The BGS Lexicon of Named Rock Units — Result Details

Wessex Formation

Computer Code: WSEX Preferred Map Code: Wx
Status Code: Full
Age range: Berriasian Age (KR) — Barremian Age (KB)
Lithological Description: Varicoloured (mainly red) mudstones with subordinate unconsolidated sandstones (generally white or pale yellow as well as red) and some ironstones. Sandstone units generally fine-upwards from basal conglomerates grading up into mudstones. The sandstone units that usually exhibit large-scale trough or planar cross-bedding, pass up into climbing ripple-laminated sandstones and are surmounted by interbedded sands and muds. Throughout the succession are plant-rich horizons (including large logs). Westward, on the mainland through Dorset, sandstone units thicken and some significant coarse sand, grit and pebble beds act as markers locally.
Definition of Lower Boundary: Base not seen at outcrop on the Isle of Wight but identified in the Arreton Borehole SZ58NW/2 [5307 8564] (Falcon and Kent, 1960). Elsewhere, on the coast at Swanage, Lulworth Cove and Worbarrow Bay, the base is rarely seen. Strahan (1898) wrote "no line can be drawn which does not either include beds of Purbeck type in the Wealden or beds of Wealden type in the Purbeck". In practice the boundary is taken at the top of the last significant limestone in the top of the underlying Peveril Point Member of the Durlston Formation. The most complete sections are at Bacon Hole [SY 841 797] west of Mupe Bay [SY 844 797]. The the former is described in Strahan (1898, pp.100-101) where he placed the boundary at the junction between a fossiliferous ironstone and an overlying clay with laminae of sand. Hesselbo and Allen (1991) placed the boundary 2.35m below a "basal" fine-grained sandstone (their unit 1) at a point where thin sand laminae first appear in the succession.
Definition of Upper Boundary: The top of the formation on the Isle of Wight is placed at the abrupt colour change from the predominantly red mudstones of the Wessex Formation up into the dull grey finely-bedded mudstone and sandstone of the Vectis Formation. This is also true in the Swanage section on the mainland. Farther west into Dorset, the Vectis Formation is absent due to pre-Lower Greensand or pre-Gault erosion, and the boundary is placed at the upward change from brightly coloured sandstones or calcareous mudstones (marls) of the undivided Wealden Group to dark siltstones, mudstones and ironstones or fine-grained sandstones of the Lower Greensand Group, or mudstones of the Gault Formation. In both cases the boundary may be marked by a pebble bed.
Thickness: 580 m on the Isle of Wight. 800 to 1000 m thick at Swanage and thins westward through Dorset to about 425 m at Worbarrow Bay and to 65 m at Durdle Cove, the formation's most westerly coastal outcrop.
Geographical Limits: Isle of Wight and south Dorset from Swanage to Durdle Door. Other outcrops occur inland in the Upton Syncline, north of Ringstead Bay, Bincombe railway cutting and Arkell (1947) records (?)Wealden as far west as Friar Waddon near Weymouth.
Parent Unit: Wealden Group (W)
Previous Name(s): Wealden Marls (-620)
Wealden Marls (Isle Of Wight) [Obsolete Name And Code: Use WSEX] (WM)
Variegated Marls And Sandstones (-4984)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Coastal and landslipped section at Bacon Hole [SY 841 797] and Mupe Bay [SY 844 797 to 847 802], Dorset. 
Reference Section  Coastal section at Swanage Bay [SZ 038 808], Punfield Cove [SZ 037 807], Dorset. 
Reference(s):
Arkell, W J. 1947. Geology of the country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the British Geological Survey. Sheets 341, 342, 343 and small portions of sheets 327, 328 and 329 (England and Wales). 
Insole, A, Daley, B, and Gale, A. 1998. The Isle of Wight. Geologists’ Association Guide. No. 60. (The Geologists’ Association.)  
White, H J O. 1921. A short account of the geology of the Isle of Wight. Memoir of the Geological Survey of Great Britain, (1994 reprint). 
Hesselbo, S P and Allen, P A. 1991. Major erosion surfaces in the basal Wealden Beds, Lower Cretaceous, south Dorset. Journal of the Geological Society of London, Vol.148, 105-113. 
Daley, B and Stewart, D J. 1979. Weekend field meeting: The Wealden Group in the Isle of Wight. Proceedings of the Geologists' Association, Vol.90, 51-54. 
Strahan, A. 1898. The Geology of the Isle of Purbeck and Weymouth. Memoir of the Geological Survey of England and Wales, 278pp. [HMSO.] 
Falcon, N L and Kent, P. 1960. Geological results of petroleum exploration in Britain, 1945-1957. Geological Society of London, Memoir, 2, 1-56. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable