The BGS Lexicon of Named Rock Units — Result Details

Cook Formation

Computer Code: COOK Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Pliensbachian Age (JE) — Toarcian Age (JT)
Lithological Description: Typically, dominantly grey siltstones and silty mudstones with streaks of very fine-grained and well sorted sandstones at some levels, calcite cemented locally. Occasional thin beds of very fine- to coarse-grained, argillaceous, micaceous and chloritic sandstone occur locally (e.g. Well 3/4-8).
Definition of Lower Boundary: The lower boundary is typically defined by a downward change from siltstone (Cook Formation) to mudstone (Burton Formation) and a distinct down-section increase in gamma values and decrease in velocity. However, locally, where the Cook Formation is less silty or where the Burton Formation is more sandy, log signatures in the boundary interval are not clear.
Definition of Upper Boundary: The upper boundary is defined by a downward change from mudstones (Drake Formation) to siltstones with subordinate sandstones (Cook Formation) and a down-section decrease in gamma values and, generally, an increase in velocity.
Thickness: Up to c. 80 m, but may be absent on structural highs.
Geographical Limits: East Shetland Basin and Viking Graben (North Sea quadrants 2, 3, 210, and 211).
Parent Unit: Dunlin Group (DUNL)
Previous Name(s): Dunlin Siltstone Member (-5175)
Cook Sub-Unit (-5182)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea Well 211/29-3 between 2887 and 2950.5 m below KB (Deegan and Scull, 1977). 
Reference Section  North Sea Well 211/18a-24 between 3661.5 m and 3704.5 m (Richards et al., 1993). 
Reference Section  North Sea Well 211/13-7 between 3822 and 3865 m (Richards et al., 1993). 
Reference(s):
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. 
Bowen, J M. 1975. The Brent Oil-Field. In: Woodland, A W (ed.) Petroleum and the contineental shelf of North-West Europe, 353-361. [Heyden & Son, London] 
Richards, P C, Lott, G K, Johnson, H, Knox, R W O'B. and Riding, J B. 1993. 3. Jurassic of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G. Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable