The BGS Lexicon of Named Rock Units — Result Details

Charmouth Mudstone Formation
Computer Code: CHAM Preferred Map Code: ChM
Status Code: Full
Age range: Sinemurian Age (JS) — Pliensbachian Age (JE)
Lithological Description: Dark grey laminated shales, and dark, pale and bluish grey mudstones; locally concretionary and tabular limestone beds; abundant argillaceous limestone, phosphatic or ironstone (sideritic mudstone) nodules in some areas; organic-rich paper shales at some levels; finely sandy beds in lower part in some areas.
Definition of Lower Boundary: Taken at the top of Blue Lias Formation or Scunthorpe Mudstone Formation. The top of the Blue Lias coincides with a marked upward decrease in frequency, thickness and lateral persistence of limestone beds. 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 the 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 there is no return, within the Lias Group, to limestone-mudstone spacing on that previous scale. In the type section, at the top of the Grey Ledge (bed 49 of Lang, W D, 1924). In the East Midlands Shelf (northwards), where the Blue Lias is absent but the Scunthorpe Mudstone occurs, the base of the Charmouth Mudstone is taken at the base of thin pebbly ferruginous oolite (Glebe Farm Bed) or, in north Lincolnshire, at the top of the Frodingham Ironstone.
Definition of Upper Boundary: At marked or gradational upward change to coarser siliciclastic deposits of the Dyrham Formation. In the type section, this is at the base of the Three Tiers Sandstone. Where the Dyrham Formation is absent (East Midlands Shelf northwards) it is taken at the base of the Marlstone Rock Formation.
Thickness: To c.335m
Geographical Limits: Wessex Basin to East Midlands Shelf, i.e. Dorset Coast to c.Market Weighton area, Yorkshire.
Parent Unit: Lias Group (LI)
Previous Name(s): Blue Marl (-123)
Lower Lias (-124)
Brant Mudstone Formation (-774)
Obtusum-Oxynotum Clays (Upper Part) (-2691)
Lower Lias Clay Formation (-4536)
Coleby Mudstone Formation (Part) (-3923)
Lower Lias Clay(S) (-3924)
Middle Lias Mudstones And Siltstones (East Midlands Shelf North) (-1425)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Lyme Regis to Charmouth coastal area, Dorset, where the Formation is well exposed in the cliffs and foreshore. 
Type Section  Cliff and foreshore exposures between Seven Rock Point and Golden Cap, Dorset. Hesslebo, S P and Jenkyns, H C, 1995. 
Reference Section  British Geological Survey Stowell Park Borehole, Gloucestershire (SP01SE/1). 276.51-560.48m depth. Green, G W and Melville, R V, 1956; Melville, R V, 1956; Spath, L F, 1956; Ivimey-Cook H C, 1993; Sumbler, M G and Barron A J M, 1995; Sumbler, M G, et al. Primary Reference Section. For other reference sections see Cox, B M, et al, 1999. 
Reference(s):
Green, G W and Melville, R V, 1956. The stratigraphy of the Stowell Park Borehole (1949-1951). Bulletin of the Geological Survey of Great Britain, No.11, p.1-66. 
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). 
Lang, W D, 1914. The geology of Charmouth cliffs, beach and fore-shore. Proceedings of the Geologists' Association, Vol.25, 293-360. 
Melville, R V, 1956. Stratigraphical palaeontology, ammonites excluded, of the Stowell Park Borehole. Bulletin of the Geological Survey of Great Britain, No.11, 67-139. 
Spath, L F, 1956. The Liassic ammonite faunas of the Stowell Park Borehole. Bulletin of the Geological Survey of Great Britain, No.11, 140-164. 
Sumbler, M G, Barron, A J M and Morigi, A N, 2000. Geology of the Cirencester district. Memoir of the British Geological Survey, Sheet 235 (England and Wales). 
Sumbler, M G and Barron, A J M, 1995. Geological notes and local details for 1: 10 000 Sheet SP01SE (Chedworth). British Geological Survey Technical Report, WA/95/73. 
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. 
Ivimey-Cook, H C, 1993. The biostratigraphy of the Lower Jurassic of the Stowell Park Borehole. British Geological Survey Technical Report WH/93/174R. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E235 E199 E279 E102 E080 E142 E281 E341 E342 E343 E217 E264 E326 E171
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