Lithostratigraphy for mapping the Chalk of southern England

Abstract

Recent British Geological Survey (BGS) mapping in Dorset, Wiltshire, Hampshire, Sussex and Kent has shown that the Lower, Middle and Upper Chalk formations of the Chalk Group, can be subdivided into mappable units of member status. The members are recognizable by their lithology and topographic expression, and can be followed readily across open country. Some members have distinctive wireline log signatures. These members will be shown on future BGS maps. In the above areas, the Lower Chalk (Formation), with its traditional boundaries, is retained and divided into two members, a lower, West Melbury Marly Chalk , comprising, in part, the Glauconitic Marl, most of the Chalk Marl up to and including the Tenuis Limestone; and a higher, Zig Zag Chalk , comprising the top of the Chalk Marl, the Grey Chalk and the Plenus Marls. These members can be readily traced from Dorset and Wiltshire into Sussex and Kent. In the Chiltems, the Glauconitic Marl (s.s.) forms a third, basal, mappable member to the Lower Chalk. The Middle Chalk (Formation) consists of two members: the redefined Holywell Nodular Chalk , comprising the Melbourn Rock and overlying Mytiloides shell-detrital chalks; and the New Pit Chalk , a massively bedded chalk with conspicuous marl seams. The traditional concept defining the base of the Upper Chalk (Formation) at the entry of common flint is too variable and unreliable for mapping. Instead, the base of the Upper Chalk is coincident with a revised base to the Lewes Nodular Chalk , defined by the entry of hard nodular 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 Nodular Chalk is succeeded by very finegrained, smooth chalks, marl-free except at the base, with conspicuous bands of large flints, the Seaford Chalk . The overlying Newhaven Chalk is characterized by firm, marly chalk with numerous marl seams and regular, but fewer, bands of flint. The marl seams locally thin or disappear over tectonic highs. In central 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 parts 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 Chalk are defined by the crests of prominent scarp features Ftl and Ft2 in central Dorset. The origin of the features, due to lack of exposure of the feature-forming beds, remains uncertain. These members comprise uniform, firm, white, flinty chalks and collectively, e.g. where not mapped separately, constitute the Culver Chalk of the existing classification. The lower limit of the Portsdown Chalk , with numerous marl seams, is taken at a pronounced negative feature break in central Dorset (base of Ft3 scarp), that approximates to the base of the member as originally defined. The terminal member in Dorset and the Isle of Wight ( 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.