INTRODUCTION:

Access

To see the Osmington Oolite at Osmington Mills you can park a car at the Smugglers Inn car park (there is a charge for this). Osmington Oolite is actually in the back bank of the car park, but there are better exposures on the shore. It is well-exposed to the east, towards Ringstead, and particularly at Bran Point. In the opposite direction the Osmington Oolite can be seen at Black Head, but low tide is needed to see the rocks which are sea-washed at high tide.

For more information please go to the:

Osmington Mills Introduction webpage

Osmington Mills to Ringstead webpage

on one of the other webpages listed above.

INTRODUCTION:

Safety and Risk Assessment

Some general information regarding safety on Dorset geological field trips is provided and you are requested to read this if going to localities described here.

Almost everywhere on the Dorset coast care must be taken with regard to hazard of falling rocks. The stretch of coast from Osmington Mills to Bran Point must be considered carefully with regard to cliff and weather conditions before proceeding along it. In dry weather in summer it is much like any other Dorset cliffs and there is no excessive risk, although, of course, a rock can fall at any cliff anywhere.

On rare occasions, particularly during heavy rainfall or following it, or after frost, the high cliff between Osmington Mills and Bran Point can be dangerous, especially the steep western part (the oil sand exposure). Debris may fall and the foot of the cliffs should not be approached in these conditions. Bran Point and eastward is rather safer than the main cliffs. While debris can fall at Black Head, the cliffs there are not so steep. Obviously, assessment will be made at the time of a visit. Safety helmets should be worn when adjacent to a cliff. A minor hazard is that of falling on slippery sea-weed covered rocks on the shore.

The tidal range is not great and in normal weather conditions the coast is nearly always accessible. Low tide is better for geology because the shore reefs are exposed. Obviously beware of rare storm wave conditions.

INTRODUCTION:

Osmington Oolite Formation

This ledge is formed by the Chlamys qualicostata Bed (A2) of the Osmington Oolite Formation of the Corallian Group. Notice that it has been undercut by wave action and blocks have collapsed on the updip side. A similar feature is seen in the Kimmeridge Clay dolomite ledges at Kimmeridge Bay. They are also eroded by wave-undercutting and subsequent collapse. This is also seen at Portland Bill , where the Portland Stone is undercut.

SUCCESSION

Osmington Oolite - Succession

.

Osmington Oolite Formation
(Osmington Oolite Series of Arkell, 1936, 1947)

A12. Nodular Rubble Member. A bed of bioturbated nodular limestone, composed of minute calcitised kidney-shaped, Rhaxella sponge spicules (visible only microscopically, in thin-section). In the field it appears grey, rough, nodular and is fossiliferous. It is easily recognised in the field because it is not separated into clearly distinct beds but forms a very steep section of the cliff of nodular limestone, reaching the shore at Bran Point. It does show division into two courses, though. There are many fragments of shells and spines of echinoderms. Fossils include the small echinoidNucleolites scutatus, the small oyster Nanogyra nana, moulds of the bivalve Pholadomya and the gastropods Pseudomelania (a large turreted form) and the smaller Natica. The lowest 0.30m (1 foot) is oolitic and clayey. Total thickness 3.35m (11 feet).

A11. Upper White Oolite (upper half). Clay with laminae of fissile white oolite, full of small Ostrea cf. dubiensis. 0.30m (1 foot).

A10. Clay, grey, the lower part oolitic, with oolitic while nodules in the lower 0.3m (1 foot). Thickness: 0.99m (3 feet, 3 inches).

A9. Upper White Oolite (lower half). Cross-bedded oolite with vertical burrows. 0.60m (2 feet).

A.8. Clay with three bands of nolular white mudstone in the highest 1.22m (4 feet). Thickness: 2.51m (8 feet, 3 inches). A.7. Marl and soft rubbly marlstone, in several bands, strongly oolitic, with Thalassinoides burrows. The small oyster Nanogyra nana is common. Chlamys qualicosta is present.

A.6. The Middle White Oolite. At the west end of the cliff towards Osmington Mills, this is 2.29m (7 feet, 6 inches) of solid, cross-bedded while oolite, overlying 1.06m (3 feet, 6 inches) of more thinly-bedded, cross-bedded white oolite. Eastward the whole becomes more marly from the base up, until at Bran Point only the highest 0.60m (2 feet) is solid white oolite. Vertical burrows are a conspicuous feature in the oolite; also cross-lamination, clay partings and some lignite. Thickness: 3.05m (10 feet).

A5. (Littlemore Clay Beds facies of Arkell). Clays and bands of nodular white mudstone. Ammonites of the genus Perisphinctes can be found. Thickness: 3.81m.

