The BGS Lexicon of Named Rock Units — Result Details

Stroma Member

Computer Code: STRM Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Oxfordian Age (JO) — Oxfordian Age (JO)
Lithological Description: The Stroma Member comprises predominantly interbedded sandstones, carbonaceous silty mudstones and coals. Sandstones are subarkosic (Maher and Harker, 1987). Reworked volcaniclastics are locally present east of the Piper Field (e.g. North Sea Blocks 15/17 and 15/18). Six lithofacies have been described (Harker et al., 1993): 1. Laminated silty mudstones with a thin, poorly sorted, basal lag sandstone. 2. Thinly interbedded sandstones and silty mudstones with abundant plant remains. 3. Fine-grained, ripple and planar bedded sandstones. 4. Upward-coarsening, cross-bedded and massive sandstones. 5. Upward fining, cross-bedded sandstones with gravel lag bases and common plant rootlets in the upper part. 6. Silty coals grading to carbonaceous silty mudstones. In the Claymore area these facies are arranged in upward coarsening cycles, 3-10 m in thickness.
Definition of Lower Boundary: The boundary is variable. In Block 15/21 (in the type section) the boundary is defined by a downward change from mudstone to tuffs and lava flows (Rattray Volcanic Member). Elsewhere in the Witch Ground Graben, coals sandstones and mudstones (Stroma Member) rest on tuffaceous mudstones and sandstones (Pentland Formation), grey-green and brown mudstones and siltstones (Skagerrak Formation) or red-brown mudstones (Smith Bank Formation). Where the Stroma Member overlies paralic sediments of the Pentland Formation, the boundary cannot be defined with certainty. In the South Halibut Basin, interbedded sandstones, mudstones and coals (Stroma Member) rest on sandstones (Lossiehead Formation), reddish brown and grey Triassic mudstones (Smiths Bank Formation) or, locally, Permian carbonates and evaporites.
Definition of Upper Boundary: In the type section and in the South Halibut Basin, the top is defined by a sharp, down-section transition from silty mudstones (Heather Formation) to coal. There is a sharp down-section decrease in gamma ray values, a decrease in velocity and an increase in resistivity. In the Witch Ground Graben, the top of the member is marked by a down-section transistion from mudstones (Heather Formation) to interbedded sandstones, mudstones and coal, accompanied by a downward decrease in gamma-ray values. Locally thin, glauconitic, marine sandstones (Pibroch Member, Piper Formation) rest on carbonaceous sandstones (Stroma Member).
Thickness: Up to c. 40 m.
Geographical Limits: Witch Ground Graben and South Halibut Basin (North Sea Quadrants 13-15, 20 and 21). The precise limits are uncertain as similar, but older sediments of the Pentland Formation may prove to belong to the Stroma Member.
Parent Unit: Pentland Formation (PNTL)
Previous Name(s): Skene Member (-5143)
Coal Marker Member (-5145)
Coal Unit (-5146)
Paralic Unit (-5147)
Pentland Formation (PNTL)
Piper Formation (PIPR)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea Well 15/21a-15 between 3584.5 and 3607 m below KB (Richards et al., 1993). 
Reference Section  North Sea Well 14/19-2 between 2745.5 and 2770.5 m (Richards et al., 1993). 
Reference Section  North Sea Well 15/17-9 between 4144 and 4160.5 m (Richards et al., 1993). 
Reference Section  North Sea Well 20/2-6 between 3995.5 and 4035 m (Richards et al., 1993). 
Reference(s):
Boldly, S A R and Brearly, S. 1990. Timing, nature and sedimentary result of Jurassic tectonism in the Outer Moray Firth. In: Hardman, R F P and Brooks, J (eds.) Tectonic events responsible for Britain's oil and gas reserves. Special Publication of the Geological Society, London, No.55, 259-279. 
Harker, S D, Gustav, S H and Riley, L A. 1987. Triassic to Cenomanian stratigraphy of ther Witch Ground Graben. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 809-818. [Graham & Trotman, London] 
Harker, S D, Mantel, K A, Morton, D J and Riley, L A. 1993. The stratigraphy of Oxfordian-Kimmeridgian (Late Jurassic) reservoir sandstones in the Witch Ground Graben, UK North Sea. American Association of Petroleum Geologists Bulletin, 77, 1693-1709. 
O'Driscoll, D, Hindle, A D and Long, D C. 1990. The structural controls on Upper Jurassic and Lower Cretaceous reservoir sandstones in the Witch Ground Graben, UK North Sea. In: Hardman, R F P and Brooks, J (eds.) Tectonic events responsible for Britain's oil and gas reserves. Special Publication of the Geological Society, London, No.55, 191-205. 
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 (eds.). Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham] viii + 219 pp. 
Coward, R N, Clark, N M and Pinnock, S J. 1991. The Tartan Field, Block 15/16, 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, 377-384. 
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. 
Maher, C E and Harker, S D. 1987. Claymore OIlfield. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 835-845. [Graham & Trotman, London] 
Turner, C C, Richards, P C, Swallow, J L and Grimshaw, S P. 1984. Upper Jurassic stratigraphy and sedimentary facies in the Central Outer Moray Firth Basin, North Sea. Journal of Marine and Petroleum Geology, 1, 105-117. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable