Northern Ireland Chalk nomenclature (Ulster Cretaceous Province) - Ulster White Limestone Group: Pre-Larry Bane Chalk Subgroup: Cloghfin Sponge Formation: Difference between revisions
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Latest revision as of 13:11, 11 June 2014
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).