The BGS Lexicon of Named Rock Units — Result Details

Portland Stone Formation

Computer Code: POST Preferred Map Code: PoSt
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: Dorset and adjacent offshore: the lower part of the formation comprises pale brown or grey bioturbated calcilutite with peloids, sponge spicules, shell material and common black nodular chert (Portland Chert Member). The upper part of the formation is thick bedded, ooidal, shell-rich calcilutite (Portland Freestone Member). Bioclastic material is abundant towards the top of the formation, with small patch reefs formed by the red alga Solenopora. On the Isle of Portland, the formation is capped by a highly fossiliferous limestone in which the shells have been dissolved out to leave a rock full of bivalve and gastropod moulds (the 'Roach'). Vale of Wardour, Wiltshire: the lower part of the formation comprises lime mud-rich limestone and sandy, glauconitic, bioclastic limestone, with subordinate siltstones, fine-grained, calcareous sandstone and sandy limestone (Tisbury Member). Above this are shell-beds and interbedded calcareous mudstones overlain by white, poorly bedded, locally sandy, lime mud-rich limestone with scattered bivalves (mostly Laevitrigonia)(Wockley Member). Nodular chert occurs in the lower and upper parts of this succession. Above are beds of fine-grained ooid-limestone and cross-bedded ooidal bioclastic sands (Chilmark Member) with tabular and nodular chert intervals. Vale of Pewsey, Wiltshire: the lower part of the formation comprises fine- to medium-grained sandy, bioclastic packstone limestone with glauconite grains and black chert (lydite) pebbles, bivalves, serpulids, oysters and crustacean fragments; the upper part is calcareous mudstone with shell fragments and other carbonate grains. No formal members are recognised. In the Weald, Portland Stone Formation has not been separately recognised within the Portland Group but may be represented by calcareous and glauconitic sandstone (Gallois, 1965) and may extend offshore (Hamblin et al., 1992).
Definition of Lower Boundary: Dorset: Isle of Portland - the base of the formation is defined by a facies change from mudstone ('Portland Clay') to lime mud-rich limestone with common chert nodules, and is marked by a shell-rich horizon (Basal Shell Bed) above a slight unconformity. Isle of Purbeck - the base of the formation is marked by the conformable upward facies change from finely crystalline dolomitic limestone of the Portland Sand Formation to chert-bearing bioturbated calcilutite. Vale of Wardour, Wilts: siltstone and fine-grained sandstone, commonly glauconitic or clayey (Wardour Formation), overlain by lime mud-rich limestone and sandy, glauconitic, bioclastic limestone, with subordinate siltstones, fine-grained, calcareous sandstone and sandy limestone (Bristow et al. 1999) or (Vale of Pewsey) shelly, sandy limestone (packstone) or irregularly cemented fine- to medium-grained sandstone of the undivided Portland Stone Formation.
Definition of Upper Boundary: In Dorset, the upper boundary is the facies change from massive-bedded, ooidal and bioclastic limestones, with abundant marine fossils, of the upper part of the Portland Stone Formation, to white-weathering calcareous mudstones, algal-laminated micrites, evaporites and dark, carbonaceous mudstones of the Lulworth Formation (basal Purbeck Group). Vale of Wardour, Wilts: the boundary is marked by the facies change from bioclastic limestone of the upper Portland Stone Formation, to dark grey calcareous mudstone with thin argillaceous limestones (Oakley Marl Member, Lulworth Formation); or unconformity overlain by glauconitic sand/sandstone (Lower Greensand Group) or grey mudstone (Gault Formation). Vale of Pewsey, Wilts: unconformity - sharp change up from calcareous mudstone to medium to coarse-grained ferruginous quartz sand (Lower Greensand Group) or dark grey sandy mudstone (Gault Formation).
Thickness: Dorset stratotype area: up to about 38m; Vale of Wardour: 20 to 35m where the Portland Stone Formation includes intervals previously considered as part of the Portland Sand Formation; Vale of Pewsey: up to 17m. English Channel (south of Hampshire-Dieppe High) up to 28m (Hamblin et al., 1992).
Geographical Limits: Outcrop onshore: South Dorset; Vale of Wardour and Vale of Pewsey, south Wiltshire. Possibly present in Weald and adjacent offshore (see Lithology). Offshore: Dorset sequence extends into English Channel south of the Hampshire-Dieppe High. Absent over the High, possibly present in offshore area adjacent to Weald (see Lithology).
Parent Unit: Portland Group (PL)
Previous Name(s): Portland Rock [Obsolete Name and Code: Use POST] (-1128)
Portland Limestone Formation [Obsolete Name and Code: Use POST] (-4270)
Portland Oolite [Obsolete Name and Code: Use POST] (-2982)
Portland Stone [Obsolete Code: Use POST] (PLST)
Alternative Name(s): none recorded or not applicable
Reference Section  Winspit and St Alban's Head, Isle of Purbeck, Dorset. Fully exposed in sea cliffs, 37 m thick, divided into Portland Chert and Portland Freestone members. Overlies Black Sandstones of Portland Sand Formation with sharp contact, and sharply overlain by the Mupe Member of the Lulworth Formation (Coe, 1996; Barton et al., in press). 
Type Area  South Dorset coast. 
Reference Section  Ringstead Bay, west of Holworth House, Dorset. Not at [762 813] as in Coe (1996) which is too far south. Fully exposed, totalling 18 m thick; from base of Lower Cherty Beds resting on Portland Clay of Portland Sand Formation to top of Portland Freestone Member (Beds 14 to 27 of Arkell, 1947, pp.111-113), overlain by Mupe Member of Lulworth Formation (Coe, 1996). 
Type Section  Clay Ope, West Weare Cliff, Isle of Portland, Dorset. Fully exposed in sea cliffs, 25 m thick, divided into Portland Chert and Portland Freestone members. Non-sequentially overlies Portland Clay of Portland Sand Formation, and sharply overlain by the Mupe Member of the Lulworth Formation (Coe, 1996; Barton et al., in press). 
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. 
Cope, J C W. in press. Bolonian-Portlandian Correlation Chart. in A correlation of Jurassic rocks in the British Isles: Middle and Upper Jurassic. Cope, J C W (editor). Special report of the Geological Society of London. 
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). 
Gallois, R. 1965. The Wealden district (4th edition). British Regional Geology. (London: HMSO for British Geological Survey.) 
Smith, W. 1816. Strata identified by organized fossils. (London.) 
Smith, W. 1817. Stratigraphical system of organized fossils (folding table). (London.) 
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. 
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
Webster, T, 1816. Geological observations on the Isle of Wight. 117-238 in Englefield, H. Description of the principal picturesque beauties, antiquities and geological phenomena of the Isle of Wight. [London: Payne and Foss.] 
Callomon, J H and Cope, J C W. 1995. The Jurassic geology of Dorset. 51-103 in Taylor, P D (Editor), Field Geology of the British Jurassic. (Bath: The Geological Society.) 
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. 
Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, and Woods, M A. 1999. The Wincanton district - a concise account of the geology. Memoir of the British Geological Survey, Sheet 297 (England and Wales). 
Hamblin, R J O, Crosby, A, Balson, P S, Jones, S M, Chadwick, R A, Penn, I E, and Arthur, M J. 1992. The geology of the English Channel. British Geological Survey United Kingdom Offshore Regional Report. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E282 E297 E298 E327 E328 E341 E342 E343