The BGS Lexicon of Named Rock Units — Result Details

Westray Group

Computer Code: WESY Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Oligocene Epoch (GO) — Miocene Epoch (NM)
Lithological Description: The Westray Group is characterized by a relatively symmetrical basin fill, with no sharp differentiation into shelf and basin lithofacies. Two formations are recognized: the Skade Formation (Isaksen & Tonstad 1989), dominated by shelf sands, and the Lark Formation, dominated by basin mudstones. The succession shows an overall progradation, such that the shallow marine sandstones of the Skade Formation extend basinwards over mudstones of the Lark Formation. In proximal sections, a thin, basal unit of highly glauconitic siltstone is overlain by relatively thick succession of shelly, glauconitic sandstone. Both lithologies are referred to the Skade Formation. The argillaceous section is assigned to the Lark Formation where the glauconitic siltstone unit passes basinwards into a relatively thick unit of largely non-glauconitic mudstone.
Definition of Lower Boundary: The base of the Westray Group is marked by a downward change from brownish mudstones to dominantly green-grey mudstones of the underlying Stronsay Group.
Definition of Upper Boundary: The top of the Westray Group is marked by a downward change from predominantly grey and silty mudstones of the overlying Nordland Group to brownish mudstones.
Thickness: The Westray Group reaches thicknesses of over 800 m.
Geographical Limits: The Westray Group extends throughout the North Sea area (Knox & Holloway 1992; Lott & Knox 1994). Knox et al. (1997) noted that Westray Group sediments extend onto the Shetland Platform and are in direct continuity with sediments of equivalent age in the West of Shetland area. The geographical range of the Westray Group was therefore expanded to include the West of Shetland area.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
none recorded or not applicable
Reference(s):
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. 
Lott, G K and Knox, R W O'B. 1994. 7. Post-Triassic of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. [British Geological Survey, Nottingham]. 
Isaksen, D and Tonstad, K. 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No.5 
Knox, R W O'B, Holloway, S, Kirby, G A and Baily, H E. 1997. Stratigraphic nomenclature of the UK North West Margin. 2. Early Paleogene lithostratigraphy and sequence stratigraphy. British Geological Survey, Nottingham. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable