The BGS Lexicon of Named Rock Units — Result Details

Blue Lias Formation
Computer Code: BLI Preferred Map Code: BLi
Status Code: Full
Age range: Rhaetian Age (TR) — Sinemurian Age (JS)
Lithological Description: Thinly interbedded limestone (laminated, nodular, or massive and persistent) and calcareous mudstone or siltstone (locally laminated). Individual limestones are typically 0.10-0.30m thick. In some areas, intervening mudstone units with relatively few limestone beds. Also includes littoral limestone facies of the Radstock Shelf - Mendip area and South Wales.
Definition of Lower Boundary: At the base of grey limestone or mudstone sharply overlying irregular surface of pale grey or bluish and greenish-grey or reddish brown mudstone of the Cotham Member (Lilstock Formation) or eroded, commonly bored, locally conglomeratic surface of pale porcellanous limestone of Langport Member (Lilstock Formation). Coincides with the base of the Lias Group, which is commonly markedly non-sequential.
Definition of Upper Boundary: At the base of the Charmouth Mudstone Formation. In any one section, this coincides with marked upward decrease in abundance of limestone beds, locally associated with marked decrease in their individual thickness and lateral persistence. In many cases, the horizon to be taken is obvious. Where there is doubt; the thickness and vertical frequency of limestone beds should be considered, and the following guidelines used: at top of a more or less persistent, thick (>0.10m) limestone above which the overlying mudstone (beneath any subsequent similar limestone) is at least twice the thickness typical of the underlying limestone-mudstone interbedded unit and above which is no return, within the Lias Group, to limestone-mudstone spacing on that previous scale. In type section, top formerly taken at top of Scipionianum Bed but characteristic interbedded limestone/mudstone sequence continues above that Bed, and that upper boundary in type area should be drawn at a higher stratigraphical level. In Dorset coastal sections, top is at top of Grey Ledge (Bed 49 of Lang, W D, 1924); in Somerset coastal sections, top is at the top of Bed 238 of Whittaker, A and Green, G W, 1983.
Thickness: To c.140m
Geographical Limits: Wessex Basin, Central Somerset Basin, Worcester Basin and adjoining marginal areas (Radstock-Bristol Shelf and South Wales), southern part of the East Midlands Shelf ie. Dorset coast to approximately Leicester.
Parent Unit: Lias Group (LI)
Previous Name(s): Blue Lias [Obsolete Code:Use BLI] (-701)
Blue Lias [Obsolete Code: Use BLI] (BLS)
Lyme Regis Beds (-3856)
Blue Lias (-1991)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  North Somerset Coast (St Audrie's). Proposed for base of Jurassic (GSGP). 
Type Section  Saltford railway cutting, near Keynsham, Avon. Bristow, H B, Etheridge, R and Woodward, H B, 1873, col.9: Donovan, D T, 1956; Torrens, H S and Getty, T A, 1980; Donovan, D T and Kellaway, G A, 1984. 
Type Area  Bath and Bristol area, Somerset. Torrens, H S and Getty, T A, 1980. 
Reference Section  British Geological Survey Twyning Borehole, 24.2m - 121.7m depth. 
Reference Section  Lyme Regis, Dorset. 
Reference Section  Boreholes drilled on the site of Draycote Reservoir, Warwick district.(Nos SP46NE/1-19; SP47SE/1-13). Summarised as composite section (Old et al. 1987, figure 17(5)). 
Reference Section  Primary Reference Section: Cliff sections between Pinhay Bay and Lyme Regis, Dorset. Lang, W D, 1924; Hallam, A, 1960; Hesselbo, S P and Jenkyns, H C, 1995; Collomon, J H and Cope, J C W, 1995. 
Reference Section  Glamorgan coast. Type section for preceeding Penarth Group (q.v.) 
Reference(s):
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. 
Cope, J C W and others, 1980. A correlation of Jurassic rocks in the British Isles. Part 1: Introduction and Lower Jurassic. Special Report of the Geological Society of London, No.14. 
Hallam, A, 1960. A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan. Philosophical Transactions of the Royal Society, London, Vol.B243, 1-44. 
Hesselbo, S P and Jenkyns, H C A, 1995. A comparison of the Hettangian to Bajocian successions of Dorset and Yorkshire. 105-150 in Taylor, P D (Editor). Field geology of the British Jurassic (London Geological Society). 
Ivimey-Cook, H C, Warrington, G, Worley, N E, Holloway, S and Young, B, 1995. Rocks of Late Triassic and Early Jurassic age in the Carlisle Basin, Cumbria (north-west England). Proceedings of the Yorkshire Geological Society, Vol.50, 305-316. 
Torrens, H S and Getty, T A, 1980. The base of the Jurassic System. Special Report of the Geological Society of London, No.14, 17-22. 
Callomon, J H and Cope, J C W, 1995. The Jurassic geology of Dorset. 51-103 in Field Geology of the British Jurassic. Taylor, P D (Editor). (Bath: The Geological Society). 
Old, R A, Sumbler, M G and Ambrose, K, 1987. Geology of the country around Warwick. Memoir of the British Geological Survey, Sheet 184 (England and Wales). 
Cox, B M, Sumbler, M G and Ivimey-Cook, H C, 1999. A Formational framework for the Lower Jurassic of England and Wales (Onshore Area). British Geological Survey Research Report No. RR/99/01. 
Lang, W D, 1924. The Blue Lias of the Devon and Dorset coasts. Proceedings of the Geologists' Association, Vol.35, 169-185. 
Whittaker A and Green G W. 1983. Geology of the country around Weston-super-Mare. Memoir of the Geological Survey, England and Wales, Sheet 279 with parts of sheets 263 and 295. 
Bristow, H B and Etheridge, R and Woodward, H B, 1873. Vertical sections of the Lower Lias and Rhaetic or Penarth Beds of Somerset and Gloucester- shires. No 9 Saltford. Vertical sections Sheet No 46. Geological Survey of England and Wales. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E216 E184 E169 E263 E249 E183 E199 E200 E201 E279 E250 E251 E264 E265 E268 S043 S044 S044 S043 S070 S060 E341 E342 E343 E217 E326
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