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

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

The formation is up to 60 m thick, and present over most of the Southern North Sea, except on some structural highs and where removed by erosion following structural inversion. It comprises chert-rich, hard, dense chalky glauconitic limestone with interbedded argillaceous chalks and mudstones. Dark, carbonaceous mudstone (the 'Black Band') occurs at the base of the formation.

The top of the formation is marked by the downward lithological change from the soft, typically more argillaceous chalks of the overlying Lamplugh Formation to harder, less argillaceous chalky limestone. Geophysically this boundary corresponds with a sharp downward reduction in gamma values and an increase in sonic velocity.

The base of the formation is lithologically marked by the downward change from dark mudstone to chalky limestone at the top of the underlying Hidra Formation. Geophysically this corresponds with a sharp reduction in gamma log values and an increase in sonic velocity.

Age: Early to mid Turonian

Biozonation:

Foraminifera:

• BGS Biozones: inferred to equate with BGS biozones 8 - 11 (pars)

• BGS Biomarkers: contains Biomarker 13

Dinoflagellates: BGS zones 3 (pars) - 6 (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.