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Bibliography and References

(This is a limited list. More will be added later.)

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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.
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Anonymous , 1950. Development of the Dukhan Field, Qatar. Petroleum Engineer USA, 22, No.5, pp. B37-B42.
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Al-Yousef , M. 2003. Mineralogy, Geochemistry and Origin of Quaternary Sabkhas in the Qatar Peninsula, Arabian Gulf. Unpublished Ph.D. Thesis, School of Ocean and Earth Sciences, Faculty of Science, Southampton University. By Mariam Al-Yousef of Qatar University.
Abstract:
This thesis represents the first comprehensive study of Holocene sabkhas in Qatar. It presents detailed analysis of surface morphology~ recent evaporite and clastic sediments, and ground water 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 Si02, CaO, SO3, MgO and A2O3. 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 than 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 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 presenl in small amounts (mean 0.06-0.14%), except in 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 semielliptical. and pseudo-tetragonal 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 Urnm Said Sabkha. The generally finer grain size of sediments 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 other 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 7-8000y 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) seawaler 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 205 ppt and 113 ppt respectively).
In summary, Dukhan Sabkha is here presented as a good type example of an inland siliclastic 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 ground water 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 worldwide. 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 number of halophytes, especially in marginal zones. The margin of the sabkha has clearly moved a few meters outwards 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 saIinization is leading to sabkharization in Qatar. Umm Said Sabkha is also increasing in size. This is a 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.
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An Zen , E. 1965. Solubility measurements in the system CaS04, NaCl, H20 at 35°, 50° and 70° C and one atmosphere pressure. Journal of Sedimentary Petrology, vol. 6, p. 124-164.
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Bernier , P., Dalongeville,R.,Dupuis, B. and de Medwwecki,V. 1995. Holocene shoreline variations in the Persian Gulf: example of the Umm Al Qowain Lagoon, U.A.E. Quaternary International, v. 29-30, p. 95-103.
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Brankamp , R.A. and Ramirez, L.F. 1961. Geologic map of the Central Persian Gulf Quadrangle. Kingdom of Saudi Arabia. Map 1-209A. U.S. Geological Survey.
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Butler , G.P. 1969. Modern evaporite deposition and geochemistry of coexisting brines, the sabkha, Trucial Coast, Arabian Gulf. Journal of Sedimentary Petrology, vol. 39, no 1, p. 70-89.

Butler, G.P., Kendall, C.G.St.C., Kinsman, D.J.J., Shearman, D.J. and Skipwith, A.d'E. 1964. Recent anhydrite from the Trucial Coast of the Arabian Gulf. Geological Society of London, Circular, 120, p.3.
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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.
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Chowns , T.M. and Elkins, J.E. 1974. The origin of quartz geodes and cauliflower cherts through the silicification of anhydrite nodules. Journal of Sedimentary Petrology, vol. 44, No. 3, pp. 885-903.
Abstract: Quartz geodes from the dolostones of the Fort Payne and Warsaw" formations near Woodbury, Tennessee have proved to be pseudomorphs after early diagenetic anhydrite nodules. Their lithological association suggests that the anhxdrite developed in an arid tidal-flat environment by a process similar to that currently operating in the sabkhas of the Persian Gulf. Silicification took place prior to compaction and lithification of the sediments, the most likely source of silica solutions being the abundant sponge spicules which characterize the peritidal dolostones. Similar geodes from other Mississippian localities share the same lithological association and mode of origin. They help to define a recurrent sabkha facies and serve as important shoreline. indicators which may be used in the reconstruction of regional patterns of marine transgression and regression.
[These nodules have replaced nodules of anhydrite like those of the Dukhan Sabkha, and shown above]
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Curtis, R., Evans, G., Kinsman, D.J.J. and Shearman, D.J. 1963. Association of dolomite and anhydrite in Recent sediments of the Persian Gulf. Nature, London, 197, 697-680.
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El Khoriby , E.M. 2005. Origin of the gypsum-rich silica nodules, Moghra Formation, Northwest Qattara depression, Western Desert, Egypt. Sedimentary Geology, 177, 41-45. Abstract: Gypsum rich-silica nodules appear in two shale horizons of the Moghra Formation (early Miocene) northwestern Qattara Depression, Western Desert, Egypt. These nodules are gray to milky white in colour, mostly botroidal and rose-like in shape and range in diameter from 2 to 7.5 cm. The silica nodule-bearing shale is composed mainly of smectite with a little minor kaolinite. The silica nodules consist mainly of quartz and are composed of gypsum-free matrix and gypsum-rich megacrystalline quartz. The matrix consists of microflamboyant quartz (less than 36 µm in diameter) and chalcedony. The megacrystalline quartz occurs as lenticular and prismatic forms (length: 90–250 µm; width: 30–90 µm). The microprobe, petrographic and SEM examinations confirmed the occurrence of gypsum relics (diameter; 2–16 µm) within the megacrystalline quartz. The chalcedony and mosaic microcrystalline quartz occurs as pore-lining and pore-filling cements. The structure of the silica nodules begins with quartzine in its outer rim, then gypsum-free microcrystalline quartz in the middle part and ends with gypsum-rich lenticular to prismatic megaquartz in the center. Field study, petrographic examination and microprobe analysis reveal that the silica nodules were formed by silicification of precursor gypsum nodules deposited in a marginal sabkha environment under an arid climate. The silicification selectively affected the gypsum nodules rather than the surrounding shale and occurred both through gypsum replacement and void filling. Transformation of isopachous chalcedony into mosaic microcrystalline quartz also occurred. The texture of the silica minerals reflects the different physico-chemical conditions under which they crystallized. Spherical nodules grew chiefly by the diffusive supply of the silica, and elongated ones grew by pore water advection. The integrated effect of climate, pH, salinity, crack systems within the sediment and oscillation in the groundwater level and its chemical composition contributed to the formation of the nodules.
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Evans, G. 1965. The recent sedimentary facies of the Persian Gulf region. Philosophical Transactions of the Royal Society, 259, No. 1099, pp. 291-298. By Graham Evans.

Evans, G., Bush, P.R. and Temple, P.H. 1980. The coastal plain and offshore islands. In: Doornkamp, J.C., Brundsden, D. and Jones, K.C. (Eds.), Geology, Geomorphology and Pedology of Bahrain. Geo Abstracts Ltd., Norwich, UK, p. 269-327.

Evans, G., Kendall, C.G., Kinsman, D.J.J., Shearman, D.J. and Skipwith, P. A. d'E. 1964. In discussion of I.M. West, 1964. Proceedings of Yorkshire Geological Society, 34 (3), p. 15.

Evans, G., Kendall, C.G.S.C. and Skipwith, A. d'E. 1964. Origin of the coastal flats, the sabkha, of the Trucial Coast, Persian Gulf. Nature, London, 202, 759-761.

Evans, G., Kirkham, A, and Carter, R.A. 2002. Quaternary Development of the United Arab Emirates Coast: New Evidence from Marawah Island, Abu Dhabi. GeoArabia, Vol. 7, No. 3, 441-457. By Graham Evans, Southampton University, Anthony Kirkham, Technoguide and Robert A. Carter, University College London
Abstract: Marawah is one of a chain of barrier islands off the coast of Abu Dhabi that separates the Khor Al Bazm lagoon from the open waters of the Arabian Gulf. The island consists of several rocky cores of Pleistocene limestone linked by areas of unconsolidated Holocene carbonates. It has the most complete Quaternary outcrop sequence in the region and the lowest exposed unit, a coralline limestone, had not been recorded previously. The Pleistocene deposits accumulated partly in a shallow-marine environment and partly under eolian conditions. The Marawah sections have revealed new data about the history of the southern Gulf in the late Pleistocene, a time interval of which little was known. The survey has shown that there were periods when sea level was close to present-day levels and other times when it was approximately 4 to 5 m higher than today. A phase of deflation and the development of a field of eolian sand dunes separated these two sealevel highstands. The unconsolidated sediments have accumulated around the Pleistocene rock cores since about 4,500 years BP to give the island its present form. Accumulation occurred because of wave action driven by the northwesterly 'Shamal' winds during periods of slightly falling or almost stationary sea level.

Evans, G., Murray, J.W., Biggs, H.E.J., Bate, R. and Bush, P.R. 1973. The oceanography, ecology, sedimentology and geomorphology of parts of the Trucial Coast barrier island complex, Persian Gulf. In: Purser, R.H. (Ed.), The Persian Gulf, Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea. Springer Verlag, Berlin, p. 233-277.

Evans, G., Schmidt, V., Bush, P. and Nelson, H. 1969. Stratigraphy and geologic history of the sabkha, Abu Dhabi, Persian Gulf. Sedimentology, 12, 145-159.
The sabkha of Abu Dhabi was formed during the past 7,000 years by wind erosion of pre-existing dunes and progradation of subaqueous, intertidal, and supratidal carbonate sediments. Marine transgression began in this area about 7,000 years ago and reached an apparent high about 1 m above its present level somewhat prior to 4,000 years B.P. Since then progradation of intertidal and supratidal sediments has taken place; this began 3,750 years ago. Arid conditions over the sabkha have produced large amounts of gypsum and anhydrite and lesser amounts of dolomite, magnesite, celestite and halite.

Evans, G. and Shearman, D.J. 1964. Recent celestite from the sediments of the Trucial Coast of the Persian Gulf. Nature, London, 202, 385-386.
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Goudie , A.S. and Cook, R.U. 1980. Aeolian landforms and deposits. In: Doornkamp, J.C., Brundsden, D. and Jones, K.C. (Eds.), Geology, Geomorphology and Pedology of Bahrain. Geo Abstracts Ltd., Norwich, UK, p. 269-327.
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Houbolt , J.J.H.C. 1957. Surface sediments of the Persian Gulf near the Qatar Peninsula. (Published) Thesis, Leiden, Monton and Co., Den Haag, Editor.
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Illing , L.V., Wells, A.J. and Taylor, J.C.M. 1965. Penecontemporaneous dolomite in the Persian Gulf. In, Pray, L.C. and Murray, R.C. (Eds.), Society of Economic Palaeontologists and Mineralogists Special Publication, 13, p. 89-111.
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Jauzein , A. 1972. Les donnees sur le systeme CaS0 ₄ , H ₂ 0 et leurs implications geologiques. Rev. Geogr.
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Jameson , J., Puls, D.D. and Kozar, M.G. 2009. Holocene sabkha and coastal systems of Qatar: process models for the interpretation of ancient Arabian Plate carbonate evaporite reservoirs. International Petroleum Technology Conference - 2009 . IPTC 13679. By Jeremy Jameson, David D. Puls (ExxonMobil Qatar Inc), and Michael G. Kozar (ExxonMobil Exploration Company). (also pdf). See also Puls et al. (2009), IPTC 13629 on Dukhan Sabkha.
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Johnstone , T.M. and Wilkinson, J.C. 1960. Some geographical aspects of Qatar. The Geographical Journal,126, 442-450.
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Kassler , P. 1973. The Structural and Geomorphic Evolution of 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. 11-32.
Abstract: The origin of the present-day morphology of the Persian Gulf has been studied and is summarized in figure 3. The Gulf is a tectonic basin of late Pliocene to Pleistocene age, whose morphology is greatly influenced by the tectonic style. The topography of Iran and the Iranian coastal islands is controlled by the intense folding of the Zagros orogeny, on NW-SE to E-W trends. The much more subdued relief of the Arabian side is the result of gentler tectonic movements: Plio-Pleistocene folding, faulting and salt diapirism, superimposed on older, predominantly northsouth-trending growth structures ("Arabian folds"). There is evidence over much of the Gulf of interference between Arabian and Zagros folds. This tectonically controlled morphologic pattern was subdued by sedimentation of Pleistocene limestones, but locally rejuvenated by Quarternary tectonic adjustments. The sea level fell by as much as 120 m during the Pleistocene, emptying the Gulf; river valleys were eroded down the slopes. The sea then cut a series of platforms, at its level of maximum retreat and at times of relative standstill during the post-glacial rise. However, in spite of these Pleistocene physiographic modifications, the underlying tectonic control of morphology is still apparent, and there is a partial correlation between bathymetric and structural highs and lows.
The topography controls the type and the thickness of the marine sediments. Sediment type is largely a function of the biological communities which give rise to skeletal material; these vary in vary in character from shoals to depressions. Sediment thickness is shown by sparker records to be least on topographic highs, and greatest in depressions.
The Recent unconsolidated sediments are the product of the post-glacial Flandrian transgression, which, according to Fairbridge (1961), began about 18 000 years B.P. and reached its present level about 5000 years B.P. These sediments are expected eventually to smooth out the pre-Recent topography by filling up the depressions and extending over the highs. The thickest Recent sediments are found in the Gulf axis.
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Kendall , C.G.St.C. and Skipwith, A. d'E. 1969. Geomorphology of a recent shallow water carbonate province: Khor Al Bazm, Trucial Coast, Southwest Persian Gulf. Geological Society of America Bulletin, v. 80, p. 865-892.
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Kenig , F., Huc, A.Y., Purser, B.H. and Oudin, J.-L. 1990. Sedimentation, distribution and diagenesis of organic matter in a recent carbonate environment, Abu Dhabi, U.A.E. Organic Geochemistry, v. 16, p. 735-747.
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Kinsman , D.J.J. 1965. Gypsum and anhydrite of Recent age, Trucial Coast, Persian Gulf. In: Rau, L.A. (Editor), Second Symposium on Salt, 1, North Ohio Geological Society, Cleveland Ohio, pp. 302-326.

Kinsman, D.J.J. 1965. Dolomitization and evaporite development, including anhydrite in lagoonal sediments, Persian Gulf. Geological Society America Special Papers, 82, 108-109.
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Kirkham , A. 1997. Shoreline evolution, aeolian deflation and anhydrite distribution of the Holocene, Abu Dhabi. GeoArabia, v. 2, no. 4, p. 403-416.

Kirkham, A. 1998a. Pleistocene carbonate seif dunes and their role in the development of complex past and present coastlines of the U.A.E. GeoArabia, v. 3, no. 1, p. 19-32.

Kirkham, A. 1998b. A Quaternary proximal foreland ramp and its continental fringe, Arabian Gulf, U.A.E. In: V.P. Wright, V.P. and Burchette, T.P. (Eds.), Carbonate Ramps. Geological Society, London, Special Publication no. 149, p. 15-41.
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Lambeck , K. 1996. Shoreline reconstructions for the Persian Gulf since the last glacial maximum. Earth and Planetary Science Letters, 142, p. 43-57.
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Lashab , M.I., Daluob, H.S. and Saqer, N.H. 2004. Geological and geochemical studies on Recent sabkha of Karkurah, northeastern Libya. 7th International Conference on the Geology of the Arab World, Cairo University, Egypt, February, 2004. Pp. 1-10 with plates.
Abstract: The Karkurah salt-pan is regularly flooded by sea water from the adjacent Mediterranean Sea. It represents a back-barrier sabkha. The brine supplied the salt-pan by springs and seepage through the ridge from the sea. Sand and silt with halite deposits are the major component of the Karkurah Sabkha sediments. The sabkha around the salt-pan is a semi-vegetated with halophyte plants in nebkhahs. The major mineral as of all large evaporites, is halite. The other minerals revealed through X-ray diffraction (XRD) analysis are quartz, calcite, gypsum and clays. The salt crust consists mainly of NaCl (about 94.97%), while the presence of MgCl does not exceed 2.94%, KCl 0.39% and S0 ₃ does not exceed 5.25%. [end of abstract]

Lashhab, M.I., West, I.M. and El Manaai, M.A. 1999. Study of recent sediments at Zuwarah Salt Pan on the northwestern coast of Libya, formation of anhydrite. In: The First Earth Conference , Univ. of Ghar Yonis, Benghazi, Libya. 12pp. with three plates. (paper accepted, but not necessarily published).

Lashhab, M.I., West, I.M. and El Zarough, R. 2002. Origin and diagenesis of the evaporites in the Jir Formation, Jabal Waddan and Western Sirt Basin, Libya. 6th International Conference on the Geology of the Arab World, Cairo University, February 2002, pp. 623-632. (Written by Dr. Mokhtar Lashhab mainly on the basis of his thesis on this topic in 1992, and supervised by Ian West.)

Abstract:
The Eocene evaporites of the Jir Formation, consist of gypsum, anhydrite, halite, celestite with dolomite. The thickest sequence is developed in and adjacent to a major graben system in the western part of the Sirt Basin. The area of the Sirt Basin was open to the sea via the Sirt Gulf at the very beginning of the Eocene, and then became isolated lagoons, when the Dahra Ridge began to function as a platform or barrier bar with a lagoon to the southwest. The barrier permitted a regular influx of warm and sulphate enriched brines to maintain continuous evaporite precipitation. At the basin margin in the hills of the Jabal Waddan at Wadi Faras, the Jir Formation consists of at least 52m of secondary gypsum and dolomite. In the subsurface, in the Zalla Trough or Meulagh Graben, to the southeast, the formation comprises up to 1000 m of anhydrite and halite with dolomite. The evaporites at Wadi Faras are of shallow water lagoon and sabkha origin, with foraminifer-rich carbonates indicating the dominant marine conditions. In the Zalla Trough laminated anhydrite has been formed also in a shallow lagoon. Halite was deposited in a closed salt-lake in the central part of the basin. Celestite in the Jir Formation was formed by replacement of calcium sulphate.

INTRODUCTION
The area of study is located in the western part of the Sirt Basin, in central northern Libya (Fig. 1). The Jir Formation evaporites of Eocene age are extensively developed in the subsurface of the Zalla Trough (Meulagh Graben) in the western part of Sirt Basin and exposed at Jabal Waddan in the southeastern edge of Hun Graben (Fig. 1). During the Eocene Epoch, the palaeolatitude of the study area was between 15° and 20° N (Smith & Briden, 1977), and climate was thus probably characterized by semiarid conditions. In the Meulagh Graben, the Jir Formation is very thick and consists of anhydrite and halite with thin beds of dolomite. The evaporites of the surface and subsurface are clearly linked as one lithological unit (Lashhab & West, 1996). The mode of occurrence, rifting and depositional models and factors controlling evaporite precipitation are considered. Evaporites from the study area have been analyzed chemically, and the results used to infer the environments in which they were deposited. Trace element content can be used as a measure of the evaporation stage of the original solution, as once crystals are removed from the system there is increased segregation of trace elements either into the crystals or into the residual brine (Holser, 1979). The diagenetic minerals produced after evaporite precipitation and by the reaction between brines and sediments in the study area are similar to those of recent sabkha deposits which occur in the Abu Dhabi area (Bush, 1973), excluding the major beds of halite found in the Zalla Trough. Gypsum, celestite, anhydrite, halite and dolomite have been formed by evaporation and by brine-sediment interactions in the study area.

PETROGRAPHY AND GEOCHEMISTRY
The gypsum crystals are commonly subhedral; some are pseudomorphs after anhydrite. The crystals are well interlocked with irregular interfaces. A few irregular lenses of anhydrite occur in the Jir Formation in samples from the Wadi Faras type-section. This is an indication that the Jir Formation gypsum is secondary and is a near surface replacement of anhydrite. Most of the gypsum nodules in the Jir Formation at Wadi Faras are randomly aligned and surrounded by microcrystalline dolomite (plate 1A). The nodules are mostly small in size, ranging up to approximately 3 cm in length. Some of the gypsum nodules are composed of crystals exhibiting porphyrotopic and granotopic secondary gypsum with minor amounts of anhydrite laths. It is possible that primary gypsum nodules were converted into anhydrite by burial diagenesis and were later transformed into secondary gypsum. As erosion re-exposed the Jir Formation, rainwater and dew dissolved gypsum and reprecipitated it as the water evaporated to form gypcrete. [continues]
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Longhurst , H. 1959. Adventure in Oil: The Story of British Petroleum. Sidgwick and Jackson Ltd., London, 286 pp. By Henry Longhurst, with a Foreword by the Right Honorable Sir Winston Churchill.
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McDonald , G. J. F. 1953. Anhydrite-gypsum equilibrium relations. American Journal of Science, vol. 251, p. 884-898.
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Milliken , K. 1979. The silicified evaporite syndrome—two aspects of silicification history of former evaporite nodules from southern Kentucky and northern Tennessee. Journal of Sedimentary Petrology, 49 (1979), pp. 245–256
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Orti , F. Orti, Rosell, L., Salvany, J. and Ingles, M. 1997. Chert in continental evaporites of the Ebro and Calatayud basins (Spain): distribution and significance. In: A. Ruinos-Millan and M. Bustillo, Editors, Siliceous Rocks and Culture, Universidad de Granada (Spain), Monografica, Arte y Arqueologia vol. 42 (1997), pp. 78–89.
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Perthuisot, J-P. 1977. La sebkha de Doukhane (Qatar) et la transformation: gypse - anhydrite plus water. [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.]

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

The Profile of the Borehole:

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

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

1. A fine crust of halite of millimetre thickness.

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

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

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

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

Interpretation:

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

gypsum -> anyhydrite + water

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

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

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

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

The Conditions of Transformation

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

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

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

There remains one irritating problem: it is the exclusive localisation, at least at present, of the anhydritisation of gypsum, in the conditions of the at the surface at the borders of the Arabian Gulf, even though there exist in other regions of the globe some conditions that are similar in temperature and salinity in the sabkhas of North Africa for example [there are traces of anhydrite in a coastal salt lake of Libya, but I am not aware of any significant quantities]. Now there is one marked difference between sahkhas of the two regions: most of the sahkha of North Africa have an environment that is extremely reducing, rich in organic matter and producing significant quantities of hydrogen sulphide. The sabkhas of the Arabian Gulf are in comparison much more oxygenated, lacking in odour and generally with light-coloured sediments. The intuitive hypothesis is that the transition from gypsum to anhydrite is improbable or at least more difficult to take place in a reducing medium in which S ^2- is the stable form of 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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Powers , R.W., Ramirez, L.F., Redmond, C.D. and Elberg, E.L. Jr. 1966. Geology of the Arabian Peninsula. Sedimentary Geology of Saudi Arabia. U.S. Geol. Surv. Prof. Paper 560 D. V.S. Government Printing Office, Washington, 147 p.
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Puls , D.D., Jameson, J., Kozar, M., Hussain, A-A, LeBlanc, J. 2009. The Dukhan Sabkha: a modern analog for the Arab C carbonate reservoir, Dukhan Field, Qatar. By David D. Puls, Jeremy Jameson (ExxonMobil Qatar Inc), Mike Kozar, Al-Ansi Hussein and Jaques LeBlanc (Qatar Petroleum). International Petroleum Technology Conference - 2009. Held in Doha, Qatar, 7-9th December 2009. (also pdf). See also Jameson (2009) on sabkhas, same conference.
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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.

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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Sharma , V.K. 1986. Geomorphology: Earth Surface Processes and Forms. Tata McGraw-Hill Publishing Company Limited. New Delhi, 244 pp.
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Shearman , D.J. 1966. Origin of marine evaporites by diagenesis. Transactions of the Institution of Mining and Metallurgy, Section B, Applied Earth Science, vol. B75, 207-215. By the late Professor Douglas Shearman of Imperial College, London. (See also: Shearman, D.J. 1967. Report of Discussion of this paper in December 1966 at a General Meeting of the Institution of Mining and Metallurgy, published in 1977, Transactions of the Institution of Mining and Metallurgy, Section B, pp. B82-B86.) [This is an early classic and very original paper on the Abu Dhabi Sabkha, with a controversial theory of origin of the evaporites.]
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Shinn, E.A. 1969. Submarine lithification of Holocene carbonate sediments in the Persian Gulf. Sedimentology, 12, pp. 109-144.

Shinn, E.A. 1973. Sedimentary accretion along the leeward, SE coast of Qatar peninsula, Persian Gulf. Pp. 199-209 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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Siedlecka A. , 1972 Length-slow chalcedony and relicts of sulphates—evidence of evaporitic environments in the Upper Carboniferous and Permian beds of Bear Island, Svalbard, Journal of Sedimentary Petrology, 42 (1972), pp. 812–816.

Siedlecka, A. 1976. Silicified Precambrian evaporite nodules from northern Norway: a preliminary report, Sedimentary Geology, 16, pp. 161–175.
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Smout , A.H. 1954. Lower Tertiary foraminifera of the Qatar Peninsula. British Museum (Natural History), London, IX + 96pp.
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Sugden ,W. 1963a. Some aspects of sedimentation in the Persian Gulf. Journal of Sedimentary Petrology, vol. 33, No.2, pp. 355-364.

