Ian West, with contributions by Maryam Mustafa Al-Mulla, 2013. Qatar - geology, sabkhas, evaporites and desert environments. Internet Webpage. http://www.southampton.ac.uk/~imw/Qatar-Sabkhas.htm. version: 19th December 2013.
Qatar - Sabkhas, Salt Lakes and Arid Environments
Dr. Ian West,
Romsey, Hampshire

and Visiting Scientist at:
Faculty of Natural and Environmental Sciences,
Southampton University,

and

Dr. Maryam Mustafa Al-Mulla,
University, Doha, Qatar


Website hosted by courtesy of Information Systems Services, Southampton University
Website archived at the British Library

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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(You can download this educational site to SurfOffline or similar software to keep an offline copy, but note that updating of the live version takes place periodically.)

Barchan sand dune south of Umm Said Sabkha, Qatar

Windward to left horn side of a barchan, Umm Said, Qatar. Note the wind-rippled, convex surface.

Umm Said supratidal sabkha, Qatar, with use of mobile phone




Click on images to enlarge!

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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.

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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.

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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.

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INTRODUCTION:

Arabian Gulf - General

The shallow Arabian Gulf at Doha, Qatar, with seawater of raised salinity

Arabian Gulf near Qatar, with motor Dhow

General morphology of the Arabian Gulf, based on old map

The Arabian Gulf is of particular geological interest in being an example of a rather restricted sea in an arid climate. It therefore has a great variation in salinity (and temperature) in different areas. Clarke and Keij (1973).

These authors have recognised within the Arabian Gulf an arbitary three-fold subdivision of the degree of restriction of the basin. For full details refer to their work, which is a section (p. 27 et seq.) in the book of Purser (1973): The Persian Gulf. Here is a summary regarding the three division:

Normal Marine Environment; salinities up to 50 ppt.

Salinities in this range (cf. about 35 ppt. in the open ocean) occur throughout the greater part of the Arabian Gulf. The associated fauna and flora is an impoverished Indo-Pacific one lacking many characteristic and volumetrically important elements, e.g. the bivalve Tridacna (the Giant Clam), the gastropod Lambis, and the algae Halimeda and Penicillis. Although listed here as "normal", the bulk of "normal" Arabian Gulf water is already slightly altered, salinity everywhere being over 39 ppt. within the Straits of Hormuz.

Within the range of the "normal" environment, it was found that at approximately 45 ppt salinity, especially at the entrance to the Gulf of Salwa, several important groups disappear. These include certain foraminifera and gastropods of the genera Strombus, Conus and Xenophora, all pectinid bivalves and all echinoids, with the exception of the infaunal Clypeaster.
[Incidently re the British Jurassic; thus the Portland oolite, the Corallian oolites, the Inferior Oolite with pectinids and ooids, were probably not deposited in seawater above 45 ppt. The ooid area off the UAE is probably mostly above 45 ppt. Of course the British Jurassic was at higher latitude, about 35 degrees, and near the northern ooid limit.]

Restricted Environment, salinities 50 to 70ppt.

Faunas are dominated by imperforate foraminifera and gastropods. There is widespread synsedimentary lithification of the Holocene sediments (i.e. hardground development); the rock substrate supports a prolific growth of brown algae which are the favoured habitat of many imperforate foraminfera and certain bryozoans. The hard substrates are unfavourable for bivalves. Cerithium-type (turreted) gastropods are particularly abundant in shallow subtidal and intertidal environments.

Highly Restricted Environment, salinities ca. 70 ppt.

Little is known regarding the faunas inhabiting restricted lagoonal areas with salinities in excess of 65 ppt. Certain isolated lagoons, including the north side of Khor Odaid in SE Qatar, seem to be faunal deserts containing little more than cyprideid ostracods.
[Compated to British late Jurassic palaeoenvironments, this is basal Purbeck-type, hypersaline facies with ostracods.]

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INTRODUCTION:

Topographic Maps

A general location map for Qatar, showing the position of some sabkhas and other features

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INTRODUCTION:

Aerial Photographs and Satellite Photographs

Satellite view of Qatar, showing the Dukhan Sabkha with a gypsum bourrelet to the south-southeast

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INTRODUCTION:

Geological Maps

A simplified geological map of the Qatar peninsula

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INTRODUCTION:

Qatar - General Introduction

* Qatar forms an exposed part of the Arabian shelf between the Arabian shield and the mobile belt of Iran. It is centred at about 23 degrees N, 51 degrees E. Topgraphically, Qatar has a low relief landscape with a maximum elevation of 103m. above sea level. Structurally, Qatar appears as elliptical anticlinal arch with a N-S main axis (LeBlanc, 2008).

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INTRODUCTION - STRATIGRAPHY OF QATAR MESOZOIC AND TERTIARY STRATA:

Stratal Sequence

Tertiary stratigraphy of Qatar, simplified

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INTRODUCTION - Qatar

Climate - Rainfall and Flash Flooding

Flash-flooding in Doha, Qatar, 1997

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.

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PETROLEUM GEOLOGY OF ARABIAN GULF:

Simple Introduction

The general structure of the Arabian Gulf region in terms of plate tectonics and generation of oil

The simple, broad-scale map above shows how oil generation occurs with progressive burial and heating in the subducting Interior Platform. It is then trapped there in the broad anticlinal traps of Saudi Arabia, Qatar, Kuwait etc. and in the Mobile Belt of Iran and Iraq with its many parallell, tighter and faulted anticlines. The Interior Platform reservoirs are in the Jurassic Arab Formation, a very shallow or sabkha carbonate facies, and in Cretaceous marine sandstones. In the Mobile Belt the oil has often risen to a higher level and is present in the Tertiary Asmari Limestone and other high-level strata. Generally it is absent beyond the Zagros Crush Zone.

Thus, overall the picture is very simple and here is the simplified explanation (although in detail and in reality it is obviously more complex):

A shallow tropical to subtropical sea, the Arabian Gulf and its predecessors, receives strong sunlight and is highly productive of organic life, including small organisms in microbial mats and plankton. Since the Jurassic the organic-rich sediment has been accumulating in the strata here. More recently, in the Tertiary there was development of the Red Sea. At the same time the Arabian Plate was slowly subducted under the Zagros Mountains to the northwest. The organic-rich sediments have been moved northwest and down with this subduction. When they were buried to depths of about 3km they started to generate oil. Some of this moved up into the broad gentle anticlines of the Interior Platforms (Saudi Arabia etc) and some into the tight folds of the Mobile Belt. Upward movement (halokinetic) movement of the Hormuz salt has formed salt dome traps in places and elsewhere has assisted in the development of broader anticlinal traps.

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PETROLEUM GEOLOGY:

Oilfields of the Arabian Gulf

Introductory map showing oilfields, oil refineries and pipelines of the The Arabian or The Persian Gulf, based on old data

Following the simple geological interpretation, here is a simple map is to set the scene regarding the Arabian or Persian Gulf oilfields. It is an old map and there are now new or enlarged oilfields etc, but the general pattern is the same. Oil or refined oil products leave the Gulf by pipeline or by ships which have to pass through the narrow Strait of Hormuz.

A general and simplified map of the Middle East showing how oil reservoirs young on the subducting Arabian Plate towards the Zagros Front

Another very simpified and old map shows the general locations and geological ages of the reservoirs in the Middle East. The Arab Formation of northeastern Saudi Arabia and adjacent area is an Upper Jurassic shallow carbonate facies. Cretaceous Sands form the reservoir in the Safaniya (Saudi Arabia) and Burgan (Kuwait) fields. The narrow, faulted anticlines of Iran and Iraq have mainly Tertiary reservoirs, but also some that are of Cretaceous age. The greater tectonism and faulting is the main reason why the reservoirs are higher there than in the structurally gentle Saudi Arabian and Qatar areas.

An introductory map showing the locations of the main oil fields of The Gulf, with emphasis on the Qatar and adjacent region

The map above shows the major oil and gas fields in the central and northern parts of the Gulf. It is, however, based on a 1977 map and does not the new oil and gas fields. However, it is intended to reveal to some extent the relationship of the smaller, but still quite large, Dukhan Field of Qatar to the famous Ghawar Field of Saudi Arabia. The Arab Formation (Upper Jurassic, Kimmeridgian) contains the reservoir horizons. The Hith Anhydrite (uppermost Jurassic) or equivalent is the seal or cap rock in both oil fields.

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PETROLEUM GEOLOGY:

Qatar - Dukhan Field

Petroleum exploration in the Dukhan Oilfield, western Qatar

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 65km long and 5km wide. Oil was first produced at Dukhan No.1 Well in 1949 (Stacey International).

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.


Footnote: the history of the Dukhan Field by Henry Longhurst (1959). - "The New Middle East" [p. 210]
"The Iraq Petroleum Group also operates through associated companies in Qatar,the Trucial Coast and South-Eastern Arabia. Qatar juts into the Persian Gulf between Bahrain and the Trucial Coast. Its oilfield, run by the Qatar Petroleum Company, was discovered before the war but was not brought into production until 1950. It lies on the west side near the Jebel Dukhan, the Mountain of Smoke. The wells and installations of the Dukhan field were put out of action in 1942 and after the war all had to be done again. Communications and services had to be restarted too, and within, a remote and waterless peninsula and masses of material and equipment imported. Drilling began again in 1947 and the first oil shipment went out in the last week of 1949. Production in 1950 was 1,617,000 tons. In 1956 it was 5,784,000 tons, and the annual income has grown from £400,000 in 1950 to nearly £ 15 million. As there are 35,000 people on the peninsula, the revenue of this barren little patch of sand, of which not ten people in a million had heard before the coming of the oil, works out at about £450 a year for every man, woman and child."



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PETROLEUM GEOLOGY:

North Field, Qatar (Large Offshore Gas Field)

The large North Field is offshore to the NNE of Qatar and almost as large at the peninsula of Qatar itself. The boundary of the field just comes onshore in the northeastern corner of the Qatar land area and extends about 150km offshore. There are ten or more producing gas wells within it and many abandoned wells. A good location map, showing wells, is Fig. 2 in Alsharhan and Nairn (1994). Details are not shown in the present account.

The gas is in the Khuff Formation of Permo-Triassic age. There is gas and condensate production from carbonates of this formation. There is also some oil in the thin (Cretaceous) Albian, Nahr Umr clastic reservoir (Alsharhan and Nairn, 1994).

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PETROLEUM GEOLOGY:

Halul Island Gas Field (Qatar)

The Eastern and Western Hormuz Salt Basins of the Arabian Gulf, and the relationship to Qatar and to Halul and Shraouh Islands of Qatar

Offshore gas reserves lies in four fields around Halul Island, to the east of the country: Id al-Shargi, Maydan Mahzam, Bul Hanine and Al Bunduq (shared with Abu Dhabi) (Stacey International, 1986). Nearby is Shraouh Island.

Southwestern Iran and adjoining parts of the Arabian Gulf region are characterised by large numbers of emergent salt plug. Halul Island and the Shraouh Island are two such salt plugs situated offshore to the east of Qatar. For past references on the salt domes or salt plugs in the region see Nasir et al. (2008), a detailed paper on the geology and petrology of the Hormuz dolomite (Infra-Cambrian) of Halul and Shraouh Islands. The Hormuz Salt Series, the cause of the diapirism, is regarded as Infra-Cambrian but the date is not well-established. It is a very thick sequence, ranging in thickness from about one kilometre to two kilometres in the Zagros Range of Iran. There are two major salt basins, as shown on a map here, and Halul and Shraouh Islands are near the (western) edge of the Eastern Salt Basin.

Halul Island is located at the eastern part of Qatar offshore Nasir et al. (2008). It covers an area of about two square kilometres and consists of small hills that reach a maximum elevation of about 55m. Beneath it a dome of Hormuz salt. At the surface the salt itself is not a feature but associated dolomite and clastic rocks of the Hormuz sequence are exposed. Lying unconformably above them are Miocene dolomitic limestones. There are also Pliocene to Recent sediments and sedimentary rocks made of carbonate sand and ranging in colour from light brown to white.

Nasir et al (2008) reported that the Infra-Cambrian, Hormuz dolomites of both Halul and Shraouh Islands include sequences of carbonate-evaporite sabkha cycles. Stromatolitic bindstones are present. Dolomitisation was attributed mainly to a sabkha setting with brine reflux. Organogenic dolomitisation could not be ruled out. The fabric of the Hormuz dolomite exposed on the islands was described as generally equigranular hypidiotopic to xenotopic. In spite of the equigranular statement, there is comment about polymodality of dolomite, perhaps due to multiple dolomitisation. There are relicts of allochems and both limpid and inclusion-rich areas within crystals. There is also some alkali feldspar and gypsum. Sr isotope data is also provided but does not give simple results. It would seem that these dolomites are complex in origin and relatively difficult to interpret. Perhpas this is not suprising for a halokinetic structure. See the Nasir et al (2008) paper for more details. The emplacement of the salt plug is believed to be Pliocene-Pleistocene, very recent compared to the antiquity of the salt!

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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.

In addition to the large quantity of published data, there is also a large amount of unpublished 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. An associated geological bibliography of sabkhas etc (in early stages at present) will help direct the reader to more technical information. Some specific references regarding the geology of Qatar are progressively being added to the end section of the present webpage.

Bay of Zekrit, west coast of Qatar

Luncheon on the Sabkha, Bay of Zekrit, Qatar

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INTRODUCTION:

Sabkhas - Introduction

Um Said Sabkha, southeast Qatar, with large barchan dunes in the distance

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).

Car in Umm Said Sabkha, Qatar

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.

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LOCATIONS:

ZEKRIT - THE INLAND CLIFFS
Introduction

A low dry plain in western Qatar with distant escarpments of Dammam carbonates of Eocene age

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.

Limestone plateau near Zekrit, Qatar, showing old stabilised slopes and white areas where there is rejuvenated erosion

Old slope with reg or hamada at the margin of a carbonate plateau, near Zekrit, Qatar

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.

An scarp of Dammam Formation Limestone, near Zekrit, Qatar, with new white, concave erosion surfaces of developed on an older slope - continues on to next picture Continues from last picture - gullies on the apron of a Zekrit inland cliff join to form a narrow dry stream bed heading across a bajada, desert near Zekrit Qatar

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.

A dry wash extending out across the plain from a carbonate scarp near Zekrit, Qatar

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.

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LOCATION:

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

Gully development in a desert plateau, near Zekrit, Qatar

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.

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LOCATION:

ZEKRIT - THE INLAND CLIFFS
Pedestal Rocks

Dry cliffs of limestone near Dukhan, Qatar, with degradation in the hot sun and occasional erosion by flash floods

Pedestal rocks of Tertiary limestone, at a scarp near Dukhan and Zekrit Bay, Qatar

Pedestal Rock at at a limestone scarp, between Dukhan Sabkha and Zekrit Bay, Qatar, with geologist for scale

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".

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LOCACATION:

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

The surface of a plateau of Dammam limestone (or dolomite) near Zekrit, Qatar

A reg or hamada with desert varnish seen in a close view of the surface of the Dammam carbonate plateau near Zekrit, Qatar

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.

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LOCATION:

DUKHAN SABKHA (DEPRESSION IN WESTERN QATAR)
Introduction

An aerial view of Dukhan Sabkha in the western part of Qatar

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.

Simplified geological map of the region around the Dukhan Sabkha, Qatar

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.

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LOCATION:

DUKHAN SABKHA continued
Anydrite Nodules

Soft nodules and enterolithic veins of anhydrite in muddy sand of the Recent Dukhan Sabkha, Qatar

A pit at the margin of a salt lake on the Dukhan Sabkha, Qatar, reveals anhydrite 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).

Recent anhydrite nodules at margin of the Dukhan Sabkha, Qatar

Recent anhydrite nodules, Dukhan Sabkha, Qatar

Recent nodules of anhydrite, one with chicken wire structure, Dukhan Sabkha, Qatar

Close-up view of the Recent anhydrite nodules of the Dukhan Sabkha, Qatar

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.




The Dukhan Sabkha (Qatar) and the Transformation: Gypsum - Anhydrite + Water.

By Jean-Pierre Perthuisot

1. The Geological Environment of the Dukhan Sabkha

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. 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.
In fact, it is probable that certain indurated beds, generally dolomitic, attributed to the Tertiary, correspond to pedogenic crusts developed in the course of Quaternary climatic cycles. Furthermore there are present within the depression and on its borders some modern aeolian sands.
Then, on the borders of the sea, are vast plains resulting from recent sedimentation (with carbonate and shell sediments) deposited during the Flandrian Transgression (with some recent epierogenic movements) and attaining a surface of about 2 metres above present sea level (see the map above).

2. The Dukhan Sabkha:

Simplified geological map of the region around the Dukhan Sabkha, Qatar

The Dukhan Sabkha is large and occupies a superficial area of about 130 square kilometres. Some of the surface is actually at about 2 metres below sea-level, with a large part at zero in relation to sea-level. On the borders are Eocene carbonates (with Quaternary crusts - see photograph) which often have the form of small cliffs. To the north of the Dukhan Sabkha is a straight depression at about plus 3 metres and this extends to the Bay of Zekrit.

a. The Origin of the Basin

Although the depression as a whole is clearly structural, the origin of the sabkha basin itself remains hypothetical. One could invoke a karstic origin resulting from the action of superficial water; there are numerous depressions on the Qatar peninsula which can be regarded as poljes [the Qatar solution depressions are often referred to as dolines, and associated with collapse after dissolution of gypsum beds]. It may be necessary to envisage a much wetter climate in the past than that of the present day.
Probably the formation of the basin has taken place in several stages. The valley which contains Dukhan Sabkha and the Bay of Zekrit corresponds to an ancient outlet of the basin. It does not seem that the Flandrian Transgression invaded it. There are no signs of marine sediments along the margin of the sabkha. Apart from possible subterranean supply there has been no direct a feeding of the sabkha from the sea.

b. The Sedimentary Fill of the Sabkha:

In the absence of a major borehole the thickness of sedimentary fill cannot be determined and only the upper part is accessible to current investigations. On the borders there is clayey-sandy colluvium with angular blocks of limestone at the base of cliffs. This detrital material is succeeded towards the centre of the basin by a sediments that is essentially gypsiferous and very poor in detrital particles. This fills most of the basin and is at least 1 to 2 metres thick as shown by the trench of a pipeline which crosses the sabkha in a north-south direction.
Finally, in the northeastern part of the sabkha the gypsum is covered by a crust of halite attaining several decimentres in thickness. This zone is more often covered by residual concentrated brine and is a salt lake. Elsewhere halite forms only a thin crust and is ephemeral.

Analyses of brine from Dukhan Sabkha, Qatar, after Perthuisot (1977)

c. The Origin of the Salts

One may think that a large part of the salts left by evaporation in the basin have come from the refluxing of the basin borders (clays with gypsum in particular) but a partly marine origin cannot be excluded a priori, considering the low altitude of the sabkha and the proximity of the sea. One must, however, expect a quantity of halite greater than that which is observed if the totality of salt coming from the marine environment was precipitated in the basin. Later studies may, perhaps, give more precise information on this point.

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.

Shallow pit showing the profile through the top sabkha sediments with anhydrite, Dukhan Sabkha, Qatar

Profile through the uppermost part of Dukhan Sabkha, northern part, about 50 metres from the edge of the salt lake, Qatar, modified after Perthuisot (1977)

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).

Probable conditions in the Dukhan Sabkha where the anhydrite occurs in relation to gypsum-anhydrite stability curves; after Perthuisot

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]

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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.)

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[DUKHAN SABKHA] COMPARISON WITH EGYPTIAN SABKHAS

Gypsum Nodules - First Depression Sabkha, Northern Egypt

A reg or hammada, stony desert, south of the Qattara Depression, Western Desert, Egypt, with party stranded by a puncture

A silicified Miocene tree in the Western Desert, Egypt, south of the Qattara Depression, and probably exposed by deflation

A part of the Mediterranean coastal zone of northern Egypt, between Alexandria and El Alamein, and a site of gypsum nodule precipitation

Yehia Ali at a pit in the First Depression sabkha, between Alexandria and El Alamein, northern Egypt, 1978

A pit in the First Depression sabkha, Egypt, showing the development of gypsum nodules by per ascensum capillary mechanism

A cross-section of an impregnated hand specimen of recent gypsum nodules from the First Depression sabkha, northern Egypt between Alexandria and El Alamein

Evidence of precipitation of nodular gypsum in a sabkha of 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 anhydrite to gypsum and back again is likely to lead to lithification by crystal growth.

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LOCATION:

Dukhan Sabkha - Enterolithic Veins

Enterolithic veins developing in muddy sand of Dukhan Sabkha, Qatar

Displacive gypsum forming incipient enterolithic veins, sabkha of the First Depression, El Hammam, between Alexandria and El Alamein, northern Egypt, 1978

Comparison of ancient and modern 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.)

Early enterolithic veins now preserved in late secondary, porphyrotopic gypsum, Worbarrow Bay, Dorset Small entolithic veins showing details of growth, Purbeck, Worbarrow Bay, Dorset

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?)

