The BGS Lexicon of Named Rock Units — Result Details

Portland Formation (Wiltshire, Oxfordshire And Buckinghamshire) [Use Not Recommended In BGS: Prefer PL]

Computer Code: PLND Preferred Map Code: Pl
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: Fossiliferous marine limestones, calcareous and commonly glauconitic, sandstones and sands.
Definition of Lower Boundary: Conglomeratic and glauconitic coarse-grained sand, sandstone and limestone (Upper Lydite Bed) resting disconformably on mudstones, siltstones or fine-grained argillaceous sands and/ sandstones of the Kimmeridge Clay Formation.
Definition of Upper Boundary: Upwards change from fully marine limestones with large molluscan fossils to lagoonal fine-grained limestones and marls of the Purbeck Formation, the base of which is commonly marked either by laminated, ostracod-bearing 'Pendle' or oyster-bearing marl, or by a sharp nonsequential junction overlain by generally non-calcareous and commonly ferruginous sands and sandstones of the Lower Greensand or Whitchurch Sand Formation, or by calcareous mudstones of the Gault Formation.
Thickness: Up to 13 m thick.
Geographical Limits: Outliers from Swindon to Aylesbury in north Wiltshire, Oxfordshire and Buckinghamshire, and possibly an outlier in the Vale of Pewsey near Devizes, though this is currently rather poorly known (Wimbledon, 1980). A separate description applies to the Portland of the Vale of Wardour in south Wiltshire and Dorset.
Parent Unit: Portland Group (PL)
Previous Name(s): Portland Beds (-948)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  BGS Hartwell Borehole, 5.16 - 17.95 m. Cox, Gallois and Sumbler, 1994; Horton et al., 1995. 
Reference Section  Bugle Pit, Hartwell. Horton et al., 1995. 
Reference Section  Swindon Old Town Railway Cutting. Wimbledon, 1976 
Reference Section  Town Gardens Quarry, Swindon. Wimbledon, 1976 
Type Area  Thame to Aylesbury area. 1:50 000 Sheet 237. 
Reference(s):
Horton, A, Sumbler, M G, Cox, B M and Ambrose, K, 1995. Geology of the country around Thame. Memoir of the British Geological Survey, Sheet 237 (England and Wales). 
Cox, B M, Gallois, R W and Sumbler, M G. 1994. The stratigraphy of the BGS Hartwell Borehole, near Aylesbury, Buckinghamshire. Proceedings of the Geologists' Association, Vol. 105, 209-224. 
Cox, B M, 1996, Portland and Purbeck Formations. In: British Regional Geology: London and the Thames Valley. Fourth Edition. Sumbler, M G, (Editor), 58-60. (London HMSO.) 
Wimbledon, W A and Cope, J C W. 1978. The ammonite faunas of the English Portland Beds and the zones of the Portlandian Stage. Journal of the Geological Society of London, Vol.135, 183-190. 
Wimbledon, W A. 1976. The Portland Beds (Upper Jurassic) of Wiltshire. Wiltshire Archaeological and Natural History Magazine, Natural History, Vol.71, 3-11. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E219 E220 E237 E252 E282