A4. The Pisolite. An oncolite or oncoid bed (oncolites are pea-sized objects of microbial or algal origin often formed around a piece of shell), coarse-grained, purplish-grey, fairly hard although prone to disintegrate into individual oncolites. It forms a small ledge both west and east of Bran Point (it is repeated by a fault). It is shelly with shell fragments and also specimens of the bivalves: Chlamys qualicosta, Chamys fibrosa, Myophorella hudlestoni (another "Trigonia"). Fragments of the ammonites Perisphinctes sp. and Cardioceras (Cawtoniceras) sp. etc. have been found. 0.46m (1 foot, 6 inches).

A3. Clay, black, full of fragile compressed shells, especially small Trigonia (Myophorella?) bivalves of clavellate form. Also the bivalves Chlamys qualicosta, Cucullaea, Grammatodon, Lucina. Locally this bed is a marl. 0.60m.

A2. Chlamys qualicosta Bed. Limestone, hard, oolitic, sparsely pisolitic, gritty, shelly, dark-grey, weathering brown. The highest 0.15m (6 inches) forms a separate course. Crowded with Chlamys qualicosta, Chlamys fibrosa, Nanogyra nana etc. Forms Bran Ledge at Bran Point and the second ledge to the west. 0.76m.

A1b. Marl, oolitic, sandy, with Nanogyra nana, passing down into the hard band of the same material beneath (A1a). Thickness: 1.37m.

A1a. The First Limestone. Sandy, argillaceous limestone with Nanogyra nana variable in thickness but thickening westward. Bioturbated with Thalassinoides burrows. This helps to form Bran Ledge and also causes the third and largest ledge on shore west of Bran Point. Thickness: variable upto 0.60m (2 feet) (Note - Arkell originally put these two beds together as A1 with a total thickness of 1.98m (6 feet, 6 inches) but it is sensible to label them separately.)

OOLITE BEDS

The Osmington Oolite Formation is about 20m in thickness. It is divided into the Upton, Shortlake and Nodular Rubble Members. It contains a variety of lithologies, including much ooid grainstone. It can be seen in the cliffs east of Osmington Mills (top), and particularly where it descends eastward to beach level at Bran Point (lower photographs). Shown in the right-hand lower photograph is the Middle White Oolite in the centre of the Osmington Oolite Formation, underlain by cyclical nodular limestones alternating with bioturbated, heterolithic carbonate-clay beds. The Middle White Oolite here has many narrow, U-shaped Skolithos burrows.

The Osmington Oolite is also well-exposed at Black Head, west of Osmington Mills, particularly at low tide.

The Middle White Oolite, part of the Osmington Oolite Formation, is about 3m thick. It is a conspicuous oolite varying from oosparite to oomicrite and shows cross bedding. This image provides detail and is a higher resolution, unretouched, photomosaic forming part of the cliff photograph shown above.

West of Osmington Mills at Black Head the Middle White Oolite (central part of the Shortlake Member) is easily seen. It has increased in thickness westwards. The general setting is also shown with the remains of a very large mudslide in the middle distance and Upper Greensand debris brought down to the beach by it in the past. The Middle White Oolite shows cross-bedding and rip-up clasts or mudclasts. Both of these features are evidence of the the high-energy conditions in which this carbonate sand was deposited.

Here is a clean, sea-washed surface through an oolitic bed in the Osmington Oolite at Black Head, exposed on the beach. Left is west, right is east; the pen gives a scale; the pebbles are mostly of subangular flint from the nearby Cretaceous outcrops and one may be of chalk. (photograph taken in November, 1999).

Have a close look at this photograph. What sedimentary structures can you see? Can you give a name to the trace fossils? What has been the sequence of events just here in the warm, almost subtropical Jurassic sea with its shoals of white lime-sand? The banks of ooid sand do not seem to have been very thick at Osmington Mills, only about 1 metre in general. Oolite banks in the Portland Stone are thicker than this. Why were the Osmington banks so thin? Have you any comments on the cross-bedding? Why are there no burrows in the upper part? (See Sellwood and Wilson (1990) for discussion of the ooid shoals, including the question of whether the carbonate sand between banks was stabilised by sponges, rather than sea-grass which had not evolved at this time.)

Osmington Oolite was quite well-exposed in a cutting made for the construction of the Weymouth Relief Road at Southdown Ridge, south of Littlemoor Estate. The photograph above shows it faulted against Bencliff Grit (which lies beneath).

OOLITE PETROGRAPHY

Introduction

Ooids are present not only in the Osmington Oolite but also in overlying limestones. One image provides an examples of oolitic limestone from Osmington Mills, as seen in the field. This example is from the Red Beds, higher in the succession than the Osmington Oolite, but the brown sideritic matrix makes the ooids more clearly visible. The example is from Black Head. Polished surfaces from septarian nodules found loose on the beach at the eastern Osmington Oolite cliff section provide good views of ooids in a relatively fine, carbonate matrix. These may have come from the Osmington Oolite or perhaps from an overlying part of the Corallian. These are modified from Tanner (1993) .

The Corallian ooids have a good concentric structure when best developed. In some cases the ooids have formed around small quartz sand or silt grains.

Many of these rocks in the Osmington Oolite Formation are unusual in that they are oomicrites, not the well-washed oosparites that are in general more common in Jurassic strata (eg. Portland Stone). Examples of such oomicrites occur at about the top of the Shortlake Member. These oomicrites are oolites with fine-grained, carbonate matrices. In the termology of Folk, 1962 these show textural inversion. They are not well-washed assemblages of ooid sand grains but instead consist of anomalous mixtures of extremely well-sorted ooids in a micrite matrix (Sellwood and Wilson, 1990.) An examples of this oomicrite are shown in thin-section.

It is clear from a photomicrograph that the matrix and cement is ferroan. It has been stained by potassium ferricyanide which gives a blue colour when significant ferrous iron is present in the carbonate. The example shown here is unusual, however, because fibrous fringe cements usually form in the pore spaces around ooids that are normally free from a lime-mud matrix. The micritic matrix crystals, probably of calcite, are roughly rhomb shaped. There seem to be slightly curved flakes of clay minerals scattered between them.

OOLITE-PETROGRAPHY

SEM Images - Osmington Mills

Shown above are SEM images of Osmington Oolite specimens from Osmington Mills, taken some years ago. The images shows that the ooids have concentric structure. Although the ooids are clearly well-preserved there are also signs of diagenesis here. There seems to be evidence of pressure solution in the SEM image above. Associated with the ooids is a fibrous carbonate. Careful examination indicates fracture in an oblique manner. This suggests the oblique cleavage present in calcite but not in aragonite. This could be investigated further by measuring the strontium content of these crystals under the SEM. Calcite usually has a lower strontium content than aragonite.

The fibrous carbonate may be an isopachous (equal thickness), fibrous fringe cement around the ooids, of the type described by Chowdhury (1982a) and Sun (1990) from the Osmington Oolite. Chowdhury, dealing with Oxfordshire and Berkshire and Sun, studying the Dorset area, both found them to be of ferroan calcite (and this accords with a thin-section image shown above).

Important early work on the Osmington Oolite is that of Wilson (1968), who shows lateral facies changes. This paper distinguishes between a "True Oolite" (or "Coastal Oolite") facies at the coast at Osmington Mills and a "Oolite Freestone Facies" in North Dorset. In the True Oolite facies the ooids have many concentric layers and are typical ooids. In the Oolite Freestone facies the allochems are said to be merely lumps of carbonate with a thin exterior oolitic coating. However, the matter is complicated on a local scale. Oosparite at Bird's Quarry, Todber is of classic type, as shown by Chowdhury (1980), and figured here. At Sturminster Newton Road Cutting section a mixed bioclastic and oolitic calcarenite is present. Particularly abundant are small, flake-like, shell fragments with micrite envelopes.

OOLITE-PETROGRAPHY

Thin-section Petrographic and SEM Images - Todber Stone - Osmington Oolite, Todber, near Marnhull, North Dorset

Compare the images of aragonitic Bahamian ooids after Bathurst (1975) with those from the Osmington Oolite. They seem remarkably similar. Dorset is, of course, an area of shallow continental shelf that was formerly off the American continent, and much like the Bahamas. Its Jurassic palaeolatitude (about 35 degrees north) was higher than the Bahamas (about 25 degrees north) but there were no ice caps in the Jurassic and the climatic belts were wider. Osmington Mills and Todber have rather Bahamian histories.

Dr. Richard Pearce of Southampton University has investigated an oolitic stone used in the facade of the Methodist Church at Hedge End, near Southampton. The nearby Church of England also seems to have similar oolitic limestone in its walls. A visit was made with a sample to Todber Quarry, North Dorset and the rocks compared. It seems very likely that the stone from the Methodist Church at Hedge End is either Todber Stone or similar Corallian oolite from a nearby locality. The SEM photographs above by Richard Pearce show details.

OOLITE PETROGRAPHY:

Ooids in Expanded Septarian Nodules

The mode of development of septarian nodules is a controversial matter and some form of contraction is sometimes assumed as the process that produces the internal cracks. In fact, field observations often show that septarian nodules contain fossils that have been broken by expansion processes of the peripheral area. Expansion was wisely put forward as a mechanism by Todd (1903), a long time ago, but for some reason this theory has been much ignored. Expansion of the periphyry of a nodule by continued crystal growth could produce cracking of the interior. It is interesting to note that the Corallian septarian nodules of Osmington Mills show " expansion shadows ", apparently formed as expansion of the periphyry has pulled apart the interior and separated the micritic matrix from the ooids. Of course, some will disagree and consider that these shadows have been produced by a compaction process. However, expansion rather than compaction explains the features more satisfactorily.

Further examples of expanded septarian concretions can be seen in the Kimmeridge Clay, not far away at Ringstead. Concretions in the Lower Lias in the Lyme Regis area and in the Barton Clay (Eocene) of Barton-on-Sea, at the Hampshire/Dorset boundary and elsewhere provide obvious evidence of expansion.

OTHER LOCATIONS - for comparison:

Todber and Marnhull, North Dorset

The Corallian provides building stones at several localities. Most of these are in Osmington Oolite. Near to the coast it has been quarried at Linton Hill, southeast of Abbotsbury. Blocks could be obtained up to about 0.9m (3ft) by 0.6m (2ft) by 0.3m (1ft) Howe (1910). The oolitic limestone is medium grained and, unlike Portland Stone, is warm, yellowish-brown in colour. The ooids are in a matrix of sparite and some micrite. It has been used in Abbotsbury. The ancient building, St. Catherine's Chapel, on a very exposed position, shows that it wears well.

There are quarries inland in North Dorset at Marnhull (Marnhall) and Todber (Todbere), near Sturminster Newton. At both these localities the Osmington Oolite is a well-developed oosparite, similar to that on the coast. It slightly buff in colour at Todber, where the quarries are now abandoned. In Whiteway Lane, Marnhull there is an active quarry in Osmington Oolite. This is the Whiteway Quarry producing Marnhull Stone. Howe (1910) described it as follows:
"a buff marly oolite, in parts shelly, occurring in regular beds, one foot to one and a half feet thick, with marly partings between; 15 feet (4.6m) or more of the beds are shown in the quarries. Blocks up to 20 cubic feet have been obtained. It has been used in Gillingham Church, Hinton St. Mary, Sutton Waldron etc."

At Todber the full thickness of the "lower freestones" (Osmington Oolite) is 14 feet (4.26m) according to Howe (1910). There are two horizons of good building stone, near the top of the Osmington Oolite and near the base of this unit.

Other localities for Corallian building stones, are Steeple Ashton, Wiltshire (oolite and pisolite), Goat Acre, north of Hilmarton, and at Preston and Purton. At Faringdon, Oxfordshire, a blue-hearted oolite, 3 feet thick, has been used as a building stone. Headington Stone (Shotover Limestone) and Wheatley Stone are Oxfordshire Corallian limestones, more shelly than oolitic Howe (1910).

PLANT DEBRIS

Small piece of foliage in the Osmington Oolite

A small piece of carbonised conifer foliage is present in the uppermost part of the Osmington Oolite at Bran Point, beneath the Nodular Rubble. The rock consists of argillaceous micrite (marl) with some ooids. Land was probably not far away because plant material is fairly common in the Corallian section at Osmington Mills.

To continue the Osmington Corallian Field Guide go to one of the following related webpages:

- Osmington - Osmington Mills Introduction

- Osmington - Osmington Mills to Ringstead.

- Osmington - Bencliff Grit

- Osmington - Osmington Oolite (this webpage)

- Osmington - Black Head

- Osmington - Corallian Fossils

- Osmington - Bibliography

ACKNOWLEDGEMENTS

I am grateful to Louise Tanner for her help in investigating some aspects of the petrography of the Corallian at Osmington Mills. Ivailo Grigorov kindly provided an SEM image of Corallian ooids. More recently Dr.Richard Pearce has taken several high quality SEM images of an Osmington Oolite specimen and I am very grateful for the opportunity to use these above. I particularly thank Richard Pearce, and Alan Holiday, Jo Thomas and Jeremy Cranmer of the Dorset DIGS group for their kind help with regard to Todber and Whiteways Quarries in North Dorset. The staff of Whiteway, Marnhull Stone quarry were particularly helpful when the site was visited with member of the DIGS group. Emma Tugwell examined the Osmington Oolite with me in the field in 2004 and her help is much appreciated.

|Home and List of Webpages