Northern Ireland Chalk nomenclature (Ulster Cretaceous Province) - Ulster White Limestone Group: Post-Larry Bane Chalk Subgroup: Portrush Chalk Formation

Name

The Portrush Chalk Member was first proposed in Fletcher (1967, 1977) and adopted in BGS publications subsequently. It is regarded as a formation herein.

Type section

Taken at the south-facing wall of the older and deeper Ballymagarry Quarry [C 890 408], i.e. immediately above the main cave which passes beneath the Portrush – Bushmills Road.

Sliddery Cove Cliffs [C 890 410] east of Portrush

Primary Reference Section

Garron Point [D 301 2390 in East Antrim Basin.

Formal subdivision

Includes the South Antrim Hardgrounds (of Member status) and informally divided into units A to D from the base.

Lithology

Limestone (chalk) with many tabular flint bands and Inoceramus debris in upper part. In the south Antrim area (Southern Uplands High) there are important hardground non-sequences of green-coated chalk pebbles called the South Antrim Hardgrounds (of member? status).

Definition of upper boundary

At a prominent bedding plane above Inoceramus-rich chalk with tabular flint.

Definition of lower boundary

Conformable, on the Garron Chalk Formation, at a bedding plane beneath a prominent continuous tabular flint.

hickness

14.28m in the type section.

Distribution

Found consistently throughout the province but is generally absent from much of the East Antrim (Midland Valley) Basin due to pre-Palaeogene basalt erosion. Oversteps onto older strata over the Londonderry Shelf.

Previous names

Portrush Chalk of Fletcher (1967)

Portrush Chalk Member.

Is equivalent to the Cardiaster Beds described by Wood (in Wilson, 1972). The term ‘Green Beds’ is utilised as a mappable unit in Manning et al. (1970) for the South Antrim Hardgrounds.

Parent

Ulster White Limestone Group.

Age and biostratigraphy

Upper Cretaceous, Campanian. Belemnitella mucronata Zone sensu lato.

References

Fletcher (1967, 1977).