The BGS Lexicon of Named Rock Units — Result Details

Durlston Formation

Computer Code: DURN Preferred Map Code: Durn
Status Code: Full
Age range: Berriasian Age (KR) — Berriasian Age (KR)
Lithological Description: Dominated by interbedded, shelly limestones and dark mudstones. The shell material commonly contains Unio and Viviparus. Detrital quartz is present through much of the formation in West Dorset (west of Stair Hole), but is confined to the Stair Hole Member in East Dorset (Worbarrow and Durlston). The Durlston Formation comprises two members in Dorset, which, in ascending order are: the Stair Hole Member and the Peveril Point Member. Elsewhere the formation is undivided, or is itself incorporated into an undivided Purbeck Limestone Group. The formation includes the informal Upper Purbeck Beds, and the upper part of the Middle Purbeck Beds, above the base of the Cinder Bed, of the traditional scheme. Within the Weald, an equivalent of the Cinder Bed has been identified, thus allowing the succession there to be considered within the formal terminology.
Definition of Lower Boundary: The lower boundary is conformable and marked by the first upward occurrence of very shelly micrite (Cinder Bed) above purer micrites at the top of the Lulworth Formation.
Definition of Upper Boundary: The boundary varies from unconformable to disconformable and is marked by the last occurrence of significant limestone below monotonous Wealden Group sandy mudstones.
Thickness: 18 to 57m.
Geographical Limits: The unit can be recognised in the Wessex Basin and southeastern England, where it is separated from the underlying Lulworth Formation by the Cinder Bed. Away from the type area of Dorset, the unit has been described from the Weald (Lake et al., 1987) and the Vale of Wardour in Wiltshire. Mapped within the South Dorset area, covered by British Geological Survey 1:50 000 scale Sheets 341, 342 and 343.
Parent Unit: Purbeck Group (PB)
Previous Name(s): Durlston Beds [Obsolete Name And Code: Use DURN] (DSB)
Alternative Name(s): none recorded or not applicable
Type Section  Northern half of Durlston Bay. Westhead, R K and Mather, A E. 1996. 
Reference Section  BGS Broadoak Borehole TQ62SW/4 [TQ6195 2214]. Between the surface and 45.45m depth, which marks the base of a limestone that grades down into a shelly mudstone with "Praeexogyra" bivalves and considered to be equivalent to the "Cinder Bed" of Sussex and Dorset. See Lake and Holliday (1978). 
Reference Section  East Cliff at Stair Hole, Dorset. Westhead, R K and Mather, A E, 1996. 
Howitt, F. 1964. Stratigraphy and Structure of the Purbeck inliers of Sussex (England). Quarterly Journal of the Geological Society of London, Vol.120, 77-113. 
Lake, R D, Young, B, Wood, C J and Mortimore, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey, Sheet 319 (England and Wales). 
Lake, R D and Holliday, D W. 1978. Purbeck Beds of the Broadoak Borehole, Sussex. Report of the Institute of Geological Sciences, No.78/3, 1-28. 
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. 
Clements, R G. 1993. Type-section of the Purbeck Limestone Group, Durlston Bay, Swanage. Proceedings of the Dorset Natural History and Archaeological Society, Vol.114, 181-206. 
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E219 E220 E237 E252 E270 E271 E283 E284 E285 E286 E287 E288 E289 E297 E298 E299 E300 E301 E302 E303 E304 E305 E306 E316 E317 E318 E319 E320 E321 E327 E328 E332 E333 E341 E342 E343