Category:Northern Ireland Chalk nomenclature (Ulster Cretaceous Province) - Ulster White Limestone Group: Difference between revisions

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The White Limestone of Hancock (1961). Ulster White Limestone Formation proposed in Fletcher (1967, 1977). Elevated to Group status herein.
The White Limestone of Hancock (1961). Ulster White Limestone Formation proposed in Fletcher (1967, 1977). Elevated to Group status herein.


======''Type section''=====
=====''Type section''=====
Numerous exposures of the constituent formations (see formal subdivisions below for reference to those divisions) with no one site containing all of the stratigraphical elements included within the group.
Numerous exposures of the constituent formations (see formal subdivisions below for reference to those divisions) with no one site containing all of the stratigraphical elements included within the group.



Revision as of 12:18, 10 February 2014

Name

The White Limestone of Hancock (1961). Ulster White Limestone Formation proposed in Fletcher (1967, 1977). Elevated to Group status herein.

Type section

Numerous exposures of the constituent formations (see formal subdivisions below for reference to those divisions) with no one site containing all of the stratigraphical elements included within the group.

Primary Reference Section

Many individual type sections for the component formations of the group.

Formal subdivisions

Fletcher (1967, 1977) proposed fourteen members which are herein considered as formations, in ascending order; the Cloghfin Sponge Formation, Galboly Chalk Formation, Cloghastucan Chalk Formation, Creggan Chalk Formation, Boheeshane Chalk Formation, Larry Bane Chalk Formation, Ballintoy Chalk Formation, Glenarm Chalk Formation, Garron Chalk Formation, Portrush Chalk Formation, Ballymagarry Chalk Formation, Tanderagee Chalk Formation, Port Calliagh Chalk Formation and Ballycastle Chalk Formation. He informally divided the succession into a Pre-Larry Bane Chalk White Limestone, Larry Bane Formation and Post-Larry Bane Chalk White Limestone parts. The two divisions above and below the Larry Bane Chalk Formation are regarded as subgroups herein but their name is shortened to the by the omission of the white limestone descriptor.

Lithology

Principally limestones (hardened chalks) with subordinate flint, marl, sponge bed, hardground, conglomeratic and fossil acme bands, with glauconite grains where formations become condensed over structural highs.

Definition of upper boundary

Eroded contact where the group is overlain by the Palaeogene Antrim Basalts or Quaternary deposits.

Definition of lower boundary

Rests disconformably on various members of the Hibernian Greensands Formation and oversteps (unconformable) onto older pre-Cretaceous rocks. Internal overstep within the group principally at the level of the Post-Larry Bane White Limestone Subgroup (informal term defined below) places successively higher formations at the base of the group over structural highs at the margins of the principal basins.

Thickness

Highly variable dependant on the position within structurally controlled basins (environments) of which five were named by Fletcher (1977) as the Hebridean Basin (presumably incorporating his Londonderry Shelf and North Antrim Basin), the Highland Border (also termed the Dalradian Massif), the Midland Valley (usually termed the East Antrim Basin) and the Southern Uplands. Up to 150m+ of ‘White Limestone’ was encountered in the Aughrimderg Borehole [H 880 685] (Wilson , 1972, p 56) but the thickest and most complete development at outcrop is found on the North Antrim coast where 120m of beds are present, although not at any one locality. The Geological Society Special Report on the Cretaceous (Rawson et al., 1978) gives a value of 130m.

Distribution

Throughout Northern Ireland. Fullest development in the North Antrim Basin with significant successions in the Midland Valley (East Antrim Basin) and the Southern Uplands as defined in Fletcher (1977).

Previous names

Upper Chalk (White Limestone) and Lower Chalk (Glauconitic Chalk) by Portlock (1843); Upper Chalk divided into an upper White Limestone (or Hard Chalk) and a lower Chloritic Chalk (or Basement-bed of the White Limestone) by Tate (1865); Reid (1958) included a ‘Basal Conglomerates’ within Tate’s definition of the Chloritic (correctly termed Glauconitic) Chalk. Ulster White Limestone Formation of BGS publications and Fletcher (1977).

Parent

None

Age and biostratigraphy

Upper Cretaceous, Santonian to Maastrichtian. Uintacrinus socialis to Belemnitella occidentalis occidentalis (see Wood, 1967) zones.

References

Fletcher (1967, 1977); Griffith and Wilson (1982).

The following entries, in stratigraphical order from the base, for the subgroups and formations of the Ulster White Limestone Group form part of the Lexicon derived from its precursor Dic_Strat (as code only entries). They are not shown individually upon maps of Northern Ireland but are described in various publications from the Province (see note above concerning the adoption of Ulster White Limestone Group as the formal notation for this succession thus raising the rank of each of the following).