The basal sequence boundary is the Navigation Hardground in basinal areas and the lowest of the suite of hardgrounds that comprise the Top Rock in basin marginal areas (Grant et al., 1999). In the thick basinal stratotype Sussex succession, a thin lowstand systems tract overlies the Navigation Hardground, and includes the paired Navigation Marls (Grant et al., 1999). In the Sussex succession, a transgressive surface is rather cryptically indicated by an interval of nodular burrow-fill flints, but at the basin margin this surface merges with the underlying sequence boundary (Grant et al., 1999). The maximum flooding surface is represented by the Cliffe Hardground in Sussex, and this is overlain by a few metres of rhythmically bedded flinty chalk, inferred to represent a highstand. In marginal areas, the correlative of the Cliffe Hardground merges with the underlying transgressive surface and sequence boundary to form a composite hardground (= Top Rock) (Grant et al., 1999).
GRANT, S F, COE, A L, ARMSTRONG, H A. 1999. Sequence stratigraphy of the Coniacian succession of the Anglo-Paris Basin. Geological Magazine, 136, 17–38.