The Wealden Group and it's Subdivisions
The term Wealden Group is applied to a suite of non-marine sediments of Lower Cretaceous age that crop out in the Weald Sub-basin of Sussex, Kent and Surrey and in the Wessex Sub-basin on the Isle of Wight and in South Dorset. The term 'Wealden' is also used for similar age strata of marginal facies in Wiltshire, Oxfordshire and Buckinghamshire, and is also used for non-marine strata of Early Cretaceous age in basins in Belgium, France and Germany. The Wessex Sub-basin Wealden Group is dominated by arenaceous successions of fluvial origin with intervening clay-dominated successions of lacustrine and lagoonal origin.
The Wealden Supergroup group has been the subject of numerous palaeoecological, palaeoclimatological and sedimentological studies (e.g. Allen 1959, 1975, 1981,1998; Stewart 1981, Stewart et al. 1991, Radley et al. 1988a, b; Radley 1994c, Radley and Barker 1998, Ruffel 1988, Ruffell et al. 1996) and has become famous for its vertebrate fossils (Insole 1980; Insole and Hutt I994a, b;Benton and Spencer 1995). The base of the Wealden Group rests conformably on freshwater limestones and clays of the Purbeck Formation, althaugh this boundary is only rarely exposed in the Weald of Sussex and on the Isle of Purbeck. It can be seen relatively easily on the South Dorset Coast at Worbarrow Bay, Lulworth Cove and Durdle Dore, but here the rocks have been intensely folded and there is undoubtedly some slippage at the boundary. The upper boundary of the Wealden Group is marked by a widespread erosion surface at the base of the Lower Greensand Group. On the Isle of Wight this boundary is marked by a bed of highly fossiliferous sandstones and sandy clays known as the Perna Beds Member (Casey 1961, Simpson 1985). There is sometimes a basal bone bed suggesting that there is a minor discontinuity associated with the marine transgression that brought Wealden Group conditions to an end (Simpson 1985).
In the Weald of Kent, Sussex and Surrey the Wealden Supergroup is divisible into a lower group (Hastings Group) comprising large scale alternations of sand and clay dominated formations and an upper, argillaceous succession, the Weald Clay Group. However, these subdivisions cannot easily be recognised on the Isle of Wight or in Dorset, but the Wealden Group of the Isle of Wight is in at least part a time equivalent of the Weald Clay Group of the mainland. On the Isle of Wight the Wealden Group is subdivided into the lower Wessex Formation, of which only the upper part crops out, and the upper Vectis Formation, of which the full sequence can be seen. Exposures of the Wealden Group are found at Yaverland, near Sandown where it is exposed in the cliffs and foreshore, and between Atherfield Point and Compton Bay on the southwest coast of the Island. Inland the Wealden Group is mainly concealed under a cover of Aptian to Cenomanian strata and exposures are few.
Figure 3. Stratigraphic nomenclature for the Wealden Group. Wessex Basin, Isle of Wight.
