The BGS Lexicon of Named Rock Units — Result Details

Purbeck Group

Computer Code: PB Preferred Map Code: PB
Status Code: Full
Age range: Tithonian Age (JI) — Berriasian Age (KR)
Lithological Description: Interbedded mudstones, limestones and evaporites of marginal freshwater, brackish and marine origin; detrital quartz occurs in parts. The group (Purbeck Beds), was traditionally divided into Lower, Middle and Upper units ("beds") with a key marker, the Cinder Bed placed within the middle part of the Middle Purbeck Beds.
Definition of Lower Boundary: The lower boundary is taken at the first occurrence of finely laminated, ostracod-rich limestones above the more massive, shelly limestones of the Portland Group.
Definition of Upper Boundary: The upper boundary is taken at the last occurrence of significant limestone before monotonous Wealdon Group sandy mudstones. Highest limestone is rich in Viviparus and Unio.
Thickness: 45 to 120m in south Dorset. 77 to 186m in the Weald. Succession incomplete in the Vale of Wardour.
Geographical Limits: Occurs in a number of sub-basins across north Europe. In England, occurs in south Dorset (between Weymouth and Swanage), in the Weald (largely in sub-crop), and as several inliers in the Vale of Wardour and Vale of Pewsey. Likely also to occur as inliers beneath cover across much of southern England. Occurs in the subcrop throughout southern Britain (Wessex Basin) as far west as Wiltshire and Dorset, and as far north as Berkshire, Oxforshire and Buckinghamshire. It outcrops within the core of the Weald and as outliers in Oxfordshire, Buckinghamshire and Bedfordshire.
Parent Unit: Not Applicable (-)
Previous Name(s): Purbeck Formation (-1114)
Purbeck Stone (-1115)
Purbeck Beds (-441)
Purbeck Strata (-4243)
Purbeck (-4830)
Purbeck Limestone Formation (-2229)
Purbeck Group (PB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Isle of Purbeck, Dorset (coastal cliffs, and quarries). 
Type Section  Cliff section in Durlston Bay, south of Swanage, Dorset. Clements, R G, 1993. 
Type Area  Thame to Aylesbury area. Horton et al. (1995, p.79-83). 
Reference(s):
Phillips, W, 1818. A selection of facts from the best authorities, arranged so as to form an outline of the geology of England and Wales. (London: W Phillips). 
Buckland, W. 1818. Order of superposition of strata in British Isles, 'In' Phillips, W. 1818. Outline of the geology of England and Wales. 
Lake, R D and Holliday, D W. 1978. Purbeck Beds of the Broadoak Borehole, Sussex. Report of the Institute of Geological Sciences, No.78/3, 1-28. 
Fitton, W H, 1836. Observations on some of the strata between the Chalk and the Oxford Oolite, in the south-east of England. Transactions of the Geological Society of London, Vol.4, 103-378, 379*-388*. 
Arkell, W J. 1947. The Geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain, Sheets 341, 342, 343 and parts of 327, 328 and 329 (England and Wales). 
Forbes, E. 1851. On the succession of strata and distribution of organic remains in the Dorsetshire Purbecks. Report of the British Association for 1850, 79-81. 
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). 
Damon, R, 1860. Handbook to the geology of Weymouth and the Island of Portland. [London: Edward Stanford.] 
Howitt, F. 1964. Stratigraphy and Structure of the Purbeck inliers of Sussex (England). Quarterly Journal of the Geological Society of London, Vol.120, 77-113. 
Lake, R D, Young, B, Wood, C J and Mortimore, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey, Sheet 319 (England and Wales). 
Clements, R G. 1993. Type-section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of the Dorset Natural History and Archaeological Society, Vol.114, 181-206. 
Arkell, W J. 1933. The Jurassic System in Great Britain. [Oxford: Clarendon Press.] 
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. 
Cope, J C W, Clements, R G and West, I M. 1969. The Purbeck Beds. A57-A64 in Torrens, H S (Editor), International Field Symposium on the British Jurassic. Excursion No.1 Guide for Dorset and south Somerset. (Keele University). 
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
Westhead, R K and Mather, A E. 1996. An updated lithostratigraphy of the Purbeck Limestone Group in the Dorset area. Proceedings of the Geologists' Association, Vol.107, 117-128. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E219 E220 E237 E252 E270 E271 E283 E284 E285 E286 E287 E288 E289 E297 E298 E299 E300 E301 E302 E303 E304 E305 E306 E316 E317 E318 E319 E320 E321 E327 E328 E331 E332 E333 E341 E342 E343