Bencliff Grit and its oil-sand, Dorset, by Dr Ian West

West, Ian. 2013. Bencliff Grit of the Corallian, Osmington Mills, Dorset; Geology of the Wessex Coast of southern England. Internet site: wessexcoastgeology.soton.ac.uk/osben.htm. By Dr. Ian West, Romsey, Hampshire and School of Ocean and Earth Sciences, National Oceanography Centre Southampton Southampton University, UK. Version 11th June 2012.
Osmington Corallian - Part 3 - Bencliff Grit

Osmington Corallian - Part 3 - Bencliff Grit

By Ian West,
Romsey, Hampshire
and Visiting Scientist at:
Faculty of Natural and Environmental Science
Southampton University,

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Aerial photographs by courtesy of The Channel Coastal Observatory , National Oceanography Centre, Southampton.
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Spherical concretions or doggers are being eroded out of the Bencliff Grit, to the east of Osmington Mills, Dorset, 2nd March 2009

Compare the Bencliff Doggers to the Moeraki Boulders of New Zealand, but, of course, they have nothing to do with electric arcs!

Go to another Osmington Guide?

Osmington - Pt. 1 - Introduction
Osmington - Pt. 2 - Osmington Mills to Ringstead
Osmington - Pt. 3 - Bencliff Grit
Osmington - Pt. 4 - Osmington Oolite
Osmington - Pt. 5 - Black Head
Osmington - Pt. 6 - Corallian Fossils
Osmington - Pt. 7 - Bibliography

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INTRODUCTION:

General

This map shows the location of the exposures of the Bencliff Grit in the Osmington Mills area. The best section is in the cliffs to the east of Osmington Mills.

INTRODUCTION:

Safety

The major risk at the exposure of the Bencliff Grit east of Osmington Mills is that of being hit by falling rock. Safety helmets should be worn. Keep back from the cliff as much as possible. Try to minimise approach to the cliff, although detailed study will inevitably require some close proximity to the Bencliff Grit. It can be reached under the shore ledge of the Osmington Oolite Bed 1 (limestone) which swings out away from the cliffs. This gives easy access to the upper part. In the area of the oil sand there is some risk and the cliff should be looked at carefully before study. In good dry weather there may be no rock fall. In certain weather conditions the cliffs may become dangerous. This is more likely during or after heavy rain or when frost is melting. Phases of erosion and undercutting may increase the risk of cliff falls.

There is no overall generalisation with regard to safety here; the state of the cliffs must be assessed by the field leader or visitor at the time of study with full consideration given to weather and tide conditions. Risk can be reduced by looking out for places where there have been recent rockfalls or where there is obvious instability in the rocks above the beach.

There may be other risks but they are not common. In storm conditions there may be problems with waves approaching the beach. Prolonged stay on the coast in wet and cold conditions might on occasions make students liable to problems of hypothermia, although this is a not a common problem in this area. The tidal range is limited but it might cause minor difficulties, particularly in pushing a party close to the cliffs. It is therefore better to study the section at low tide but this is not essential.

INTRODUCTION:

Bencliff Grit

The Bencliff Grit Formation is, cross-bedded sandstone with very large, ovoid, carbonate concretions. It is oxidised to brown at the surface but underground, as in the Poxwell Borehole it is grey and reduced. It takes its name from Bincleaves, a locality that is now built up, and situated on Portland Harbour between the Nothe and Sandsfoot Castle. Bincleaves or Bencliff is at the northwestern end of the Portland Harbour breakwater (here the Bencliff Grit is 5.5m. thick and overlain by 0.46m. of carbonate-cemented sandstone - it has large doggers or nodules of carbonate-cemented sandstone in the lower part Strahan (1898)). It is 5.8 metres thick (vertical thickness) in the nearby Poxwell Borehole

The Bencliff Grit is outcrops at various places in the Weymouth area. It is present on the shores of the Fleet at Wyke Regis. It is present at Redcliff Point. The best exposure is between Osmington Mills and Bran Point; this is the section discussed here.

The Bencliff Grit is mostly composed of fine-grained and well-sorted sands with finely broken plant debris. It does not appear to be very fossiliferous, partly because aragonitic fossils are easily dissolved in a permeable sandstone. Under the microscope there is some echinoid debris present. The sands form 80% to 90% of the formation, a mudstone facies about 5%, and a heterolithic facies of mixed mud and sand about 5-10% (Allen and Underhill, 1989).

These diagrams show the setting of the Bencliff Grit. Details from the succession log are given below:

Bencliff Grit Formation
(c. 3.96m or 13 feet in total at Osmington Mills)

BG c. Almost continous band of indurated, calcite-cemented, sandstone nodules (doggers). 0.41m (1 foot, 4 inches).

BG b. Yellow, brown and white sands, locally cross-bedded, with thin sandy clay beds (heterolithic units) and a few calcite-cemented concretions (nodules or doggers). The upper part is strongly impregnated with oil. 3.05m (10 feet).

BG a. Huge calcite-cemented concretions (nodules or doggers) with cross-bedding. Some are more than 1.83m (6 feet) in diameter. In some cases they pass into a more or less continuous band of thin-bedded (flaggy) calcite-cemented sandstone with the large, dark-coloured, round oyster Gryphaea dilatata and worm tubes (Serpula). The nodules weather out as prominant features on the beach.

Goldring et al. (1998) have carried out an integrated sedimentological, micropalaeontological, palynological and trace fossil study of the Bencliff Grit. They found evidence for back-barrier lagoon or bay conditions. The main sand units may be storm wash-over sands accumulated in about 5 m of water, but there has been much argument about the origin of the Bencliff Grit and the matter is not settled.

The general appearance of the Bencliff Grit is clear from this image. Shaly horizons with a mixture of mudstone, siltstone and sandstone, the heterolithic facies of Allen and Underhill (1989), are clearly visible within the sandstone. There is some concretionary development in the lower part. Most of the darker brown staining is by water but some is by oil, particularly above the level of the students' heads.

These photographs show the uppermost metre of the Bencliff Grit, with the First Limestone Bed of the Osmington Oolite above. The blue-grey beds are heterolithic facies of clay and sand. Note the details of the cross-bedding in the Bencliff Grit and the abrupt change in lithology to the bioturbated (see the holes) First Limestone. The junction is probably an erosion surface.

SEDIMENTARY STRUCTURES

Nodules or Concretions in the Bencliff Grit

Large calcitic concretions or " doggers " are common in the Bencliff Grit, especially the lower part . This photograph was taken on a misty morning after heavy rainfall when the cliffs were smeared with clay washed down from above. Much debris has fallen and the far block has slid out of position a little. The concretions show quite well some sedimentary structures that are, of course, also present in the weakly lithified sandstone. Cross-stratification is visible and is of a particular type here known as swaley cross-stratification and characterised by numerous troughs or swales (Allen and Underhill 1989). Notice that the concretions seem to have well-defined bases at a particular level, the top of the Nothe Clay beneath. Consider possible reasons for this

This concretion has fallen a short distance from the cliff but is probably still the right way up. It may be the same nodule as the one in the middle ground in the photograph above, but now, a few years later, the weakly-cemented sandstone above has been washed over the back. Again, cross-bedding is obvious. There is a thin wedge shaped bed of heterolithic material, that is clay and sand, which has clearly had some effect on the growth of the nodule. You can see where the nodule development started and how it grew into this form.

The calcite cement of these nodules is often poikilotopic, that it it consists of large calcite crystals containing and including many sand grains. Cleavage surfaces of these large crystals glint in the sun when freshly broken material is examined (Sellwood et al. 1990). This is a feature like the "lustre mottling" of the famous Fontainebleau Sandstone, in France, and it is also known from the Spilsby Sandstone in Lincolnshire, and in Quaternary raised beach sandstones of southwest England (West, 1973). The development of large poikilotopic calcite crystals is probably dependent on the relative deficiency of calcite nuclei (i.e. carbonate sand grains) which if available would have initiated numerous smaller cement crystals. Thus, it raises some interesting questions about, either initial carbonate grain rarity or, alternatively, a possible phase of carbonate loss prior to cementation.

