The BGS Lexicon of Named Rock Units — Result Details

Orrin Formation

Computer Code: ORRN Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Pliensbachian Age (JE) — Toarcian Age (JT)
Lithological Description: Pale to dark grey, argillaceous siltstone, coarsening upwards into fine-grained, bioturbated, carbonaceous, kaolinitic sandstone and massive, fine to medium-grained (occasionally coarse), silica cemented sandstone with occasional cross-bedding. Gamma log character suggests that there are two major upward coarsening sandstone units overlain by thin, persistent, fossiliferous mudstone that is, in turn, capped by a major cross-bedded sandstone unit. Thin coals and rootlet horizons occur.
Definition of Lower Boundary: The lower boundary is taken at the base of the lower, upward-coarsening siltstone-sandstone unit. There is a minor, but distinct, downward increase in mud content at the boundary with Lady's Walk Formation, reflected in the downward increase in gamma values. In Well 12/26-1, where the Orrin Formation rests unconformably on either the Mains Formation or a remnant of the Lady's Walk Formation, there is a downward change from sandstone to mudstone and a sharp down-section increase in gamma values.
Definition of Upper Boundary: The upper boundary is marked by a downwards change from interbedded mudstones, sandstones and coal (Brora Coal Formation) to predominantly clean mudstones and a down-section decrease in gamma values and an increase in velocity. The Orrin Formation has a more uniform wireline-log response compared to the overlying Brora Coal Formation.
Thickness: Up to c. 60 m.
Geographical Limits: Inner Moray Firth, North Sea UK Quadrants 11, 12, 17, 18.
Parent Unit: Dunrobin Bay Group [See Also DRB] (DUNR)
Previous Name(s): Beatrice Formation (BETR)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 11/30-2 at 2085.5-2143 m depth below KB. Richards et al. (1993). 
Reference Section  North Sea well 12/21-2 at 2283-2339 m depth. Richards et al. (1993). 
Reference Section  North Sea well 11/25-1 at 3141-3189 m depth. Richards et al. (1993). 
Reference(s):
Andrews, I J and Brown, S. 1987. Stratigraphic evolution of the Jurassic, Moray Firth. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 785-795. [Graham & Trotman, London] 
Stevens, V. 1991. The Beatrice Field, Block 11/30a, UK North Sea. In: Abbotts, I L (ed.) United Kingdom oil and gas fields 25 years commemorative volume. Memoir of the Geological Society, London, 14, 245-252. 
Neves, R and Selley, R C. 1975. A review of the Jurassic rocks in north-east Scotland. In: Finstead, K.G. & Selley, R.C. (eds) Proceedings of the Jurassic Northern North Sea Symposium, Stavanger 28-30 September 1975, 5/1-5/21. 
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