The BGS Lexicon of Named Rock Units — Result Details

Yoredale Formation [Offshore Areas]

Computer Code: YOF Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Asbian Substage (CR) — Namurian Stage (CN)
Lithological Description: The Yoredale Formation is characterized by marine limestones and mudstones occurring within a dominantly deltaic association of mudstones, siltstones, sandstones, and occasional thin coal seams. The sandstone beds are white, pale grey, or pale brown, mostly less than 15 m thick, and constitute between 10% and 30% of the formation. Sand grain size is mainly very fine or fine, but the thickest beds are often coarser grained and are moderately or poorly sorted. Some sand beds are argillaceous; others contain abundant carbonaceous detritus. Minor pale grey siltstones occur within the argillaceous sections, but such sections tend to be dominated by dark grey and black carbonaceous mudstones, and are up to 30 m thick. Mudstones in the uppermost part of the formation have been affected by secondary reddening where they subcrop the base-Permian unconformity. The limestone beds are generally thin but exceptionally are up to 15 m thick, and comprise between 1% and 10% of the Yoredale Formation in different wells. The limestones are mainly microcrystalline or cryptocrystalline, white, pale grey, or brownish grey, and are argillaceous. Many limestone beds are fossiliferous; others are black, foetid, and unfossiliferous. Coal seams constitute less than 1% of the formation, and are generally less than 1 m thick. In northern England, a typical Yoredale cycle comprises a basal marine limestone overlain by marine shales and a variety of delta-related sedimentary facies, often including seat earths and coals (Elliott 1975). Such cycles are between 15 m and 90 m thick in Northumberland (Leeder et al. 1989). Yoredale cycles can also be identified in the North Sea wells (e.g. 41/24a-2) by their gross upward coarsening log responses, and are between 30 m and 130 m thick.
Definition of Lower Boundary: The base of the lowest high-velocity marine limestone bed defines the boundary between the Yoredale and Scremerston formations in wells 39/7-1 and 43/2-1. However, the downward transition from Yoredale facies is gradational, with prominent coal seams occurring for up to 65 m above the boundary in well 43/2-1. The base of the Yoredale Formation has not been penetrated in other North Sea wells.
Definition of Upper Boundary: The top of the highest marine limestone bed defines the boundary between the Yoredale Formation and Millstone Grit Formation in wells 39/2-1 and 39/7-1, and between the Yoredale Formation and Bowland Shale Formation in well 41/24a-2. In well 39/7-1, the boundary occurs within a secondarily reddened section, and the highest limestone bed marks a sharp downward transition from large scale (50 < 100 m) to relatively small scale upward-coarsening cyclicity. The Yoredale Formation is unconformably overlain by Lower Permian desert sediments or Upper Permian marine and evaporitic sediments in all other wells.
Thickness: 420 m thick in well 39/7-1 and more than 704 m thick in well 41/24a-2.
Geographical Limits: The Yoredale Formation is likely to be widespread in the UK sector between 54°N and 55°N. An outlier has been proved at the northeastern flank of the Mid North Sea High, and the formation may also be distributed patchily across western crestal areas of the High. The formation is 420 m thick in well 39/7-1, and more than 704 m thick in well 41/24a-2. In northern England, equivalent sediments are 1219 m thick at Seal Sands in the eastern Stainmore Trough (Dunham & Wilson 1985), and 827 m thick at Harton on the southern margin of the Northumberland Trough (Ridd et al. 1970).
Parent Unit: Farne Group [Offshore] (FRNE)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  North Sea well 39/7-1: 3076.5-3496 m (10094-11470 ft). 
Reference Section  North Sea well 42/10a-1: 2528-2998 m TD (8294-9836 ft TD). 
Reference Section  North Sea well 43/2-1: 2683.5-2984 m (8804-9790ft). 
Type Section  North Sea well 41/24a-2: 2501-3205 m TD (8205-10515 ft TD) below KB. 
Reference(s):
Cameron, T D J. 1993. Carboniferous and Devonian of the Southern North Sea. In: Knox, R W O'B & Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. (British Geological Survey, Nottingham). 
Elliott, T. 1975. The sedimentary history of a delta lobe from a Yoredale (Carboniferous) cyclothem. Proceedings of the Yorkshire Geological Society 40, 505-536. 
Leeder, M R, Fairhead, D, Lee, A, Stuart, G, Clemmey, H, El-Haddaheh, B and Green, C. 1989. Sedimentary and tectonic evolution of the Northumberland Basin. Proceedings of the Yorkshire Geological Society 47, 207-223. 
Ridd, M F, Walker, D B and Jones, J M. 1970. A deep borehole at Harton on the margin of the Northumbrian Trough. Proceedings of the Yorkshire Geological Society 38, 75-103. 
Dunham, K C, and Wilson, A A. 1985. Geology of the Northern Pennine Orefield. Volume 2 Stainmore to Craven. Economic Memoir of the British Geological Survey, Sheets 40, 41 and 50, and parts of Sheets 31, 32, 51, 60 and 61(England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable