The BGS Lexicon of Named Rock Units — Result Details

Peak Mudstone Member

Computer Code: PMDS Preferred Map Code: PM
Status Code: Full
Age range: Toarcian Age (JT) — Toarcian Age (JT)
Lithological Description: Mudstone, dark grey, micaceous and silty, fining upwards, with red-weathering sideritic concretions and layers of phosphate nodules, many containing the bivalve Meleagrinella substriata.
Definition of Lower Boundary: Conformable: sharp change up from grey shaly mudstone with common layers of calcareous concretions (Alum Shale Member) into dark grey micaceous and silty mudstone with red-weathering sideritic concretions: Bed 44b on Bed 44a at Blea Wyke (bed numbers of Dean, 1954).
Definition of Upper Boundary: Conformable: sharp change up from dark grey mudstone into muddy siltstone (Fox Cliff Siltstone Member), taken at base of semi-continuous sideritic mudstone bed (Bed 57 of Dean, 1954) in Peak Trough, or into micaceous siltstone (Yew Grain Member of Blea Wyke Sandstone Formation of Young, 1994) in the central North York Moors (see Supplementary Information); or unconformably overlain by ferruginous, pebbly sandstone of the Dogger Formation (Middle Jurassic).
Thickness: 12.6m at type section.
Geographical Limits: Cleveland Basin: complete in Peak Trough and presence inferred further west in Rosedale, Farndale, Fryup, Glaisdale, Westerdale and Danby Dale area.
Parent Unit: Whitby Mudstone Formation (WHM)
Previous Name(s): Peak Shales (-3616)
Striatulum Shales (Lower Part) (-3617)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Fully exposed at the foot of Fox Cliff, Blea Wyke, Ravenscar, North Yorkshire coast, where it is 12.6m thick, Beds 44b to 56 of Dean (1954). Knox, 1984; Simms et al., 2004, pp 271-281. 
Reference(s):
Knox, R W O B. 1984. Lithostratigraphy and depositional history of the late Toarcian sequence at Ravenscar, Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 45, 99-108. 
Buckman, S S. 1915. A palaeontological classification of Jurassic rocks of the Whitby district; with a zonal table of Lias ammonites. The geology of the country between Whitby and Scarborough. Memoir of the Geological Survey of the United Kingdom, (2nd edition). Fox-Strangways, C, and Barrow, G (editors). 
Simms, M J, Chidlaw, N, Morton, N, and Page, K N. 2004. British Lower Jurassic Stratigraphy. Geological Conservation Review Series. No. 30. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.) 
Dean, W T. 1954. Notes on part of the Upper Lias succession at Blea Wyke, Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 29, 161-179. 
Young, T P. 1994. The Blea Wyke Sandstone Formation (Jurassic, Toarcian) of Rosedale, North Yorkshire, UK. Proceedings of the Yorkshire Geological Society, Vol. 50, 129-142. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable