The BGS Lexicon of Named Rock Units — Result Details

Selborne Group
Computer Code: SELB Preferred Map Code: notEntered
Status Code: Full
Age range: Albian Age (KA) — Cenomanian Age (KE)
Lithological Description: Thick succession of glauconitic sandstones predominantly in the Hampshire and western Weald (Upper Greensand Formation) passing laterally into mudstones (siltstones and claystones) in the Wealden area and northwards into the East Anglia region (Gault Formation); which in turn passes laterally northwards into interbedded limestones and marls (calcareous clays and clayey limestones, commonly red in colour) of northern Norfolk, Lincolnshire and southern and eastern Yorkshire (Hunstanton Formation).
Definition of Lower Boundary: The lower boundary is generally taken at the base of a phosphatic pebble bed or gritty mudstone at the base of the Gault Formation where this overlies the Carstone in East Anglia, the Monk's Bay Sandstone Formation on the Isle of Wight or the Folkestone Formation of the Weald. Elsewhere in southern England the base of the Gault Formation rests on an unconformity and oversteps onto undivided Lower Greensand Group sandstones and ironstone, Wealden or Purbeck groups or onto various units of the Jurassic. In the extreme west of the outcrop the overstep places the Gault Formation on Triassic mudstones and sandstones. On parts of the Sidmouth (Sheet 326) and Wellington (Sheet 311) districts the lower boundary at the base of the constituent Upper Greensand Formation is unconformable on the Mercia Mudstone Group, Penarth Group and Lias Group. Farther west, in the Newton Abbot (349) Sheet area, there is an unconformable contact with Permian, Carboniferous and Devonian strata. In north Norfolk, Lincolnshire and south Yorkshire the boundary is placed at the base of the calcareous mudstones ("marls", commonly of a red or pink colour) of the Hunstanton Formation where they overstep various formations (generally mudstones) within the Jurassic.
Definition of Upper Boundary: The upper boundary is defined at the boundary between mudstones sandstones or interbedded limestones and marls, and the basal unit of the overlying Chalk Group. The erosion surface characterised by glauconitic sand and phosphatic nodules at the base of the Glauconitic Marl Member of the West Melbury Marly Chalk Formation or at a glauconitic chalk horizon stratigraphically higher in uplifted areas.
Thickness: Up to 110m in the Weald where the full succession is present. In marginal areas to the west and north the group may be a little as 5m thick.
Geographical Limits: The group is known at outcrop and at depth throughout southern England, from Devon to the Weald and on the Isle of Wight, throughout the Chilterns into East Anglia and in Lincolnshire and South Yorkshire.
Parent Unit: Not Applicable (-)
Previous Name(s): Selbornian [Stage] (-3026)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Type area and partial type sections designated in the three Selborne Boreholes: SU73SW22 [73200 34940], SU73SE39 [75400 34350] and SU73SE40 [75830 34000] (Hopson et al., 2001). 
Reference Section  East Anglia, Mundford C Borehole (TL79SE13) [7670 9132] (Gallois and Morter, 1982; Owen, 1972). 
Type Area  The Weald [TQ 59 00 to TQ 52 55] of Sussex and Hampshire. 
Reference Section  Copt Point Cliff Section [TR 241 364] at Folkestone Kent (Price, 1874; Topley, 1875; Jukes-Browne and Hill, 1900; Owen, 1971). Entirely in Gault Formation. 
Reference Section  South Ferriby Quarry [SE 9915 2045], Lincolnshire (Gaunt et al., 1992). 
Type Area  Type area of the district around Selborne, Hampshire. The succession is demonstrated here by the three overlapping Selborne Boreholes. See Hopson et al., 2001. 
Reference Section  Speeton Cliffs [TA 162 752 to TA 192 744], Yorkshire (Mitchell, 1995). 
Reference Section  Hunstanton Cliff [TF 6725 4130 to TF 6786 4238], North Norfolk (Owen, 1995; Gallois, 1994). 
Reference(s):
Hopson, P M, Farrant, A R and Booth, K A. 2001. Lithostratigraphy and regional correlation of the basal Chalk, Upper Greensand, Gault and Uppermost Folkestone formations (Mid-Cretaceous) from cored boreholes near Selborne, Hampshire. Proceedings of the Geologists' Association, Vol.112, 193-210. 
Gallois, R W and Morter, A A. 1982. The stratigraphy of the Gault of East Anglia. Proceedings of the Geologists' Association, Vol.93, 351-368. 
Gaunt, G D, Fletcher, T P and Wood, C J. 1992. Geology of the country around Kingston Upon Hull and Brigg. Memoir of the British Geological Survey, Sheets 80 and 89 (England and Wales). 
Mitchell, S F. 1995. Lithostratigraphy and biostratigraphy of the Hunstanton Formation (Red Chalk, Cretaceous) succession at Speeton, North Yorkshire, England. Proceedings of the Yorkshire Geological Society, Vol.50, 285-303. 
Owen, H G. 1971. Middle Albian stratigraphy in the Anglo-Paris basin. Bulletin of the British Museum (Natural History): Geology, Supp.8, 1-164. 
Owen, H G. 1995. The upper part of the Carstone and the Hunstanton Red Chalk [Albian] of the Hunstanton Cliff, Norfolk. Proceedings of the Geologists' Association, Vol.106, 171-181. 
Price, F G H. 1874. On the Gault of Folkestone. Quarterly Journal of the Geological Society of London, Vol.30, 342-366. 
Woods, M A, Wilkinson, I P and Hopson, P M. 1995. The stratigraphy of the Gault Formation (Middle and Upper Albian) in the BGS Arlesey Borehole, Bedfordshire. Proceedings of the Geologists' Association, Vol.106, 271-280. 
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). 
Jukes-Browne, A J and Hill, W. 1900. The Cretaceous rocks of Britain, Vol.1. The Gault and Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. 
Owen, H G. 1972. The Gault and its junction with the Woburn Sands in the Leighton Buzzard area, Bedfordshire and Buckinghamshire. Proceedings of the Geologists' Association, Vol.83, 287-312. 
Topley, W. 1875. The geology of the Weald. Memoir of the Geological Survey of England and Wales. 
Wilkinson, I P. 2006. The Holostratigraphy of the Albian Stage (Lower Cretaceous) of the United Kingdom and its continental shelf. British Geological Survey Research Report, RR/06/01. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable
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