The BGS Lexicon of Named Rock Units — Result Details

Osmington Oolite Formation

Computer Code: OSS Preferred Map Code: Os
Status Code: Full
Age range: Oxfordian Age (JO) — Oxfordian Age (JO)
Lithological Description: The lower part predominantly comprises a regularly bedded series of ooidal limestones, sandy limestones and argillaceous limestones, overlain by mudstones with calcareous concretions, becoming sandy upwards (Upton Member of Wright and Cox, 2001). These are overlain, locally with an erosional contact by laterally variable cross bedded and bioturbated ooidal limestones with generally subordinate mudstone beds (dominant at Sandsfoot)(Shortlake Member of Wright and Cox, 2001). Shelly calcilutites are also developed at East Fleet. The Shortlake Member is overlain sequentially by irregularly bedded limestone and calcareous mudstone, with locally developed calcareous concretions (Nodular Rubble Member of Wright and Cox, 2001). The Formation contains a diverse ichnofauna, including Thalassinoides, Diplocraterion, Gyrochorte and Rhizocorallium, and macrofossils are represented particularly by ammonites, bivalves and gastropods.
Definition of Lower Boundary: An erosive, burrowed junction marks the boundary between the argillaceous sandstones and siltstones of the Bencliffe Grit Member (uppermost Redcliff Formation) and the argillaceous, sandy and ooidal limestones of the basal Osmington Oolite Formation.
Definition of Upper Boundary: An irregular unconformity marks the contact of the uppermost Osmington Oolite Formation with the overlying Clavellata Formation. A change, from nodular limestone (below) to sandy limestone (above) occurs at this unconformity.
Thickness: Variable, but up to about 22m in the cliffs and foreshore between Bowleaze Cove and Ringstead. About 12m at East Fleet.
Geographical Limits: South Dorset. Equivalent to the upper part of the Stour Formation of north Dorset. Probably no physical continuity due to pre-Cretaceous erosion.
Parent Unit: Corallian Group (CR)
Previous Name(s): Osmington Oolite Series (-1450)
Alternative Name(s): none recorded or not applicable
Reference Section  Cliffs and foreshore at East Fleet, Weymouth. 12m of beds visible, only base unexposed. Wright and Cox, 2001. 
Type Section  200m of coastal cliffs and wave-cut platform between Frenchman's Ledge and Bran Ledge, near Osmington Mills, Dorset. Formation fully exposed, between about 19 and 22m thick. Wright, 1986; Wright and Cox, 2001. 
Wright, J K. 1986. A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset. Proceedings of the Geologists' Association, Vol. 97, 1-21. 
Wright, J K, and Cox, B M. 2001. British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian). Geological Conservation Review Series. No. 21. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.) 
Blake, J F and Huddleston, W H 1877. On the Corallian rocks of England. Quarterly Journal of the Geological Society of London, Vol.33, 260-405. 
Coe, A L. 1995. A comparison of the Oxfordian successions of Dorset, Oxfordshire and Yorkshire. 151-172 in Taylor, P D (Ed.) Field Geology of the British Jurassic. [Bath: The Geological Society.] 
Newall, A J. 2000. Fault activity and sedimentation in a marine rift basin (Upper Jurassic, Wessex basin, UK). Journal of the Geological Society, London, Vol.157, 83-92. 
Arkell, W J. 1947. Geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the British Geological Survey. Sheets 341, 342, 343 and small portions of sheets 327, 328 and 329 (England and Wales). 
Arkell W J, 1933. The Jurassic System in Great Britain [Oxford: Clarendon Press.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E341 E342 E343 E327