The BGS Lexicon of Named Rock Units — Result Details

Atherfield Clay Formation

Computer Code: AC Preferred Map Code: AC
Status Code: Full
Age range: Aptian Age (KP) — Aptian Age (KP)
Lithological Description: Generally massive yellowish brown to pale grey sandy mudstone throughout most of its outcrop, with an impersistent phosphatic pebble bed with vertebrate bones, gritty sandstone or very shelly sandy mudstone with glauconite, at the base. At the type site on the Isle of Wight, the predominant lithology is blue grey mudstone, variably sandy with calcareous concretions; the formation includes beds of sandstone, clay ironstone and phosphatic nodules. Weathers to a chocolate brown, bluish grey and brown, mottled pinkish brown to orange.
Definition of Lower Boundary: The lower boundary is disconformable on the Wealden Group throughout the Wessex Basin, on the Vectis Formation in the Vectian Basin, and on the Weald Clay Formation in the Wealden Basin. This contact marks the Early Aptian marine transgression. On the Isle of Wight, this boundary is placed at the sharp non-sequence where the dark grey mudstones of the Vectis Formation are overlain by coarse quartz grit passing up into grey brown and dark blue-grey, fossiliferous sandy mudstone containing glauconite, bone fragments, fish teeth, phosphatic nodules and rolled ammonites (the Lower Clay and Atherfield Bone Bed (Bed 1) of Simpson (1985)). In the Wealden Basin, the base of the Atherfield Clay Formation is infrequently exposed. The junction is marked by a sharp colour change from yellow-brown mudstones of the Weald Clay to grey and dark blue-grey sandy mudstones of the Atherfield Clay Formation. This boundary is locally marked by a sandy, fossiliferous, glauconitic and pebbly clay, with or without concretionary sandstone. Unconformable to disconformable contact on Weald Clay (orangish brown/yellow or steely grey mudstone).
Definition of Upper Boundary: The upper boundary is conformable between the Atherfield Clay Formation and the Ferruginous Sand Formation on the Isle of Wight, and between the Atherfield Clay Formation and the Hythe Formation in the Weald. On the Isle of Wight the boundary is placed at the change from interbedded mudstones with subordinate sandstones of the Upper Lobster Member (Simpson, 1985) and the fine-grained ferruginous sandstones of the Ferruginous Sand Formation. In the Wealden Basin, the passage from the increasingly sandy and glauconitic mudstones of the Atherfield Clay Formation into the interbedded sandy limestones and calcareous argillaceous sandstones of the Hythe Formation is indistinct. On the ground this contact is obscured for the most part by weathered and landslipped debris derived from the Hythe Formation scarp.
Thickness: Between 55 and 60m on the Isle of Wight. In the Weald, the formation is 18m thick in West Sussex and Surrey and thins eastward to about 10m around Sevenoaks and Maidstone.
Geographical Limits: The formation occurs at outcrop in the Weald of Kent, Surrey and Sussex and on the Isle of Wight. In the subcrop the formation is known to be overstepped and cut out towards the west and north of the Wessex Basin and to the north of the Wealden Basin against the London-Brabant landmass.
Parent Unit: Lower Greensand Group (LGS)
Previous Name(s): Atherfield Clay (-2555)
Atherfield Group (-2556)
Atherfield Clay 'Group' (-4)
Atherfield Clay Series (-630)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  At the type site in Chale Bay (Shepherd's Chine to Whale Chine) on the Isle of Wight [SZ 4466 7982 to SZ 4684 7825] the formation is divided into five units referred to as members in Simpson (1985). These members have not been tested in terms of their mappability on the Isle of Wight and are not formally adopted. The type section includes Atherfield Point. 
Type Area  In the Weald, the Atherfield Clay is poorly exposed. The most complete sections are in boreholes the locations of which are given by Simpson (1985, figure 7). Simpson (1985) summarised the most significant localities although the majority of these are no longer available for inspection (e.g. the Haslemere railway cutting described by Salter (in Bristow, H W, 1862). 
Reference(s):
Rawson, P F. 1992. Cretaceous, 355-388 in Duff, P McL D and Smith, A J (editors), Geology of England and Wales. (London: Geological Society.) 
Aldiss, D T. 2002. Geology of the Chichester and Bognor district. Sheet Explanation of the British Geological Survey, 1:50 000 Series Sheet 317 and Sheet 332 (England and Wales). 
Young, B and Lake, R D. 1988. Geology of the country around Brighton and Worthing. Memoir of the British Geological Survey, Sheet 318 and 333 (England and Wales). 
Casey, R, 1961. The stratigraphical palaeontology of the Lower Greensand. Palaeontology, Vol.3, 487-621. 
Thurrell, R G, Worssam, B C and Edmonds, E A. 1968. Geology of the country around Haslemere. Memoir of the British Geological Survey, sheet 301 (England and Wales). 
Topley, W. 1875. The geology of the Weald. Memoir of the Geological Survey of England and Wales. 
Worssam, B C. 1963. Geology of the country around Maidstone. Memoir of the Geological Survey of Great Britain, Sheet 288 (England and Wales). 
Osborne White, H J. 1921 [1994 reprint]. A short account of the geology of the Isle of Wight. Memoirs of the Geological Survey of Great Britain. 235pp. [HMSO.] 
Bristow, H W. 1862. Geology of the Isle of Wight. Memoir of the Geological Survey of the United Kingdom. 
Drew, F. 1864. The geology of the country between Folkestone and Rye, including the whole of Romney Marsh (Sheet 4). Memoir of the Geological Survey of the United Kingdom. 
Fitton, W H. 1847. A stratigraphical account of the section from Atherfield to Rocken-end on the south-west coast of the Isle of Wight. Quarterly Journal of the Geological Society of London, Vol.3, 289-327. 
Lake, R D, Young, B, Wood, C J and Mortimore, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey, Sheet 319 (England and Wales). 
Rawson, P F, Curry, D, Dilley, F C, Hancock, J M, Kennedy, W J, Neale, J W, Wood, C J and Worrsam, B C. 1978. A correlation of Cretaceous rocks in the British Isles. Geological Society of London, Special Report No.9. 
Smart, J G O, Bisson, G and Worssam, B C. 1966. Geology of the Country around Canterbury and Folkestone. Memoir of the Geological Survey of Great Britain, Sheets 289, 305 and 306 (England and Wales). 
Dines, H G, Buchan, S, Holmes, S C A and Bristow, C R. 1969. Geology of the country around Sevenoaks and Tonbridge. Second edition. Memoir of the Institute of Geological Sciences, Sheet 287 (England and Wales). 
Simpson, M I. 1985. The stratigraphy of the Atherfield Clay Formation (Lower Aptian, Lower Cretaceous) at the type and other localities in southern England. Proceedings of the Geologists' Association, Vol.96(1), 23-45. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E285 E286 E287 E288 E289 E300 E301 E305 E317 E318 E319 E331 E332 E333 E334 E342 E343 E344 E345