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

Name

The Cloghfin Sponge Beds Member was first proposed in Fletcher (1967, 1977) and adopted in BGS publications subsequently. It is regarded herein as a formation and the name altered to the Cloghfin Sponge Formation.

Type section

As defined in Fletcher (1967) on the shore [J 4760 9110] near Whitehead and Hillsport, Island Magee.

Primary Reference Section

Restricted to the southeast Antrim region between Little Deer Park near Glenarm and Castle Dobbs. Other principal exposures near Kilcoan [J 461 985] and Cloghfin Port [J 483 938].

Formal subdivisions

None but divided informally into Bed A B and C by Fletcher (1967).

Lithology

Chalk and chalk conglomerate/breccia, glauconitic, with conspicuous hexactinellid sponge psuedomorphs. Three separate beds recognised (A, B, C) each with characteristic shape-variants of the irregular echinoid Echinocorys.

Definition of upper boundary

The exact relationship with the overlying Galboly Chalk Formation is not certain (Fletcher, 1978 p. 8 para. 2). It is taken a well-developed separation plane below which the beds contain arenaceous ‘contamination’.

Definition of lower boundary

Everywhere the base rests unconformably upon eroded Hibernian Greensand Group successions. In the most complete Hibernian Greensands Formation sequences (Kilcoan [J461 985]) the base of the Clochfin Sponge Formation is 4.27m above the conspicuous ‘Inoceramus Bands’.

Thickness

As defined in the type-site the beds are 1.47m thick.

Distribution

Limited to the East Antrim Basin Northern Ireland.

Previous names

Cloghfin Sponge Beds Member (note the term Beds is removed from the new name designation to remove ambiguity).

Equivalent to the ‘Spongiarian Zone’ of Hume (1897) and later called the ‘Sponge Bed’ by Hancock (1961).

Parent

Ulster White Limestone Group.

Age and biostratigraphy

Upper Cretaceous, Santonian. Uintacrinus socialis Zone in part.

References

Fletcher (1967, 1977).