The BGS Lexicon of Named Rock Units — Result Details

Jet Rock

Computer Code: JRB Preferred Map Code: JR
Status Code: Index Level
Age range: Toarcian Age (JT) — Toarcian Age (JT)
Lithological Description: Mudstone, dark grey, bituminous and pyritic, with common layers of grey calcareous concretions, ovoid and spherical and ranging up to 4.5m in diameter; also hosts the horizon bearing the black highly compressed fossil wood (jet); fossils including ammonites and belemnites common throughout.
Definition of Lower Boundary: Conformable: base of bituminous mudstone with large concretions resting upon non-bituminous shaly mudstone (Grey Shale Member). Bed 33 on Bed 32 at Hawsker Bottoms and Port Mulgrave (bed numbers of Howarth, 1955).
Definition of Upper Boundary: Conformable: change up from finely-laminated bituminous shaly mudstone with numerous concretion layers to less finely-laminated bituminous mudstone (Bituminous Shales of Mulgrave Shale Member) with fewer concretion layers (e.g. Ovatum Bed). Bed 41 on Bed 40 at Hawsker Bottoms, Port Mulgrave and Whitby (bed numbers of Howarth, 1955).
Thickness: 7.1m thick on the Yorkshire coast, thickens west and south to between 9 and 10m (Felixkirk and Brown Moor boreholes).
Geographical Limits: Cleveland Basin as far south as Acklam area.
Parent Unit: Mulgrave Shale Member (MULG)
Previous Name(s): Jet Rock Member (-336)
Jet Rock (JRB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
none recorded or not applicable
Reference(s):
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.) 
Powell, J H, 1984. Lithostratigraphic nomenclature of the Lias Group in the Yorkshire Basin. Proceedings of the Yorkshire Geological Society, Vol.45, p.51-57. 
Hemingway, J E. 1974. Jurassic. 161-233 in Rayner, D H and Hemingway, J E (eds), The geology and mineral resources of Yorkshire. (Leeds: Yorkshire Geological Society.) 
Rawson, P F and Wright, J K. 1995. Jurassic of the Cleveland Basin, North Yorkshire. 173-208 in Taylor, P D (Ed.) Field Geology of the British Jurassic. (London: Geological Society.) 
Fox-Strangways, C. 1892. The Jurassic Rocks of Britain, Vol.1. Yorkshire. Memoir of the Geological Survey of the United Kingdom. 
Howarth, M K, 1955. Domerian of the Yorkshire coast. Proceedings of the Yorkshire Geological Society, Vol.30, 147-175. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E043