The BGS Lexicon of Named Rock Units — Result Details

Mains Formation

Computer Code: MANS Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Hettangian Age (JH) — Sinemurian Age (JS)
Lithological Description: The formation comprises interbedded white to pale grey sandstone, muddy siltstone and grey to black organic mudstone. Sandstones are upward fining from coarse to fine-grained and locally very calcareous. They are cross-bedded and current-ripple bedded. The interbedded mudstone is fossiliferous, bioturbated and plant and root horizons occur. The proportion of sandstone to mudstone varies from borehole to borehole. Cores and gamma-ray logs indicate that the formation is composed of two upward fining units, each about 12 m thick.
Definition of Lower Boundary: The lower boundary is defined by a downwards passage from sandstone with interbedded mudstone to varicoloured mudstone and thin sandstone (Golspie Formation). The boundary may be sharp (e.g. Well 11/25-1) or gradational (e.g.Well 12/21-2). There is usually a down-section increase in average gamma values and a decrease in average velocity.
Definition of Upper Boundary: The upper boundary is marked by a downward change from dark, organic-rich, fossiliferous mudstone (Lady's Walk Formation) to sandstone with mudstone interbeds. There is a sharp down-section decrease in average gamma values and an increase in velocity. Where the Lady's Walk Formation is absent (e.g. Well 12/26-1) the boundary is placed at a downward change from upward-coarsening sandstones (Orrin Formation) to sandstones with interbedded mudstone.
Thickness: Up to 60 m.
Geographical Limits: Inner Moray Firth Basin, North Sea UK Quadrants 11, 12, 17, 18.
Parent Unit: Dunrobin Bay Group [See Also DRB] (DUNR)
Previous Name(s): Grant Sand Member (-5109)
White Sandstone Unit (-5110)
Alternative Name(s): none recorded or not applicable
Type Section  North Sea well 11/25-1 at 3210.5-3250 m depth below KB. Richards et al. (1993). 
Reference Section  North Sea well 12/21-2 at 2363.5-2417 m depth. Richards et al. (1993). 
Reference Section  North Sea well 12/27-1 at 1132-1159 m depth. Richards et al. (1993). 
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. 
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] 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable