Chalk Group Lithostratigraphy: Central & Northern North Sea (UK Sector) - Ekofisk Formation

The following details are based on the account of Johnson & Lott (1993)

The formation, which reaches over 100 m in thickness, comprises variably cemented chalky limestone, with horizons of limestone with thin, discrete clay layers. Locally horizons of sandstone and reworked chalk also occur.

The lower boundary is lithologically defined by the downward change from a basal argillaceous chalky limestone to cleaner, chalky limestone at the top of the underlying Tor Formation. Geophysically this boundary corresponds to a downward change to lower gamma log values.

The upper boundary is lithologically marked by the downward change from calcareous mudstones of the overlying Montrose Group to chalky limestone. Geophysically this corresponds with a a downward decrease in gamma log values and an increase in sonic velocity. Where the basal Montrose Group comprises sandstones and reworked limestone, then the corresponding geophysical response is a downward change from a rather erratic gamma and sonic log signature, to a more consistent pattern. A downward change from scattered to consistent dips can be used to distinguish the upper boundary where the base of the Montrose Group comprises relatively pure re-worked limestones.

Within the formation, argillaceous chalky limestone and horizons of sandstone are common in the basal part, and there is a unit of clean chalky limestone at the top.

Age: Early Palaeocene

Bizonation:

Dinoflagellates: equates with BGS zones 19-20

References

JOHNSON, H & LOTT, G K. 1993. 2. Cretaceous of the Central and Northern North Sea. In, KNOX, R W O'B & CORDEY, W G. Lithostratigraphic nomenclature of the UK North Sea. (British Geological Survey)