This nodule is an approximately symmetrical concretion. It is in the uppermost part of the Bencliff Grit (between Osmington Mills and Bran Point), not far below the First Limestone Bed of the Osmington Oolite Formation. The symmetry of the nodule is interesting. Possible explanations for this involve regularity of flow of groundwater and transport of ions, a regular but higher rate of precipitation on near vertical surfaces and a regular but lower rate of precipitation on horizontal surfaces.

Incidently, the light patches in cliff are where the Bencliff Grit is dry and not darkened by water descending from above. Notice how it is light under an overhanging ledge. Lower than the nodule is a "heterolithic" bed, that is sandy clay. Higher in the top right hand part of the photograph is the Chlamys qualicosta Bed, the Middle White Oolite and at the top, brown stained, the Nodular Rubble.

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Footnote: Possible Student Project on the Nodules

The nodules may be suitable for undergraduate, postgraduate or other project work. Some or all of the following aspects may need to be investigated, although there is too much listed here in total for a typical student project, and some procedures may be too advanced or too costly to undertake. There is a danger that the nodules may be relatively uniform internally and, therefore, do not yield good results. Clearly some reconnaisance study would be needed before major work was undertaken.

Start with a study of the previous literature. Then after taking 10 or more samples from centre to exterior of a typical nodule make some of the following studies:
1. Field morphology of nodules (quantitative and statistical assessment).
2. Evidence of relative compaction of strata around to the strata in the nodules.
3. Relationship of nodules to pyrite, burrows, clay or heterolithic horizons.
4. Field evidence, if any, of early cementation (any borings, rather than burrows; any erosion or reworking?).
5. Carbonate (and clastic) mineralogy by XRD (low-Mg calcite, high-Mg calcite, aragonite, dolomite, ankerite etc.).
6. Petrography, including changes from centre to exterior: descriptive petrography, staining, petrographic image analysis or point-counting, percentage clasts, percentage cement, percentage porosity, cement stratigraphy, catholdoluminescence, SEM etc.
7. Major and trace element geochemistry, considering changes in Fe, Mg, Sr and Rb (for clays) from centre to exterior.
8. Isotopic studies, if available.
9. Other methods, if required and considered relevant, such as clay mineralogy.

Further notes: A full search of the literature has not been made, and key published work on these nodules may have been missed. If this is the case corrections will be made later. Try Scotchman (1991) for a start into the concretion literature. Do an online search for later literature.

Here are some ideas to consider. It is not expected that these particular nodules will show evidence of penecontemporaneous development - but they might (compare with the Bridport Sands concretionary horizons). An initiation in late Oxfordian to early Kimmeridge times with the ending of concretion growth by at the latest Mid-Cretaceous would seem to be likely, but this may be wrong. It seems unlikely the centre to exterior difference will be very great (try the Liassic Birchi nodules for this), but might be detectable. The change from the core to the exterior may mark a diagenetic boundary, possibly from the Sulphate-Reduction Zone to the Fermentation Zone (think how you would check on this). It might just mark the Mid-Kimmerian phase but that seems unlikely; it that is the case, the latter date suggested above is wrong.

Particularly think about source of carbonate and migration of carbonate. Study of the petrography of the Bencliff Grit away from the nodules is needed to understand this.

If time permits, a better project would involve comparing the Bencliff Grit Doggers with some other nodules, such as in some part of the Lias. There are "cannon-ball concretions" in the Nothe Grits. There are some broadly similar nodules in Bracklesham (Middle Eocene) sands, but they differ in petrographic detail. The "lustre mottling" may lead to comparitive discussion of the Fontainebleau Sandstone.
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This concretion can be considered in more detail. It has a core which a common feature of nodules and provides information on the history of formation. In some other formations (e.g. Birchi Nodules of the Lower Lias - calcite and dololomite) the core and exterior may have very different mineral compositions. It is likely that both parts are calcite-cemented sandstone but with some geochemical differences. Notice that is not a septarian nodule; it has no calcite-filled septarian cracks.

