The BGS Lexicon of Named Rock Units — Result Details
Ampthill Clay Formation
| Computer Code: | AMC | Preferred Map Code: | AmC |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Oxfordian Age (JO) — Oxfordian Age (JO) | ||
| Lithological Description: | Mudstone, mainly smooth or slightly silty, pale to medium grey with argillaceous limestone (cementstone) nodules; some rhythmic alternations of dark grey mudstone in the lower part; topmost beds are typically pale grey marls with cementstone. | ||
| Definition of Lower Boundary: | Base of mudstones (as described under Lithology), resting on darker more silty mudstone of the West Walton Formation or on limestone/sandstone of that formation's members (Upware Limestone, Elsworth Rock) or of the Corallian Group; generally a marked colour change and break of slope at outcrop; change commonly well-marked on borehole geophysical (particularly gamma-ray) logs. Base of unit AmC1 in Fenland (Cox and Gallois, 1979). | ||
| Definition of Upper Boundary: | Upward change to darker mudstone of the Kimmeridge Clay Formation; commonly an erosive non-sequence marked by phosphatic pebbles; generally a marked colour change and break of slope at outcrop; change comonly well-marked on borehole geophysical (particularly gamma-ray) logs. Top of unit AmC42 in Fenland (Cox and Gallois, 1979), but incomplete and succeeded unconformably by Cretaceous rocks in the vicinity of the type section. | ||
| Thickness: | 0 to c.90m; typically c.25m in Oxon-Bucks., c.10m in the Bedfordshire type section; c.50m in Fenland, c.90m in north Lincolnshire. | ||
| Geographical Limits: | Oxon/Wilts, where it passes progressively southwestwards into the Corallian Group (Horton et al., 1995; Gallois and Cox, 1994), to the Market Weighton area, where it is overstepped by Cretaceous rocks (Gaunt et al., 1980); also present in the Cleveland Basin (Powell et al., 1992). | ||
| Parent Unit: | Ancholme Group (AMG) | ||
| Previous Name(s): | Gamlingay Clay [Obsolete Name and Code: Use AMC]
(-2563)
Bluntisham Clay [Obsolete Name and Code: Use AMC] (-3798) Ampthill Clay [Obsolete Name and Code: Use AMC] (-4426) Corallian [Obsolete Name and Code: Use AMC, WWB] (-4427) Tetworth Clay [Obsolete Name and Code: Use AMC] (-3164) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Ampthill railway cutting. Seeley 1869; Woodward, 1895; Shephard-Thorn et al., 1994. | ||
| Reference Section | North Wootton Borehole (Borehole 23) TF62SW/96. Description of standard succession; Gallois and Cox , 1977; Cox and Gallois, 1979; Gallois 1994. | ||
| Type Area | Ampthill area of Bedfordshire to "Fenland" ie. Boston-Wisbech area adjoining the Wash. Gallois and Cox, 1977. | ||
| Reference Section | British Geological Survey Ampthill Borehole (TL03NW/4). Shephard-Thorn et al., 1994. | ||
| Reference(s): | |||
| Seeley, H G. 1869. Index to the fossil remains of Aves, Ornithosauria and Reptilea from the secondary System of Strata, arranged in the Woodwardian Museum of the University of Cambridge. (Cambridge.) | |||
| Gallois, R W and Cox, B M. 1977. The stratigraphy of the Middle and Upper Oxfordian sediments of Fenland. Proceedings of the Geologists' Association, Vol.88, 207-228. | |||
| Gallois, R W, and Cox, B M. 1994. The Kimmeridge Clay and underlying strata (Upper Jurassic) at Swindon, Wiltshire. Proceedings of the Geologists' Association, Vol.105, 99-110. | |||
| Seeley, H G. 1861a. Notice of the Elsworth and other new rocks in the Oxford Clay, and of the Bluntisham Clay above them. Geologist, Vol.4, 460-461. | |||
| Shepard-Thorn, E R, Moorlock, B S P, Cox, B M, Allsop, J M and Wood, C J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England and Wales). | |||
| Seeley, H G. 1861b. On the Fen Clay Formation. Annals and Magazine of Natural History (3), 8, 503-505. | |||
| Gallois, R W, 1994. The geology of the country around King's Lynn and The Wash. Memoir of the British Geological Survey, Sheet 145 and part of 129 (England and Wales). | |||
| Seeley, H G. 1867. Remarks on the Potton sands. Annals and Magazine of Natural History (3), 20, 8. | |||
| Woodward, H B. 1895. the Jurassic Rocks of Britain. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Vol.5. Memoir or the Geological Survey of the United Kingdom. | |||
| Cox, B M, and Gallois, R W. 1979. Description of the standard stratigraphical sequences of the Upper Kimmeridge Clay, Ampthill Clay and West Walton Beds. 68-72 in Geological investigations for the Wash Water Storage Scheme. Gallois, R W. Report of the Institute of Geological Sciences, No.78/19. | |||
| Gaunt, G D, Ivimey-Cook, H C, Penn, I E and Cox, B M. 1980. Mesozoic rocks proved by the Institute of Geological Sciences boreholes in the Humber and Acklam areas. Report of the Institute of Geological Sciences, No. 79/13. | |||
| Powell, J H, Cooper, A H C and Benfield, A C. 1992. Geology of the country around Thirsk. Memoir of the British Geological Survey, Sheet 52 (England and Wales). | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E052 E102 E103 E104 E115 E116 E128 E129 E145 E144 E159 E172 E187 E204 E220 E236 E237 E064 E081 E114 E173 E188 E254 E219 | |||