. They have moved from north to south with the prevailing wind, but the Holocene rise in sea level has cut off the sand supply so that now northern Qatar is an area of deflation. Photographs above shows part of this deflated area and which lacks sand dunes, although it has some residual sand.

Sand dunes, particularly barchans are very well-developed in southeast Qatar. Some details of these are given below. Much of southern Qatar with dunes and sabkhas is very different from the northern part.

INTRODUCTION
Qatar - Petroleum Geology

This old map is given to show general geographical feature of the Arabian Gulf area, and at the same time to show distribution of some of the most well-known oilfields and their reservoir rocks. Later maps will show more oilfields, but in the same general regions. The study of sabkhas which started seriously in the 1960s was given by much impetus by the oil exploration in the region. Sabkha cycles occur in association with the ancient oil reservoirs and particularly the evaporite seals and the understanding of them is of economic importance.

Qatar is notable for both gas and oil production. The Dukhan Sabkha is immediately to the east of the well-known Dukhan oil field. This field is 65 km long and 5 km wide. The search for oil and gas in Qatar began in 1931 when the first detailed geological survey was made by BP. A consortium consisting of Petroleum Development (Qatar), Iraq Petroleum Company and BP struck oil in 1938 (Dukhan No. 1). There are now three crude oil reservoirs and one gas reservoir (the Khuff Reservoir). The main oil reservoir has been the Upper Jurassic Zekrit Formation of limestone and dolomite; it is the equivalent of the Arab Formation of Saudi Arabia. Production started at Dukhan field in 1947 and the first shipment of its oil was exported in December 1949. The output capacity of crude oil at Dukhan field attains 335,000 barrels per day.

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Qatar is a limestone and dolomite peninsula of subdued topography and extreme desert conditions. It projects northward from the Saudi Arabian shores of the Arabian Gulf because of north-south anticlinal structures. It has a major oil field on the west side. The coasts are generally low, with marine terraces and with sabkhas (salt flats) in several places. Major sand dunes (barchans) are situated in the southeastern part. They have moved progressively southward, under the effects of the prevailing wind. There are no sand dunes in the north because the sand supply was cut off by a rising sea level flooding the former valley of the Arabian Gulf. The capital city is Doha which has an airport and modern buildings, hotels, shops and Qatar University. It is easy to travel away from the city and see the natural environment. Most of it is quiet, uncultivated and scenically attractive. Relatively few people live away from the main towns.

The Government organisations, the university and the oil industry have a huge amount of data on the relatively small area of Qatar. All types of satellite maps, geological and geophysical maps etc on a large scale exist. This web site provides only a simple introduction with with a little more detail about sabkhas. It is hoped that an attached geological bibliography of sabkhas etc (in early stages at present) will help direct the reader to more technical information.

QATAR - INTRODUCTION Rainfall and Flash Flooding

Qatar is of course a desert region. As a consequence it has aeolian features of erosion and sedimentation. The influence of water erosion should not be underestimated though.

Annual rainfall in desert areas is usually less than 250 mm. In Qatar, which has well-developed desert and sabkhas it is less than 100 mm per annum. Rainfall is very irregular in desert areas though and there may be years with little or no rain and years with heavy falls. The illustration above shows torrential rain in Doha, the capital of Qatar. This was in April 1997. The modern city has been built at the seaward edge of desert and is normally very dry, with desalinization plants being necessary to supplement the very limited water supplies. Flash flooding occurs from time to time in desert area, only causing temporary inconvenience in cities such as Doha and Cairo, but presenting a serious hazard to travellers camping in dry valleys or wadis. There have been fatalities to people staying in or near a dry river bed and unaware that there has been a rainstorm higher up the valley.

The importance in terms of sedimentology of flash floods is that much of the desert sediment is reworked by water at distant intervals of time. Thus aeolian deposits may be less conspicuous in the geological record than expected, or at least cut into by fluvial sediments. Look for wadi channels and broad finer-grained alluvial fan and outwash deposits.

QATAR - LOCALITIES:

ZEKRIT - THE INLAND CLIFFS
Introduction

Between Dukhan and Zekrit on the west side of Qatar is an interesting dissected plateau of Tertiary limestone or dolomite standing out in the desert about 10 or 15 metres above the low plain. The rock is probably part of Dammam Formation of Middle Eocene age and is almost horizontal. The plateau stands above a plain which is only a short distance above sea level. The plain which links to the Bay of Zekrit is sandy but without surplus sand and with no large sand dunes. It provides a smooth surface to drive over. When photographed in Spring 1997 there had recently been some rain, althought this area can be extremely arid. Because of the rain some xerophyte plants are well-developed on the plain.

An old, almost planar, slope is present at the margin of the plateau in many places. This is of moderate angle and has a cover of residual carbonate rocks, so that it is effectively an extension of the reg or hamada. It is brownish in colour and it is clear that it is fairly old, probably Pleistocene.

There is clear evidence of occasional water erosion on the marginal scarp of this plateau. White concave alcoves lead down to small gullies on the scarp apron. A group of these gullies merge to a single dry stream bed which extends out across the low-angle bajada.

This photograph shows a larger dry wash extending out across the plain from a carbonate scarp of the Zekrit area. This stream bed was completely dry when photographed, but showed signed having had flood water quite recently.

QATAR - LOCATION:

ZEKRIT (ZEKREET) - THE INLAND CLIFFS
Ravine or Gully Development

In places there is development of ravines or large gullies at the margin of the upland. One of these is shown in the photograph above. The marginal slopes are white and show cleanly eroded limestone (or dolomite). These are characterised by a concave slope often with a cantilever overhang. There is very little debris on them. They clearly result from a new phase of erosion. It is interesting that the gully shown above has white curved surfaces of this type. It is likely that occasional flash floods are responsible.

QATAR - LOCATION:

ZEKRIT - THE INLAND CLIFFS
Pedestal Rocks

The scarp of Damman Limestone near Zekrit shows some unusual and interesting features. As large gullies, alcoves or concave hollows enlarge they can coalesce. Thus residual pedestal rocks can be left between them. The hard brown caliche or calcrete cap projects with cantilever overhang. Thus is formed a desert "pedestal rock".

QATAR - LOCACATION:

ZEKRIT - THE INLAND CLIFFS
Reg or Hamada of the Top Surface

The top surface of the plateau has almost no sand but has a stony desert, a reg or hamada. There are angular blocks of carbonate rocks on the surface and probably some calcrete or caliche cementation beneath this. The clasts have some brown desert varnish but this is not as well-developed as in deserts elsewhere (e.g. Egypt) which has siliciclastic clasts. Xerophyte bushes are more scattered on the reg or hamada of the plateau.

QATAR - LOCATION:

DUKHAN SABKHA (DEPRESSION IN WESTERN QATAR)
Introduction

Dukhan sabkha has long been know for its anhydrite occurrence. Good anhydrite nodules occur. An excellent introduction and description of anydrite development has been given, in French, by Perthuisot (1977). The notes which follow make use of his contribution.

The Dukhan Sabkha is situated in the western part of the peninsula of Qatar, in the extension of the Bay of Zekrit, between the oil-producing Duhkan Anticline and the main Qatar Dome (Al-Yousef, 2003). It occupies a synclinal depression of recent origin. The strata in the Dukhan region are essentially limestones and dolomites of the lower and middle Eocene with some beds of gypsiferous marls.

QATAR LOCATION:

DUKHAN SABKHA continued
Anydrite Nodules

A notable feature of the Dukhan Sabkha is the occurrence there of Recent nodules of anhydrite. Nodules of this type are common in ancient strata, and are often replaced by quartz geodes or chert or chalcedonic nodules. See for example Chowns and Elkins (1974).

This sabkha has been briefly but clearly described by Perthuisot (1977) in French. It is a good account and thus I have translated the main text of this paper, and this follows. It relates to the map shown above, and comparison can be made to the photographs of the sabkha pits given here, and which were taken by Dr. Mariam Al-Yousef and myself in 1997.

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Comparison of the major ions in terms of milliequivalents for surface water of the sabkha and for that in borehole shows that Mg ²⁺ and SO ₄ ^2- are both noticeably low. This leads to comparison with the classic sabkha dolomitisation of the coasts of Arabian Gulf. Overall the origin of the salts precipitated in the basin remains hypothetical. Probably there was a mixed origin of both marine and continental brine sources.

The Profile of the Borehole:

A shallow borehole (or excavation?) was made at about 50 metres from the edge of the halite crust to a depth of almost 1.2 metres [Perthuisot's Fig 2 provides a schematic section diagram of this].

In the upper 10 cm the following succession (downward) was found:

1. A fine crust of halite of millimetre thickness.

2. A thin superficial reworked zone with a mixture of very fine quartz, some gypsum, some halite and some microcrystalline anhydrite.

3. A compact crust, 2 or 3 centimetres thick, essentially consisting of microcrystalline anhydrite forming very flattened lenticles of light colour. These were separated by streaks of dark anhydrite mixed with detrital matter (very fine quartz and clay). Locally there are some air bubbles.

4. Lenticular pockets of a mixture of water and microcrystalline anhydrite forming a soft whitish paste. It had the consistancy of chocolate mousse and with numerous bubbles. There were some crystals of gypsum present. [This may be the equivalent of the anhydrite nodules discussed below, but they are harder.]

5. Gypsum in yellowish millimetre-sized crystals. These are mostly as flattened lenticles [lenticular gypsum] with crystal faces not obvious. It may perhaps be recrystallised gypsum. Below the gypsum continues but there is a change of colour and it becomes more bluish. The water table at the time was about 10 centimetres down from the surface.

Interpretation:

The mixture - water plus anhydrite is found in pockets above the bed which is full of gypsum sediment. It is in a zone where water only occurs as a film [capillary?] on the surface of the gypsum crystals. The simplest explanation is that the lenticular pockets [equivalent of anhydrite nodules?] of anhydrite and water originated by the reaction:

gypsum -> anyhydrite + water

The distribution of the mixture as dispersed pockets results from the endothermic character of the reaction. This probably took place in temperature conditions much higher than equilibrium conditions and this could have been attained by reducing the temperature of the surrounding milieu.

Moreover, the many crystals of gypsum which exist in the whitish mixture are intact. This suggests that each crystal of gypsum remained in unaltered totality until an almost instant reaction of dehydration. Finally, the presence of the bubbles in the mixture is consistant with the imprisonment of gas which occupied the cavities between the intact gypsum crystals.

Also there exists in the upper part of the sediment profile a true "front of anhydritisation" which progressed towards the base.

The pockets of the mixture - water and anhydrite - progressively lose their water by evaporation and is incorporated in the anhydrite crust already formed.

The Conditions of Transformation

This takes place in the presence of water; the crystals of gypsum attacked by anhydritisation always occur in the humid capillary zone of the profile.
The composition of this capillary water is very similar to that of the water table, but having perhaps concentrations a little higher (see table).

The temperature of the groundwater was 24 degrees C on 31 st December, 1976. But the temperature of the ground would have attained much higher values in summer; meteorological instruments registered a soil temperature of 40 degrees C at the surface and 30 degrees C at 50 cm. depth. Butler (1969) reported some temperatures of more than 50 degrees C at the surface of the sabkha in the Trucial Coast (UAE).

Thus the conditions of temperature and salinity at the start of the reaction are clearly beyond the conditions of equilibrium. However, this reaction stops a certain time because of the lowering of the temperature and the dilution of the brine which it causes. Without doubt the lowering of the temperature and the winter rains contribute equally to this cessation.

There remains one irritating problem: it is the exclusive localisation, at least at present, of the anhydritisation of gypsum, in the conditions of the at the surface at the borders of the Arabian Gulf, even though there exist in other regions of the globe some conditions that are similar in temperature and salinity in the sabkhas of North Africa for example [there are traces of anhydrite in a coastal salt lake of Libya, but I am not aware of any significant quantities]. Now there is one marked difference between sahkhas of the two regions: most of the sahkha of North Africa have an environment that is extremely reducing, rich in organic matter and producing significant quantities of hydrogen sulphide. The sabkhas of the Arabian Gulf are in comparison much more oxygenated, lacking in odour and generally with light-coloured sediments. The intuitive hypothesis is that the transition from gypsum to anhydrite is improbable or at least more difficult to take place in a reducing medium in which S ^2- is the stable form of sulphur.

Thus the particular geochemical conditions of the region explain the localisation of the gypsum-anhydrite transition in the present-day environment of the Arabian Gulf. [end of main text]

Here in the the salt-encrusted margin of a salt lake of the Duhhan Sabkha of Qatar, a continental sabkha, anhydrite was found by Perthuisot in 1977. If we dig through the thin white salt crust (there has been recent rain and the salt has only just start to crystallise again) into muddy sand below some white calcium sulphate can be seen.

In the clayey sand a few centimentres below the surface there are these white nodules with sutures between them. These are Recent anhydrite nodules like those described in previous literature from the Abu Dhabi area of the Emirates. These are not crumbly and prone to distintegrate as is usually the case and very much resemble ancient nodules of anhdyrite or secondary gypsum. (In some ancient sequences where there has been replacement of quartz from the outside and dissolution of the calcium sulphate these can form geodes.)

[DUKHAN SABKHA] COMPARISON WITH EGYPTIAN SABKHAS

Gypsum Nodules - First Depression Sabkha, Northern Egypt

In the First Depression of the semi-arid coastal zone of northern Egypt gypsum nodules are forming. The diagram above shows a profile through the sabkha sediments with brine chemistry data. Salinity increases upwards from the shallow water-table. The gypsum nodules are forming by the "per ascensum" mechanism in the capillary zone above it. There is an obvious loss of calcium from the interstitial brines at the level of the nodules.

The nodules in the Dukhan Sabkha of Qatar may have originated as gypsum and been dehydrated to anhydrite by the very concentrated brines and the relatively high temperature, or they may forming as anhydrite in that high-salinity setting.

Further into the salt lake the anhydrite is soft and creamy and solid nodules cannot be extracted. The nodules may be more solid in this marginal area because of incoming runoff water which may from time to time convert the anhydrous calcium sulphate into gypsum. Diagenesis from anydrite to gypsum and back again is likely to lead to lithification by crystal growth.

QATAR - LOCATION:

Dukhan Sabkha - Enterolithic Veins

Photographs above show enterolithic veins in secondary gypsum (after anhydrite, after primary gysum) in Lower Purbeck Formation at Worbarrow Tout , Dorset, UK. With them are photographs of similar enterolithic veins forming at the present day in a sabkha of desert loess (blown wind-blown silt) between El-Alamein and Alexandria on the Mediterranean coast of Egypt, and at Dukhan Qatar where they are of anhydrite. (See - West, Ali and Hilmy, 1979.)

Nodules, chicken-wire structure and enterolithic veins are all closely related. They are early displacive fabrics formed by continued growth of calcium sulphate from capillary water in sabkhas or salt-flats. Within them the sulphate is extremely pure because this displacive material has grown in place and contains hardly any sediment. The pure gypsum within nodules is known as alabaster and is used for carving. Enterolithic veins are over-developed displacive nodules of white soft gypsum or anhydrite which have burst out and pushed on into the associated soft sediment of a sabkha (West, 1965) . They commonly occur approximately parallel to the sediment surface, although in the Purbeck Formation there has often been a tendency towards obliquely-upward movement. Their lithification into strong but soluble rocks is the result of later diagenesis, i.e. gypsum-anhydrite-gypsum or anydrite-gypsum.

(Some questions: Why are the modern examples from Egypt and Qatar in a browner matrix than the ancient Purbeck ones (buff at the surface and grey underground)? The Purbeck veins have no porosity - why? The Purbeck enterolithic veins are porphyrotopic (have large crystals scattered through them) particularly at the margins of the veins - what is the reason for this?)

QATAR - LOCATION:

Umm Said Sabkha - Introduction

QATAR LOCATION:

Umm Said Sabkha - Northern Part - Introduction

The northern part of Umm Said Sabkha, north of the Industrial area does not have the huge barchan sand dunes which characterise the southern part. This is because the rise in sea level in the Holocene, the Flandrian Transgression, flooded the Arabian Gulf and thereby cut off the sand supply by progressive dune movement from the north. Older sand dunes have moved slowly southward and occupy the southern part of the Umm Said Sabkha. The dunes, though, have travelled southward past the northern part when sea-level was a little lower than at present. The latest rise in sea-level has produced an embayment of shallow water in the northern Umm Said sabkha area. Since then there has been longshore drift in a southward direction, as would be expected with the Shamal winds (from the north). Several recurved spits of beach sand from the north have developed at different times. These spits have built up the northern part of Umm Said Sabkha. However there has been some coastal retreat which has resulted in truncation of the older recurved spits. These features can be seen clearly in the aerial photograph above (based on a modified version of a Google Earth image). Go to Google Earth to see the details of this interesting area.

There has, incidently, been some minor progradation in the area of mangroves in the most northerly part of the sabkha. The small mangrove plants have been artificially introduced here, and they successfully produce a locally green environment in a rather barren sandy region. There are some low-salinity springs from the groundwater just here which favour the mangrove survival. Seaward of the mangroves there is a small barrier beach.

QATAR LOCATION:

Umm Said Sabkha - Intertidal Zone

QATAR LOCATION: UMM SAID SABKHA

Waste Disposal and Pollution

QATAR - LOCALITIES:

UMM SAID SABKHA
Desert Roses - Gypsum Crystals

Large desert roses or gypsum sand crystals in the sand of a supratidal sabkha at Umm Said in eastern Qatar. These large crystals are sublenticular and flattened at about right-angles to the c-axis. They include sand grains (poikilotopically) within the crystals and cleave readily along the 010 (side pinacoid) direction. They split and fan open along these characteristic cleavage planes of gypsum. The large gypsum sand crystals are best developed when the water-table is between about half a metre (one and a half feet) and a metre beneath the surface, and not when it is closer. These were found while returning across the sabkha in the late afternoon when the low sun shows them up clearly with good shadows.

It is interesting that displacive nodules of gypsum or anhydrite do not seem common in this sabkha and the gypsum sand crystals form instead. It is not clear as to what causes the splitting of the crystals; perhaps it is temperature changes at the surface. Buried examples are not so severely split.

QATAR - LOCATION:

UMM SAID SABKHA
Cyanobacterial Mats

Polygonally cracked cyanobacterial mats ("algal mats" in old literature) occur in hypersaline water at the intertidal margin of a lagoon near Umm Said, Qatar. This part of the lagoon seems too saline for browsing molluscs and there are none of the cerithid gastropods which are abundant in the intertidal zone elsewhere in Qatar. Because this lagoon, unlike many ancient ones, is close to the sea and has a direct narrow connection with it true lunar tides are significant (tidal range - 0.5 to 1 m along the coast of Qatar). Note that these polygons are rather small, although it is not clear as to what is the reason for this.

Here a closer view of the mat. It has a brown soft leathery surface with green filaments and you can pull up polygons of this with your hand. Some washed-in leaves of sea-grass are incorporated. Underneath is sand which contains both siliciclastic (quartz etc.) and carbonate (aragonite and calcite) grains. The water which is close to the surface is milky white with fine carbonate.

