The BGS Lexicon of Named Rock Units — Result Details

Portland Chert Member

Computer Code: POCH Preferred Map Code: PoCh
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: Buff-coloured lime-packstones and lime-wackestones with conspicuous black nodular chert, and locally on the Isle of Purbeck, horizons of tabular chert. Correlative shell-rich intervals occur at the base of the member on the Isle of Portland (Basal Shell Bed) and in the middle of the member on the Isle of Purbeck (Bed J and J'), and consequently the base of the member on Portland is an unconformity. Ooids appear towards the top of the member.
Definition of Lower Boundary: On the Isle of Purbeck, the lower boundary is the conformable upward facies change from fine-grained dolomitic limestone of the underlying Portland Sand Formation, to the chert-bearing wackestones and packstones of the Portland Chert Member. On the Isle of Portland, the lower boundary is an unconformity, underlain by dolomitic mudstone of the topmost Portland Sand Formation and overlain by a shell-rich limestone of the basal Portland Chert Member.
Definition of Upper Boundary: The upper boundary of the Portland Chert Member is the upward facies change from chert-bearing lime-wackestones and packstones to massive ooid-grainstones of the Portland Freestone Member. The facies change is sharp on the Isle of Portland, and identified as an unconformity by Coe (1996). On the Isle of Purbeck the facies change is conformable, the correlative surface of the unconformity seen on the Isle of Portland occurring some distance above the base of the Portland Freestone Member (Coe, 1996).
Thickness: 22.6 m on the Isle of Purbeck; 17.7 m on the Isle of Portland (Wimbledon, 1980)
Geographical Limits: South Dorset
Parent Unit: Portland Stone Formation (POST)
Previous Name(s): Cherty Beds (-1127)
Cherty Series (CHTS)
Basal Shell Bed (-5242)
Dungy Head And Dancing Ledge Members (Isle Of Purbeck) (-5243)
Portland Cherty Member (-5244)
Alternative Name(s): none recorded or not applicable
Type Area  Winspit to St Alban's Head, Isle of Purbeck, Dorset: member fully exposed about 20 to 23 m thick in vertical sea cliffs, Upper Cherty Beds on Bed J and J' on Lower Cherty Beds Coe (1996); Barton et al. (2011) 
Type Section  Clay Ope, West Weare Cliff, Isle of Portland, Dorset: member fully exposed 15 to 17 m thick in vertical sea cliffs, Cherty Beds on Basal Shell Bed Coe (1996); Barton et al. (2011) 
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. 
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. 
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