The BGS Lexicon of Named Rock Units — Result Details

Hidra Formation

Computer Code: HIDR Preferred Map Code: notEntered
Status Code: Full
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: The Hidra Formation consists of fine-grained chalky limestones with interbedded mudstones. The limestones are white to pale grey and occasionally pink, red brown and pale green and variably argillaceous. The mudstones are dark grey and reddish brown. Traces of pink waxy tuff were reported by Deegan and Scull (1977). Thin, pale grey, glauconitic and calcareous, quartzose sandstones are locally present in the Inner Moray Firth (Andrews et al., 1990) and the South Viking Graben (e.g. 16/28-7). Traces of chert are also present locally. The Hidra Formation becomes more argillaceous northwards, and in the North Viking Graben it grades laterally into the more argillaceous Svarte Formation.
Definition of Lower Boundary: The base of the Hidra Formation is normally defined by a subtle downward change from interbedded pale to dark grey and pink limestones, argillaceous chalky limestones and calcareous mudstones to similar, but generally less calcareous strata of the Rødby Formation. It is usually marked by the base of a thin, but widespread limestone bed and corresponds to a slight downward increase in gamma-ray values and downward decrease in velocity (e.g. 22/1- 2A). Locally, however, wireline-log signatures suggest that the basal beds of the Hidra Formation are absent (e.g. 14/20-6A). As noted by Crittenden et al. (1991), care is needed in identifying the base of the Hidra Formation, because the wireline-log character of its lower part is closely similar to that of the upper Rødby Formation. Consequently, detailed biostratigraphic analysis is required in some sections before the boundary can be accurately placed. On intrabasinal highs, the Hidra Formation locally rests unconformably on the Carrack Formation (e.g. 16/28-7) or pre-Cretaceous rocks.
Definition of Upper Boundary: The top of the Hidra Formation is normally marked by a sharp downward change from the basal black mudstones of the Herring Formation (Black Band) to white or pale grey, chalky limestones. It is marked on wireline logs by a sharp downward decrease in gamma-ray values and an increase in velocity. Locally, where the Black Band is absent, limestones of the Herring Formation directly overlie the Hidra Formation (e.g. 21/1-7ST, see fig. 5 in Burnhill and Ramsay, 1981 and 14/26-1).
Thickness: The Hidra Formation is typically about 30-70 m thick, but reaches up to about 160 m in the Central Graben. Outside the graben areas, it is usually less than 35 m thick.
Geographical Limits: The Hidra Formation occurs throughout much of the Central North Sea and South Viking Graben. It is absent, however, over some contemporary structural highs (30/1c-2A). Whilst seismic data suggest onlap onto these structures (Gatliff et al., 1994), condensed sections are also present and the absence of the Hidra Formation may be the result of local intra-Cretaceous erosion, as on the Glenn Horst (see fig.5 in Burnhill and Ramsay, 1981).
Parent Unit: Chalk Group (CK)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Norwegian North Sea well N1/3-1 (Deegan and Scull, 1977, p.26, fig.30): 4371-4441 m (14340-14570 ft) below KB (metric conversion slightly amended). 
Reference Section  North Sea well 15/22- 4: 3115-3266.5 m (10219-10717 ft) (Johnson and Lott, 1993). 
Reference Section  North Sea well 22/01- 2A: 3738.5-3782.5 m (12265-12410 ft) (Johnson and Lott, 1993). 
Reference Section  North Sea well 29/9a- 1: 3791-3907 m (12438-12819 ft) (Johnson and Lott, 1993). 
Reference(s):
Lott, G K and Knox, R W O'B. 1994. 7. Post-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. 
Ritchie, J D, Gatliff, R W and Riding, J, 1996. 1. Pre - Tertiary lithostratigraphy. In: Stratigraphic Nomenclature of the UK North West Margin. British Geological Survey, Nottingham. 
Johnson, H and Lott, G K. 1993. 2. Cretaceous 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. 
Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) 
Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. 
Andrews, I J, Long, D, Richards, P C, Thomson, A R, Brown, S, Chesher, J A, and McCormac, M. 1990. The geology of the Moray Firth. (London: British Geological Survey.) 
Burnhill, T J, and Ramsay, W V. 1981. Mid-Cretaceous palaeontology and stratigraphy, Central North Sea. In: Illing, L V, and Hobson, G D (eds) Petroleum geology of the continental shelf of North-West Europe, 245-254. Heyden and Son, London. 
Crittenden, S, Cole, J, and Harlow, C. 1991. The early to 'middle' Cretaceous lithostratigraphy of the Central North Sea (UK Sector). Journal of Petroleum Geology, 14, 387-416. 
Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. 
Gatliff, R W, Richards, P C, Smith, K, Graham, C C, McCormac, M, Smith, N J P, Long, D, Cameron, T D J, Evans, D, Stevenson, A G, Bulat, J, and Ritchie, J D. 1994. United Kingdom offshore regional report: the geology of the central North Sea. (London: HMSO for the British Geological Survey.) 
Nygaard, E, Lieberkind, K, and Frykman, P. 1983. Sedimentology and reservoir parameters of the Chalk Group in the Danish central graben. Geologie Minjbouw, 62, 177-190. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable