The BGS Lexicon of Named Rock Units — Result Details

Portland Freestone Member

Computer Code: POFR Preferred Map Code: PoFr
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: The member comprises ooidal grainstones and packstones with Thalassinoides burrows. On the Isle of Purbeck, there is a local horizon of cherty packstone / wackestone, and nodules of chert occur in the lower part of the member at Ringstead Bay. On the Isle of Portland, the ooidal grainstones that form the lower and upper parts of the member are separated by an interval of shelly lime mud-rich limestone. Here, small patch reefs are also developed in the top of the member, formed by masses of oysters and bryozoans cemented around a massive calcareous laminated framework built of the red alga Solenopora. The topmost part of the member on Portland is a highly fossiliferous unit known as the 'Roach', in which shells are dissolved out to leave a rock full of the moulds of gastropods and bivalves. On the Isle of Purbeck, the uppermost bed is a massive calcilutite known as the Shrimp Bed, yielding remains of Callianassa.
Definition of Lower Boundary: The base of the member is the upward facies change from chert-bearing lime-packstones and wackestones of the underlying Portland Chert Member, to massive ooidal grainstones. On the Isle of Portland, this facies change is marked by a sharp unconformity, but on the Isle of Purbeck the facies change occurs at a slightly lower stratigraphical level, and is apparently conformable.
Definition of Upper Boundary: The top of the Portland Freestone, and also the top of the Portland Group, is marked by the conformable upward facies change from massive ooidal, bioclastic grainstones or lime mud-rich limestone, with common marine fossils, to a succession of stromatolitic limestones and palaeosols of the basal Purbeck Group.
Thickness: 9m on the Isle of Portland to 15 m on the Isle of Purbeck (Wimbledon, 1980; Barton et al. 2011).
Geographical Limits: South Dorset.
Parent Unit: Portland Stone Formation (POST)
Previous Name(s): Freestone Series (*518)
Winspit Member (-5238)
Freestone Series [Obsolete Name And Code: Use POFR] (FRES)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Clay Ope, West Weare Cliff, Isle of Portland, Dorset: member about 9m thick fully exposed in vertical sea cliffs. Coe (1996); Barton et al. (2011). 
Type Area  Winspit to St Alban's Head, Isle of Purbeck, Dorset: member about 15 to 16m thick fully exposed in vertical sea cliffs. Coe (1996); Barton et al. (2011). 
Reference(s):
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
Coe, A L. 1996. Unconformities within the Portlandian Stage of the Wessex Basin and their sequence-stratigraphical significance. In Sequence Stratigraphy in British Geology. Hesselbo, S P & Parkinson, D N. (eds). Geological Society Special Publication, 103, 109-143. 
Barton, C M, Woods, M A, Bristow, C R, Newell, A J, Westhead, R K, Evans, D J, Kirby, G A, and Warrington, G. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey, Sheets 328, 341/342, 342/343 and parts of 326/340, 327, 329 and 339. 
Wimbledon, W A. 1980. Portlandian correlation chart. 85-93 in Cope, J C W (Editor), A correlation of the Jurassic rocks of the British Isles. Part Two: Middle and Upper Jurassic. Geological Society of London Special Report, No.15. 
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). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E341 E342 E343