The BGS Lexicon of Named Rock Units — Result Details

Portland Sand Formation

Computer Code: POSA Preferred Map Code: PoSa
Status Code: Full
Age range: Tithonian Age (JI) — Tithonian Age (JI)
Lithological Description: The lower part of the formation comprises mixed siliciclastic and carbonate sediments, including bioturbated clay-rich siltstones, fine-grained sandstones and silty mudstones with variable amounts of calcite and dolomite. Some beds are shell-rich, especially with oysters. The upper part of the formation comprises purer carbonate lithologies, including finely crystalline, bioturbated dolomite (usually described as sandstone in lithological accounts). A number of local informal stratigraphic divisions are named (Barton et al., in press), but no formal members are recognised. At depth in the Weald, the formation is much more clayey than in Dorset and difficult to distinguish from the Kimmeridge Clay Formation below (Gallois, 1965). The continuation of the formation offshore into the Channel is doubtful (Hamblin et al., 1992).
Definition of Lower Boundary: The lower boundary is recognised by the upward change from the silicate mudstone to finely sandy siltstone of the Kimmeridge Clay Formation to the coarser siliciclastic clayey siltstone or sandstone and limestone facies of the basal Portland Sand Formation. In most parts of south Dorset this change in facies appears to be gradational.
Definition of Upper Boundary: On the Isle of Portland (Dorset), the upper boundary is marked by the facies change from mudstone ('Portland Clay') to lime mud-rich limestone with common chert nodules, here coincident with a slight unconformity and a shell-rich horizon (Basal Shell Bed) at the base of the Portland Stone Formation. On the Isle of Purbeck (Dorset), the boundary is marked by the conformable upward facies change from finely crystalline dolomitic limestone (Black Sandstones) to chert-bearing bioturbated calcilutite (Lower Cherty Beds) of the Portland Chert Member of the Portland Stone Formation.
Thickness: Up to about 39m (Isle of Purbeck), 30m (Isle of Portland), 8.5m (Ringstead Bay).
Geographical Limits: South Dorset. Is overstepped northwards by Cretaceous strata, but is equivalent to the Wardour Formation and basal part of the Portland Stone Formation of south Wiltshire. The continuation of the formation offshore into the Channel is doubtful (Hamblin et al., 1992).
Parent Unit: Portland Group (PL)
Previous Name(s): Portland Sand Formation [Obsolete Code: Use POSA] (PLS)
Portland Sand (-2373)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  South Dorset coast, Portland Bill to Durlston Head. 
Type Section  Clay Ope, West Weare Cliff, Isle of Portland, Dorset. Fully exposed in cliff section, about 30m thick, six informal named units: Black Nore Sandstone, Upper Black Nore Beds, Exogyra Bed, Cast Beds, West Weare Sandstones and Portland Clay (ascending order). Overlies 'Lower Black Nore Beds' of Kimmeridge Clay Formation with sharp contact, and overlain non-sequentially by 'Basal Shell Bed' of Portland Chert Member of Portland Stone Formation. Arkell, 1947; Coe, 1996; Barton et al. in press. 
Reference Section  Ringstead Bay, west of Holworth House, Dorset. Not at 762 813 as in Coe (1996) which is too far south. Upper 8.5m exposed, from within Upper Black Nore Beds to top of Portland Clay (Beds 1 to 13 of Arkell, 1947, p.113), overlain by Lower Cherty Beds of Portland Stone Formation. Coe, 1996. 
Reference Section  Winspit to St Alban's Head, Isle of Purbeck, Dorset. Sea cliff section, about 39m thick, six informal named units: Massive Bed, Emmit Hill Marls, White Cementstone, St Alban's Head Marls, Parallel Bands and Black Sandstones (in ascending order). Overlies 'Upper Hounstout Silt' of Kimmeridge Clay Formation with sharp contact, and overlain non-sequentially by 'Lower Cherty Beds' of Portland Chert Member of Portland Stone Formation. Arkell, 1947; Coe, 1996; Barton et al. in press. 
Reference(s):
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. 
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. 
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. 
Buckland, W, and de la Beche, H T. 1835. On the geology of the neighbourhood of Weymouth and the adjacent parts of the coast of Dorset. Transactions of the Geological Society, Vol. 4, 1-46. 
Gallois, R. 1965. The Wealden district (4th edition). British Regional Geology. (London: HMSO for British Geological Survey.) 
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. 
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.) 
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:
E327 E328 E341 E342 E343