The BGS Lexicon of Named Rock Units — Result Details

Millfield Member

Computer Code: MFLD Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Sinemurian Age (JS) — Sinemurian Age (JS)
Lithological Description: The dominant lithology is a creamy micrite in beds up to 0.3 m thick; large ammonites common, interbedded with thick (up to 10m) calcareous mudstones
Definition of Lower Boundary: The lower boundary is taken at the base of the first significant limestone above the Saltford Shale.
Definition of Upper Boundary: The upper boundary is not exposed, but is presumed to be at the upward change from a dominantly limestone/medium grey mudstone sequence to the dark grey mudstones of the Charmouth Mudstone Formation.
Thickness: Approximately 20m.
Geographical Limits: The member has been mapped south-east and north-west from the village of Street. It dies out southwards around the village of Butleigh where individual units of the Blue Lias Formation merge into a single unit.
Parent Unit: Blue Lias Formation (BLI)
Previous Name(s): Division D (-4996)
Alternative Name(s): none recorded or not applicable
none recorded or not applicable
Donovan, D T and Kellaway, G A 1984. Geology of the Bristol district: Lower Jurassic rocks. Memoir of the British Geological Survey, Bristol Special Sheet (England and Wales). 
Donovan, D T, 1956. The zonal stratigraphy of the Blue Lias around Keynsham, Somerset. Proceedings of the Geologists' Association, Vol.66, p. 182-212. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: