For information and photographs of the Blackstone west of Kimmeridge Bay at Brandy Bay, please go to:
Oil Shale or Blackstone at Brandy Bay - West of Kimmeridge Bay

Other Kimmeridge Field Guides

Kimmeridge - Kimmeridge Bay
Kimmeridge - West of Kimmeridge Bay
Kimmeridge - East - Hen Cliff, Yellow Ledge and Cuddle
Kimmeridge - Blackstone, Oil Shale at Clavell's Hard
Kimmeridge - Burning Beach, Burning Cliffs and the Lyme Volcano
Kimmeridge - Rope Lake Head to Freshwater Steps
Kimmeridge - Egmont Bight to Chapman's Pool
Kimmeridge Clay Fossils
Kimmeridge - Bibliography - Start
Kimmeridge - Bibliography Continued

SAFETY; the Hazards of Kimmeridge Cliffs

There is considerable risk in this area of being cut off by the tide, and of being hit by falling rocks. See the main Kimmeridge webpage for more details regarding safety. Sadly someone has been killed by a falling rock in Kimmeridge Bay. In general, keep out as far from the cliffs as possible. Some debris falls every day and if you are close to the cliff you are at risk of being hit by a rock-fall.

The significant danger with regard to the Kimmeridge cliffs is perhaps matched only by Liassic shale cliffs of West Dorset. Although the main danger is risk of being hit by a rock fall, there is a second hazard is that of being cut off by the tide. This applies particularly to the cliffs east of Kimmeridge, although it could also happen at Brandy Bay and elsewhere.

Because of the hazard of rock falls and it is a dangerous place to visit unless it is well-understood and proper precautions are undertaken. The cliffs are vertical and high and subject to erosion by the sea at the base. The shale and mudstone is full of joints and fissures and not stable. Small pieces will tumble off from time to time as you walk along.

More serious are substantial falls like the one shown above. These may occur without warning; suddenly there will be a loud crash and a plume of debris and dust. These happen particularly in certain weather conditions, such as when there is frost or rain and sometimes when the shale has dried in the hot sun. If you are out on the low-tide ledges falling debris would not usually reach you but there is no guarantee of safety. The risk is greatest where the cliffs are highest, where there is joint-separated shale above and where there is evidence of a recent fall in the form of shattered debris. Sadly, someone has been killed recently by a rock fall in Kimmeridge Bay.

For more on the hazard from falling debris at Kimmeridge see photograph of Yellow Ledge.

In terms of route, from the east side of Kimmeridge Bay it is always possible to go east by the cliff top. It is only possible to go east by the beach when the tide is low and conditions are favourable.

Watch the tide carefully, because a rising tide may push you back towards the dangerous cliffs, and there is no access up them. It is easy to be cut off on the beach here and only proceed if the tide and weather conditions are safe.

For study, examine the low-tide ledges instead of the cliffs whenever this can be done. Take care because the ledges are very slippery and this is especially the case where there is algal slime, as shown above. Do not attempt to climb the slippery waterfall at Clavell's Hard. A way up the cliffs and access down at Clavell's Hard was at one time possible but it is now too difficult to be attempted. There is no route up the cliffs at Freshwater Steps (once actually steps at the end of the path down from Encombe House) or anywhere east of Kimmeridge Bay. There have been several rescues of people trapped to the east of Clavell's Hard.

If you are able to get onto the Clavell's Hard mining ledge (and this should not be done without a safety rope), do not enter the old mine adit (in any case the entrance has recently collapsed). It extends in about 100 metre horizontally but the shale is unstable and unsafe.

Kimmeridgian strata are very well-know to petroleum geologists. Much of the world's oil comes from strata of Kimmeridgian age. The Ghawar Field, discovered in 1948, is in the Kimmeridgian reservoir limestone known as the Arab Formation. It produces 62.5 percent of Saudi Arabia's crude production, 8 million barrels of oil per day. The source rock is Upper Jurasic, but probably Oxfordian (Oxford Clay and Corallian equivalent - beneath the Kimmeridge Clay). The Kimmeridge Clay, seen in the cliffs of Dorset is not a reservoir rock; it is not porous and permeable. It is a potential oil source rock. On land in Britain it is mostly immature (not sufficiently deeply buried and heated) but under the North Sea and elsewhere it has been more deeply buried (and therefore more heated) and has resulted in the formation of large oilfields. Dorset is the type locality and the cliffs around Kimmeridge form the type-section, giving excellent exposures of the potential oil source rocks. In this webpage we go to Clavell's Hard where the Blackstone, the oil shale, was recognised in Victorian times as a great energy resource and was mined in the cliffs.

(To read more on the Kimmeridgian Arab Formation in the Ghawar Field see:
Ghawar, Saudi Arabian: The King of Giant Fields. Geo Expro, September 2010, pp. 24-29.)

INTRODUCTION

General and Clavell's Hard Location

INTRODUCTION:

Maps, Aerial Photographs and Publications

Geology of the Isle of Purbeck shown on an old map (part of a map modified from Damon, 1884). Modern changes in the geological mapping of the area have only been of detail. Some place names have changed since Victorian times and Clavell's Hard has been added by the present author. Note an older spelling of "Kimeridge" with one "m".

The map, modified after Cox and Gallois (1981) shows the solid geology of the Kimmeridge area in particular and locations referred to in this and associated Kimmeridge field trip guides. Initial information is in the Introduction to the Kimmeridge Clay section.

INTRODUCTION:

The Blackstone, Oil Shale, East of Kimmeridge

The Blackstone, the main Kimmeridge oil shale is best seen at and near Clavell's Hard, east of Kimmeridge. It was worked on what is now a rather inaccessible cliff quarry. Underground mining took place from levels or adits, one of which still remains. The oil shale was also mined inland of here.

As noted elsewhere, at Kimmeridge Bay the lower Kimmeridge Clay with dolomite beds, crushed ammonites and cycles of sedimentation is seen. By walking along the coast eastward from here progressively higher parts of the Kimmeridge Clay are seen. The Blackstone occurs in the Upper Kimmeridge Clay above the level of the strata at Kimmeridge Bay. Proceeding eastward firstly Hen Cliff, Yellow Ledge and Cuddle is seen. The oil shale is then reached at Clavell's Hard or Blackstone Point about one kilometre east of Kimmeridge Bay. Here and in the adjacent bay to the west there have been fires in the oil-shale . The cliffs around Clavell's Hard are described in some detail in this webpage. Reference is made to geochemistry of the oil shale and the history of mining. This is then followed eastward by a web page on Rope Lake Head to Freshwater Steps section.

Cliff Section

Beneath this diagram is shown a low-resolution version of one of many continuous coastal photographs from sea taken by Richard Edmonds of the Jurassic Coast, Dorset and East Devon World Heritage Coast. The full images are very high quality.

A cliff section is shown here for a large part of the Kimmeridge Coast. This is based on Arkell (1933), Cox and Gallois (1981) and other information. Amongst other things, it shows the position of the Kimmeridge oil-shale, Clavell's Hard and the location of the mining in the cliffs.

BLACKSTONE:

The Blackstone or Oil Shale in the Succession

For reference the Kimmeridge zonal succession is given. The "Bolonian" is the Upper Kimmeridgean in traditional British terms, but is not part of the Kimmeridgean in France. The oil shale is in the Upper Kimmeridge Clay in the hudlestoni zone. Other thinner oil shales occur near this in the sequence.

STAGE	CHRONOZONE	ZONAL AMMONITE
Bolonian
	Fittoni	Virgatopavlovia fittoni
	Rotunda	Pavlovia rotunda
	Pallasioides	Pavlovia pallasioides
	Pectinatus	Pectinatites pectinatus
	Hudlestoni	Pectinatites hudlestoni
	Wheatleyensis	Pectinatites wheatleyensis
	Scitulus	Pectinatites scitulus
	Elegans	Pectinatites elegans
Kimmeridgian
	Autissiodorensis	Aulacostephanus autissiodorensis
	Eudoxus	Aulacostephanus eudoxus
	Mutabilis	Aulacostephanus mutabilis
	Cymodoce	Rasenia cymodoce
	Bayliei	Pictonia baylei

Here is the succession seen directly east from the small promontory of Clavell's Hard towards Rope Lake Head. The main oil-shale, the Blackstone, clearly projects in the cliffs. It is unsafe to approach it here in the cliff and it is best to look for loose material on the shore or an outcrop amongst the low-tide ledges.

The base of the Blackstone marks the boundary between the Pectinatites wheatleyensis Zone below and the Pectinatites hudlestoni Zone above.

OCCURRENCE:

The Oil-Shale - The Blackstone

KIMMERIDGE BLACKSTONE AND OTHER OIL SHALES:

Listing of Main Beds in Clavell's Hard Area

The diagram, shown higher above, and based on the work of Coe et al. 1999) shows how oil shales might be indicated by inverse magnetic susceptibility. On this basis the following might be worth further investigation for AOM, amorphous organic matter (mostly kerogen). Some of these beds might have been the source material for archaeological artificacts, although the Blackstone is obviously the most likely source. There are about 12 oil shale bands, east of Kimmeridge Bay.

Sequence - top, downwards to bottom, mostly wheatleyensis and hudlestoni zones.

45/11 (White Stone Band, with some oil shale)
45/19 (thin)
45/17 (thin)
45/6 (thin)
45/6 (thick, hudlestoni - worth investigation)
43/4 (thin)
42/29 (base - thin)
42/12 (Blackstone - in two major parts, upper and lower with calcareous clay in the centre; with calcite-pyrite nodules)
42/10 (Bubbicum, significant)[or Rudicum, see Pringle in Whitaker and Edwards (1926)]
42/5
41/10
39/1
38/2

(there are few others)

Note that the maximum gamma peak, with relatively high uranium and potassium is in 42/6, calcareous mudstone beneath the Bubbicum.