Sugden, W. 1963b. The hydorology of the Persian Gulf and its significance in respect to evaporite deposition. American Journal of Science, vol. 261, pp. 741-755.
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Taylor , J.C.M. and Illing, V.C. 1969. Holocene intertidal calcium carbonate cementation, Qatar, Persian Gulf. Sedimentology, 12, pp. 69-107.
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Teller , J.T., K.W. Glennie, K.W., Lancaste, N. and Singhi, A.K. 2000. Calcareous dunes of the United Arab Emirates and Noah's Flood: the post glacial reflooding of the Persian (Arabian) Gulf. Quaternary International, 68-71, p. 297-308.
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Vogel, M.B., Marais, D.J.D., Parenteau, M.N., Jahnke, L.L., Turk, K.A and Kubo, M.D.Y. 2009 (or 2010 in print?). Biological influences on modern sulfates: Textures and composition of gypsum deposits from Guerrero Negro, Baja California Sur, Mexico. Sedimentary Geology, vol. ?, pp. ?. By Marilyn B. Vogel, David J. Des Marais, Mary N. Parenteau, Linda J. Jahnke, Kendra A. Turk and Michael D.Y. Kubo. Available online 3 December 2009.
Abstract:
Gypsum (CaSO4.2H2O) deposits from a range of sedimentary environments at Guerrero Negro, Baja California Sur, Mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent. Gypsum deposits were classified according to their sedimentary environment, textures, crystal habit, brine composition and other geochemical factors. The environments studied included subaqueous sediments in anchialine pools and in solar salterns, as well as subsurface sediments of mudflats and saltpans. Gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines. Subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles. Water column precipitates had euhedral prismatic habits and extensive penetrative twinning. Gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds. Gypsum precipitating within benthic biofilms, and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits. This evidence included irregular, high relief surface textures, accessory minerals (S°, Ca-carbonate, Sr/Ca-sulfate and Mg-hydroxide) and distinctive crystal habits such as equant forms and crystals having distorted prism faces.
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Wells, A.J. 1962. Recent dolomite in the Persian Gulf. Nature, London, 194, 274-275.
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West , I.M. 1964. Evaporite diagenesis in the Lower Purbeck Beds of Dorset. Proceedings of the Yorkshire Geological Society, 34, 315-330. [Worbarrow evaporitic strata, lutecite, celestite, pseudomorphs etc.]

West, I.M. 1965. Macrocell structure and enterolithic veins in British Purbeck gypsum and anhydrite. Proceedings of the Yorkshire Geological Society, 35, 47- 58. [Nodular and enterolithic evaporitic structures from Worbarrow Tout and elsewhere. These were recognised as early displacive features at a time when they were not understood and they were subsequently found to be similar to evaporite structures, such as nodular anhydrite and gypsum of modern sabkhas. The word "macrocell" did not receive general use, and should be replaced now by nodular or chickenwire structure.]

West, I.M. 1975. Evaporites and associated sediments of the basal Purbeck Formation (Upper Jurassic) of Dorset. Proceedings of the Geologists' Association, London, 86, 205-225. Abstract: Four facies of limestones, each with particular contents of calcitised evaporites and of skeletal debris were recognised. They are compared with sediments of modern evaporite-depositing environments. The lowermost limestones, stromatolitic and pelletoid with foraminifera, probably originated in intertidal to shallow subtidal, moderately hypersaline, water. Overlying pelletoid limestones with algal-mats and some gypsum are products of high-intertidal flats. The main evaporite beds were originally gypsum, probably formed in supratidal to intertidal, very hypersaline, palaeoenvironments. The gypsum was converted to anhydrite and later brecciated in part, forming the Broken Beds. Extensive calcitisation produced porous unfossiliferous limestones. Ostracodal limestones above probably originated in shallow, only moderately hypersaline water. All the basal Purbeck strata were formed in and around a large shallow gulf with extensive tidal flats and with water of varying but predominantly high salinities. At times of uplift, thin soils developed on the former margins of the gulf. Forests were able to exist there because, although the area was within the semi-arid zone, it was probably very near to the boundary of the warm-temperate zone. End of Abstract. [Additional notes on topics discussed: Palaeosalinity origins of the basal Purbeck facies and lateral correlation. Mostly hypersaline to varying extents, including the stromatolite horizons. Fossil trees 'pickled' in a salt lake. Details of the basal Purbeck strata at all the main localities, studied petrographically. Depositional environments of the dirt beds and marls. Palaeoenvironmental significance of sedimentary cyles. Thickness variations of the facies. Relationship of the Broken Beds to the evaporitic facies. Local uplift. Penecontemporaneous fault movement. The Mupe Bay oil sand.] Some diagrams that are based on this paper are given below.

West, I.M. 1979a. Sedimentary Environments and Diagenesis of Purbeck Strata (Upper Jurassic - Lower Cretaceous) of Dorset, U.K. Unpublished Ph.D. Thesis, Southampton University, 181 p. Abstract: Twelve papers, notes and a contribution to a book, all either published or accepted for publication, constitute this thesis. All parts of the classic, shallow-water, schizohaline Purbeck Formation of the type area are discussed but emphasis is on Lower Purbeck evaporites. Diagenesis of these involved much conversion of initial small lenticular crystals of gypsum to anhydrite with net-texture. The anhydrite was extensively replaced by calcite and celestite in the Broken Beds, a tectonic evaporite breccia at the base of the Purbecks. Evaporites were almost completely lost in solution from this breccia leaving characteristic relics of "vanished evaporites". Elsewhere, in the more argillaceous parts of the formation the sulphate remains, mainly as porphyrotopic secondary gypsum. Nodules and enterolithic veins are abundant in both the calcium sulphate and in the replacements. The similarity to those in Holocene sabkhas of the Trucial Coast (Shearman, 1966) suggested an origin on supratidal sabkhas, but there is a lack of desert sediments and instead the evaporites are interbedded with forest soils. Analogous Carboniferous evaporites show evidence of sabkha origins but no sign of desert conditions [West, Brandon and Smith, 1968. A tidal flat evaporitic facies in the Visean of Ireland. Journal of Sedimentary Petrology, 38, 1079-1093.]. New evidence has come from sabkhas in Northern Egypt where gypsum nodules develop in partly vegetated environment, dry but not excessively so, and supports other evidence for a semi-arid origin for the Lower Purbeck evaporites [West, Ali and Hilmy. 1979. Primary gypsum nodules in a modern sabkha on the Mediterranean coast of Egypt. Geology, 7, 354-358.]. The relatively dry climate was temporary and facies of higher parts of the Purbecks seem to result from sub-humid conditions. Throughout the formation lagoonal, 'intertidal' and supratidal deposits can be recognised but in the Middle and Upper Purbecks the lagoonal sediments have abundant brackish shelly faunas and, there, 'tidal-flat' deposits consist of shell-sand with dinosaur footprints but usually without evaporites. Progressively the proportion of land-derived clastics such as kaolinite and quartz sand increases as the continental Wealden is approached and final Purbeck sediments contain debris eroded from the underlying Portland Stone Formation, then uplifted at the western margin of the basin.

