The stratigraphy of the Chalk Group (Upper Cretaceous) of the Gipping Valley, near Ipswich, Suffolk, UK

Abstract

The Chalk outcrops in the Gipping Valley, northwest of Ipswich are of great significance in providing a rare glimpse of the often Drift-covered inland East Anglian Chalk succession. The exposures show an unusually thick development of virtually flintless and virtually marl-free chalk, unlike any chalk seen elsewhere in the UK. Macrofossil and microfossil data show that this chalk belongs to the Marsupites testudinarius and O ffaster pilula macrofossil zones, and is coeval with the Newhaven Chalk Formation of southern England. The Uintacrinus anglicus Zone appears to be absent. This chalk is herein named the Blakenham Chalk, provisionally regarded as a Member of the Newhaven Chalk Formation. Similar, virtually flintless chalk, up to 63.5 m thick, occurs in boreholes in Ipswich, beneath a variable cover of Cenozoic and Quaternary strata. This chalk belongs to the basal Gonioteuthis quadrata Zone, and extends down through the O. pilula, M. testudinarius, Uintacrinus socialis and topmost Micraster coranguinum zones, equivalent to the Newhaven Chalk and topmost Seaford Chalk formations of southern England. At outcrop and in boreholes, the virtually flintless chalk is overlain by regularly flinty chalk, up to 8.5 m thick. Macrofossil data show that this chalk belongs to the lower part of the G. quadrata Zone, and that records of Belemnitella from this interval probably correlate with an acme of this belemnite in the lower G. quadrata Zone of southern England. Microfossil data are more equivocal, but this may reflect unusual environmental conditions. Lithostratigraphically, the flinty chalk succession is correlated with the lower part of the Culver Chalk Formation of southern England. The unusual lithology of the Blakenham Chalk Member may reflect the influence of local structural highs rejuvenated by Late Santonian-Early Campanian (Wernigerode Phase) tectonism.