BLACKSTONE:

Further Details

The Blackstone or Kimmeridge Coal, 42/12, the main oil shale bed descends to the shore east of the Clavell's Hard mining ledge. It is about 86cm thick. It forms a projecting band in the cliff but it is not as conspicuously dark as might be expected because it is often coated with white sulphate, carbonate and clay encrustations washed down from the cliff above. In the field you will notice by looking at the Blackstone in the cliff that there are quite large nodules of pyrite, visible in cross-section.

Debris frequently falls from the cliffs here and it is safer to study the oil shale as loose blocks on the shore rather than in the cliff. It is a dark chocolate brown, sometimes appearing almost black. When polished by the beach debris or by human activity is a waxy-looking material. When fresh surfaces are broken is it is a mat dark grey, well-laminated and quite strong. As noted above, joint surfaces are often coated in contrasting white calcite, as in the photograph here. It can weather to a paper-shale.

The oil-shale is of relatively low density and rather tough and resistant to disintegration. As a result small, flat, ovoid pebbles of oil-shale with a rather brownish colour are commonly washed along the shore and appear on the beach at many places where Kimmeridge Clay outcrops nearby.

The Blackstone can be seen as a smooth surface between the beach boulders, a short distance to the west of Rope Lake Head. It is strange that the oil-shale occupies this narrow gap between the blocks, but probably it is projecting upwards to some extent above the level of the adjacent shale. The boulders are from the Rope Lake Head Dolomite Bed. In the distance, on a wet and rather misty day, is St. Aldhelm's Head, a peninsula of Portland Stone.

BLACKSTONE

Pyrite Nodules

(see also the cross-sections of nodules shown above in photographs of the vertical exposure of oil shale at the foot of the cliff)

In the area between Clavell's Hard and Rope Lake head the large pyrite nodules of the Blackstone, mentioned above, are accessible for study. On the shore they are visible, not in section but partly eroded out of the flat surface of the oil-shale. They are quite large, up to about 60cm or more, and elongate and rather irregular in shape. Although pyrite is common throughout the Kimmeridge Clay, these are the largest pyrite nodules in the formation and, of course, it is not surprising that they are associated with the greatest accumulation of organic matter. The pyrite is the result of sulphate-reducing bacteria living on the organic matter of the oil-shale shortly after deposition and reducing the sulphate of seawater to sulphide. This then has reacted with available iron to produce the ferrous sulphide - pyrite (FeS₂). The oil-shale gives an impression of curving a little against the nodules and this probably because there were of early origin and compaction has taken place after their formation. In section they can often be seen to have flanges or "wings". Presumably there was some soft sulphide precursor devopment in the very soft, organic-rich mud prior to compaction.

Sulphate reduction (diagenetic zone 2) is typically an early process taking place in marine sediments before burial to the fermentation zone (3) where the Kimmeridge dolomite beds originated Irwin et al. (1977).

BLACKSTONE:

Organic Content General

The Kimmeridge Blackstone or the main Kimmeridge Oil Shale is not a uniform bed. Some old analyses have been published on the basis of random samples from it, without reference to their detailed stratigraphic horizon. Percentages of organic matter given are not consistent. The reason is that it is quite variable vertically on a scale of a few centimetres. In total the bed is about 64 cm thick in the main section between Clavell's Hard and Rope Lake Head. The maximum total organic matter (TOM, not TOC) is about 70%. The diagram above, based on the work of Ioannides et al. (1976) and of Gallois (1978) shows some of the variation well. See also the magnetic susceptibility log of Coe et al. (1999) because this reveals a good inverse relationship to the TOM of the Blackstone and Bubbicum. The data above should be of some use in determining the source of archaeological samples of Blackstone.

BLACKSTONE AND KIMMERIDGE CLAY IN GENERAL - ORGANIC CONTENT

TOC - Total Organic Carbon

Note that TOC is the Total Organic Carbon in a source rock. Note that it is not the TOM, total organic matter, which is higher. For the Kimmeridge Clay figures for both TOM and TOC have been given by various authors. Carbon constitutes 75 to 95 wt % of hydrocarbons by molecular weight, with an average value near 83 weight % ( Jarvie, 1991). Thus regard the TOC, the total organic carbon, as indicating a large proportion, about 83 wt% of the total organic matter, when considering the meaning of the data given (or to be given) below.

To understand the Kimmeridge Clay Formation in broad terms the zonal averaging scheme of Tyson (2004) is very useful. Bulk geochemical data from the NERC Rapid Global Geological Events (RGGE) project were used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1-2% at the top and base to a peak of 8-9% in the middle of the formation. (Note that these averaged values do not seem very high in comparison with the extreme figure of 70% TOM for part of the Blackstone.)

