The BGS Lexicon of Named Rock Units — Result Details

Offa Formation

Computer Code: OFFA Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Sinemurian Age (JS) — Pliensbachian Age (JE)
Lithological Description: The Offa Formation consists of a rhythmically interbedded sequence of dark grey mudstones with moderately thick beds of argillaceous limestone and, locally, thin ferruginous developments.
Definition of Lower Boundary: The lower boundary is characterised by a sharp downward change from dark grey mudstones to paler, harder, argillaceous limestones (Penda Formation), together with a sharp down-section increase in velocity and slight decrease in gamma values.
Definition of Upper Boundary: The upper boundary is marked by a downward change from hard, silty and sandy limestones (Ida Formation) to mudstones. This corresponds to a gradual or weak increase of gamma-ray values, but a marked decrease in velocity.
Thickness: Up to c. 190 m.
Geographical Limits: Southern North Sea Basin (quadrants 41-43, 46-49, 51-53).
Parent Unit: Lias Group (LI)
Previous Name(s): Lias Group (LI)
Alternative Name(s): Charmouth Mudstone Formation
Redcar Mudstone Formation
Stratotypes:
Type Section  North Sea Well 47/14a-9 between 1108 and 1169 m (Lott and Knox, 1994). 
Reference Section  North Sea Well 42/24-1 between 559 and 753 m (Lott and Knox, 1994). 
Reference Section  North Sea Well 48/11b-4 between 1242.5 and 1412.5 m (Lott and Knox, 1994). 
Reference Section  North Sea Well 49/12a-7 between 1837 and 1899 m (Lott and Knox, 1994). 
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]. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable