School of Ocean and Earth Science ,
National Oceanography Centre, Southampton
Southampton University,
Website hosted by courtesy of Information Systems Services, Southampton University
| Field Guide Introduction & Geological Maps|
Triassic Red Beds of Budleigh Salterton, Devon, England.
1. GENERAL INTRODUCTION
This is an introduction to sabkhas, evaporites, salt lakes and other desert environments. It is in progress, and from time to time more illustrations and more detailed descriptive text will be added.
Sabkha is an arabic name for a salt-flat that has come into general use in sedimentology following classic research in the United Arab Emirates of the Arabian Gulf in the 1960s and later. They are flat and very saline areas of sand or silt lying just above the water-table and often containing soft nodules and enterolithic veins of gypsum or anhydrite. A thin crust of halite and gypsum may be present in some parts. Many ancient evaporites show sedimentary feature of sabkhas, such as gypsum nodules.
A supratidal part of a large coastal sabkha at Umm Said in Qatar. This particular area is the remains of a lagoon indirectly filled with siliciclastic sand of aeolian origin, originating from some large barchan sand dunes. The flatness is controlled by the content of capillary moisture from the water-table, which is only about half a metre (one and a half feet) down, keeping the sand damp and firm and preventing it from being blown away. Any higher dryer sand can be moved away by deflation. This particular sabkha has much granular gypsum in addition to the sand but is firm enough for a four-wheel drive vehicle where the brine is not at the surface (although it is sensible to keep only to well-used car routes and to have a mobile phone and use GPS for location).
A lower part of the same sabkha where a thin halite crust is developed. The halite is empheral and easily dissolved by a rare flood of rain. The gypsum in the sand beneath is less soluble and remains. Here the water-table is almost at the surface and the sand beneath is soft with gypsum and some clay. The salt crust can sometimes support cars until they break through and rapidly rust away in the brine. The wheels and windows will eventually be the only fossil remains of this vehicle. One hopes that the driver was able to walk away on the salt crust to the solid desert beyond!
SABKHAS - Beach Ridges Separating Lagoons and Sabkhas from the Coast
White beach ridges of carbonate sand are often developed in arid or semi-arid regions where the sea water is warm and where evaporation is taking place. Such beach ridges may be discontinuous as in the Abu Dhabi area of the Arabian Gulf or almost continuous as on the northern Egyptian coast west of the Nile Delta. The left-hand photograph above shows the typical white sand of spherical ooid grains, but here the ridge is broken by an embayment. This is not typical for this stretch west of Alexandria and the beach is mostly continous. The ooids are of superficial type here and have nuclei that are frequently of quartz. They are probably less well-developed than in the Arabian Gulf. The oolitic sand is in some cases cemented into beachrock. This can be seen in the right-hand photograph, and the cross-bedding is probably of wind-blown sand dune origin in this upper part of the ridge. Beach cross-bedding probably occurs further down. Behind this white beach ridge are light brown sabhas of desert loess; this is blown dust from the Sahara desert which is trapped in the semi-arid, as opposed to the extremely arid, desert environment.
(more will be added)
For information on the sabkhas and other desert features of Qatar, Arabian Gulf please go to the new separate webpage:
SABKHAS - Stromatolites and Cyanobacterial Mats
Polygonally cracked algal-mat (more-strictly cyanobacterial mat) 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 (cerithid gastropods are abundant in the intertidal zone elsewhere). 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).
(What do you think about the tide conditions when the photograph was taken? What do you think controls the rather small size of the polygons? Do you think that many ancient cyanobacterial mats are not recognised because they do not necessarily build up laminated sediments but just some polygonal cracking like this? How could you distinguish them from the effects of the simple desiccation of mud?)
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.
Photographs above show enterolithic veins in secondary gypsum (after anhydrite, after primary gysum) in Lower Purbeck Formation at Worbarrow Tout , Dorset, UK. With them are photographs of similar enterolithic veins forming at the present day in a sabkha of desert loess (blown wind-blown silt) between El-Alamein and Alexandria on the Mediterranean coast of Egypt, and at Dukhan Qatar where they are of anhydrite. (See - West, Ali and Hilmy, 1979.)
Nodules, chicken-wire structure and enterolithic veins are all closely related. They are early displacive fabrics formed by continued growth of calcium sulphate from capillary water in sabkhas or salt-flats. Within them the sulphate is extremely pure because this displacive material has grown in place and contains hardly any sediment. The pure gypsum within nodules is known as alabaster and is used for carving. Enterolithic veins are over-developed displacive nodules of white soft gypsum or anhydrite which have burst out and pushed on into the associated soft sediment of a sabkha (West, 1965) . They commonly occur approximately parallel to the sediment surface, although in the Purbeck Formation there has often been a tendency towards obliquely-upward movement. Their lithification into strong but soluble rocks is the result of later diagenesis, i.e. gypsum-anhydrite-gypsum or anydrite-gypsum.
(Some questions: Why are the modern examples from Egypt and Qatar in a browner matrix than the ancient Purbeck ones (buff at the surface and grey underground)? The Purbeck veins have no porosity - why? The Purbeck enterolithic veins are porphyrotopic (have large crystals scattered through them) particularly at the margins of the veins - what is the reason for this?)
4.1 AKROTIRI SALT LAKE, CYPRUS - Introduction
(Note: Some of the photographs of Cyprus have been photo-edited to show only geological and geomorphological features)
The Akrotiri Salt Lake, near Lemesos (Limassol) is one of two significant salt lakes in Cyprus, the other being at Larnaca. It is a former embayment of the sea between the Episkopi Bay Tombolo Beach, a pebble beach, on the west side and a low sand barrier beach on the east. The lake is situated in a very seasonal Mediterranean environment which has appreciable rainfall in winter but is very dry in summer. Thus it only become satured for halite in the summer season and precipitates much salt by about July or August. It usually, but not always dries completely in late summer. In Spring, before it becomes very saline the lake and associated ponds are a great nuisance as a source of numerous mosquitoes.
In aerial views the relationship of the pebble beach to the original limestone island to the south is clearly visible. Old maps (see above) show that there was once a much larger body of water east of the beach. There seems to have been much natural reclamation and probably also much artificial reclamation of the original harbour or estuary.
In the aerial view notice the dry braided system of the Kouris River which has brought clasts of ultrabasic and basic igneous material from the Troodos Massif. The river mouth forms a small delta protruding seaward from the general line of the beach. On the old map a branch of this river is shown entering the salt-lake area.
The tombolo beach to the west is cemented into beachrock in places, particularly in the southern part. There is an offshore reef of beachrock which is visible in one of the aerial photographs.
Lady's Mile, the Eastern Barrier
The eastern barrier of the Akrotiri Salt Lake is Lady's Mile, a publicly-accessible beach south-southwest of Lemesos or Limassol. This is a very low, prograding series of parallel sand ridges and it might well be flooded in storms. The sand is presumably able to accumulate here because there is less wave fetch and therefore less erosion on this eastern side of the peninsula. However, the source of the sand is not obvious.
Ladies Mile has little vegetation on it because the surface is so close to the salt water-table. The sabkha without vegetation, at the southern end of the beach in the picture suggests that this is a route for either flooding by seawater or by lake water escaping seaward.
Further south at Button Beach small coastal sand dunes have been developed. Here the surface is above the limit of the capillary zone and as a result this is well-vegetated. Although the vegetation is different because of the high carbonate content in Cyprus it resembles the coastal sand dunes of Studland in Dorset, England (also a prograding eastern barrier of a coastal embayment, Poole Harbour).
My daughter Joanna Bentley, her husband Ben and my grandson Daniel were very hospitable and assisted during field work in Cyprus. I thank Tonya West for help in various ways.
Several geologists gave much help in Egypt and Libya and I will add references to them later, when this webpage is further developed. I am very grateful to all who assisted.
Bibliography and References Please go to:
Select Bibliography on Sabkha, Salt Lakes and Evaporites.
| Select Bibliography on Sabkhas, Salt Lakes and Other Desert Environments
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Webpage - written and produced by:
.
Ian West, M.Sc. Ph.D. F.G.S.