Osmington Oolite Formation - Introduction

This view shows the Osmington Oolite Formation in the cliffs east of Osmington Mills, with some details regarding the basal beds of the 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.

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 (12 feet, 6 inches).

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 (2 feet).

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 (2 feet, 6 inches).

A1b. Marl, oolitic, sandy, with Nanogyra nana, passing down into the hard band of the same material beneath (A1a). Thickness: 1.37m (4 feet, 6 inches).

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.)

Osmington Oolite - Oolitic Rocks

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 (oolite). 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.

Left: West of Osmington Mills at Black Head. Here again we can see the Middle White Oolite (central part of the Shortlake Member), which 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.

Right: A student is examining the rock of Middle White Oolite, shown in the centre of the left photograph. There is 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 for the Weymouth Relief Road at Southdown Ridge, south of Littlemoor Estate. The photograph above shows it faulted against Bencliff Grit (which lies beneath).

Petrography of the Oolites

Ooids are present not only in the Osmington Oolite but also in overlying limestones. The top left 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 photographs in an undergraduate research report by Tanner (1993) .

The Corallian ooids have a concentric structure which can be seen in the left hand and central images. In some cases the ooids have formed around small quartz sand or silt grains (see left image).

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.

To use a term of Folk, 1962 these show textural inversion in not being a well-washed assemblage of ooid sand grains but instead consisting of an anomalous mixture of extremely well-sorted ooids in a micrite matrix (Sellwood and Wilson, 1990.)

Examples of this oomicrite are shown in thin-section (left) and under the high magnification of the SEM - scanning electron microscope (centre and right) in the images above. The thin-section is by Louise Tanner and the SEM pictures have been taken by Ivailo Grigorov, to both of whom I am very grateful.

The left and central images shows that the ooids have concentric structure. Although the ooids are clearly well-preserved there are also signs of diagenesis here. Can you see any evidence of pressure solution in the central photograph?

Associated with the ooids is a fibrous carbonate. What do you think that the fibrous crystals might be? If you look carefully you can see that they fracture in an oblique manner. This suggests the oblique cleavage present in calcite (trigonal) but not the other form of calcium carbonate - aragonite (orthorhombic) . To check this we are going to measure the strontium content of these crystals under the SEM, because calcite usually has a lower strontium content than aragonite.

The fibrous carbonate is probably 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. It is clear from the left 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. (Please note, however, there are, however, some additional fabric complications in the optical photomicrograph, concerned with septarian nodule growth that I hope to discuss later).

The micritic matrix crystals, probably of calcite, are roughly rhomb shaped. Can you see the slightly curved flakes of clay minerals scattered between them?

This specimen is from almost the top of the Osmington Oolite at Bran Point beneath the Nodular Rubble. It consists of argillaceous micrite (marl) with some ooids. It not only provides an enlarged view of ooids but also the interesting piece of carbonised foliage. Land was probably not far away because plant material is fairly common in the Corallian section at Osmington Mills.

Ooids in Expanding or Contracting 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, look carefully at the photomicrograph and decide whether these structures could really have been produced by compaction.

For further study examine septarian concretions in the Kimmeridge Clay, not far away at Ringstead. Also look at 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. The evidence seems to support the theory of expansion of the periphery producing cracking.

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

- 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 the SEM illustration of Corallian ooids. Emma Tugwell examined the Osmington Oolite with me in the field in 2004 and her help is much appreciated.

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