The BGS Lexicon of Named Rock Units — Result Details

Rannoch Formation

Computer Code: RANN Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Aalenian Age (JA) — Bajocian Age (JB)
Lithological Description: Northern East Shetland Basin: The formation comprises an upward coarsening succession from mudstone (informally the 'Rannoch Mudstone unit' of Richards et al., 1993) to very fine- or fine-grained sandstone. Southern East Shetland Basin: Upward coarsening sandstones (the 'Rannoch Mudstone unit' is absent). Sandstones are often parallel laminated and hummocky cross-stratified, micaceous, and carbonaceous along the laminations. Individual beds have sharp bases and may show an upward succession from laminated or hummocky cross-stratified sandstone to ripple laminated sandstone. Bioturbation is extensive. Locally calcite-cemented. The 'Ranoch Mudstone unit' comprises a heterolithic succession of interbedded mudstones, siltstones and sandstones. Mudstones and siltstones are dark grey, carbonaceous, micaceous and laminated. Low angle scours truncate laminations in places. Sandstones are thin, very fine-grained and micaceous. Bioturbation is locally intense. South-western East Shetland Basin: Fine-grained, locally medium-grained sandstone.
Definition of Lower Boundary: East Shetland Basin: In the southern part of the East Shetland Basin the boundary is placed at a downward change from very fine-grained, micaceous sandstone to coarser sandstones (Broom Formation). In the northern part of the East Shetland Basin the boundary is marked by a downward change from micaceous mudstones ('Rannoch Mudstone unit') to coarse sandstone (Broom Formation). In both areas there is a down-section decrease in gamma values. South-western part of the East Shetland Basin: downward transition from fine-grained- to medium-grained, micaceous, laminated sandstones (Rannoch Formation) to coarse grained sandstone (Broom Formation). This corresponds to the base of an interval of increased gamma-ray values. North eastern part of East Shetland Basin: micaceous mudstones ('Rannoch Mudstone unit') overlie poorly micaceous mudstones of the Drake Formation (Dunlin Group). This corresponds with a slight down-section decrease in gamma values and increase in velocity.
Definition of Upper Boundary: The boundary is defined by a sharp downward change from coarser, less micaceous, generally structureless sandstones (Etive Formation) to finer, micaceous, laminated sandstones. It coincides with a downward increase in gamma values and and often a downward increase in velocity.
Thickness: Up to c. 100 m.
Geographical Limits: East Shetland Basin and into the Viking Graben. North Sea quadrants 2, 3, 210, 211.
Parent Unit: Brent Group (BRNT)
Previous Name(s): Rannoch Shale (-5201)
Rannoch Sub-Unit (-5202)
Micaceous Sand (-5198)
Micaceous Sand Bed (-5199)
Rannoch Mudstone Unit (-5200)
Alternative Name(s): none recorded or not applicable
Type Section  North Sea Well 211/29-3 between 2783 and 2819 m below KB (Deegan and Scull, 1977). 
Reference Section  North Sea Well 211/13-7 between 3695.5 and 3798.5 m (Richards et al., 1993). 
Reference Section  North Sea Well 211/18-7 between 3311.5 and 3360 m (Richards et al., 1993). 
Reference Section  North Sea Well 211/27-10 between 3597.5 and 3620.5 m (Richards et al., 1993). 
Reference Section  North Sea Well 3/1-2 between 3783.5 and 3801 m (Richards et al., 1993). 
Giles, M R, Stevenson, S, Martin, S V, Cannon, S J C, Hamilton, P J, Marshall, J D and Samways, G M. 1992. The reservoir properties and diagenesis of the Brent Group: a regional perspective. In: Morton, A C, Haszeldine, R S, Giles, M R and Brown, S (eds.) Geology of the Brent Group. Special Publication of the Geological Society, London, No.61, 289-327. 
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