INTRODUCTION - General

Swanworth Quarry is a very large limestone quarry, not far from Swanage, in the Isle of Purbeck, eastern part of Dorset, southern England. It is an impressively large working quarry, in very active use in 2019, with still some unquarried ground. The quarry does not have open public access without permission. It can be seen from outside the boundary fence from a footpath near Worth Matravers, Dorset.

With access permission,although the quarry is easy to interpret in broad terms of the Portland Limestone Formation, with some strata of the Lulworth Formation ("Lower Purbeck" in old terminology) above. However, the quarry it not easy to study in detail, partly because of vertical cliffs in parts, and dusty, worked rock rock exposure. A major factor is that quarrying work on a large scale might be taking place nearby and big vehicles may be moving around. etc. It is not like an abandonned quarry that you can wander around at leisure. Special permission to enter is needed and visitors have to be signed in. Geological work may only be permitted, if at all, in certain parts.

This webpage is only a introduction to Swanworth Quarry. It does not present a thorough study. There are gaps and uncertainties about thickness in the Purbeck Group. No ostracod data, geochemistry, magnetostratigraphy is given and very little petrography. Thus section is yet known in the detail of the Purbeck type section of Durlston Bay. Note that the quarry changes with time and in the long term may be "restored". The quarry is important geologically because of the Swanworth Quarry Boreholes No. 1 and 2, which are key reference boreholes for the Kimmeridge Clay, the great oil source rock of the North Sea. The Kimmeridge Clay contains the famous Kimmeridge oil shale or Blackstone beneath Swanworth Quarry and Worth Matravers village. It is about half a kilometre down.

INTRODUCTION

General Overviews

INTRODUCTION - Swanworth Quarry Environment

Worth Matravers Village

The general environment of Swanworth Quarry is very rural. It is in an area of fields and steep-sided valleys ("Bottoms") near to the small and old village of Worth Matravers The quarry is well-concealed and has earth banks at the sides. Compact Farm is a short distance to the east. The nearest large building is the church of St. Nicholas at Worth Matravers. However, the quarry is not clearly visible from this village. Road traffic from the quarry does not have to pass through the village, so it has probably been very little affected by the quarrying. The Swanworth Boreholes were only exploratory and drilled with a relatively small drilling rig, so this probably did not cause any significant disturbance to Worth Matravers.

Swanworth Quarry - Aerial Views

Swanworth Quarry of Suttle Stone Ltd.

This quarry is situated west of Swanage in the Isle of Purbeck and north of St. Aldhelm's Head. It is an old quarry, extending back to 1918, but much expanded in recent years. This large quarry supplies aggregate from limestone and chert of the Portland Group (Portland Freestone and Portland Cherty Series). The Portland Stone section here was described by Arkell in 1935. It was owned in the past by Tarmac but has changed ownership but is now part of the Suttle Stone Dorset quarrying organisation.

Go to:

Suttle Stone Quarries - Swanworth Quarry.

The webpage of Suttle Stone Quarries will provide contact information etc.

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[Regarding a restoration project at Swanworth Quarry see:
Unique restoration project at Tarmac, Swanworth Quarry
(Featured in Safety, Health and Environment, 01 August 2006.)]

The location of the quarry is NNE of the village of Worth Matravers. The map reference is SY 969783 and it is marked as Swanworth Quarries on the Ordnance Survey 1:25,000 Purbeck and South Dorset map. The quarry entrance is passed on the road from Kingston to Worth Matravers. There is no access to the quarry without permission from the owners. A public footpath (part of the Purbeck Way) passes up Hill Bottom and Coombe Bottom to the west of the quarry. In the future restoration scheme the quarry will cease working and the area will be landscaped to some extent. It is hoped that some good exposures will still be maintained for geological reference. Exposures of Purbeck marls are easily degraded and vegetated and so it is not clear how long sections of these strata will survive. This record here may thus be of some value in the long term.

INTRODUCTION:

Safety and Risk Assessment

This website simply provides geological information and gives descriptions of exposures and is not advice to visit or to collect at any specific place. It does not discuss in full the particular risks of entering and undertaking geological activity in a quarry. The safety regulations of the particular quarry must be adhered to. Quarries are potentially hazardous place and no liability is accepted.