West, I. 1979. Review of evaporite diagenesis in the Purbeck Formation of southern England. Symposium on: West European Jurassic Sedimentation - "Sedimentation Jurassique W. European", A.S.F. (Association of French Sedimentologists), Special Publication No. 1, March 1979, pp. 407-416. In English with Abstracts in English and French.
Abstract (slightly enlarged and relating to fig 2, alongside): Evaporites and remains of "vanished evaporites" are widely distributed in the Purbeck Formation of southern England. Associated sediments show that these were formed in semi-arid conditions on the extensive tidal-flats of a shallow hypersaline gulf. The primary sulphate was predominantly gypsum as lenticular crystals. Fabrics developed indicate five major stages of diagenesis. There was early recrystallisation of the initial gypsum mush (Stage I) of small lenticular crystals to a less porous anhedral fabric with the small-scale "net-texture" (Stage II), a microscopic network of impurities. The coarser nodular structure, chicken wire structure and enterolithic veins developed as the sulphate was converted to anhydrite (Stage III), a process which commenced penecontemporaneously and was completed before deep burial. The anhydrite was recrystallised so that several anhydrite fabrics now exist. Hydration is a relatively recent process resulting from contact with meteoric water near the surface after uplift and erosion had taken place. This usually commenced with a Stage IV of anhydrite containing gypsum porphyroblasts or porphyrotopes. The existing porphyroblastic or porphyrotopic gypsum represents the final Stage V. Concurrent with sulphate diagenesis there was replacement of the evaporites on an appreciable scale, particularly where they were not enclosed in impermeable clays. Calcitisation of the evaporites has produced peculiar, porous, secondary limestones and limestone breccias [resembling cargneule or rauhwacke]. Associated replacement products, including the strontium minerals - celestite and calciostrontianite, euhedral quartz, the varieties of chalcedony - lutecite and quartzine, suggest an inorganic mechanism for the calcitisation [but see also the thesis of Quest on isotopic studies of Purbeck strata which suggests that there was involvement of hydrocarbons in some cases.]. Criteria are listed that may be used for the recognition of similar replaced evaporites elsewhere. [End of abstract] There follows an extract (with minor additions):
Evidence for Former Evaporites (Vanished Evaporites). An association of several points should be sought:
1. Pseudomorphs of calcite, chalcedony or quartz (or moulds or casts) after gypsum, after anhydrite or after halite.
2. Length-slow chalcedony (quartzine).
3. Spherulites of the lutecite variety of chalcedony.
4. Euhedral crystals of authigenic quartz.
5. Celestite, sometimes with calciostrontianite (occasionally barytes or barite).
6. Net-texture, a small-scale relic of gypsum with displaced impurities, now in secondary, sparry limestone, resulting from calcitisation.
7. Chicken-wire, nodular structure or spherical vugs in limestone or dolomite.
8. Coarsely crystalline (sparry) limestone, often porous, and without skeletal debris (possibly calcitised evaporites). May be massive, laminated or contorted.
9. Small contortions that are not obviously of sub-aqueous slumping or other non-evaporitic origin.
10. Minute rectangular relics of anhydrite in quartz or other minerals such as calcite (obvious in quartz because of contrasting moderate birefringence and the rectangular cleavage - but very small).
11. Oligomict limestone breccia with a carbonate matrix (possible calcitised evaporite breccia).
12. (an addition) A crumbly, brown, porous bed of carbonate and clay (like evaporitic cargneule or rauhwacke common in the Trias of the Alps and Pyrenees).
..Such evidence is likely to be found in other formations without normal marine faunas but with features such as microbial-mats, algal stromatolitic heads, caliches, abundant hypersaline ostracods or peculiar breccias. Obviously red-bed evaporites of more arid origin can provide additional criteria resulting from desert environments.