The symmetry is not perfect. This is also shown in the image with details. It should be noted, however, that the photograph was not taken at exactly right-angles to the surface, so that this a small perspective effect. Notice that the approximate plane of symmetry is not parallel to bedding. Notice too that very little compaction has occurred since the nodule developed; the strata not being squeezed around it. Bear in mind though, that you are considering compaction in sandstone which is much less than the great compaction which occurs in clay.

This is particular concretion that is easily recognsed. This "Whale Rock" seen at very low tide. This concretion has been washed out of the Bencliff Grit and become encrusted by algae and a few marine organisms. It is on the shore at low tide about 50m east of the cascade (further west than the main cliff section with nodules). The cliffs are landslipped here and there is no cliff section. The Bencliff Grit is presumably present under the grass.

SEDIMENTARY STRUCTURES

Trace Fossils

The Bencliff Grit is not in general bioturbated. Nevertheless trace fossils are commonly seen in places. Diplocraterion is conspicuous in some of the Bencliff Grit concretions. These may contain clay as shown in a photograph above. It is interesting to note that one U-shaped burrow has been superimposed almost at right-angles on a previous one. Presumably this was made by the same crustacean. In the cliff some Diplocraterion burrows may be seen extending down into the Bencliff Grit from the overlying fossiliferous and bioturbated limestone bed (bed 1 of the Osmington Oolite Formation). These burrows are lithified by calcite in some cases and may take the form of limestone projections down from the bed above.

SEDIMENTARY STRUCTURES

Ripples and Plant Debris in the Bencliff Grit

Comminuted plant debris occurs in ripple troughs in the Bencliff Grit. Similar plant debris can sometimes be found in ripple troughs on modern beaches such as those of Bournemouth.

SEDIMENTARY STRUCTURES

Top of the Bencliff Grit

In places the uppermost few centimetres of the Bencliff Grit are cemented hard by calcite. Emma Tugwell pointed out to me some interesting features here. There are some narrow burrows into the sand and these have curved lower parts. Perhaps they are Scolithos as occur higher in the Osmington Oolite. Note the contrast between the cross-bedded, marine sandstone of the Bencliff Grit with very little bioturbation and the bioturbated, shelly bed above.

This overlying bed is the First Limestone of the Osmington Oolite. It is very bioturbated and has Rhizocorallium burrows on its top surface.

Here is a gutter cast, a scour and fill structure (see Stow, 2005) at the top of the Bencliff Grit in the main cliff section east of Osmington Mills. This is interesting because it contains similar, much bioturbated, sandy limestone like that of Bed 1 of the Osmington Oolite Formation which overlies. Talbot (1973) regarded the boundary between the Bencliff Grit and the Osmington Oolite as the transgressive start of a new cycle (his cycle 3), following the Bencliff regression. This scour structure certainly agrees well with such an interpretation. The most curious aspect of it is that the burrows do not continue into the adjacent Bencliff Grit. This is difficult to explain but can be accounted for if the Bencliff Grit was already partially compacted and already a relatively firm sediment. In other words the sand was inimical to burrowing organisms. The structure has not been thoroughly investigated and there might be some explanation other than early compaction; there might be a geochemical explanation.

A very thin, brown, ripple-marked sand bed overlies the gutter cast and separates it from Bed 1. Perhaps carbonate sediment of Bed 1 type was deposited on a scoured surface of the Bencliff Grit, subsequently eroded but preserved in the gutter. Then the main Bed 1 was deposited and consisted of the same type of sediment.

Not well-understood as yet is the occurrence of hard, cemented sandstone of cross-bedded Bencliff-type at the upper side margin of the gutter cast. The sedimentary structure requires further study, with ideally, an investigation of the thin-section petrography of various parts.

For more on gutter casts see:
Upper Stiped Beds, at Staithes, Yorkshire.

PETROLEUM GEOLOGY

Oil in the Bencliff Grit

Oil was discovered in the Bencliff Grit at Osmington Mills in 1937 by Mr. Taitt, a geologist of the D'Arcy Exploration Company, Ltd., a subsidiary of the Anglo-Iranian Oil Company, Ltd. The geologist was said to have discovered this when back on leave from his work in Iran, the major region for British oil supplies at that time (which were produced by the Anglo-Iranian or British Persian Oil Company, i.e. predecessor of BP). The exploration was much influenced by the knowledge of the Iranian (Persian) oil traps. The following is from Lees and Cox (1937):

"The presence of a rich impregnation of oil in the Corallian Bencliff Grits outcropping near Osmington Mills in Dorset was recently discovered by Mr. A. H. Taitt. A specimen of the sand from the exposed cliff face has given 12.1 per cent of oil, and where these beds pass below high-tide level at Bran Point there is a small but active seepage of free oil. This seepage is, so far as we are aware, a new discovery, and its existence does not appear to be known even to the local inhabitants."

The discovery of oil here resulted in oil exploration in the nearby anticline of Poxwell. See the Poxwell webpage for details. The oil well and the borehole in section are shown below.

The Bencliff Grit east of Osmington Mills contains oil-sand and sometimes some surface oil seepage. It is best seen near the western end of the main cliff section. Dark brown specimens can be obtained from from the Bencliff Grit exposure, but there is risk of rock fall and excessive hammering should be avoided. It is not necessary for all members of a party to take samples and this increases the rock-fall risk. It may be safer just to pass round one or two samples to a party. Notice that the oil sand smells strongly of oil.

The oil sand is not at the top of a sandstone reservoir as is usual underground, but instead it is here above the water-table (i.e. in the vadose zone). It seeps downward and is usually concentrated above clay laminae. In some weather conditions there is some small seepage in the form of small trickles from the cliff, as shown on the right, but this surface appearance varies from time to time. Recognise the oil sand by a dark brown colour and any surface oil seepage. Try breaking of a piece and smelling it; it has a strong bituminous smell.

Further east at the eastern end of the outcrop in the cliffs of the Bencliff Grit, just after it has disappeared under a ledge of the next limestone in the sea there is a small active oil seep, although it is often difficult to see. Bubbles of oil emerge in shallow water at intervals, probably rising from joints in the limestone, and the oil disperses as a film on the surface. To observe this it is best to look when there are quiet water conditions and a tide rising from low water.

This small but interesting feature is a classic aspect of Dorset geology because the finding of this in the 1930s was one of the reasons for commencement of oil-exploration in Dorset. That led eventually to the discovery of the great Wytch Farm oil-field, containing almost 500 million barrels of recoverable oil. Have a careful hunt around the east side of the ledges of limestone just east of Bran Point (not the Bran Ledge but a little distance southeast of the exposure of the Pisolite on the beach).

BENCLIFF GRIT EXPOSED IN A WEYMOUTH RELIEF ROAD CUTTING

The Corallian strata have been exposed in the cutting for the Weymouth Relief Road at Southdown Ridge, an east-west hill, directly south of the Littlemoor Housing Estate. The area is in the vicinity of Lorton House. The Bencliff Grit is quite well-exposed but the effects of scraping by heavy machinery has resulted in smearing and crushing. At present the details visible in the cliff section cannot be made out in the broken rock. Large doggers or carbonate-cemented sandstone concretions are present in place, sometimes fractured by the machinery.

ACKNOWLEDGEMENTS

I am very grateful to Emma Tugwell for helpful discussion in the field and drawing attention to some details of the cliffs. Various field trips with university staff, students and others have provided helpful information for the Osmington Mills webpages and I much appreciate discussion with the many participants out on the cliffs. I much appreciate the opportunity to participate in many field trips here with Professor Dorrik Stow. James Codd of Amey is thanked for providing the opportunity to examine the Bencliff Grit in the Weymouth Relief Road excavations.

To continue the Osmington Corallian Field Guide go to the section of interest:

- Osmington - Pt. 1 - Osmington Mills Introduction

- Osmington - Pt. 2 - Osmington Mills to Ringstead.

- Osmington - Pt. 3 - Bencliff Grit

- Osmington - Pt. 4 - Osmington Oolite

- Osmington - Pt. 5 - Black Head

- Osmington - Pt. 6 - Corallian Fossils

- Osmington - Pt. 7 - Bibliography

Poxwell Quarry and Anticline (with borehole into Corallian).

REFERENCES AND BIBLIOGRAPHY

Go to Osmington, Corallian, Bibliography

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Webpage - written and produced by:

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

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.