Swanworth or Worth Quarry is at the time of writing a working quarry, without open access, and not a place that you can just walk into. Permission for access needs to be arranged with the quarry owners and in accordance with their regulations. If it is visited the safety regulations of the quarry company must be followed strictly and these include safety helmets and high-visibility clothing. There are the usual quarry hazards at Swanworth Quarry with regard to cliffs, rocks, boulders, large dumper trucks, quarry machinery and blasting. The rock faces in working quarries are usually more hazardous than those of natural cliffs because the rock has often been loosened and the more unstable pieces have not slipped to the ground over the years. Safety helmets are essential. It is important not to hammer the chert in the Portland strata because dangerous splinters can easily penetrate the body and may cause blindness. When preoccupied in geological study take care not to walk backwards over a quarry cliff or fall down fissures in the Portland Stone. A hazard of injury exists if persons attempt to move or lift a large block of stone.

INTRODUCTION:

Geological Maps and Quarry and Borehole Locations

The geology of the area is shown on the old maps of 1895 above, but the quarry was not present at this date. See the geological map below for the location of Swanworth Quarry in relation to the geology (note that there have been some terminological changes). Purchase the new edition (2000) of the British Geological Survey, 1:50,000 Sheet 342 (East) and part of 343, Solid and Drift Edition, folded.

This large quarry is worked for crushed stone or aggregate (40mm). The Portland Stone here is very fractured, as can be seen in the illustrations. It uses almost all the Portland Stone succession, including both the Portland Cherty Series, not usable for worked blocks of stone, and the Portland Freestone above, which is quarried elsewhere for building stone. The Portland Cherty Series and Freestone Series are well-seen in the coastal sections as at Seacombe, Winspit and St. Aldhelm's Head, not far away, and the sequence is not greatly different in this quarry. The Purbeck succession of thin-bedded limestone, shale and marl is of particular interest because it is not well-exposed at the coast near here, and the best coastal sections in the region are at Durlston BayDurlston Bay and Worbarrow Bay. The Purbeck succession is Lower Purbeck, including Caps, Broken Beds, "Cypris" Freestone, Hard Cockle and Soft Cockle Members. Strata above this has been eroded away.

The quarry was at one time estimated to continue to 2017, but it is now (at the time of writing - 2019) and later landscaped with some rock faces left for geological study. (See the DIGS Presentation regarding Swanworth Quarry (and other quarries), mentioned below).

[Note in May 2019. Portland Stone fissuring. In the quarry there is significant fissuring (as on the Isle of Portland) which is unusual is being almost confined to Portland Stone. The fissures do not seem to extend up as major features into the Durlston Formation or "Lower Purbeck" which contains much clay. Thus the fissure system before quarrying was underground in this area. Because the strata are almost horizontal at the quarry site the underground fissure system may not necessarily conduct water downslope to the north or to the south. The location of the quarry at or near the anticlinal axis is probably reducing flow through fissures, but aquifer and pumping details are not known to the writer. More study of the fissuring system and its hydrogeology might be interesting (but it is a subject for specialists).]

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INTRODUCTION:

History

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The Swanworth Quarry Boreholes into the Kimmeridge Clay

The RGGE - Rapid Global Geological Events Research Project (late 1990s)
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Two boreholes were drilled at Swanworth Quarry [Worth Quarry], near Worth Matravers, East Dorset, down into the Kimmeridge Clay, and through the Kimmeridge oil shale, in the late 1990s. This was primarily for a special research project on rhythmic sedimentation. The RGGE or Rapid Global Geological Events Project was initiated in 1995. Much data was also obtained on Kimmeridge Clay sequence, sedimentation and the distribution of organic matter. The boreholes at Swanworth Quarry were supplemented by another borehole at Metherhills near Kimmeridge Bay.

For a description of the results of the RGGE programme, including the Swanworth Quarry Boreholes, see the following two papers:

Gallois (2000). The Stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) in the RGGE Project boreholes at Swanworth Quarry and Metherhills, south Dorset.

Morgans-Bell et al. 2001. Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset.

The drilling project at Swanworth Quarry was organised by Dr. Ramues Gallois, the well-known Kimmeridge Clay specialist, in conjunction with other geologists of the RGGE research group.

Swanworth Quarry was the chosen site for the borehole for very good reasons. It is stratigraphically not far above the Kimmeridge Clay. It is in many ways the quarry is very good site for exploratory Jurassic boreholes in the southern Isle of Purbeck. This large, Portland Stone quarry has near horizontal strata, only some faulting on the east side, and descends to the top of the Portland Sand. Kimmeridge Clay is not far beneath, and the Kimmeridge oil shale is only about 250m down. In addition to suitable geology, the quarry is deep, away from Worth Matravers village and an inconspicuous site for borehole operations. Technical problems were encountered with fractured rock, however. It should be noted though that the borehole system used was a smaller rig than the standard oil company exploration rigs, and very much smaller than a Extended Reach Borehole Derrick like that at Goathorn Peninsula, Wytch Farm oilfield. I do not know the details of the problems caused by fractured rocks in the Kimmeridge Clay of the boreholes at Swanworth Quarry.

The drilling rig was a Soil Mechanics drilling system. Various specialists were involved in logging procedures. Dr. Sarah Pearson, at that time at Southampton University did the onsite geological logging. Steve Etches gave palaeontological advice, according to Ramues Gallois's paper.

The first borehole at Swanworth Quarry was terminated at a depth of 505 metres, within the lower part of the Lower Kimmeridge Clay, according to Gallois (2000).

In general the borehole sections are similar to the coast section. All the zones and chrono units are present. This is summarised in Fig. 3, occupying pages 274 and 274 of Gallois (2000).

The Kimmeridge Blackstone or Oil Shale is in unit KC 42 and is at the top of the Pectinatites wheatleyensis Zone, as normal. Morgans-Bell et al. (2001) reported on the Blackstone in the Swanworth Quarry boreholes:

"The Blackstone is heterogeneous, containing organic- poor layers (less than 10% TOC [NB. TOC is Total Organic Carbon, not total organic matter]) intercalated between more organic-rich beds (40-60% TOC; Huc et al., 1992; Herbin et al., 1995). Analysis of the Blackstone in Swanworth Quarry 1 gave a value of 35wt% TOC. This comparitively low value [does not seem low] probably relates to our mode of sampling, as 10cm composites were analysed for TOC content. The Blackstone forms the most prominant bed in a package of organic-rich shales that straddle the Pectinatites wheatleyensis - Pectinatites hudlestoni zonal boundary (Bed Group 42). Immediately below the Blackstone lies a bed rich in pyritised specimens of the planktonic crinoid Saccocoma, and above rest a series of olive-grey, organic-rich, coccolithic mudstones, weakly laminated in part. Almost at the top of Bed Group 42 lies the Rope Lake Head Stone Band (Bed 42/24), a well-cemented, laminated coccolithic limestone showing evidence of burrowing in its upper surface. Above this cementstone occurs another thin carbonate band sandwiched between two oil shales (Bed 43/2), termed the Little Stone Band by Coe (A.L. Coe, unpublished D.Phil thesis, Univ. Oxford, 1992) but herein referred to as the Short Joint Coal (Strahan, 1920). "

A feature of special interest is the Hobarrow Bay Fluidised Bed, just underneath the Nannocardioceras Cementstone in KC 32 near the top of the Aulacostephanus eudoxus Zone of the Lower Kimmeridge Clay. Earthquake shock beds are rare in the Jurassic and Cretaceous of Dorset. There is a fluidised bed in the Unio Beds of the Durlston Formation (Purbeck Group - "Upper Purbeck") and this too almost certainly is the result of an earthquake shock (associated with Late Cimmerian movement).

More details of the Well Logs are given in the less readily available: Gallois, R.W. (1998). British Geological Survey Technical Report: WA/97/91.

Detailed large scale graphic logs of Swanworth Quarry No. 1 are held by the Oxford Group (Morgans-Bell et al.). There is also isotope and uranium data held [reaching about 4ppm near the Freshwater Steps Stone Band].

PORTLAND STONE FORMATION

Introduction (revised 4th May 2019)

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STRATIGRAPHY

Portland Chert Member (Portland Cherty Series)

The Portland Chert Member (Portland Cherty Series) of the Portland Stone Formation, is quarried in separate, lower rock faces. It is similar to the Portland Chert Member in the coast sections at Winspit (also shown above). It is easier to access specific beds at the cliffs.

STRATIGRAPHY:

Portland Freestone Member of the Portland Stone Formation.

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PORTLAND FREESTONE MEMBER

Down in the Quarry

The Portland Freestone is less porous and not such a well-developed oolite as on the Isle of Portland. It is similar to the Portland Stone in the coast sections of the Isle of Purbeck. It contains very little chert. Major joints with dissolution features occur at intervals. They are brown-stained with travertine and with clay debris probably derived from the Purbeck Formation above during periglacial conditions. At least partly as a result of blasting the face of Portland Freestone is very fractured and does not contain the large unfractured blocks that are characteristic of the Portland Stones quarries of the Isle of Portland. This, of course, is not of any consequence because the stone is all being used here in crushed condition for aggregate.

Portland Freestone Member - Fracturing

Fracturing is pronounced in the Portland Freestone Member. Often the highest part including the Shrimp Bed shows mainly curved fracture planes. Lower in the Portland Freestone the fractures may more often be vertical or near vertical.

STRATIGRAPHY:

Purbeck Group - Basal Purbeck

(Lulworth Formation = Lower Purbeck plus lower part of Middle Purbeck)

See the petrographic work on the basal Purbecks here by West (1975). See also the vertically more extensive but, for access reasons, incomplete field logs of Ensom (2010). See particularly his figs 2a-c, pp. 130-131. Perhaps because of changes in the quarry, after the 2003 field trip shown here, there may not have been access to the upper parts of the eastern quarry cliff section.

As noted above, the Purbeck succession seen here is entirely Lower Purbeck, including Caps, Broken Beds, "Cypris" Freestone, Hard Cockle and Soft Cockle Members. These units are visible towards the top of the quarry and strata above them have been eroded away.

STRATIGRAPHY:

Basal Purbeck Sequence

The following is based on West (1975), and is part of a petrographic study of basal Purbeck evaporites:

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Basal Purbeck Strata of Swanworth (Worth) Quarry (SY 971787)

The Purbeck overburden of this Portland Stone quarry consists of the Caps and the Broken Beds. Small folds are present like those in the breccia of the Lulworth area and these contorted beds contain evidence of evaporites. A section measured on the east side of the quarry, and not shown in Figs. 2 or 3 is described below. Beds 1 to 5 probably belong to facies A, beds 6 and 7 probably belong to facies B. beds 8 to 10 facies C, whilst the uppermost beds belong to Facies D.

(above the section described below is the Hard Cockle Member and the Soft Cockle Member. The latter contains calcitised evaporites (see DIGS field guide) but actual gypsum has not been reported, probably because the evaporite horizon would have been very close to the surface and probably dissolved.)

Bed

Cypris Freestone

13. Laminated pelletoid limestone and micrite with ostracods. 0.3m.
12. Brown shale. 0.46m.
11. Ostracodal pelletoid limestone with scattered ooids. 0.58m.

Broken Beds.
[calcitised evaporites - quite thick - 4.4m. Unlike the Lulworth Cove area the breccia is entirely in evaporites and the underlying Great Dirt Bed and Soft Cap with thrombolites of the Fossil Forest is absent, while instead much anhydrite was present. This is Basal Purbeck, evaporite basin facies.]

10. Contorted, very friable, calcitised anhydrite with abundant pseudomorphs after anhydrite. 3.8m.
9. Shale and calcitised anhydrite with net-texture, partly brecciated. 0.24m.

Caps [mostly Hard Cap]

8. Laminated, calcitised secondary anhydrite with ghosts of anhydrite crystals and also of earlier lenticular crystals of gypsum. 0.82m.
7. Laminated marl. 0.26m.
6. Thrombolites of limestone with chert passing laterally into laminated limestone. 0.16m.
5. Carbonaceous calcareous shale (Lower Dirt Bed). 0.09m.
4. Thrombolitic limestone with chert. 0.39m.
3. Calcareous shale. 0.02m.
2. Laminated, sparry calcitised anhydrite with net-texture and ghosts of anhydrite crystals. 0.08.m.
1. Laminated pelletoid [peloidal] limestone with ostracods, gastropods and foraminifera (overlying and fused to the Portland Shrimp Bed). [i.e. The widespread Transition Bed or Skull Cap.] 0.08m.

Purbeck Sequence - Broken Beds

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Blue Cypris Freestones

Purbeck Group, Lulworth Formation - Cypris Freestones, Marlstone with Archaeoniscus

There is some doubt about the horizon here. However, it is possible that the geologists are at a lower stratigraphical level, than first thought. They might be the level of Bed 7, Durlston Bay, of Bristow in Damon (1884) (p. 209). Bed 7, immediately above the Broken Beds, is an olive-brown marlstone with Archaeoniscus. The matter needs to be checked further.

These photographs above show the occurrence of the isopod Archaeoniscus brodiei in marls of the Lower Purbeck above the level of the "Cypris" Freestone Member. This lagoonal isopod occurs in various parts of the Purbeck Formation and seemed to have been able to tolerate slightly hypersaline conditions that occurred in the earlier part of Purbeck times (very early Cretaceous). It was related to the modern sea slater, also shown. It sometimes occur with fossil insect remains. The specimens in Swanworth or Worth Quarry are particularly well-preserved.

Also shown above are fish remains found in a loose block of Purbeck limestone nearby. The horizon of this is not known. (compare with fish remains at the Fossil Forest)

Purbeck Sequence - Lulworth Formation - Cypris Freestones up to Hard Cockle Member

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This hill, southeast of the working part of the quarry, is of Lulworth Formation [i.e. Lower Purbeck] shale and limestone reaches the height of 131m.(430 feet). In the images above it appears to be compressed (actually the heighest point, with the hawk, is more than 150m. distant from the camera). The good section is presumably not accessible without permission from the Quarry authorities and even if was, the cliff is obviously too steep for climbing. So it is quite a good exposure but not easily accessed. The strata seen are quite high in the Lulworth Formation, probably ranging from Cypris Freestone Member to Soft Cockle Member. The gypsum which is a notable part of the Soft Cockle Member at both Durlston Bay and Worbarrow Bay is not clearly seen (is it really present? - has it, at this high level above the water table, been leached away in solution?).

The "top beds" of Swanworth Quarry, beneath the uppermost limestones, include a very conspicuous cream (or whitish) marlstone. This is a good marker horizon. Above it is a dark shale. There is localised folding, including a small overfold, in these beds on the eastern side of Swanworth Quarry.

Purbeck Group - More High Parts of Swanworth Quarry.

Eastern Face.

ACKNOWLEDGEMENTS

I am very grateful to Ed Jarzembowski for help in measuring up and studying sections in Swanworth Quarry many years ago. I thank Professor Daniel Bosence for very kindly providing some new photographs of the Portland-Purbeck junction. Member of the DIGS group and the Open University Geological Society have kindly helped with field work in the past. The owners of the quarry have kindly allowed visits of individuals and parties and this is much appreciated. I am much obliged to Alan Holiday, Chairperson of DIGS, for a copy of the DIGS Virtual Field Guide to Swanworth Quarry and for other information. The Staff of Swanworth Quarry have been very helpful and this is much appreciated. I am particularly obliged to the Kimmeridge Clay specialist, Dr. Ramues Gallois for the photograph of his borehole operations into the Kimmeridge Clay beneath Swanworth Quarry.

REFERENCES

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Arkell, W.J. 1935. The Portland Beds of the Dorset Mainland. Proceedings of the Geologists' Association, vol. 46, pp. 301-347. No abstract. This is a classic and key paper on the Portland Stone of east Dorset. Plate 19 shows tables of strata at Gad Cliff, St. Alban's Head, Winspit, Worth Quarry [now Swanworth Quarry] and Seacombe [a modified part of this diagram is given above in the main part of this webpage]. Includes a brief discussion of the Shrimp Bed at Worth Quarry (p. 306). "At Worth Quarry, two miles inland [from St. Aldhelm's Head], which is almost as far west as St. Aldhelm's Head Quarry and half a mile further west than Winspit, the development of the beds is the same as at Seacombe (north end)." (p. 318). On page 305 he mentioned that the after a change of ownership in Summer 1935 the quarry now bears the name Swanworth Quarries.

Arkell, W.J. 1945. The names of the strata in the Purbeck and Portland stone quarries. Proceedings of Dorset Natural History and Archaeological Society, vol. 66, 158-168.

Arkell, W.J. 1947. The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain. 386pp. With Wright, C.W.and Melville, R.V. 2nd edition - 1952 with Addenda and Corrigenda. This well-known memoir also contains the diagram showing the correlation of Portland strata on the Dorset mainland, including Worth Quarry - Fig. 20, p. 100.

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Cole, D.J. 1976. Velocity/Porosity relationships in limestones from the Portland Group of southern England. Geoexploration, vol. 14, pp. 37-50. Abstract: Simple linear regression analysis of velocity/porosity data for limestones from the Portland Group of southern England is presented. These limestones are divided into four main petrographic types - sparites, micrites, microsparites and chalks - on the basis of texture and grain size of cement. Velocity/porosity inverse relationships for each of these types are significantly different. In sparites and chalks, most of the observed variation in velocity can be directly related to porosity which is also an important parameter in microsparites and, to a lesser extent, micrites. Mineral content is probably an additional factor in microsparites and pore structure is important in micrites. Velocity in the majority of limestones is isotropic. [Some of the samples for this research were collected (with me) in Swanworth Quarry in about 1971. The paper does not discuss details of Swanworth Quarry, although they may be so discussed in the 1975 Ph.D thesis of Dr. Douglas Cole]

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Damon, R. 1884.Geology of Weymouth, Portland, and Coast of Dorsetshire, from Swanage to Bridport-on-the-Sea: with Natural History and Archaeological Notes. New and Enlarged Edition (2nd Ed.), Weymouth, R.F. Damon, London, Edward Stanford. 250p. With a colour geological map of part of the Dorset coast, and including a log of the Purbeck strata of Durlston Bay, Swanage, by H. W. Bristow and Prof. E. Forbes (although note that it contains a small error).
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DIGS, Dorset's Important Geological Sites Group. 2009. The Purbeck Limestone Group.

The DIGS Presentation - The Purbeck Limestone Group

An introduction to the geology of Swanworth Quarry is provided in the DIGS CD PowerPoint Presentation - Purbeck Group in Purbeck limestone quarries, 2009 . This guide on a CD could be purchased from the DIGS organisation for 5 pounds.

Virtual geology field trips to Purbeck limestone quarries in south Dorset. A CD available from DIGS, Dorset's Important Geological Sites Group. This is a volunteer organisation whose aims are the registration and preservation of important geological exposures throughout the county. For further information on publications and membership please contact: D.I.G.S.
c/o Dorset Wildlife Trust, Brooklands Farm, Forston, Dorchester, DT2 7AA. (Telephone number given)

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Ensom, P. 2010. Lithostratigraphic sections through the Purbeck Limestone Group (Tithonian-Berriasian) at five Regionally Important Geological/Geomorphological Sites (RIGS) on the Isle of Purbeck, and at Bacon Hole near West Lulworth, Dorset, southern England. Proceedings of the Dorset Natural History and Archaeological Society, vol. 131, 2010, pp. 127-144. By Paul Ensom.
The creation of a virtual field trip principally featuring the Purbeck Limestone Group on the Isle of Purbeck, Dorset, southern England by Dorset's Important Geological/Geomorphological Sites Group, led to the production of logs of the strata exposed in the five Regionally Important Geological/Geomorphological Sites. The logged sections of these are published along with one for the Purbeck Limestone Group exposed between Bacon Hole and the southern margin of Mupe Bay near West Lulworth. Insects recovered from one of the sites are listed. Reference is made to the distribution of silicified wood based on observation in the field. The presence of microinvertebrates, dinosaur tracks, and of reddened strata at certain horizons is recorded

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Gallois, A., Bosence, D. and Burgess, P.M. 2018. Brackish to hypersaline facies in lacustrine carbonates : Purbeck Limestone Group, Upper Jurassic-Lower Cretaceous, Wessex Basin, Dorset, UK. Facies, Vol. 64, 12, 04.2018, p. 1-39. [by Dr. Arnaud Gallois and co-authors]
Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic-Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone-grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.

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Gallois, R. 2000. The statigraphy of the Kimmeridge Clay Formation (Upper Jurassic) in the RGGE Project boreholes at Swanworth Quarry and Metherhills, south Dorset. Proceedings of the Geologists' Association [London], vol. 111, pp. 265-280. By Dr. Ramues Gallois of Exeter, retired BGS geologist and author of many geological publications and, in particular, a Kimmeridge Clay specialist.
Abstract: Three continuously cored boreholes were drilled in the Kimmeridge Clay Formation in south Dorset to provide unweathered samples for multidisciplinary study of late Jurassic rhythmic sedimentation and its possible causes. Taken together, the borehole cores provide the first complete sequence through the Kimmeridge Clay and the Kimmeridge Stage in their type area. The cores have been correlated in detail with the succession exposed in the nearby Kimmeridge cliffs and other sections elsewhere in southern and eastern England. The cores have enabled the current chronostratigraphical classification of the Kimmeridge Clay to be extended to the top of the formation, covering strata that are poorly exposed at outcrop. Four types of small-scale rhythm are present within the formation, each of which can be related to the sequence stratigraphy. Only one of these is organic rich and of importance as an oil-source rock.
[end of abstract]
[Example extract from the Introduction follows:]
"1. Introduction.
Rapid Global Geological Events (RGGE) Project.
In 1995 the National Environmental Research Council (NERC) initiated the Rapid Global Geological Events (RGGE) special research topic to examine rhythmicity and its possible causes in the Kimmeridge Clay. The formation was chosen for the study because it consists of an almost unbroken sequence of relatively uniform, high fossiliferous marine-shelf mudstones that have suffered little tectonic deformation. The mudstones contain rhythmic variations in clay mineralogy, and faunal and organic content that reflect climatic and sea-level changes, some of which have been interpreted as Milankovitch precession/obliquity rhythms (House, 1995). In order to obtain enough material for the interdisciplinery study, it was proproposed that two continuously cored boreholes, about 20m apart, should be drilled through the full thickness of the formation at a single site close to the type section at Kimmeridge, Dorset.
The drilling site originally chosen, in the floor of Swanworth Quarry [or Worth Quarry] [SY 9675 7823] near Worth Matravers (fig. 1), enabled drilling to begin at a known stratigraphical horizon (the top of the Portland Sand), and it could be seen from the adjacent quarry faces to be in an unfaulted area. Examination of the geophysical logs from boreholes through all or part of the Kimmeridge Clay in this area, together with seismic-reflection profiles provided by British Petroleum Ltd, had suggested that the full thickness of the formation here was between 535 and 585m. To this was added 40m for the overlying Portland Sand and 15m to allow for over-run of the geophysical tools at the bottom of the borehole, to give an estimated total required depth of 590 to 640m if the full formational thickness was to be recovered.
[....continues]

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Gallois, R.W. 1998. The stratigraphy of and well-completion reports for Swanworth Quarry No.1 and No.2 and Metherhills No.1 boreholes (RGGE Project), Dorset. British Geological Survey Technical Report WA/97/91.
Three continuously cored boreholes were drilled in the Kimmeridge Clay Formation in south Dorset to provide unweathered samples for a multidisciplinary study of late Jurassic rhythmic sedimentation and its possible causes. Taken together, the borehole cores provide the first complete sequence through the Kimmeridge Clay and the Kimmeridgian Stage in their type area. The cores have been correlated in detail with the succession exposed in the nearby Kimmeridge cliffs and other sections in south Dorset, as well as with those proved in borehole sections elsewhere in southern and eastern England. The cores have enabled the current chronostratigraphical classification of the Kimmeridge Clay to be extended to the top of the formation, covering strata that are poorly exposed at outcrop. Four types of small-scale rhythm are present within the formation, each of which can be related to the sequence stratigraphy. Only one of these is organic rich and of importance as an oil-source rock.

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Herbin, J.P., Muller, C., Geyssant, J.R., Melieres, F, Penn, I.E. and Yorkim Group. 1993. Variation in the distribution of organic matter within a transgressive systems tract: Kimmeridge Clay (Jurassic), England. In Source Rocks in a Sequence Stratigraphic Framework (eds B.J. Katz and L.M. Pratt), pp. 67-100. American Association of Petroleum Geologists, Studies in Geology, no. 37.

Herbin, J.P., Fernandez-Martinez, J.L., Geyssant, J.R., El Albani, A., Deconinck, J.F., Proust, J.N., Colbeaux, J.P. and Vidier, J.P. 1995. Sequence stratigraphy of source rocks applied to the study of the Kimmeridgian/Tithonian in the north-west European shelf (Dorset/UK, Yorkshire/UK and Boulonnais, France). Marine and Petroleum Geology, vol. 12, pp. 177-194.

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Huc, A.Y., Lallier-Verges, E., Bertrand, P., Carpentier, B. and Hollander, D.J. 1992. Organic matter response to change of depositional environment in Kimmeridgian Shales, Dorset, UK. In: Whelan, J.K. and Farrington, J.W. (eds.). Organic Matter Productivity, Accumulation and Preservation in Recent and Ancient Sediments. Columbia University Press, New York, 469-486.

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Morgans-Bell, H.S., Coe, A.L., Hesselbo, S.P., Jenkyns, H.C., Weedon, G.P., Marshall, J.E.A., Tyson, R.V. and Williams, C.J. 2001. Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset. Geological Magazine, vol. 138, 511-539. Abstract: For the purposes of a high-resolution multi-disciplinary study of the Upper Jurassic Kimmeridge Clay Formation, two boreholes were drilled at Swanworth Quarry and one at Metherhills, south Dorset, UK. Together, the cores represent the first complete section through the entire formation close to the type section. We present graphic logs that record the stratigraphy of the cores, and outline the complementary geophysical and analytical data sets (gamma ray, magnetic susceptibility, total organic carbon, carbonate, delta 13C org). Of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area. Detailed logs are available for download from the Kimmeridge Drilling Project web-site at http://kimmeridge.earth.ox.ac.uk/. Of further interest is a mid-eudoxus Zone positive shift in the delta 13C org record, a feature that is also registered in Tethyan carbonate successions, suggesting that it is a regional event and may therefore be useful for correlation. The lithostratigraphy of the cores has been precisely correlated with the nearby cliff section, which has also been examined and re-described. Magnetic-susceptibility and spectral gamma-ray measurements were made at a regular spacing through the succession, and facilitate core-to-exposure correlation. The strata of the exposure and core have been subdivided into four main mudrock lithological types: (a) medium-dark–dark-grey marl; (b) medium-dark–dark grey–greenish black shale; (c) dark-grey–olive-black laminated shale; (d) greyish-black–brownishblack mudstone. The sections also contain subordinate amounts of siltstone, limestone and dolostone. Comparison of the type section with the cores reveals slight lithological variation and notable thickness differences between the coeval strata. The proximity of the boreholes and different parts of the type section to the Purbeck–Isle of Wight Disturbance is proposed as a likely control on the thickness changes.
This paper is available on the internet at Integrated stratigraphy of the Kimmeridge Clay Formation (Upper Jurassic) based on exposures and boreholes in south Dorset, UK.
Detailed, large-scale graphic logs of the Swanworth and Metherhill cores and of the cliff section, with geochemical data etc. are available from the British Library Document Supply Centre as Supplementary Publication No. SUP 90490 (51) pages. They can be downloaded from the Kimmeridge Drilling Project web-site . For the coast see: Coe, A.L., Hesselbo, S.P., Jenkyns, H.C., Morgans Bell, H. and Weedon, G.P. 2001. Kimmeridge Clay Formation composite graphic log for coastal exposures, near Kimmeridge, Dorset. Part of Supplementary Publication No. SUP 90490, British Library.

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Strahan, A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoirs of the Geological Survey, England and Wales. 278pp.

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Townson, W.G. 1971. Facies Analysis of the Portland Beds. Unpublished. D.Phil. thesis, Oxford University., 284pp. By Dr. Geoff Townson.

Townson, W.G. 1975 Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society of London, vol. 131, pp. 619-638. (Key Paper)

Townson, W.G. 1976. Discussion of Portlandian faunas. Journal of the Geological Society of London, vol. 132, pp. 335-336.

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West, I.M. 1975. Evaporites and associated sediments of the basal Purbeck Group (Upper Jurassic) of Dorset. Proceedings of the Geologists' Association, London, vol. 86, pp. 205-225. By Dr. Ian Michael West.

Available online at:
Paper - Evaporites and Associated Sediments of the Basal Purbeck Group (Upper Jurassic) of Dorset, by Ian M. West, 1975.

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.]

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 Group 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 Group of southern England. In 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): Evaporites and remains of "vanished evaporites" are widely distributed in the Purbeck Group 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]

West, I. M. 1992. Contribution on Purbeck Group. In: Cope, .J C.W., Ingham, J. K. and Rawson, P.F. (Editors). 1992. Atlas of Palaeogeography and Lithofacies. Geological Society of London. [With contributions from various authors. See Purbeck palaeogeographic maps - mid Portlandian (equivalent of Lower Purbeck) on page 129, and Berriasian (equivalent of Middle Purbeck etc) on p. 133].

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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, 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.

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