The BGS Lexicon of Named Rock Units — Result Details

Dyrham Formation
Computer Code: DYS Preferred Map Code: DyS
Status Code: Full
Age range: Pliensbachian Age (JE) — Pliensbachian Age (JE)
Lithological Description: Pale to dark grey and greenish grey, silty and sandy mudstone, with interbeds of silt or very fine-grained sand (locally muddy or silty), weathering yellow. Variably micaceous. Impersistent beds or doggers of ferruginous limestone (some ooidal) and sandstone, which tend to occur at the top of sedimentary cycles. Sporadic large cementstone nodules.
Definition of Lower Boundary: Marked or gradational downward change from silty mudstones to smoother argillaceous sediments of the Charmouth Mudstone Formation; commonly coincides with negative change of slope and/or line of seepage that may correspond with a sandy bed (e.g. Capricornus Sandstone in the Cotswolds; Sumbler, M G, Barron, A J M and Morigi, A N, 2000). On Dorset coast, at base of Three Tiers Sandstone (Hesselbo, S P and Jenkyns, H C, 1995) (base of Bed 6 of Howarth, M K, 1957).
Definition of Upper Boundary: At base of ferruginous limestone or ironstone of the Marlstone Rock Formation or Marlstone Member of the Beacon Limestone Formation.
Thickness: To c.125m
Geographical Limits: Wessex Basin, Worcester Basin and East Midlands Shelf (Dorset coast to c.Grantham, Lincs).
Parent Unit: Lias Group (LI)
Previous Name(s): Middle Lias Sandy Beds (-4017)
Dyrham Siltstone Formation (-3380)
Gonerby Member [Obsolete Name And Code: See DYS] (GO)
Middle Lias Sands And Silts [Obsolete Name And Code: Use DYS] (MLS)
Gonerby Member (-880)
Middle Lias Silts And Clays (-1517)
Dyrham Silt(S) (-2772)
Middle Lias (-2773)
Middle Lias Silts (-2774)
Middle Lias Clays (-223)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Tuffley Brickworks, Robin's Wood Hill, near Gloucester, Gloucestershire (SSSI). Ager, D V, 1956, 1969; Palmer, C P, 1971; Murray, J W and Hancock, P L, 1977; Simms, M J, 1990. 
Type Area  Village of Dyrham, east of Bristol, Avon. 
Reference Section  BGS (Elton Farm) Borehole, from 36.36 to 45.42m. Ivimey-Cook, 1978. 
Type Area  Bath, Malmesbury and Gloucester districts. 1:50 000 Geological Sheets 265, 251 and 234; Cave, R, 1977. 
Reference Section  Cliff and foreshore exposures between Seatown and Eype Mouth, Dorset. See also Cox, B M, et al, 1999 for additional sections. 
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). 
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. 
Ager, D V, 1969. Cotswolds and Vale of Gloucester. B27-B43 in Torrens, H S (Editor). International Field Symposium on the British Jurassic. Excursion No.2 Guide for north Somerset and Gloucestershire. (Keele Geology Department, Keele University). 
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). 
Murray, J W and Hancock, P L, 1977. The south and mid Cotswolds. 140-165 in Geological excursions in the Bristol district. Savage, R J G (editor). (University of Bristol). 
Palmer, C P, 1971. The stratigraphy of the stonehouse and Tuffley claypits in Gloucestershire. Proceedings of the Bristol Naturalists' Society, Vol.32, 58-68. 
Simms, M J, 1990. Upper Pliensbachian stratigraphy in the Severn Basin area: evidence for anomalous structural controls in the Lower and Middle Jurassic. Proceedings of the Geologists' Association, Vol.101, 131-144. 
Howarth, M K, 1957. The Middle Lias of the Dorset coast. Quarterly Journal of the Geological Society of London, Vol.113, 185-204. 
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). 
Ager, D V, 1956. Field meeting in the central Cotswolds. Proceedings of the Geologists' Association, Vol.66, 356-365. 
Ivimey-Cook, H C, 1978. Stratigraphy of the Jurassic of the Elton Farm Borehole near Dundry, Avon. Bulletin of the Geological Survey of Great Britain, No.69, p.1-9. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E216 E234 E127 E235 E251 E265 E311 E199 E279 E217 E142 E326 E340 E341 E342 E343 E171
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