Stratigraphy of the upper cretaceous white chalk of Sussex

Abstract

The Middle/Upper Chalk boundary has been removed from the geological maps in the current survey of the Chalk of Sussex by the British Geological Survey leaving an undifferentiated Middle-Upper Chalk. In place of this undivided sequence, six new Members are introduced using Rowe's (1900–1908) term ‘White Chalk’ for the Formation with stratotypes nominated in the thickest sequences along the Sussex coast between Beachy Head and Brighton and around Lewes. 1. The Ranscombe Chalk Member extends from the base of the Melbourn Rock, through a succession 70–100 m thick, to the top of a massively bedded sequence containing conspicuous marl seams that occurs within the Terebratulina lata Zone, terminating with the Glynde Marl. The basal marker is taken as the Foyle Marl at Eastbourne (boundary stratotype), and Gun Gardens Beachy Head is nominated as the holostratotype. 2. The Ranscombe Chalk is succeeded by a sequence containing regular seams of nodular chalks and the first regular seams of flint. This sequence of nodular chalks with flints is named the Lewes Chalk Member and persists for some 80 m at Lewes, terminated by the upper of the Shoreham Marls at the traditional boundary between the Micraster cortestudinarium and Micraster coranquinum Zones. The basal marker is the Glynde Marl 1 in Caburn Pit, Lewes, boundary stratotype. Caburn Pit, South Street to Southerham Cliffs Lewes, and Beachy Head are nominated as holostratotype sections. 3. Overlying the Shoreham Marls is a thick (60–80 m) sequence of soft, flint bearing, relatively featureless chalks containing seven conspicuous semi-tabular courses of flint; the Seaford Chalk Member. The basal marker is the Shoreham Marl 2 at Seaford Head, the boundary and holostratotype section. 4. Flint bearing chalks with marl seams return in the crinoid and Offaster pilula Zones and characterise the 60–80 m thick Newhaven Chalk Member. The basal marker is the Buckle Marl 1 at Seaford Head, boundary stratotype and holostratotype section. 5. Marl seams cease to be a significant lithology in the Gonioteuthis quadrata Zone where the chalk is generally very soft and featureless but contains several conspicuous courses of flint. This part of the succession is named the Culver Chalk Member and is between 60–100 m thick. The basal marker is the upper Castle Hill Marl at Seaford Head (boundary stratotype) and the only reasonably accessible, complete section through this chalk at Whitecliff Isle of Wight, is nominated as the holostratotype section. 6. Marl seams return in the uppermost part of the G. quadrata Zone and persist into the lower part of the Belemnitella mucronata Zone. These beds are found in the highest chalk on the Isle of Wight, Portsdown and the Sussex coast east of Bognor Regis and form the Portsdown Chalk Member. The basal marker is the Portsdown Marl at the Farlington British Gas Store and Whitecliff (boundary and holostratotype sections). These six members are divided into groups of beds within which numerous lithological marker horizons are identified. Conspicuous marl seams are chosen as the key markers dividing the members and beds and these seams can be recognised on geophysical borehole logs as well as in the field. The relationship of this stratigraphy to geophysical borehole logs, to the key fossil marker bands and to Upper Cretaceous Stage boundaries is shown.