Regarding the Dorset Coast, consider about 8% TOC for a favourable zone (near the middle) and about 40% TOC for the oil shale bed, the Blackstone, as the type of figures involved (generalising).

In comparison, the American Kimmeridgian, Upper Haynesville/Basal Bossier shales contain about 1.5 -5.0 wt% TOC. These figures are not very different from those of the Kimmeridge Clay basinal facies of Dorset (but the thermal maturity is much greater in the USA sequences).

West of Shetland, the offshore Kimmeridge Clay Formation has an average TOC of 7% (maximum 15%) (R. Jowitt et al. 1999, in Fleet, A.J. and Boldy, 1999. Petroleum Geology of Northwest Europe, Proceedings of the 5th Conference), Vol. 2, published by the Geological Society of London.)

In both Dorset and Somerset the Liassic black shales have TOC values between 0 and about 12%, with most figures near 3%. In general these are not very different from Kimmeridge Clay figures, excluding the exceptional Blackstone.

OCCURRENCE

Comparison - Oil Shale in the Weymouth Relief Road Cutting

Kimmeridge oil shale occurs at Burning Cliff near Ringstead, and in the inland area north of Weymouth. When weathered, as in the example from the Weymouth Relief Road above, it may appear very laminated, and, rarely, distorted by head of the sun.

BLACKSTONE:

Details of the Blackstone Bed and the Associated Oil Shales

The Blackstone is sometimes treated for simplicity as though it is a single bed and relatively homogeneous, but, in fact, it is not. As shown in the photograph, it really consists of two main beds separated by bituminous shale. In addition it contains numerous lenticular sulphide-carbonate nodules, that probably originated in the early sulphide reduction zone. The presence of the nodules means that analyses of the Blackstone should be treated with caution, because, although valid for the separated oil shale, these usually ignore the presence of the nodules. Thus they therefore grossly underestimate the real iron and sulphur content of the bed, considered in bulk.

Details at Blackstone Point or Clavell's Hard were given by Strahan (1898) and are reproduced below with modification and additions. Strahan's measurements, given in inches, have been converted to metric figures and probably not highly accurate. I have shown, though, how the bed listing ties in with the photograph. However, there is some doubt as to whether the Bastard Coal is the uppermost part of the main oil shale shown and not the base of the mudstone above. It is clear from analyses that, not surprisingly, the Best Blackstone is the richest in total organic matter and the Blackstone deteriorates upward. Note that the photograph has been taken at a point just west of the mining ledge and the oil shale cannot be reached from the beach just here. (To recognise the Blackstone easily, look for the white veining on a black rock with angular fractures)

Sequence
(with bed numbers from Coe et al. 1999)

43/3. 'Short Joint Coal', not worked but picked up by the villagers - 0.15m (6 inches). Coccolith rich limestone within oil shale (Cox and Gallois, 1981)
Shale - 3.05m (10 ft)
42/24. Hard calcareous stone band, formerly used for cement-making [Rope Lake Head Dolomite Bed]. - 0.46m (1ft, 6 ins)
Shale - 4.57m (15ft)

------------------------------------- top of Kimmeridge Coal - The Oil Shale or Blackstone
(This is given as approximately 60cm by Cox and Gallois, 1981 and by Coe et al., 1999, and does not include the Bastard Coal)
42/14 & 16). Black Dirt or Bastard Coal' - 0.15m (6 ins) (inferior bituminous shale, not always included in the Blackstone)
Shale - 0.08m (3 ins)
42/12 upper part: Blackstone, upper Blackstone - 0.30m (1 ft)
Shale - 0.08m (3 ins)
42/12 lower part: Best Blackstone or lower Blackstone (springs issue from this bed) - 0.25m (10 inches)
--------------------------------------- base of Kimmeridge Coal or Blackstone
(total Blackstone thickness - 0.86m including the Bastard Coal. True Blackstone without Bastard Blackstone totals about 63 cm, according to this list).

Shale - 1.83m (6 ft)
(end of sequence list)

Below the Blackstone (42/12), about 1.4m down is another oil shale known as the 'Bubbicum' or 'Rudicum'. This is bed 42/10. It has lower organic matter (32%) according to Dunn (1973). Like the Blackstone it contains pyritised plates of Saccocoma. Further down, Bed 42/5 is a laminated, cemented oil shale with Saccocoma. See Cox and Gallois (1981, p. 42) and Coe et al. (1999).

[Bubbicum or Rudicum - more notes. This is referred to in three boreholes (107, 108 and 109 at Kimmeridge by Whitaker and Edwards (1926). It is given as 10 inches [0.25m.] in two borehole, and at 8 inches [0.20m.] in another. The separation from the Blackstone is given as 5ft 6 inches (1.68m), 5ft 1 inch 9 (1.55m) and as 5ft 9 inches (1.75m). What is the meaning of "rudicum" and why has it been used for this bed. Is there any connection to Roman use of Kimmeridge oil shale?]

BLACKSTONE continued:

Kimmeridge Oil Shale - General Appearance

The Blackstone is actually a rock of a very dark chocolate grey colour (the ordinary Kimmeridge shales are dark grey, but lack the chocolate tinge). It is very well-laminated, and when eroded occurs on the beach as flat pebbles, often with a rather brownish rim. Oil shale density is much less than associated sedimentary rocks and can be as low as 2.2 (and isprobably lower in the case of the Best Blackstone). A result of this low density and the occurrence of thin, flat slabs is that is often thrown up high on beaches by wave action. Because it outcrops at the Isle of Portland it is often washed up at the Portland end of the Chesil Beach, and found high on the beach.

Clay minerals in Kimmeridge oil-shales are dominantly illite, but also with some kaolinite, some mixed-layer mineral and some chlorite (Gallois, 1978).

.

BLACKSTONE continued:

Composition and Geochemistry of the Kimmeridge Blackstone and Associated Shales

It is recommended to have available a pdf or printed copy of the Kimmeridge Clay Composite Graphic Log for Coastal Exposures, by Angela L. Coe, Stephen P. Hesselbo, Hugh Jenkyns, Helen Morgans Bell and Grapham P. Weedon. 1999. This is part of the REGGE, Rapid Global Geological Events Project on Anatomy of a Source Rock. This excellent log provides the following data for almost the whole of the Kimmeridge Clay cliff section:

Magnetic Susceptibility,
Total Gamma Ray,
Uranium (ppm.),
Potassium (%),
Thorium (ppm.),
Th/K,
Th/U,

The log is essential for anyone making a serious study of Kimmeridge Clay composition. It is an impressive log and remarkable that samples have been studied at 10cm intervals throughout almost all of the Kimmeridge Clay of the Kimmeridge coast. This is a remarkable achievement.

Some older, but very useful, geochemical data, modified after Dunn (1974) is now provided to give, in particular, total organic contents and trace element data for comparison with recent work and for general discussion. The paper of Dunn (1974) should be consulted for information on analytical methods and the application of the data to the study of sedimentary cycles. Dunn used a simple Fourier analysis to find harmonics. Most notable is his discovery of a dominant periodicity of about 40,000 years at 3.3 to 5 metres. This is the well-known obliquity cycle (axial tilt 21.5° to 24.5 °). [For more recent work on time-series analysis see Weedon et al. (1999) who obtained similar results in recognising a dominant 38 ka obliquity cycles.] Dunn's (1974) paper is an early and good paper of interest with regard to trace element geochemistry. It was undertaken mainly by optical emission spectrograpy and is semi-quantitative. The tables below are based on his work. Some results that are significantly different from the average or of special interest are highlighted here. Note that the sequence in the tables in inverted and they young downwards. The data given in the later and very detailed work of Coe et al. (1999) is not the same. Sampling was closer in the later work (10cm as opposed to Dunn's 25cm), and gamma ray and magnetic susceptibility have been measured, together with data for U, K, Th etc. Total gamma ray in Coe et al. corresponds quite well with V in Dunn, but otherwise there is little direct relationship or overlap, and the two works are complimentary.

Another table of trace-element geochemical data is provided by Cosgrove (1970), Table 1, p. 832. Twenty samples were analysed (including three not from the Kimmeridge Clay). Most of these are from Blakes Bed 26, a broad unit which includes the lower part of the hudlestoni Zone. Within this is the Blackstone, the Rope Lake Head Stone Band and the Short Joint Coal. The samples are not well localised within the unit. Two samples for comparison are from scitulus Zone shales. With regard to the samples collected in the field from the hudlestoni zone the sample with highest organic matter (26C1)is listed as containing 55.3% organic C% (not organic carbon).

A museum specimen of Kimmeridge Coal from Clavell's Hard, Dorset, gave, according to the table an organic carbon content (i.e. - TOC) of 71.4%. This is anomalously high for organic carbon from the Blackstone, but just feasible for total organic matter. [total organic matter - TOM can be obtained from total organic carbon - TOC by multiplying by 1.2, assuming that the organic matter is 83 wt% carbon.]. Converting the figure to TOM gives 85.68%, which is much higher than anyone else has reported. However, the details of origin of the museum specimen are not known. It remains to be seen whether samples of such high organic content can be obtained from the Blackstone in the field. It is possible that they occur at a particular level.

If the highest figure for the field samples for TOC, 55.3 and very likely from the Blackstone is converted to TOM, the figure is 66.36%. This is remarkably close to the maximum figure for TOM given by Dunn (1974) which is 61.6%. Thus the TOC figures for the field samples of Cosgrove (1970) seem to be appropriate for the bituminous strata involved. However, there is not a very good correspondence between the trace element analyses of Cosgrove and of Dunn. However, very different analytical techniques were being used, and they were presumably not as advanced as methods available now. The matter is not simple in any case since the sampling was different.

