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

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Name

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

Type section

Old quarry at Parishagh [D 304 156], half a mile northwest of Glenarm in the Midland Valley (East Antrim) Basin.

Primary Reference Section

Cliff sections at White Rocks near Portrush and in the quarry at Larry Bane Head in the northern depositional basin.

The sections around Glenarm in the Midland Valley Basin.

At Kilwaughter [D354 008] and Ballycarry [J449 943].

Formal subdivisions

Includes the North Antrim Hardgrounds (of member status) and is informally divided into units A to D from the base.

Lithology

Limestone (chalk), with flints ranging from small nodular at the base through nodular to tabular flints in the middle part of the sequence to massive nodular at the top. The Formation can be divided into four ‘Beds’ (A to D) by subdivision on bedding planes. In the North Antrim Basin the Formation includes two well-developed hardgrounds, the North Antrim Hardgrounds (of member? status) and the sequence is therefore incomplete. In the Midland Valley (East Antrim) Basin only limited wavy bedding and scattered green glauconitised pebbles indicate interrupted sedimentation. This area is regarded as the ‘standard’.

Definition of upper boundary

Weathered-out bedding plane below conspicuously shelly wavy-bedded chalk at the base of the conformable Garron Chalk Formation.

Definition of lower boundary

Base delimited by a prominent weathered-out bedding plane immediately above the Altachuile Breccia (Bed) of the Ballintoy Chalk Formation.

Thickness

8.10m at the type area.

Distribution

Maximum development in the East Antrim (Midland Valley) Basin and found over the Highland Border high and well developed in the North Antrim Basin. Becomes attenuated including several non-sequences and overstepped by younger formations across the Londonderry Shelf and in the Southern Uplands Belfast district.

Previous names

Glenarm Chalk of Fletcher (1967)

Glenarm Chalk Member.

Parent

Ulster White Limestone Group.

Age and biostratigraphy

Upper Cretaceous, Campanian. Belemnitella mucronata Zone sensu lato.

References

Fletcher (1967, 1977).