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LOCATION:

Umm Said Sabkha - Introduction

Augering into a sandy part of the Umm Said Sabkha, eastern Qatar, 1997

A small northern branch of the huge Umm Said Sabkha, Eastern Qatar, 1997

Industrial use of a raised coastal bar at the margin of the Umm Said, supratidal sabkha, eastern Qatar

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QATAR LOCATION:

Umm Said Sabkha - Northern Part - Introduction

Recurved sand spits at the northern part of Umm Said Sabkha, Qatar, aerial view, 2010

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.

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LOCATION:

Umm Said Sabkha - Intertidal Zone

A narrow zone of polygonally cracked, microbial mats, intertidal zone, Umm Said, Sabkha, Qatar

Microbial mats at the margin of a small lagoon, northern part of Umm Said, Sabkha, Qatar

A microbial mat or cyanobacterial mat dug out of the intertidal sabkha of a small lagoon in Kuwait

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LOCATION:

UMM SAID SABKHA

Waste Disposal and Pollution

Waste disposal, probably fly-tipping on the Umm Said, supratidal sabkha, Qatar, 1997

Yellow pollution in gypsum deposits of the Umm Sabkha, Qatar, 1997

Gypsum crystals grow on a plastic cup at a waste disposal location in the Umm Said supratidal sabkha, Eastern Qatar

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LOCATION:

UMM SAID SABKHA
Desert Roses - Gypsum Crystals

Larege desert rose type of gypsum sand crystals have split by thermal exfoliation on the 010 cleavage, northern Umm Said Sabkha, near Umm Said, Qatar, 1997

Desert Rose gypsum crystals, Qatar

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.

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LOCATION:

UMM SAID SABKHA
Cyanobacterial Mats

Cyanobacterial mat, Umm Said Sabkha

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.

Cyanobacterial mat, closer view, Umm Said Sabkha

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.

A microbial mat or cyanobacterial mat dug out of the intertidal sabkha of a small lagoon in Kuwait

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.

Stromatolites on an old drum in Qatar

Thrombolites on three trees at the Fossil Forest, Dorset, UK

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.

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LOCATION:

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!).

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LOCATIONS:

SOUTH OF UMM SAID

BARCHAN SAND DUNES

Aerial view of large barchans and barchanoid dunes drifting SSE across the Umm Said Sabkha, southeast Qatar

Desert dunes reach the sea at Umm Said, southeast Qatar

Windward to left horn side of a barchan, Umm Said, Qatar. Note the wind-rippled, convex surface.

Driving to the slipface of a barchan, Umm Said, Qatar

Hot work investigating the sediment in a quarry within a large barchan sand dune, Umm Said, Qatar, 1997

The slipface of a barchan, with lineation from avalaching sand, Umm Said, Qatar

The supposedly largest sand dune in Kuwait, for contrast with the giant dunes of  Qatar

The large barchan sand dune (crescent-shaped with the horns downwind) in Qatar, shown above, is about 30 m 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?

Quarry in a barchan, Umm Said Sabkha, Qatar, 1997

Quarry in a barchan, Umm Said , Qatar, 1997, with Ian West

Quarrying in a barchan, Um Said Sabkha, Qatar, 1997

Sedimentary structures in a quarried sand dune, Umm Said Sabkha, Qatar, 1997

Here a sand quarry, near Umm Said, 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.

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LOCATION:

Dukhan

Deflation Features - Qatar

Aeolian abrasion features from a deflated area, Dukhan Sabkha, Qatar - these resemble aeolian features on Mars

More aeolian abrasion features on limestone or dolomite, Dukhan Sabkha, 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).

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KARSTIC FEATURES:

Dolines, Poljes and Caves

The entrance to a doline and cave system in Tertiary dolomite and gypsum, Qatar, 1997

Cave in dolomite over gypsum, Qatar

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.

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ACKNOWLEDGEMENTS

For kind hospitality, field leading and assistance in many respects in Qatar I am particularly grateful to Dr. Mariam Mustafa-Al-Mulla 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 by Dr. Mariam Al-Yousef. 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 Nasir. I am much obliged to Jaques Leblanc, the petroleum geologist, for drawing my attention to his excellent fossil hunting guides to the Tertiary Formations of Qatar, and which are based on much detailed field study in harsh climatic conditions.

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REFERENCES AND SELECT BIBLIOGRAPHY


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Alsharhan, A.S. 1989. Petroleum geology of the United Arab Emirates. Journal of Petroleum Geology, vol. 12, 253-288.

Alsharhan, A.S. and Kendall, C.G.St.C., 1994. Depositional setting of the Upper Jurassic Hith anhydrite of the Arabian Gulf an analogue to Holocene evaporite of the United Arab Emirates and Lake Macleod of western Australia. American Association of Petroleum Geologists Bulletin, vol. 78, pp. 1075–1096.

Alsharhan, A.S. and Kendall, C.G.St.C. 1994. Depositional setting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf: an analogue to Holocene evaporites of the United Arab Emirates and Lake MacLeod of Western Australia. American Association of Petroleum Geologists Bulletin, vol. 78, 1075-1096.

Alsharhan, A.S. and Kendall, C.G.St.C., 2002. Holocene carbonate/evaporites of Abu Dhabi, and their Jurassic ancient analogs. In: Barth, H.J., Boer, B.B. (Eds.), Sabkha Ecosystems. Kluwer Academic Publishers, pp. 187–202.

Alsharhan, A.S. and Kendall, C.G.St.C. 2003. Holocene coastal evaporites of the southern Arabian Gulf and their ancient analogues. Earth Science Reviews, vol. 61, 191-243, Elsevier. A.S. Alsharhan at Faculty of Science, United Arab Emirates University, PO Box 17551, Al-Ain, United Arab Emirates. C.G.St.C Kendal at Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USA.
This is a key paper on the Arabian Gulf evaporites and othere coastal sediments.
Abstract: The Holocene sediments of the coast of the United Arab Emirates in the southeastern Arabian Gulf are frequently cited in the literature as type examples for analogous assemblages of carbonates, evaporites and siliciclastics throughout the geologic record. This paper is intended as a convenient single source for the description of sediments of this region, providing information on how to reach the classic localities and some of the analogs. The Holocene sediments of the region accumulate over an area that is 500 km long and up to 60 km wide. The sediments collecting offshore are predominantly pelecypod sands mixed with lime and argillaceous mud, with these latter fine sediments increasing as the water deepens. The pelecypod-rich sediments also collect east of Abu Dhabi Island both in the deeper tidal channels between the barrier island lagoons and in deeper portions of the protected lagoons. West of Abu Dhabi Island the shallow water margin is the site of coral reefs and coralgal sands, whereas to the east oolites accumulate on the tidal deltas of channels located between barrier islands. Grapestones accumulate to the lee of the reefs and the oolite shoals where cementation becomes more common. They are particularly common on the less protected shallow water margins of the lagoons west of Abu Dhabi Island. Pelleted lime muds accumulate in the lagoons in the lee of the barrier islands of the eastern Abu Dhabi. Lining the inner shores of the protected lagoons of Abu Dhabi and on other islands to the west are cyano-bacterial mats and mangrove swamps. Landward of these, a prograding north facing shoreline is formed by supratidal salt flats (sabkhas), in which evaporite minerals are accumulating.
This paper describes the localities associated with (1) the mangrove swamps of the west side of the Al Dhabaiya peninsula;
(2) the indurated cemented carbonate crusts, cyanobacterial flats and sabkha evaporites on the shore of the Khor al Bazam south of Qanatir Island;
(3) the reef and oolitic sand flats on the coast just east of Jebel Dhana; and (4) the marine travertine and aragonite coats associated with the beach sediments in a small bay south of Jebel Dhana; and (5) the Sabkha Mutti between Jebel Barakah and Al Sila.
Similar sedimentological associations of carbonate and evaporites to those of the Holocene of the United Arab Emirates are to be found in the Tertiary and Mesozoic sedimentary rocks of the immediate subsurface in the Arabian Gulf. Other analogs to this setting include the Paleozoic carbonates of the western USA, Europe, and Asia, Mesozoic carbonates of the Gulf of Mexico, Europe, and Middle East and Tertiary sedimentary rocks in the Middle East.
[end of Abstract].