The main point of the Cosgrove paper is not the general range of trace elements, discussed above, but the discovery of a relatively high iodine content in Kimmeridge oil shale. The museum specimen gave a value of 72 ppm I and also 16 ppm Br. A field sample (26C1) that is likely to have come from the Blackstone gave an I value of 34 ppm and a Br value of 14. The important and original conclusion of Mike Cosgrove's paper is as follows:

"The bituminous Kimmeridge shales of Dorset are particularly I-rich sediments. With an average value for I of 17 ppm and highest values in excess of 30 ppm they are amongst the most I-rich sedimentary rocks yet recorded. The Kimmeridge "Coal" with a value of 72 ppm. I must likewise be amongst the most I-rich of combustible rocks recorded.
The abundance of microplankton in those rocks, and the identifiable celular structure in the kerogen points to the organic C as being derived from these organisms. Further, the known enrichments of I and Br by various marine plants suggests that these elements are also derived from the same source. The relationship between organic C, I and Br is confirmed by the high correlation obtained between these elements during statistical analysis of the results.
Comparison with other shales indicates that while Mo enrichment can be related to the marine environment, I accumulations requires the presence of plant material during deposition, Br being enriched at the same time. Comparison with an oil shale deposited in fresh water emphasises the presence of Mo, I and high Br content as marine indicators."

Further Discussion of Blackstone Geochemistry

As a potential oil-source rock, more discussion will be given to the geochemistry of the main oil-shale, the Blackstone. Various analyses have recorded a total organic content of between 20% and 74% (e.g. Cosgrove (1973), Dunn (1974), and Legg (1984)). In a very old report, Strahan (1898) referred to an analysis which gave 61% volatile matter and 13.15 % carbon (coke). Thus the total organic content was 74.15 % with ash of 28.85 %. This seems compatible with a general impression that the purest, high quality Blackstone can be considered as consisting of about 70% organic matter in the form of kerogen (although see the report by Cosgrove (1970), discussed above, on the museum specimen.

HISTORY OF THE OIL SHALE USE:

1. Iron Age and Roman Use

The Kimmeridge oil shale was used extensively by Iron Age British tribesmen and the Romans for armlets. These were made on a primitive lathe. The residual central disks are known as Kimmeridge coal money. Large numbers have been found. With one of them is shown a beach pebble of oil shale that I polished with carborundum paper and metal polish. This gives an idea of the polished appearance. It is easy to polish and the material seems to have been much valued in antiquity.

The upper images shows stages in the manufacture of Kimmeridge Blackstone armlets, as illustrated by Calkin (1955). The lower image shows a young lady, KTICIC, the Founding Spirit or Creation, wearing something much like a Kimmeridge Blackstone armlet on her right wrist. This is from a Roman mosaic at Kourion in Cyprus, so it is not necessarily a Kimmeridge armlet that was the model or inspiration for this ancient work of art. However, perhaps something resembling the Kimmeridge Blackstone armlets was also worn in the Mediterranean area.

HISTORY OF THE OIL SHALE USE:

2. 17th Century Use of the Oil Shale or Blackstone

The Blackstone or Kimmeridge Coal has been used as fuel in local cottages in the past, up to quite recent times, but was clearly a very unhealthy means of heating. It gives off very unpleasant sulphurous fumes, including hydrogen sulphide, and leaves much ash.

----- No. 1 Level

What are probably the collapsed remains of No.1 Level can be seen in the middle of the bay between Yellow Ledge (Cuddle) and Clavell's Hard, and about half-way up the cliff. Tramway rails can project from the remains of the tunnel. To the east of this the oil shale has been worked out and only the debris of shale that is less organic-rich fills the former mining excavations.

----- No. 2 Level

The No. 2 Level is not open at present. However its location is visible in the shallow embayment a short distance to the west of Clavell's Hard. There is a stretch of worked-out Blackstone which has been backfilled by small shale mining debris. If you look closely you will see an old tramway rail hanging down from the old workings in the cliff in the area of dark shale above the right shoulder of the petroleum engineering student.

From here for a distance along the cliff of about 300 metres northwest is a stretch marked as "Line crawl underground shale taken out" on the map given by Legg (1984), p. 23, and based on the original plans of the Kimmeridge Oil and Carbon Co. Ltd. Legg mentioned that this area is "a thousand feet by a hundred and fifty feet, which worked to their standard height of seven feet would call for the extraction of a million cubic feet of shale and clay". I have not studied the working to determine the height. I wonder whether "line crawl" suggests that the working were not full height and that only the thin Blackstone and adjacent shale was extracted. The Blackstone is only about 0.86 m., less than half the seven foot working height (i.e. about the height of the main levels). Examine once more the backfilled mine in the photograph.

The cliff fall, shown, is recent and seems to have taken place in April 2006. It indicates the dangers of these cliffs. Two planar sloping surfaces in the cliff are probably fault planes.

----- No. 3 Level

No. 3 Level is now filled with debris and shows some roof collapse. The sea has eroded away the western end of the mining ledge just here. This level, when in work, is shown in an old illustration further above. This tunnel was at the western end of the mining ledge adjacent to the hut and to the ladder up the cliff. The location is not of easy access.

----- No. 4 Level

This is No. 4 Level at the east end of the ledge. It is still open, but now highly dangerous. It was said on the old plans to be not much worked, but this one alone has survived. It is not safe to go into because the roof is loose and there have several falls at the entrance. The entrance is on sloping shale debris at the edge of cliff and a close visit is not recommended. This particular adit extends horizontally into the cliff for about a hundred metres and there is nothing special to see inside. Note the Rope Lake Head Dolomite Bed, with a thin limestone beneath, just above the adit. This dolomite is 4m above the Blackstone. At the entrance of the level calcium carbonate is precipitated as calcite over moss by dripping water and "moss-tufa" is formed.

----- No. 5 Level

No.5 Level was visible in the mid-20th century as a small opening in the cliff at about the level of the Blackstone. Then it became covered by the debris of a cliff fall. This photograph shows what may be the No. 5 level in the centre of the photograph. The aperture is small and seems to be largely filled by the fallen debris. With future coast erosion it should become clear as to whether this is No.5 level.

For more on the history of the oil-shale workings see Legg (1984) and Mansell-Pleydell (1893; 1894). For more modern and technical information on the oil-shale see Gallois (1976;1976a; 1978; 1978a). See Cole (1975) for a record of a cliff fire here.

BLACKSTONE continued:

Oil Shale Working - Victorian

The Blackstone or the main Kimmeridge oil shale which is discussed here, tends now regarded as a single bed of about 60cm thickness (two feet). Study of the detail given here will show that it actually consists of two rich beds of oil shale with a band of less-bituminous mudstone in the centre, and it was regarded as such in Victorian times ( Damon, 1884, p. 60.). The mining was confined to this 60cm (or 2 feet) of Blackstone. The Short Joint Coal above, Bed 43/3 and of the lower part of the hudlestoni Zone, is 15 to 20cm thick. It was too thin to be economical to mine. It was, however, used locally by villagers.

Oil was produced from the Kimmeridge oil shale, but so too was gas. One ton yielded 11,300 ft. of 20 candle gas ( Damon, 1884) compared to 15,480 ft of 52 candle gas from Boghead Coal, torbanite (algal oil shale) from West Lothian, Scotland.

BLACKSTONE contiued

History - deodoriser etc.

When Strahan was investigating Kimmeridge for the geological survey in the 1890s he visited these works and wrote " Of late years the mineral property at Kimmeridge has been leased to the Kimmeridge Oil and Carbon Company, by whom the coal has been used for fuel, for improving the illuminating power of coal-gas and for the manufacture of paraffin. It is stated by them that the residual coke or carbon after the distillation of the oil possesses the properties of animal charcoal, and can be employed as a deodoriser, distinfectant, and decoloriser and that it also serves as a manure.

From the oil an insecticide and a preparation for the prevention of mildew, Oidium , etc, on vines and other plants has been made. The Blackstone of Kimmeridge Bay, by the same account, it the richest seam yet discovered, and in the laboratory has been known to yield 120 gallons of oil to the ton, or , when distilled on a large scale, 66 gallons to the ton, while common shale gives 33 gallons to the ton."

"Notwithstanding, however, the variety of uses to which the coal has been put, it has never paid the expense of extraction, and at the time the resurvey of the district was in progress the works were almost at a standstill. "

BLACKSTONE continued:

Oil Shale Mining History - A Summary

The sequence of mining here is given, based on information from Strahan (1898). ;Legg (1984); Arkell (1947) and other sources.

1560s. Lord Mountjoy discovered that the land at Kimmeridge was 'full of allom myne' and he obtained a patent to make alum there ('myne' means ore not mine). Mountjoy's plan was unsuccessful.

Around 1600. Sir William Clavell, owner of Kimmeridge, took up alum manufacture. When the process was perfected the alum works were seized by London merchants who had King James 1' sole patent for making alum.

1617. Abraham Bigo with Clavell used oil shale to make green drinking glass at a glass-house just north of the fishermen's huts. Royalties were unpaid and later Clavell went to prison. He had large losses and died in 1644.

Until 1620. Sir William Clavell built a pier 100 feet long in imitation of the Cobb at Lyme Regis. It was destroyed by storm in 1745. Some remains are still visible.

1848. Bituminous Shale Company built a tramway and worked oil shale. This was used for production at Weymouth of varnish, grease, pitch, naptha, dyes, wax, fertiliser and other by-products. In 1854 the Weymouth factory was condemned as a public nuisance because of the smell and the company went into liquidation.

1848. Wanostrocht and Company used Kimmeridge oil shale to light the streets of Wareham with 130 gas lamps. There was building of a new works near the Wareham railway station in 1852.

1855-1858. Ferguson and Muschamp established an oil shale plant at the site that later became the Wareham pottery works, but it closed in three years.

1858. Wanostrocht and Company reopened the Wareham shale plant. Fifty tons of shale oil were produced there each month and there was a contract to light the streets of Paris with gas from this product.

1862. Wanostrocht and Company were struggling and taken over by the new Wareham Oil and Candle Company. The works were southwest of the railway station.

1871. West of England Fireclay Bitumen and Chemical Company was incorporated. Ten thousand tons of Kimmeridge shale were to be processed annually at Calstock, Cornwall. It went into liquidation in 1876.

1872. Works of the Wareham Oil and Candle Company were burnt down and the firm closed.

1876. West of England Chemical Company went into liquidation.

1876. Sanitary Carbon Company at Wareham was established to convert shales into a coke that could filter sewage. The company later became the Kimmeridge Oil and Carbon Company.

1883. A longer tramway was constructed at Kimmeridge. The Manfield Shaft mine was dug.

By 1890. Four main levels or adits had been dug, two of these coming from the mining ledge at Clavell's Hard. Galleries branched off. No. 5 level was marked out for cutting. There were 5,000 feet of tunnels.

Late 1890s. Oil shale mining came to an end.

1898. Strahan stated that "Notwithstanding, however, the variety of uses to which the coal has been put, it has never paid the expense of extraction, and at the time the resurvey [geological survey] of the district was in progress the works were almost at a standstill."

1917-1918. A series of test boreholes were sunk around Kimmeridge and Portesham to determine the total extent of the oil shale field in case exploitation should become necessary because of shortage of other oils.

1947. Arkell stated that because of the high content of sulphur ( 6 to 7%) and the thinness of the workable seam, the cost of extraction has always been too high to allow the project to become an economic success. An annual royalty for mineral rights was said to be still paid to the landowner at Smedmore.

1959. The BP oilwell west of Gaulter's Gap was established. The reservoir is Cornbrash at 564m (1850 feet).

1978. R. Gallois published ' A pilot study of oil shale occurrences in the Kimmeridge Clay'. This was intended to determine whether a full-scale study was warranted. The oil shales in the Kimmeridge Clay in Norfolk, Lincolnshire and Dorset are comparable to the Jurassic oil shales which have been worked by opencast methods in France. There is, however, a major problem of disposing of useless (and possibly harmful) residue which is just as bulky as the original oil shale which is retorted.

2000. No plans are known, at present, for oil shale production. Opencast mining or some method of burning the shale underground seem remote possibilities in the event of an energy crisis, but even then not necessarily in Dorset since, as noted, the oil shale extends as far as Norfolk. It continues under the North Sea and the Kimmeridge Clay with its bituminous matter is the major oil source rock for North Sea oil.

BLACKSTONE continued:

Oil Shale Fracking (Fracturing)

The Kimmeridge Oil Shales are the most famous of the oil shales in Britain, and are important as the source rock for North Sea Oil. At present there is a general interest, and also concern, about possible gas-fracking, or fracturing oil shales under high pressure underground and extracting large quantities of gas. The Kimmeridge oil shale in the south of England might seem an obvious target.

However, it should be noted that there may be good reasons for not using it for gas fracking off the Dorset coast. The main problem is here that it is close to the surface. Almost everywhere in the English Channel close to the coast the Kimmeridge Oil Shale is not buried under a great thickness of strata. It is overlain in places by Portland and Purbeck strata but these are not thick, and to the east (towards the Isle of Wight) by Wealden. Faults in the Kimmeridge Clay are probably not sealing in most cases. Gas escape is quite likely unless those faults end at the unconformity at the base of the Gault (the Late Cimmerian or Sub-Albian Unconformity). If boreholes penetrate from above this unconformity for fracking purposes then the safety factor is greater. However, Upper Cretaceous strata (Chalk etc) are only present as an extensive, low-dipping cover at about 20 or 30km south of the Dorset and Hampshire coast. Only the Isle of Wight may have site where an ERD borehole could, at present, reach Kimmeridge strata to the south beneath an Upper Cretaceous cover. Complications arise because of lateral facies changes and it is not clear whether the Kimmeridge oil shale is well-developed very far south.

PALAEONTOLOGY:

Ammonites from the Clavell's Hard Area.

Left: Subplanites cf. grandis (Buckman), according to Arkell (1947). The specimen is from between Clavell's Hard and Rope Lake Head, 2 - 3m below the Blackstone or Kimmeridge Coal. Image modified after Arkell (1947), which should be seen for details and discussion.

Right: Ammonite under shallow seawater at Clavell's Hard. This is probably from the Wheatleyensis Zone between the Grey Ledge and the Blackstone.

MINERALS:

Pyrite