The BGS Lexicon of Named Rock Units — Result Details

Stair Hole Member

Computer Code: SHO Preferred Map Code: Sho
Status Code: Full
Age range: Berriasian Age (KR) — Berriasian Age (KR)
Lithological Description: Characterised by calcarenites and calcirudites. These are thicker and dominant lower in the succession; the Cinder Bed at the base of the member comprises a package of biomicrites, with abundant oysters, and can be easily correlated across the region. This passes up into a sequence of interbedded biosparites, biomicrites and mudstones (Intermarine and Scallop beds). The limestone may contain cross-stratification, particularly in the east. Thicker, carbonaceous mudstones, with numerous "beef" (fibrous calcite) veins, become more common towards the top of the member (Corbula and Chief Beef beds), again, particularly in the east. The middle part of the member, across Dorset, contains detrital quartz. The upwards decrease in shelly limestone defines a crude fining-upwards sequence.
Definition of Lower Boundary: First occurrence of very shelly micrite (Cinder Bed) above more pure micrites of the Warbarrow Tout Member.
Definition of Upper Boundary: Top of thicker mudstones and finer grained biosparites or micrites beneath thickly bedded, coarse biosparite (base Broken Shell Limestone Bed) at the base of the overlying Peveril Point Member.
Thickness: Ranges from 10m in west Dorset to 40m in the east.
Geographical Limits: South Dorset area, covered by British Geological Survey 1:50 000 scale Sheets 341, 342 and 343.
Parent Unit: Durlston Formation (DURN)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  Central part of Durlston Bay, Swanage, Dorset. Westhead, R K and Mather, A E, 1996. 
Reference Section  East cliff at Stair Hole, Dorset. Westhead, R K and Mather, A E, 1996. 
Westhead, R K and Mather, A E. 1996. An updated lithostratigraphy of the Purbeck Limestone Group in the Dorset area. Proceedings of the Geologists' Association, Vol.107, 117-128. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E341 E342 E343