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

Name

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

Type section

Fallen blocks on the foreshore north of Garron Point [D 301 245], and in Red Bay and Murlough Bay on either side of the Highland Border Ridge.

Primary Reference Section

Restricted to the North Antrim and East Antrim (Midland Valley) basins, and overlaps, and is itself overlapped (at the Benvan track [D 2000 413]) onto the Highland Border Ridge (Massif). In the North Antrim Basin the formation is continuous from Murlough Bay to the Portrush-Downhill area but diminishes between Oweynamuck and Portbraddan with a concomitant loss of flints. South of the Highland Border Ridge the Formation is first seen at Altmore Burn [D 237 228], is absent around Dooray Bridge but attains its maximum thickness on the south side of Red Bay. It overlaps in the south against the Southern Uplands and is absent south of Castle Dobbs.

Formal subdivisions

Contains within it the Oweynamuck Flint Band.

Lithology

Limestone (chalk), with an overall ‘gritty’ texture and with some flint bands (mainly small and ‘spindle-shaped’) most notably the coalescing nodular flint course termed the Oweynamuck Flint Band (of bed status defined at the eastern bluff in White Park Bay [D 029 448]). Beds of green-coated chalk pebbles at some levels. Common brachials and calyx plates of Marsupites.

Definition of upper boundary

The upper limit is taken at the top of a thin (0.15m) wavy bedded unit underlying Creggan Chalk Formation chalks rich in Inoceramus debris. The wavy bedded unit contains a marked increase in Inoceramus compared to the rest of the Cloghastucan Chalk Formation.

Definition of lower boundary

At the type site boundary is at a prominent bedding plane immediately above a wavy bedded unit at the top of the underlying Galboly Chalk Formation. Outside the type area the basal contact is disconformable and the junction is marked by an erosion surface with glauconitised chalk pebbles, which includes rolled sponge psuedomorphs.

Thickness

Thin succession of 2.33m thickness at its type-site. Absent over structural highs and overlapped by succeeding member.

Distribution

Present in the North Antrim and East Antrim basins and parts of the Highland Border and Southern Uplands areas. Absent over the Londonderry shelf.

Previous names

Cloghastucan Chalk Member.

The Marsupites White Limestone (Reid, 1964).

Parent

Ulster White Limestone Group.

Age and biostratigraphy

Upper Cretaceous, Santonian. Marsupites testudinarius Zone.

References

Fletcher (1967, 1977).