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

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

The formation comprises hard to very hard, dense, cryptocrystalline limestone with interbedded argillaceous chalky limestone and mudstone. Thin glauconitic quartzose sandstones are locally present in the lower part of the formation. Typically 25-75 m thick, the formation locally exceeds 120 m, although the lower part is locally eroded, and the whole interval is missing from some contemporary structural highs.

The base is marked by the sharp downward lithological change from black mudstones to chalky limestones at the top of the underlying Hidra Formation. Geophysically the boundary is marked by a sharp downward decrease in gamma log values and an increase in sonic velocity.

The top of the formation is marked by the downward lithological change from argillaceous chalky limestones of the Mackerel Formation to cleaner, denser chalky limestones. Locally part of the Shetland Group overlies the Herring Formation, and in these cases the lithological change is from calcareous mudstones above to dense Chalky limestones below. Geophysically, there is a downward decrease in gamma log values and increase in sonic velocity across the boundary.

In the Outer Moray Firth, the Central Graben and the South Viking Graben, there are four subdivisions of the formation,designated G0 (oldest) to G3-4 (youngest). Their characteristics are as follows:

G0: The base of this interval comprises black mudstones, named the 'Black Band', following use of this term for an allegedly coeval unit in onshore successions in northern England. The Black Band s.l. of Wood & Mortimore (1995) belongs to the Late Cenomanian and Early Turonian, the Cenomanian / Turonian boundary probably occurring above the dark mudstones that form the Black Band s.s. (Wood & Mortimore, 1995).

The Black Band is overlain by calcareous mudstones and limestones

G1: Argillaceous limestones grading up into purer limestones.

G2: Interbedded calcareous mudstones and limestones.

G3-4: Clean limestones

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

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)