West, Ian M. 2013. The Purbeck Formation of Southern England; Sedimentology of Evaporites: Geology of the Wessex Coast. Internet field guide. http://www.southampton.ac.uk/~imw/Purbeck-evaporites.htm. Version: 19th December 2013.

Sedimentology of Purbeck Evaporites .

by: Ian West
Romsey, Hampshire,
and Ocean and Earth Science ,
Faculty of Natural and Environmental Sciences,
Southampton University,

Website hosted by courtesy of Information Systems Services, Southampton University

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|Durlston Bay bibliography |Purbeck Formation Geological Bibliography |Sabkhas of Qatar and other areas

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STRATIGRAPHY:

Logs of Basal Purbecks with Evaporites

Basal Purbeck logs

Basal Purbeck logs

Basal Purbeck Facies and Breccias

This diagram shows the distribution of the breccia, the Broken Beds, in the basal Purbeck Formation of Dorset and the relationship of this breccia to the occurrence of evaporite facies. There is a clear relationship although the breccia also extends above the major evaporite horizon. Part of the overlying laminated limestone of Cypris Freestone type is involved in the brecciation.

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Pseudomorphs after Lenticular Crystals of Gypsum

Crystallography and habit of lenticular or discoidal crystals of gypsum

Early or penecontemporaneous crystals of gypsum are very commonly lenticular in form. Later, diagenetic gypsum is rarely so. The early lenticular gypsum is often replaced and present at lutecite pseudomorphs after gypsum, calcite pseudomorphs after gypsum or celestite pseudomorphs after gypsum. This a major part of the common evidence for "Vanished Evaporites". Such pseudomorphs are abundant in the Lower Purbeck Formation of Dorset, but are not normally visible in the field. Thin-section petrography is needed.

Lutecite pseudomorph after lenticular gypsum, Lower Purbeck Formation, Durlston Bay

Small lenticular crystals of gypsum in Quaternary sabkha deposits of the Northern Egypt, and similar to early gypsum in the Lower Purbeck Formation of Dorset, England

Pseudomorphs of gypsum in chert, from Upwey, Dorset, but common in the basal Purbeck evaporite beds of various locations in Dorset and Sussex, polished surface

Chalcedony (lutecite) pseudomorphs after gypsum seen on a polished surface of chert from the basal Purbeck Formation, Portesham quarry, Dorset. Such pseudomorphs are abundant in the basal Purbecks at Lulworth Cove, the Fossil Forest, Stair Hole etc. but are not always easily visible without appropriate weathering or special etching of the chert.

Pseudomorphs after gypsum in oolite of the Hard Cap

Pseudomorph after gypsum in fine-grained oolite of the Hard Cap at the Fossil Forest.

 Pseudomorphs after primary lenticular gypsum in chert, basal Purbeck Formation

Pseudomorphs after gypsum in pelsparite

Pseudomorphs after gypsum in pelsparite

Calcite pseudomorphs after gypsum in pelsparites of the "Cypris" Freestone Member. The right-hand section of the lower image is in false colours to give an indication of the original appearance of the gypsum in the creamy-coloured carbonate silt.

Penecontemporaneous gypsum crystals in peloidal fine sand and replaced by calcite, Lower Purbeck Formation, Portesham Quarry, specimen LP351

Thin-sections of lenticular gypsum from the First Depression Sabkha, between Alexandria and El-Alamein, northern Egypt

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MICROBIAL MATS

Mats and Gypsum

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

Thin section through a microbial mat associated with gypsum, celestite and lutecite, Lower Purbeck Formation, Mountfield Mine, Sussex

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EVAPORITE DIAGENESIS

Evaporite Diagenesis Scheme.

Please see the papers of West (1964; 1965 and 1979). A summary of the diagenetic scheme as given in West (1979) is reproduced here. More details will be added later.

West, I. M. 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.

A general diagenetic classification of British Purbeck calcium sulphate rocks - gypsum and anhydrite

Abstract (slightly enlarged and relating to fig 2): 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]

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EVAPORITES - GENERAL

Vanished Evaporites

Some criteria for finding evidence of vanished evaporites is reproduced here from West (1979).


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).

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EVAPORITES FACIES

Cockle Bed - Evaporite Association

Some information on a paper which discusses this topic is given here. (This section will be revised in due course and new text and photographs added.)

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 b 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:

Purbeck and modern cockles of hypersaline lagoons

Cockle shell on a dried saline lagoon of northern Egypt

"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).

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BIBLIOGRAPHY and References (Geology)

Go to Bibliography and References on the Purbeck Formation of Dorset.

Go to Bibliography and References on Durlston Bay, Purbeck type-section.

Go to Bibliography and References on the Isle of Portland.

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Copyright © 2013, Ian West, Catherine West, Tonya Loades and Joanna Bentley. All rights reserved. This is a purely academic website and images and text may not be copied for publication or for use on other webpages or for any commercial activity. A reasonable number of images and some text may be used for non-commercial academic purposes, including field trip handouts, lectures, student projects, dissertations etc, providing source is acknowledged.

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Dr Ian West, author of these webpages

Webpage - written and produced by:


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

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at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.