Alsharhan, A.S. and Nairn, A.E.M. 1994. Geology and hydrocarbon habitat in the Arabian Basin: the Mesozoic of the State of Qatar. Geologie en Mijnbouw, vol. 72, pp. 265-294.
Abstract:
The State of Qatar is situated in the southwestern Arabian Gulf and covers an area of about 12,000 sq. km. The land portion is formed by a large, broad arch, which is part of the regional, NE-SW trending Qatar-South Fars Arch, separating two Infracambrian salt basins. The Dukhan Field on the west coast of the Qatar Peninsula, with its reservoirs in Upper Jurassic limestones, was the first oil field discovered. Since this discovery in 1940, a series of other discoveries have been made and Qatar became a member of the Organisation of Petroleum Exporting Countries (OPEC) in 1973.
Hydrocarbon accumulations are widely dispersed throughout the stratigraphic column with production from Middle Jurassic to Middle Cretaceous strata. The most prolific reservoirs are in shelf carbonate sequences and minor accumulations occur in Albian clastic sediments.
Seals, mainly anhydrite and shale, occur as formations of regional extent as well as intraformationally with smaller areal distributions. There are several stratigraphic intervals which contain source rocks or potential source rocks. Upper Oxfordian - middle Kimmeridgian source rocks were formed in an extensive, starved basin during a period of sea-level rise. They contain organic matter of sapropelic, liptodetrinitic and algal origin and have a total organic content of 1 to 6%.
Both depositional environment and tectonic evolution through geologic time have influenced sedimentary facies and stratigraphic features, which controlled reservoir, source and seal characteristics and subsequent hydrocarbon generation, migration and entrapment.
[This is a key paper on petroleum geology of Qatar and with much useful information.]

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Alsharhan, A.S., Rizk, Z.A., Nairn, A.E.M., Bakhit, D.W. and Alhajari, S.A. 2001. Hydrogeology of an Arid Region: The Arabian Gulf and Adjoining Areas. Elsevier Science. Webpage information, including a summary of contents, and price.

[See also: Kendall and Alsharan (2011), Quaternary Carbonate and Evaporite Sedimentary Facies and their Ancient Analogues.]

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Al-Yousef, M. (Dr. Mariam Mustafa Al-Mulla). 2003. Mineralogy, Geochemistry and Origin of Quaternary Sabkhas in the Qatar Peninsula, Arabian Gulf. Ph.D. Thesis, 2 volumes, 438pp + Appendices. School of Ocean and Earth Science, Faculty of Science, University of Southampton, UK. A copy is available for reference in Southampton University Library. Copies are also held by Dr Mariam, Dr. Ian West, and Qatar University. This is a large thesis, of about maximum acceptable size, with in addition to the 438 pages, an Appendix of about 50pp. There are 200 figures in the text, and a further 87 in the Appendix. This is the second large thesis by Dr. Mariam, the first being a 324 page M.Sc. thesis (1994) on Geomorphological Study of Surface Drainage Patterns in Qatar Peninsula, Arabian Gulf. Supervisors of the Ph.D. Thesis study were Professor Dorrik Stow and Dr. Ian West of Southampton University. Dr. Sobhi Nasser of the Geology Department, University of Qatar, provided supervision and field guidance in Qatar. Dr. Ali Akbar, who had also completed a Ph.D. thesis, at Southampton University was amongst others who kindly provided help with this research. External Examiner was Dr. Anthony Kirkam, and Internal Examiner was Professor John Murray.

Abstract:


This thesis represents the first comprehensive study of Holocene sabkhas in Qatar. It present detailed analysis of surface morphology, recent evaporite and clastic sediments, and groundwater nature and flow, for two large sabkhas: the inland Dukhan Sabkha and the coastal Umm Said Sabkha. The fieldwork involved the taking of shallow pit and core samples, as well as brine samples from various locations within the sabkhas. In addition, sand dune samples were obtained from selected locations within and between the two sabkhas. Laboratory analyses of the mineralogy and geochemistry of the sediments included: hand specimen study, smear slide and thin section petrography, XRD, XRF, organic carbon and SEM analyses. Chemical analyses of brines were also undertaken.

The dominant sediment facies in both sabkhas are sands with variable amounts of evaporite precipitates. Of secondary importance are sandy silts, muds and algal/microbial mat deposits. Most of the evaporites occur within these detrital facies. The surface is covered with a firm duricrust of halite and gypsum. A large hypersaline lake covers part of the northeastern sector of Dukhan Sabkha. Evaporite minerals present in the sabkhas are gypsum, halite, anhydrite, and a small amount of bassanite. Carbonate minerals are dolomite, calcite, Mg-calcite and aragonite. Siliciclastic minerals are quartz, K-feldspars and plagioclase, with minor heavy minerals and clays. The major oxides found in sediment samples throughout are SiO2, CaO, SO3, MgO and Al2O3. Of trace elements: Sr, Cr, Ba and Zr, V, Rb and Ni are high in comparison with other trace elements.

The relative abundance of gypsum and anhydrite is greater in Dukhan Sabkha then in Umm Said Sabkha, whereas halite is more abundant in Umm Said Sabkha. These differences relate primarily to the greater salinity of Umm Said Sabkha. These differences relate primarily to the greater salinity of Umm Said Sabkha brines. The relative distribution of gypsum and halite within the upper meter of sediment is influenced both by grain size and marine flooding. The sandier sediments of Umm Said Sabkha tend to have gypsum above halite whereas the reverse is true of the generally finer-grained Dukhan Sabkha. Anhydrite nodules are most common in the uppermost capillary zone in the salt lake area, where they are formed as a result of gypsum dehydration. Minor authigenic dolomite also occurs in this area because of high salinities and a high Mg2+: Ca2+ ratio. Elsewhere, the dolomite is of detrital origin. Clay minerals are present only in small quantities in both sabkhas, with palygorskite as the dominant clay, followed by chlorite and illite. Palygorskite is believed to be at least partly authigenic in origin, whereas the other clays are detrital. Organic carbon is only present in small amounts (mean 0.06 - 0.14%), except in a few samples from the algal/microbial mats.

Gypsum crystal habits in Dukhan and Umm Said Sabkha sediments are acicular, lenticular and sublenticular, intergrown sublenticular, prismatic, pyramidal, elliptical and semi-elliptical, and pseudotetragonal shapes. Fine gypsum crystals in Dukhan Sabkha sediments are more abundant than in Umm Said Sabkha. Lenticular and sublenticular habits are dominant in Dukhan Sabkha, while prismatic crystals are dominant in Umm Said Sabkha. The generally finer grain size of sediment in Dukhan Sabkha results in the greater variety of crystal habits observed than in Umm Said Sabkha. The crystals of both sabkhas are euhedral, simple and tabular on (010), and the cleavage (010) is very good on the crystal surface.

Most of the detrital sands throughout both sabkhas, in sand dunes across parts of the sabkha surface, as well as in other parts of Qatar, have been derived primarily as windblown sediments from an Arabian Peninsula source. Quartz sands are dominant. Carbonate minerals include calcite and dolomite. The percentage of gypsum in sand dunes in the western part of Qatar is high, whilst it is lower elsewhere. No halite is found in sand dune samples. The heavy minerals present in the sand dunes and in the surface sand from within the sabkhas are very similar. Opaque and semi-opaque grains are dominant, followed by abundant garnet, common epidote, tourmaline, hornblende, zircon and pyroxene, and a range of minor minerals. Transport is believed to have taken place across the Gulf of Salwa from the Arabian Peninsula during the last sea-level lowstand (prior to 7 - 8,000 y BP). This would suggest that the dominant wind direction in the last glacial period was rather more northerly than the present northwesterly winds.

The sources of groundwater in Dukhan Sabkha are complex and mixed. They include: (a) fresh groundwater, particularly from the Rus and Umm Er Raduma aquifers in the north, which are above the level of Dukhan Sabkha; (b) seawater infiltrating from the north and/or west and (c) runoff water during rainy periods. In Umm Said Sabkha, seawater forms the main source for groundwater by marine flooding and seepage reflux. There is no major barrier between sea and sabkha so that regular flooding occurs during each high tide, extending still further inland across the sabkha during exceptionally high tides and storm conditions. Freshwater provides a more minor secondary source from land aquifers and rainwater. The pH of brines in both sabkhas averages 6.8. The mean salinity of Umm Said Sabkha is higher than that of Dukhan Sabkha (mean Total Dissolved Solids 205ppt and 113 ppt respectively).

In summary, Dukhan Sabkha is here presented as a good type example of an inland siliciclastic sabkha. The topographic depression in which it occurs formed in several stages as a result of tectonic folding followed by karstic dissolution of underlying gypsum and carbonate deposits. This depression, which included one or more large inland lakes that formed sometime during the Pleistocene, was progressively infilled by detrital sand, silt and clay. Gradually Dukhan Sabkha formed and further built upwards as evaporite deposits were precipitated within the siliciclastic sediments. Deposition is now kept in balance by wind deflation. The nature and distribution of sediment facies and mineralogy, the influence of climate, winds and regional setting, together with the groundwater source and movement, and the brine geochemistry, as documented in this study, can be used to construct a new facies-environmental model for this type of sabkha world-wide. This is considered more appropriate than the standard model that exists for a prograding coastal sabkha with carbonate affinities. Umm Said Sabkha is more typical of a prograding coastal sabkha, although its principal difference from existing models is the dominance of siliciclastic sands of wind-blown origin.

Dukhan Sabkha is seen to be increasing in size because of the death of large numbers of halophytes [plants], especially in marginal zones. The margin of the sabkha has clearly moved a few meters outward in historical times, leaving a zone of old dead halophytes. This is partly the result of increased salinity of the groundwater as a result of aquifer draw down due to excessive pumping for human use. This progressive salinization is leading to sabkharization in Qatar. Umm Said Sabkha is also increasing in size. This is the result of seaward progradation of windblown sands, so that the increase in area is at the expense of the marine environment rather than the land.

[End of Abstract}

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Cavelier , C. 1970. Geological Description of the Qatar Peninsula (Arabian Gulf). Department of Petroleum Affairs, Government of Qatar. B.R.G.M. (Bureau de Recherches Geologique et Minieres) 39pp. Explanation of the 1:100,000 geological maps of Qatar. By Claude Cavelier, Abdullah Salatt and Yves Heuze.
Example extract (p. 3):
"1 - General Geographic Situation
The Qatar peninsula covers some 10,000 km 2. It constitutes the eastern appendix to the Arabian peninsula crossed by the 25th parallel, and jutting northwards into the central zone of the Arabian Gulf.
The independent Arab state of Qatar is a Sheikdom with about 100,000 inhabitants, most of whom live in Doha, the capital. Qatar governs the offshore islands of Halul, Shra Auh, and Las Hat, of which only the first is inhabited.
Both geographically and geologically, Qatar is a quite distinct entity, the political boundaries of which are also natural ones. To thc north, west and east, the borders with Saudi Arabia, Bahrain Islands, Iran and Abu Dhabi are maritime ones. To the south, Qatar is separated from Saudi Arabia and Trucial Sheikdom of Abu Dhabi by an almost continuous belt of salt flats which, in recent times, were still marine.
Geologically, Qatar may be defined as a wide anticlinal dome gently warped and slightly folded. The general roughly elliptic configuration, with north-south main axis, is at present underlined by underlined by a wide spread outcrops of Eocene rocks, raised above younger Miocene deposits which surround them.
Several excellent works wcre published about the different gcographical aspects of Qatar by Melamid (1953) - political geography -, Johnstone and Wilkinson (1960) - history of discovery, tribes and population, glossary of arabic local topographic words - Meigs (1966) - general geographical description -. But the most complete and recent work is the book headed "Qatar 1968", published by the Government.

2. - The North of Qatar
Traditionally, Qatar is classified by geographers among desert countries. The southern half is sparsely populated but the northern region, especially the NE, is comparatively populated and agricultural development has commenced. The population is sedentary for the most part; relatively abundant water is extracted from generally shallow well; the constantly developing road network and the numerous tracks in good condition are quite frquented. In fact, it is an arid region, for there, as over the whole qatari country, the tolal volume of rainfall rainfall - essentially that of winter - is quite low.
North of the road from Doha to Dukhan, the ground is quite flat and pebbly, showing often extensive, hardly marked depressions, in which sills and muds accumulated , carrying natural pasture, in winter,on which graze camels, sheep, goats and oxen. Agriculture is constantly improving: orchards and kitchen-gardens, but also cereals (rye). Fishing is still carried out in an artisanal way.
The villages are rather numerous inland, but the small harbours along the shore-line are being deserted; the main city, however, is Khor on the eastern const.
Bctween the Doha-Dukhan and Doha-Umm Bab roads, the ground becomes more contrasted. The plateau is essentially rocky and uneven especially to the west. The depressions are generally deeper and sharper; their soil is essentially silty and muddy, but small accumulations of eolian sand, held by trees, occur especially to the west. The wells, located in important depressions, are still numerous and deeper.
To the west, along the Salwah Gulf, the rocky massive of Djebel Dukhon carries strongly marked depressions where the accumulations of eolian sand are considerable. Two important centres are implanted there: Dukhan, with the Qatar Petroleum Company (Q.P.C.) plant and Umm Bab with its cement factory. The road network which connects the numerous oil drill holes exploited in this area since 1949, is broadly developed, but generally in bad condition.
Footnote: (l) The oral tradition which reports that Qatar was formerly an Island separated from the Saudi province of Al Hasa is practically confirmed by geology. In particular by the discovery of extensive deposits of calcareous sands with a mollusc fauna comparahle to the present in the sebkha located South of Sauda Nathil."
[continues]

Cavelier, C., Salatt, A. and Heuze, Y. 1970. - Qatar Geological map. 1/100,000. Important basic geological map of Qatar.

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Clarke, M.W.H. and Keij, A.J. 1973. By M.W. Hughes Clark and A.J. Keij. Organisms as producers of carbonate sediment and indicators of environment in the southern Persian Gulf. Pp. 33-56 in: Purser, B.H. (Editor) 1973. The Persian Gulf: Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea.. Springer-Verlag, Berlin, Heidelberg, New York. 471pp.

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LeBlanc, J. 2008. A Fossil Hunting Guide to the Tertiary Formations of Qatar, Middle East. First Edition, March 2008. By Jaques LeBlanc, a Canadian petroleum geologist and fossil hunting specialist; he is with Qatar Petroleum as Senior Geologist and Data Management Specialist for their Dukhan Oil Field Division. See also 2009 edition which has additional material. This has the form of a book but is available online (and does not seem to be obtainable as a printed book). It can be downloaded. 81pp.
Example extract from p. 11: 2.
The Surface Geology and Stratigraphy of Qatar.
2A. Geological Setting.
Qatar forms an exposed part of the Arabian shelf between the Arabian shield and the mobile belt of Iran. It is centred at about 23 degrees N, 51 degrees E. Topgraphically, Qatar has a low relief landscape with a maximum elevation of 103m. above sea level. Structurally, Qatar appears as elliptical anticlinal arch with a N-S main axis.
The exposed geological succession is made up of Tertiary limestones and dolomites with interbedded clays, shale, gypsum and marls, covered in places by a series of Qaternary and recent deposits.
The Tertiary sedimentation started with a marine transgression in the Paleocene (Umm Er Rhaduma Formation, not exposed in Qatar). Later, shallow marine to sabkha conditions prevailed until the end of the Eocene, a carbonate-evaporite sequence (Rus and Dammam Formation) was deposited during this period. A sea regression at the end of the Eocene is marked by a widespread unconformity, causing the absence of Oligocene deposits over most ot the area.
In Qatar, the Lower and Middle Eocene outcrops are represented by the Rus and Dammam formations. The Dammam Formation is part of the Hasa Group which consists of the Umm Er Radhuma, Rus and Dammam Formations. Vertically upward in the stratigraphic sequence, the Middle Eocene Dammam Formation is succeeded by the Miocene Dam Formation and the Pliocene to Late Miocene Hofuf Formation (Nasir et al. 2003).
[continues with Table 1 - stratigraphic column for Tertiary strata..]

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Nasir, S., Al-Saad, H., Alsayigh, A and Weidlich, O. 2008. Geology and petrology of the Hormuz dolomite, Infra-Cambrian: Implications for the formation of the salt-cored Halul and Shraouh islands, Offshore, State of Qatar. Journal of Asian Earth Science, Elsevier, vol. 33, pp. 353-365. By Sobhi Nasir, Hamad Al-Saad, Abudlrazak Alsayigh, and Oliver Weidlich.
Abstract:
Geological investigations of the Halul and the Shraouh islands, offshore Qatar, indicate that most of their calcareous rocks, which display abundant stromatolitic bedding, belong to the Infra-Cambrian Hormuz Series. Mineralogical, petrological, and geochemical analyses show that these calcareous rocks consist dominantly of dolomite and have formed in a reducing depositional environment. Faint laminations and small streaks of organic matter furnish evidence for the involvement of algal mats in their genesis and indicate their formation in an intertidal to supratidal setting. The Halul and Shraouh dolomites experienced extensive recrystallization and sulfatization during the emplacement of the Halul and Shraouh salt domes that form the cores of the islands. During mobilization and ascent of the salt, the dolomite recrystallized, and its Sr initial ratios were abnormally enhanced by the incorporation of 87Sr from a source, which is more radiogenic than the attendant seawater at the time of the dolomite formation near the Proterozoic–Cambrian boundary. Geochemical analysis show that Si, Al, Ti Zr, and % of insoluble residue are highly correlative, suggesting the presence of detrital minerals such as rutile and zircon. A paleosabkha model may well agree with this chemical signature. However, the Infra-Cambrian age of the Hormuz rocks and the presence of stromatolitic layers containing organic materials in the studied rocks, suggest that organogenic dolomitization could be an alternative dolomitization model.

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Perthuisot, J-P. 1977. La sebkha de Doukhane (Qatar) et la transformation: gypse - anhydrite plus eau. [The sabkha of Dukhan (Qatar) and the the transformation: gypsum to anhydrite plus water.] Bulletin de la Societe geologique de France, vol. 19, No. 5, 1145-1149. By Jean-Pierre Perthusiot.
Abstract: The gypsum to anhydrite transformation occurs at the present day in the Dukhan Sabkha of western Qatar. It begins within temperatures and salinity conditions clearly beyond the theoretical conditions at equilibrium, and this is because the reaction is endothermic. In addition, it is suggested that this reaction must be facilitated by the oxidising environment of the medium. [end of abstract]

[The text is in French; because this is a very useful paper I have provided an English translation of all the text - below.]

The Dukhan Sabkha (Qatar) and the Transformation: Gypsum - Anhydrite + Water.

By Jean-Pierre Perthuisot

1. The Geological Environment of the Dukhan Sabkha

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. 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.
In fact, it is probable that certain indurated beds, generally dolomitic, attributed to the Tertiary, correspond to pedogenic crusts developed in the course of Quaternary climatic cycles. Furthermore there are present within the depression and on its borders some modern aeolian sands.
Then, on the borders of the sea, are vast plains resulting from recent sedimentation (with carbonate and shell sediments) deposited during the Flandrian Transgression (with some recent epierogenic movements) and attaining a surface of about 2 metres above present sea level (see the map above).

2. The Dukhan Sabkha:

Simplified geological map of the region around the Dukhan Sabkha, Qatar

The Dukhan Sabkha is large and occupies a superficial area of about 130 square kilometres. Some of the surface is actually at about 2 metres below sea-level, with a large part at zero in relation to sea-level. On the borders are Eocene carbonates (with Quaternary crusts - see photograph) which often have the form of small cliffs. To the north of the Dukhan Sabkha is a straight depression at about plus 3 metres and this extends to the Bay of Zekrit.

a. The Origin of the Basin

Although the depression as a whole is clearly structural, the origin of the sabkha basin itself remains hypothetical. One could invoke a karstic origin resulting from the action of superficial water; there are numerous depressions on the Qatar peninsula which can be regarded as poljes [the Qatar solution depressions are often referred to as dolines, and associated with collapse after dissolution of gypsum beds]. It may be necessary to envisage a much wetter climate in the past than that of the present day.
Probably the formation of the basin has taken place in several stages. The valley which contains Dukhan Sabkha and the Bay of Zekrit corresponds to an ancient outlet of the basin. It does not seem that the Flandrian Transgression invaded it. There are no signs of marine sediments along the margin of the sabkha. Apart from possible subterranean supply there has been no direct a feeding of the sabkha from the sea.

b. The Sedimentary Fill of the Sabkha:

In the absence of a major borehole the thickness of sedimentary fill cannot be determined and only the upper part is accessible to current investigations. On the borders there is clayey-sandy colluvium with angular blocks of limestone at the base of cliffs. This detrital material is succeeded towards the centre of the basin by a sediments that is essentially gypsiferous and very poor in detrital particles. This fills most of the basin and is at least 1 to 2 metres thick as shown by the trench of a pipeline which crosses the sabkha in a north-south direction.
Finally, in the northeastern part of the sabkha the gypsum is covered by a crust of halite attaining several decimentres in thickness. This zone is more often covered by residual concentrated brine and is a salt lake. Elsewhere halite forms only a thin crust and is ephemeral.

Analyses of brine from Dukhan Sabkha, Qatar, after Perthuisot (1977)

c. The Origin of the Salts

One may think that a large part of the salts left by evaporation in the basin have come from the refluxing of the basin borders (clays with gypsum in particular) but a partly marine origin cannot be excluded a priori, considering the low altitude of the sabkha and the proximity of the sea. One must, however, expect a quantity of halite greater than that which is observed if the totality of salt coming from the marine environment was precipitated in the basin. Later studies may, perhaps, give more precise information on this point.

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.

Shallow pit showing the profile through the top sabkha sediments with anhydrite, Dukhan Sabkha, Qatar

Profile through the uppermost part of Dukhan Sabkha, northern part, about 50 metres from the edge of the salt lake, Qatar, modified after Perthuisot (1977)

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).

Probable conditions in the Dukhan Sabkha where the anhydrite occurs in relation to gypsum-anhydrite stability curves; after Perthuisot

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 sulpher.

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]

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Purser , B.H. (Editor) 1973. The Persian Gulf: Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea.. Springer-Verlag, Berlin, Heidelberg, New York. 471pp. A key book on the Arabian or Persian Gulf, with many useful papers.

Purser, R.H. and Evans, G. 1973. Regional sedimentation along the Trucial Coast, SE Persian Gulf. In: Purser, R.H. (Ed.), The Persian Gulf, Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea. Springer Verlag, Berlin, p. 211-231.

Purser, B.H. and Seibold, E. 1973. The Principal Environmental Factors Influencing Holocene Sedimentation and Diagenesis in the Persian Gulf. In: Purser, R.H. (Ed.), The Persian Gulf, Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea. Springer Verlag, Berlin, p. 1-9. Abstract: The Persian Gulf is a marginal sea with an average depth of 35 m, and a maximum depth of 100 m near its narrow entrance. Its elongate bathymetric axis separates two major geological provinces - the stable Arabian Foreland and the unstable Iranian Fold Belt - which are reflected in the constrasting coastal and bathymetric morpho10gies of Arabia and Iran. The Persian Gulf has a gently inclined sea floor lacking "shelf edges" comparable with those of modern Caribbean carbonate provinces. The arid, sub-tropical climate with summer temperatures attaining 50° C, and frequent winds, stimulate the formation of evaporitic minerals and the delivery of aeo1ian dust to the basin. F1uviati1e influx is limited to the Tigris-EuphratesKarun delta and to the mountainous Iranian coast where terrigenous sediments contrast with the relatively pure carbonates forming in the shallow seas in front of the low deserts of Arabia. Excessive evaporation and partial isolation from the adjacent Indian Ocean provoke abnormal sa1inities throughout most of the basin, which attain a maximum of ca 700/00 in remote Arabian lagoons.Because the prevailing "shama1" wind blows down the axis of the gulf from the NW, most coastal environments are swept by waves and surface currents which favour the formation and dispersal of carbonate sands on the Arabian side and terrigenous material on the Iranian. Tidal currents influence sediment textures, even in the deepest parts of the gulf, while extensive rock bottoms influence the biota and skeletal composition of Ho10cene sediments. These are mixed with significant amounts of relict sediment, especially in the deeper parts of the basin.

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Stacey International. 1986. Qatar. 160pp. with many photographs. (A general, introductory, information book on Qatar, not geological).

Stephens, N., Puls, D., Albotrous, H., Al-Ansi, H. and and Al-Tamimi, F. 2009. Sequence Stratigraphic Framework of the Arab Formation Reservoirs, Dukhan Field, Qatar. International Petroleum Technology Conference, 7-9 December 2009, Doha, Qatar. ISBN 978-1-55563-264-9.
Authors: Nat Stephens, ExxonMobil Production Company; Dave Puls, ExxonMobil Qatar Inc.; Hilal Albotrous, Hussain Al-Ansi, and Fahad Al-Tamimi, Qatar Petroleum.
Abstract
In the Upper Jurassic Arab C and D reservoirs, three 3rd-order sequences are interpreted in Dukhan Field, Qatar and are refered to as the Kim1, Kim2, and Kim3 sequences (after Hardenbol et al., 1998). At Dukhan, the Arab C and D reservoirs have produced since 1949. Over 800 wells have been drilled in Dukhan, 180 of which have been cored. Based on sedimentological analysis of the cores, environments of deposition (EODs) have been assigned to cored wells. Twenty three EODs were identified and grouped into five facies tracts: Sabkha, Beach, Ramp Interior, Ramp Crest, and Outer Mid Ramp. Parasequences and parasequence sets were recognized in the core data, and sequences were interpreted from the parasequence stacking patterns as well as evidence for exposure. Log-based correlation provides a method for extending the sequence stratigraphic framework from cored to uncored wells across the field.
Within a sequence stratigraphic framework, the spatial positions of facies tracts and corresponding EODs are predictable. Based on lateral changes in EODs and onlap patterns, we have determined that deposition dip is towards the northeast. Capping the Kim1 sequence, the Arab D consists of a 20 foot thick interval of cross-bedded, coated-grain grainstone in the southern portion of the field. To the north, the grainstone package of the Kim1 sequence passes into a deeper-water EOD with a higher percentage of packstone and wackestone. Above the Kim2 sequence boundary, a transgressive wedge thins toward the south. The Kim2 sequence contains Ramp Interior and Beach facies tracts that prograde to the northeast. The top of this sequence occurs in the Sabkha facies tract. The Kim3 sequence is initiated by the Lower Qatar Anhydrite, a regional feature that forms a seal between the Arab D and C reservoirs. The transgressive system tract of the Kim3 sequence is capped by coarse grain beaches and shallow subtidal lagoon EODs of the lower Arab C reservoir. The maximum flood is indicated by distinctive thrombolitic horizons in core. The upper Arab C represents the highstand of the Kim3 sequence and is composed of upward thinning and increasingly restricted EODs of the Sabkha facies tract.
In the Arab C and D, reservoir quality is highly influenced by depositional texture. Well-sorted, cross-bedded grainstones in the shoal and beach EODs consistently have excellent interparticle porosity with permeability over 100 mD and commonly exceeding 1000 mD. Deeper-water EODs consists of a greater percentage of packstone and wackestone with intraparticle porosity and microporosity. These low-energy EODs typically exhibit permeability less than 20 mD. Sabkha EODs are of low to moderate reservoir quality except where dolomitization has improved permeability. Placing EODs in a sequence stratigraphic framework provides a method by which we can predict lateral changes in rock properties and, within 3D geologic models, distribute rock properties insightfully.

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