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

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Name

Proposed as the Pre-Larry Bane Chalk White Limestone by Fletcher (1977) as an informal term to cover the Santonian and lower Campanian Chalks below the lowest of three distinctive and traceable erosion surfaces marking the Larry Bane Chalk Formation.

Type section

Defined at the type sections of its constituent formations. East Antrim Depositional Basin around Glenarm [D 310 150] and marginally less well developed in the North Antrim Basin in the White Park Bay [D 027 440] area.

Primary Reference Section

White Park Bay [D 027 440].

Formal subdivisions

Fletcher (1977) proposed five members that are herein considered as formations. They are in ascending order the Cloghfin Sponge Formation, Galboly Chalk Formation, Cloghastucan, Creggan Chalk Formation, and the Boheeshane Chalk Formation.

Lithology

Chalk with flints and a basal glauconitic arenaceous member.

Definition of upper boundary

Taken at the base of the Larry Bane Chalk Formation where it is marked by the lower of three persistently strong erosion surfaces.

Definition of lower boundary

Rests unconformably on the Hibernian Greensands Formation within the deep depositional basins and on a variety of older deposits as the subgroup oversteps onto the bounding structural highs.

Thickness

37.19 m from type-sites of constituent members. 26 m in the East Antrim Depositional Basin south of Glenarm.

Distribution

Present in the East Antrim and North Antrim basins overstepped by the Ballintoy Chalk Formation and higher units over structural highs.

Previous names

None formally but may be considered as equivalent to those defined as ‘Beds earlier than the zone of Belemnitella mucronata’ by Hancock (1961). This group of sediments are usually easily recognised by the presence above of the characteristic Larry Bane Chalk Formation and is reported (Fletcher, 1977) to be recognisable within downhole geophysical logs.

Parent

Ulster White Limestone Group.

Age and biostratigraphy

Upper Cretaceous, Santonian to Campanian. Uintacrinus socialis to Gonioteuthis quadrata and basal Belemnitella mucronata zones.

References

Fletcher (1977); Griffith and Wilson (1982).