West, I.M., Ali, Y.A. and Hilmy, M.E. 1979. Primary gypsum nodules in a modern coastal sabkha on the Mediterranean coast of Egypt. Geology, 7, 354-358. Abstract: Nodules of anhydrite in Holocene sabkhas of the Arabian Gulf and Baja California have been used as analogues to interpret calcium sulphate nodules in ancient rocks to be of sabkha origin. Nodules and incipient enterolithic veins of gypsum occur in a modern sabkha in Egypt about halfway between Alexandria and El Alamein, in a depression between a modern and a Pleistocene beach ridge. The displacive gypsum is apparently being precipitated from hypersaline calcium sulphate-saturated interstitial water that increases in salinity as it rises by capillarity from the water table to the surface. Calcium and sulphate ions seem to be derived mainly from dissolution of pre-existing lagoonal gypsum beneath the water table. The nodules occur within a supratidal sand unit of a sabkha sequence capped by a gray, saline soil on which grow clumps of halophytes, separated by salt-encrusted flats. This discovery shows that calcium sulphate nodules can develop (1) within sediments of a region where the climate is almost semiarid rather than very arid, (2) as primary gypsum rather than as anhydrite, and (3) as a consequence of redistribution of calcium sulphate. End of abstract. West, I.M., Ali, Y.A. and Hilmy, M.E. 1983. Facies associated with primary gypsum nodules of northern Egyptian sabkhas. Sixth International Symposium on Salt, 1983, vol. 1, Salt Institute, 171-183. Abstract: Modern sabkha and lagoonal, evaporitic environments are well-developed in the Mediterranean coastal zone of Egypt between Alexandria and El Alamein. It is a semi-arid region with an annual rainfall of about 19cm. Landward of the modern, ooid beach ridge is a narrow depression. This is occupied by partially vegetated sabkhas of desert loess and some small lagoons. Those which are moderately hypersaline contain the cockle [common edible, estuarine bivalve], Cardium glaucum; very hypersaline lagoons are precipitating gypsum. The sabkhas are usually underlain by the following sequence of Holocene sediments. At the base are lagoonal shelly silts, locally with Cardium. Lagoonal gypsum follows. Then comes desert-loess, within which gypsum nodules are developing by precipitation from capillary water. These sabkha deposits are being gradually covered by ooid sand. The present marine transgression should ultimately produce a sequence in which an oolitic limestone is overlain by a cockle bed, followed by laminated gypsum, overlain, in turn, by a red bed siltstone with gypsum nodules. This would be capped by oolitic limestone. Ancient strata resembling certain of these facies include cockle beds associated with evaporites in the British late-Jurassic [or early Cretaceous, Purbeck] strata. The facies association of desert loess with gypsum nodules, halite, caliche, palygorskite and scorpions can be matched in the British Trias. End of Abstract. [The paper is partly concerned with Purbeck analogues in northern Egypt.] Extracts on analogue for the Purbeck cockle beds, such as parts of the Hard Cockle Member and Soft Cockle Member with the ancient "cockle"Protocardia purbeckensis:

"A modern, moderately hypersaline environment with Cardium glaucum is a lagoon (AL.1) east of El Alamein [ The Second World War battlefield]. The brine is of approximately 55% salinity in summer. There are other similar lagoons nearby. The bivalves here are dwarfed (Figure 4), with a mean length of 12.6mm, and are associated with turreted gastropods. Elsewhere, Cardium glaucum occurs in hypersaline lagoons of the south of France and of the Sea of Azov (Rygg, 1970). The abundant Fragum (Cardiaceae) fo hypersaline Shark Bay, Western Australia (Hagan and Logan, 1974) may be analogous. ... Perhaps the best known examples of ancient "cockle beds" with evaporites are in the lagoonal Lower Purbeck Formation (Upper Jurassic-Lower Cretaceous) of southern England (Arkell, 1947). The "cockles" are bivalves of the species Protocardia purbeckensis (Figure. 4). They occur in members known as the "Hard Cockle Beds" and the overlying "Soft Cockle Beds" (Bristow and Forbes in Damon, 1884; Clements, 1969; Ali, 1981). In the Soft Cockle Member there is secondary gypsum that has replaced anhydrite, which in turn is a replacement of primary gypsum (West, 1964). It contains well-developed nodules and enterolithic veins (West, 1965). Calcitized gypsum occurs in the Hard Cockle Member. The usual association of the small cockle Protocardia purbeckensis with evaporites suggests that the species was tolerant of hypersaline conditions. Comparison with modern analogues suggests that it might have been able to live in brine of up to about 60 parts per thousand salinity. The lack of desert sediments, the presence of coniferous forests and the characters of the insect, molluscan and ostracod faunas is evidence, however, for a climate that was semi-arid and Mediterranean type (West, 1975; 1979; Francis, 1983). This is confirmed by the palaeolatitude of about 37 degrees N (Smith and Briden, 1977).

West, I.M., Lashhab, M.I. and Muhan, I.M. 2000. North African sabkhas and lagoons compared to those of the Arabian Gulf. Pp. 512-530 in: Proceedings of the Sixth Mediterranean Petroleum Conference and Exhibition, November 23-25th, 1999, Tripoli, Libya (G.S.P.L.A.J.), 845 pp. Abstract: The most arid part of the southern Mediterranean coast extends from Tunisia in the west to the Sinai in the east. Here, major wave-action has produced straight coastlines with beaches. Behind these are widespread sabkhas and some lagoons. These environments and their evaporites are compared to the coastal sabkhas, lagoons and evaporites on the Arabian side of the Arabian Gulf... continues.

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Copyright © 2006 Ian West and Tonya West. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged. No permission can be given for reproduction of any images of the Lulworth Cove area in books or in other websites, for special reasons.

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

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Webpage - written and produced by:

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

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at his private address, kindly supported by the School of Ocean and Earth Science, the National Oceanography Centre, Southampton (NOCS),and web-hosted by courtesy of the iSolutions of Southampton University.