Knockrushen Formation is part of the Great Scar Limestone Group
The formation comprises limestone, with interbedded subordinate claystone and siltstone at the top. Limestones vary from wackestone to fine-grained packstone; they are commonly nodular in form and argillaceous. Thin beds of black fissile claystone are also present. Siltstone is common near the base of the formation.
A partial type section is a foreshore exposure east of Knockrushen House (SC 2584 6627 to 2653 6727) where 21.m of irregularly bedded to nodular, dark-grey, argillaceous spicular limestones (wackestones) are interbedded with black, fissile shales. There is a reference section on the foreshore by the lime kilns at Scarlett (SC 2300 6860) where the top 6 m of the formation are exposed. See Dickson et al. (1987).
Lower and upper boundaries
The formation overlies the Skillicore Member of the Derbyhaven Formation (Figure 8, Column 8). The basal boundary is taken immediately above a bryozoan bed, which is exposed to the north-east of the airport light gantry at Ronaldsway at about (SC 28 68), at Port St Mary at about (SC 21 67), and at Ballasalla at about (SC 28 70) (see Dickson et al., 1987 who quoted no grid references for these localities, so the boundary can be regarded as not well defined).
The top of the formation is also poorly defined. The wackestones and packstones of the Knockrushen Formation are overlain by the massive lime mudstones of the Hodderense Limestone Formation, Craven Group. Dickson et al. (1987) described the base of the Hodderense Limestone Formation as occurring close (SC 2580 6620) to the Visitor Centre at Scarlett south of the old lime kilns, where its base is taken at the lowest mottled horizon. This mottling is a result of the presence of blue or grey micrite nodules up to 3 cm in size.
The formation is 21 m thick.
Distribution and regional correlation
The south Isle of Man, in the Castletown area.
Age and biostratigraphical characterisation
Holkerian. The formation contains the large caninioid coral Siphonophyllia benburbensis and the brachipod Megachonetes papilionensis. Zoophycos burrows are common near the base.
- Chadwick, R A, Jackson, D I, Barnes, R P, Kimbell, G S, Johnson, H, Chiverrell, 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.The 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, Vol. 86, 234–290.
- Dickson, J A D, Ford, T D, and Swift, A.1987.The strati-graphy of the Carboniferous rocks around Castletown, Isle of Man.Proceedings of the Yorkshire Geological Society, Vol. 46, 203–229.