LITHOSTRATIGRAPHY FOR MAPPING THE CHALK OF SOUTHERN ENGLAND

Recent British Geological Survey (BGS) mapping in Dorset, Wiltshire, H ampshire, Sussex and Kent has shown that the Lower, Middle and Upper C halk formations of the Chalk Group, can be subdivided into mappable un its of member status. The members are recognizable by their lithology and topographic expression, and can be followed readily across open co untry. Some members have distinctive wireline log signatures. These me mbers will be shown on future BGS maps. In the above areas, the Lower Chalk (Formation), with its traditional boundaries, is retained and di vided into two members, a lower, West Melbury Marly Chalk, comprising, in part, the Glauconitic Marl, most of the Chalk Marl up to and inclu ding the Tenuis Limestone; and a higher, Zig Zag Chalk, comprising the top of the Chalk Marl, the Grey Chalk and the Plenus Marls. These mem bers can be readily traced from Dorset and Wiltshire into Sussex and K ent. In the Chilterns, the Glauconitic Marl (s.s.) forms a third, basa l, mappable member to the Lower Chalk. The Middle Chalk (Formation) co nsists of two members: the redefined Holywell Nodular Chalk, comprisin g the Melbourn Rock and overlying Mytiloides shell-detrital chalks; an d the New Pit Chalk, a massively bedded chalk with conspicuous marl se ams. The traditional concept defining the base of the Upper Chalk (For mation) at the entry of common flint is too variable and unreliable fo r mapping. Instead, the base of the Upper Chalk is coincident with a r evised base to the Lewes Nodular Chalk, defined by the entry of hard n odular chalk in basinal successions, by the base of the Chalk Rock in condensed marginal successions and the base of the Spurious Chalk Rock in south Dorset and the Isle of Wight. The Upper Chalk is subdivided into a basic framework of 8 members. The coarse-grained, rough Lewes N odular Chalk is succeeded by very fine-grained, smooth chalks, marl-fr ee except at the base, with conspicuous bands of large flints, the Sea ford Chalk. The overlying Newhaven Chalk is characterized by firm, mar ly chalk with numerous marl seams and regular, but fewer, bands of fli nt. The marl seams locally thin or disappear over tectonic highs. In c entral Dorset, the base of Newhaven Chalk cannot be mapped, leaving an undivided Seaford and Newhaven Chalk, later renamed Blandford Chalk; the latter name is herein discontinued. In pai ts of east Kent (Thanet ), the Seaford Chalk is overlain by very soft, nearly flint-free chalk , the Margate Chalk. The bases of the Tarrant Chalk and Spetisbury Cha lk are defined by the crests: of prominent scarp features Ft1 and Ft2 in central Dorset. The origin of the features, due to lack of exposure of the feature-forming beds, remains uncertain. These members compris e uniform, firm, white, flinty chalk?; and collectively e.g. where not mapped separately, constitute the Culver Chalk of the existing classi fication. The lower limit of the Portsdown Chalk, with numerous marl s eams, is taken at a pronounced negative feature break in central Dorse t (base of Ft3 scarp), that approximates to the base of the member as originally defined. The terminal member in Dorset and the Isle of Wigh t (Studland Chalk), comprising soft, white, marl-free chalk with very large flints, is mapped with the Portsdown Chalk as it is not readily separable from the latter unit.