The BGS Lexicon of Named Rock Units — Result Details

Knockrushen Formation

Computer Code: KNRN Preferred Map Code: notEntered
Status Code: Full
Age range: Holkerian Substage (CQ) — Holkerian Substage (CQ)
Lithological Description: Limestone, with interbedded subordinate claystone and siltstone at the top. Limestones very from wackestone to fine-grained packstone; they are often nodular in form and argillaceous. Thin beds of black fissile claystone are also present. Zoophycos burrows and siltstone are common near the base of the formation. The formation contains the large caninoid coral Siphonphyllia benburbensis and the brachipod Megachonetes papilionensis.
Definition of Lower Boundary: The formation overlies the Skillicore Member of the Derbyhaven Formation. The basal boundary is taken as immediately above a byozoan bed which is exposed to the north-east of the airport light gantry at Ronaldsway, at Port St Mary and Ballasalla (Dickson et al.1987). No grid references are quoted in the Dickson et al.(1987) paper for these localities so the boundary can be regarded as not well defined.
Definition of Upper Boundary: The top of the formation is poorly defined. The Knockrushen Formation is overlain by the Hodderense Limestone Formation.This boundary is marked by a change from the wackestones and packstones of the Knockrushen Formation to massive lime mudstones of the Hodderense Limestone Formation. Dickson et al. (1987) describe the base as occurring close to the Visitor Centre at Scarlett south of the old lime kilns [SC 2580 6620], where the base of the Hodderense Limestone Formation 'is taken at the lowest mottled horizon'. This mottling is a result of the presence of blue or grey micrite nodules up to 3cm in size.
Thickness: c.21m
Geographical Limits: Southern part of the Isle of Man, in the Castletown area, from Ballasalla [SC 2817 6990] to Scarlett [SC 2598 6664] and across to Strandhall [SC 2322 6864].
Parent Unit: Great Scar Limestone Group (GSCL)
Previous Name(s): Castletown Formation-Knockrushen Member (-1630)
Alternative Name(s): none recorded or not applicable
Partial Type Section  Foreshore exposure east of Knockrushen House Dickson et al. (1987) describes the section on the foreshore east of Knockrushen House as consisting of up to 21m of irregularly bedded to nodular, grey, argillaceous spicular wackestones interbedded with black fissile mudstones. 
Reference Section  Foreshore section by the lime kilns at Scarlett where the top 6m of the formation are exposed. Dickson et al.(1987) 
Lamplugh G W. 1903. The geology of the Isle of Man. Memoir Geological Survey of Great Britain 
Dickson, J A D, Ford, T D and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. Proceedings of the Yorkshire Geological Society, Vol.46, 203-229. 
Chadwick, R A, Jackson, D I, Barnes, R P, Kimbell, G S, Johnson, H, Chiverell, R C, Thomas, G S P, Jones, N S, Riley, N J, Pickett, E A, Young, B, Holliday, D W, Ball, D F, Molyneux, S G, Long, D, Power, G M and Roberts, D H. 2001. Geology of the Isle of Man and its offshore area. British Geological Survey Research Report, RR/01/06. 
Lewis, H P. 1930. The Avonian succession in the south of the Isle of Man. Quarterly Journal of the Geological Society of London, 86, 234-290. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: