The BGS Lexicon of Named Rock Units — Result Details

Basalanhydrit Formation

Computer Code: BSHY Preferred Map Code: notEntered
Status Code: Full
Age range: Guadalupian Epoch (PUG) — Lopingian Epoch (PUL)
Lithological Description: In marginal, shelf sections, the Basalanhydrit Formation typically consists of white to pale grey, amorphous, microcrystalline anhydrite and argillaceous anhydrite with local intercalations of pale grey to pale brown microcrystalline dolomite and argillaceous dolomite (e.g. 49/26-4). Near the southern limit of the Basalanhydrit Formation, where it passes laterally into the Blakeney Formation, red mudstone is interbedded with anhydrite (e.g. 53/1-1). In more basinal sections, the Basalanhydrit Formation is represented by a thin unit of white, amorphous anhydrite and dolomitic anhydrite.
Definition of Lower Boundary: In shelf sections, the base of the Basalanhydrit Formation is generally marked by a downward change from white or pale grey anhydrite and dolomitic anhydrite to pale grey and pale brown dolomite and anhydritic dolomite (e.g. 49/26-4) of the Hauptdolomit Formation. In basinal sections, the anhydrite rests on dark grey, finely laminated, bituminous limestone or dolomite of the Hauptdolomit Formation (e.g. 49/21-2 and 44/21-1 see Cameron et al., 1992, fig. 35). On wireline logs, the base of the Basalanhydrit Formation normally coincides with a downward decrease in density. Commonly, a downward increase in velocity is also recorded (e.g. 49/26-4). Locally, the boundary is gradational due to the interbedded nature of the anhydrite/ dolomite sequence and here it is taken at the top of the dolomitedominated section (e.g. 48/30-4). Beyond the southern depositional limit of the Hauptdolomit Formation, the base of the Basalanhydrit Formation is taken at the downward change from white to grey anhydrite with interbedded red mudstone to pale brown dolomite of the Zechsteinkalk Formation (e.g. 48/30-1 and 53/1-1).
Definition of Upper Boundary: The top of the Basalanhydrit Formation is normally defined by a downward change from halite of the Stassfurt Halite Formation to white and pale grey anhydrite and dolomitic anhydrite. It is marked on wireline logs by a sharp downward increase in velocity and density. Locally, in the basin centre, the top is marked by a downward change from high-gamma polyhalite or interbedded anhydrite and polyhalite of the Stassfurt Halite Formation to low-gamma, amorphous, white anhydrite (e.g. 44/21-1 see Cameron et al., 1992, fig. 35 and 48/7b-4). In sections at the southern and western margins of the basin, where the Stassfurt Halite Formation is absent, the Basalanhydrit Formation is directly overlain by grey dolomite of the Plattendolomit Formation (e.g. 41/8-1).
Thickness: The Basalanhydrit Formation is best developed near the margins of the Zechstein Basin, where it is typically 10 to 55 m thick. It is often thin or absent from the central basin floor, where the Stassfurt Halite Formation rests directly upon the Hauptdolomit Formation (e.g. 44/11-1).
Geographical Limits: The Basalanhydrit Formation is best developed near the margins of the Zechstein Basin.
Parent Unit: Zechstein Group (ZG)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  North Sea well 49/21- 2: 2262.5-2267 m (7422-7437 ft) (Johnson et al., 1994). 
Reference Section  North Sea well 49/26- 4 (Rhys, 1974, table 2, fig. 3): 1772-1807 m (5813-5928 ft) (revised depths) (Johnson et al., 1994). 
Reference(s):
Johnson, H, Warrington, G and Stoker, S J. 1994. 6. Permian and Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Cameron, T D J, Crosby, A, Balson, P S, Jeffery, D H, Lott, G K, Bulat, J, and Harrison, D J. 1992. The geology of the southern North Sea. United Kingdom offshore regional report. (London: HMSO for the British Geological Survey.) 
Colter, V S, and Reed, G E. 1980. Zechstein 2 Fordon Evaporites of the Altwick No. 1 borehole, surrounding areas of NE England and the adjacent southern North Sea. In: Fuchtbauer, H, and Peryt, T M (eds.) The Zechstein Basin with emphasis on carbonate sequences. Contributions to Sedimentology 9, 115-129. 
NAM and RGD (Nederlandse Ardolie Maatschappij and Rijks Geologische Dienst) 1980. Stratigraphic nomenclature of the Netherlands. Transactions of the Royal Dutch Geological and Mining Society of Delft. 
Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. 
Smith, D B. 1980. The evolution of the English Zechstein basin. In: Fuchtbauer, H, and Peryt, T M (eds.) The Zechstein basin with emphasis on carbonate sequences. Contributions to Sedimentology 9, 7-34. 
Smith, D B, Brunstrom, R G W, Manning, D I, Simpson, S and Shotton, F W. 1974. A correlation of Permian rocks in the British Isles. Geological Society of London Special Report No. 5. 
Clark, D N. 1986. The distribution of porosity in Zechstein carbonates. In: Brooks, J, Goff, J C, and Van Hoorn, B (eds.) Habitat of Palaeozoic gas in NW Europe. Geological Society, London, Special Publication No. 23, 121-149. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable