Chalk Group Lithostratigraphy: Southern North Sea (UK Sector) - Jukes Formation

The following details are based on the account of Lott & Knox (1994).

The Jukes Formation, up to 500 m thick, comprises moderately hard greyish-white chalky limestones with relatively few nodular flint horizons. There are thin argillaceous horizons and the top of the formation is often marked by a thin, indurated unit. It occurs widely in the Southern North Sea, although its thickness is affected by tectonics and post-Cretaceous erosion.

The upper boundary of the formation is lithologically marked by the downward change from soft, flinty argillaceous chalk of the overlying Rowe Formation to harder, less flinty chalk. Geophysically, this boundary is marked by a downhole decrease in gamma log values and a relatively sharp downward increase in sonic velocity.

The lower boundary of the formation is lithologically marked by a downward change from moderately hard, poorly flinty chalk, to consistently harder, more argillaceous and flinty chalks of the underlying Lamplugh Formation. Geophysically, this boundary is marked by a downward increase in sonic velocity.

Age: Santonian - Early Campanian

Biozonation:

Foraminifera:

• BGS Biozones: inferred to equate with BGS biozones 16 (pars) - 21 (pars)

• BGS Biomarkers: contains Biomarker 5, 6, 7, 8 & 9

Dinoflagellates: equates with BGS zones 9 (pars) - 13 (pars)

References

LOTT, G K & KNOX, R W O'B. 1994. Post-Triassic of the Southern North Sea. In: KNOX, R W O'B & CORDEY, W G. (eds) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham.