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

Name

As first published called the Ballykeel Chalk in Fletcher (1967). The Creggan Chalk Member was first proposed in Fletcher (1977) and adopted in BGS publications subsequently. It is regarded as a formation herein.

Type section

Gortin Quarry, Creggan [D 277 187]. The type section of the Ballykeel Chalk in Fletcher (1967) is on the southeast coast of Island Magee at [J 9710 4860].

Primary Reference Section

Best developed in the North Antrim and Midland Valley Basins particularly in the ‘down-warp areas around Garron and Hillsport. Other localities within these basins to note are at White Park Bay, Boheeshane Bay and Murlough Bay in North Antrim, in Church Bay on Rathlin Island and along the East Antrim coast (Midland Valley Basin). This formation is more widely spread than all older formations overlapping significantly onto the Londonderry Shelf to near Dungiven, onto the Highland Border Ridge as seen at Carneighaneigh Quarry, and onto the Southern Uplands as far south as Collinwell in south Belfast. The stromatolitic surface becomes very prominent in the Southern Uplands area where it was formerly known by the quarrying term ‘mulatto’.

Formal subdivisions

None

Lithology

Limestone (chalk), with common Inoceramus fragments (up to 20%) and many small burrow-fill flints scattered throughout. The upper part contains several green-coated pebble horizons that coalesce towards the basin margins to form a single conspicuous hardground with rolled and mineralised fossils most notably belemnites and echinoid steinkerns. Glauconitic in marginal overlap facies where the uppermost beds/surfaces are attenuated, gritty, glauconitic and algal-topped (stromatolitic).

Definition of upper boundary

Taken at the highest of several green-coated chalk pebble beds within the Inoceramus-rich limestone. This unit is termed the Bendoo Pebble Bed (of member? status) in the North Antrim Basin. Overlain by the Boheeshane Chalk Formation.

Definition of lower boundary

Point in the succession where there is a sudden increase in Inoceramus fragments. At Creggan this occurs 0.23m above the Oweynamuck Flint Band within the top of the Cloghastucan Chalk Formation.

Thickness

3.45 m at the type locality.

Distribution

North Antrim and East Antrim basins and the Southern Upland area. Oversteps onto the Highland border area and a limited northern part of the Londonderry Shelf.

Previous names

Creggan Chalk Member.

Parent

Ulster White Limestone Group

Age and biostratigraphy

Upper Cretaceous, Campanian. Lower part of Offaster pilula Zone.

References

Fletcher (1967, 1977).