The BGS Lexicon of Named Rock Units — Result Details

Forties Sandstone Member

Computer Code: FORS Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Paleocene Epoch (GP) — Paleocene Epoch (GP)
Lithological Description: The Forties Sandstone Member consists of fine to coarse sandstone interbedded with medium to dark grey siltstone and mudstone. The sandstone is moderately to poorly sorted, and locally pebbly; beds of muddy, extremely poorly sorted, pebbly sandstone are also encountered. The pebbles include intraformational mudstone clasts of Sele and Lista type. Primary sedimentary structures include planar lamination and sporadic small-scale cross-lamination, but much of the sandstone is apparently homogeneous or displays soft- sediment deformation, including dish structure, convolute lamination, microfaulting and sandstone dykes. Sandstone bed thicknesses are highly variable: distal facies display superimposition of successive units that rarely exceed two metres in thickness, whereas proximal facies often include units of 20 m or more. The interbedded mudstones and siltstones are generally carbonaceous, micaceous, poorly sorted and crudely laminated. Thin beds of finer grained, well-laminated mudstone also occur. In the southern Central Graben, beds of reddish or greenish, often slumped, mudstone are present.
Definition of Lower Boundary: Where mudstone is present at the base of the Sele Formation, the base of the Forties Sandstone Member is taken at a distinct downward increase in the proportion of sandstone, usually reflected in both the gamma-ray and sonic logs. In some sections, the basal sandstone bed contains intraformational mudstone pebbles. Where the Forties Sandstone Member rests directly on the Mey Sandstone Member (e.g. 22/02-2), it may be difficult to locate the boundary without the aid of biostratigraphic data (Bathysiphon acme, S. spectabilis acme).
Definition of Upper Boundary: The top of the Forties Sandstone member is taken at a downward change from mudstone to sandstone, reflected on wireline logs by a sharp upward increase in gamma values and decrease in sonic velocity. The sandstone is commonly overlain by a thin unit of low-gamma mudstone (e.g. 22/10a-4) that represents the uppermost part of Sele unit 1 (refer to Sele Formation).
Thickness: It reaches 200 m along the NW-SE depositional axis of the Central Graben. (Stewart 1987, fig.13).
Geographical Limits: The Forties Sandstone Member is restricted to the Central Graben. The northwestern limit is defined arbitrarily by the basinward limit of the Dornoch Formation (refer to Moray Group overview). To the south and east, the Forties Sandstone Member pinches out within the laminated mudstone of Sele unit 1.
Parent Unit: Sele Formation (SELE)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  North Sea well 21/10-1 (Deegan & Scull 1977, p.31, fig.36): 2131.5-2314 m (6993-7592 ft) below KB (revised depths). 
Reference Section  North Sea well 21/10-2: 2217-2479 m (7274-8133 ft). 
Reference Section  North Sea well 22/8a-2: 2617.5-2716 m (8588-8910 ft). 
Reference Section  North Sea well 22/10a-4: 2567.5-2609.5 m (8423-8562 ft). 
Reference(s):
Stewart, I J. 1987. A revised stratigraphic interpretation of the Early Palaeogene of the Early Palaeogene of the central North Sea. In: Brooks, J and Glennie, K W (eds.) Petroleum geology of North West Europe, 557-576.Graham & Trotman, London. 
Knox, R W O'B and Holloway, S. 1992. 1. Paleogene 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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable