The BGS Lexicon of Named Rock Units — Result Details

Lulworth Formation

Computer Code: LULW Preferred Map Code: Lulw
Status Code: Full
Age range: Tithonian Age (JI) — Berriasian Age (KR)
Lithological Description: Dominated in the lower part by pale-weathering, ostracod-rich calcarenites that are commonly rippled. Stromatolitic or algal, micritic limestone, with evaporitic material, is also common (particularly in east Dorset). The upper part of the formation comprises monotonous, interbedded dark grey mudstones and white-weathering micrites. Detrital quartz is common at several levels. The Lulworth Formation comprises three members in Dorset, namely in ascending order the Mupe Member, the Ridgway Member and the Worbarrow Tout member. Elsewhere the formation is undivided or is itself incorporated into an undivided Purbeck Limestone Group. The formation includes the informal Lower Purbeck Beds and the lower part of the Middle Purbeck Beds (up to 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 boundary is marked by the first occurrence of finely laminated, ostracod-rich limestones above the more massive, shelly limestones of the Portland Limestone.
Definition of Upper Boundary: The top of the formation is taken at the top of the Cherty Freshwater Bed, comprising thick-bedded, chert-rich micrites. Overlain by the Cinder Bed at the base of the Durlston Formation.
Thickness: In Dorset, 27 to 63m. In the central Weald area boreholes show the formation to be about 86m thick (Broadoak Borehole TQ 62 SW/4 [6195 2214]). In the Vale of Wardour in Wiltshire the formation is up to 15m thick.
Geographical Limits: Mapped within the South Dorset area, covered by British Geological Survey 1:50 000 scale sheets 341, 342 and 343. The unit can be defined elsewhere in southern England (e.g. Wessex Basin and southeast England), separated from the Durlston Formation above by the Cinder Bed. Occurs in the subcrop throughout southern Britain (Wessex Basin). Away from the type area of Dorset, the presence of the formation can be interpreted from described outcrops from the Weald and to the west in Wiltshire (Vale of Wardour).
Parent Unit: Purbeck Group (PB)
Previous Name(s): Lulworth Beds [Obsolete Name And Code: Use LULW] (LWB)
Lower Purbeck Beds [Obsolete Name And Code: See Alternative Name Details] (LPB)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  West side of Worbarrow Tout. Westhead, R K and Mather, A E, 1996. 
Reference Section  Durlston Bay, between Durlston Head and the centre of the bay. Westhead, R K and Mather, A E, 1996. 
Reference Section  BGS Broadoak Borehole TQ62SW/4 near Heathfield, Sussex, [6195 2214]. The Lulworth Formation equivalent is encountered between 44.29 and 133.67m depth. 
Reference(s):
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. 
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. 
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). 
Townson, W G. 1975. Lithostratigraphy and deposition of the type Portlandian. Journal of the Geological Society, London, Vol. 131, 619-638. 
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 and Holliday, D W. 1978. Purbeck Beds of the Broadoak Borehole, Sussex. Report of the Institute of Geological Sciences, No.78/3, 1-28. 
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