This mat in Qatar has not built-up a good laminated structure beneath, as occurs elsewhere such as Abu Dhabi and Kuwait (shown above). Note that the open sea of the Arabian Gulf is not far away. It is just over the beach ridge that can be seen at the level of the hat of the researcher in the upper photograph. Perhaps the mat is not permanent because it is periodically destroyed by storms. Alternatively, the mobile, wind-blown sand in this area may cause some periodic destruction.

Left: Very small stromatolites growing at the present day on an old steel drum that has sunk into a soft sabkha at Umm Said, Qatar. The iron does not seem to inhibit the cyanobacterial growth. A hard or raised surface for attachment seems favourable for stromatolite growth. Right: Similarly, tree stumps submerged in a hypersaline lake of the Lower Cretaceous Purbeck Formation of Dorset, UK, have provided attachment areas on which stromatolites have formed.

LOCALITIES:

UMM SAID SABKHA
Fish Remains

At the present day some fish are being fossilised in hypersaline lagoons and on sabkhas. A notable example is the Kara Bogaz lagoon which has a constant supply of fish from fish from the Caspian Sea brought in by the inflowing current. Here, on the Umm Said Sabkha in Qatar dead fish, like this sting ray, are being buried in the sands near the seaward margin of the sabkha. ? The remains of it are be preserved in a red sabkha sandstone. Some Devonian fish of Scotland may have been washed into lagoons and sahkhas in a similar manner (imagine this is a Cephalaspis or Drepanaspis!).

QATAR - LOCALITIES:

BARCHAN SAND DUNES
south of Umm Said

The large barchan sand dune (crescent-shaped with the horns downwind) in Qatar, shown above, is about 30 m (a hundred feet) high. It contrasts with the very small sand dunes of Kuwait, an area of deflation. The windward side and the horns are convex and have a wind-rippled surface. In one photograph the four-wheel drive car, has driven over one low horn and is now heading for the high avalance lee face. Some sand seems to have avalanched recently so as to change the slope from the initial angle to the slightly lower "residual angle after shearing". This has produced a conspicuous lineation.

Depending upon how near they are to the coast these large sand-dunes in southern Qatar either migrate into the sea, extending a sabkha, or move on into Saudi Arabia and the great Rub' Al Khali or Empty Quarter. Dunes can migrate at a rate of several metres a year.

Questions: The flat area to the left of the car is a sand sabkha. How will this appear when the dune has moved over it ? When the dune has gone will it have left any indications of its former presence? How will you know that there has been aeolian action here and not just a sabkha facies?

Let us have a look at a section through part of a similar barchan but in a different desert locality. We have arrived at a sand quarry in an unconsolidated dune. The floor of the quarry is the damp sabkha surface, on which I am standing. Surprisingly, the sand when quarried will stand with a very steep face, because of a small moisture content or some incipient mineral cementation. I am holding my compass-climometer roughly parallel to the bedding of the foreset beds (behind the left side truck is the avalanche lee face of the barchan) to show you that the dip is at about 30 degrees. You can tell the approximate direction from my shadow (it is about mid-day). This ties in with the direction of the prevailing Shamal winds blowing down the Arabian Gulf from the north-west or north-northwest. The relative stability of these strong winds is the reason for the formation of well-shaped barchan dunes.

This sand is clearly suitable for some purposes without further investigation. It just could be unwise, however, to use it in concrete reinforced with steel without first checking for the presence of evaporite minerals. Some gypsum and to a lesser extent halite can be incorporated in dunes as a result of these minerals being blown off the interdune sabkhas or salt-flats. Near to sabkhas small lenticular crystals of gypsum are sometimes abundant constituents of small sand-dunes. It is easy to get a rough indication of whether gypsum is present. The late Professor Douglas Shearman used to place sand on a hotplate and any gypsum grains would turn into obvious white plaster of paris. For semi-quantitative determination X-ray diffraction is quick. Chemical analysis may be needed for accurate measurements of gypsum and/or halite content. These evaporite minerals can be serious causes of defects in buildings in the Middle East and North Africa. They are not always easily washed out because much freshwater is needed and gypsum does not dissolve readily.

It is surprising that in Qatar it is possible to quarry dunes at a steeper angle than the angle of repose of loose sand! You would not expect this. The quarry has even produced some small vertical faces over which sand dribbles. There has to be some adhesion of the sand for this to be possible. There might be very light cementation by carbonates, or by gypsum or halite. Some moisture can occur under sand dunes, but it would probably soon dry out in this warm desert environment.

Notice in one of the photographs the foresets (planar-tabular and wedge-tabular) which are at the usual angle of repose of about 30 to 34 degrees. The bounding surface shows that there have been at least two separate phases of accretion.

Deflation Features - Qatar

There has been severe deflation in the depression of the Dukhan Sabkha. There are abrasion features in limestone or dolomite that resemble abrasion marks on the rocks of Mars. The wind direction was from north to south (right to left in the photographs).

Karstic Features - Dolines, Poljes and Caves

Large cave or duhul in Qatar, Arabian Gulf. Karstic features are widespread on the peninusula of Qatar because Tertiary gypsum under dolomite and limestone has been extensively dissolved in previous wetter climatic conditions. Only about 10 substantial caves are known but many have probably been filled with blown sand and many have collapsed to produce some of the thousand of depressions or dolines of northern Qatar. This particular one, named in Arabic, the dark cave, is about 30 metres deep but filled with sloping loose sand at the bottom. Pools of freshwater at the bottom of such caves were once used by Beduins for drinking water. There has been geological work on these caves (Embabi and Ali, 1990. Geomorphology of Depressions in the Qatar Peninsula. In Arabic).

It is probable that many similar dolines exist in southern Qatar but that is an area of major sand dunes. The sand dunes are travelling southward and their northern limit is at about the centre of the peninsula, because of closure of sand supply across the Arabian Gulf area by the Holocene rise in sea-level. The dunes and interdune sabkhas of the south will cover the dolines so nothing of them is visible at the surface.

ACKNOWLEDGEMENTS

For kind hospitality, field leading and assistance in many respects in Qatar I am particularly grateful to Dr. Mariam Al-Yousef and members of her family. Reference should be made to her Ph.D. Thesis - Mineralogy, Geochemistry and Origin of Quaternary Sabkhas in the Qatar Peninsula, Arabian Gulf. See this thesis for more details on the sabkhas of the area. Dr Ali Akbar has discussed Qatar with me over some years and was very helpful when I was in Doha. I particularly appreciate the kind help in the field by Dr. Sobhi.

Copyright © 2011 Ian West, Catherine West, Tonya West and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged. No permission can be given for reproduction of any images of the Lulworth Cove area in books or in other websites, for special reasons.

Disclaimer: Geological fieldwork involves some level of risk, which can be reduced by knowledge, experience and appropriate safety precautions. Persons undertaking field work should assess the risk, as far as possible, in accordance with weather, conditions on the day and the type of persons involved. In providing field guides on the Internet no person is advised here to undertake geological field work in any way that might involve them in unreasonable risk from cliffs, ledges, rocks, sea or other causes. Not all places need be visited and the descriptions and photographs here can be used as an alternative to visiting. Individuals and leaders should take appropriate safety precautions, and in bad conditions be prepared to cancell part or all of the field trip if necessary. Permission should be sought for entry into private land and no damage should take place. Attention should be paid to weather warnings, local warnings and danger signs. No liability for death, injury, damage to, or loss of property in connection with a field trip is accepted by providing these websites of geological information. Discussion of geological and geomorphological features, coast erosion, coastal retreat, storm surges etc are given here for academic and educational purposes only. They are not intended for assessment of risk to property or to life. No liability is accepted if this website is used beyond its academic purposes in attempting to determine measures of risk to life or property.

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Webpage - written and produced by:

Ian West, M.Sc. Ph.D. F.G.S.

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at his private address, under the auspices of the National Oceanography Centre, Southampton (NOCS),and web-hosted by courtesy of the Information System Services (ISS) of Southampton University.