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

Name

The name was first proposed by Tate (1865) as Hibernian Greensand. Hancock (1961) used the term Hibernian Greensands. Hibernian Greensands Formation was adopted in BGS publications and in Fletcher (1967). Elevated to Group status herein.

Type section

Numerous exposures of the constituent formations (see formal subdivisions below for reference to those divisions) in the Midland (East Antrim) Basin and Southern Uplands district.

Primary Reference Section

Cloghfin Port [J 483 938], Magheramorne borehole [J 433 975 and area, Collin Glen [J 269 702] district southwest of Belfast, Kilcoan Old Quarry [J 461 985] on Island Magee.

Formal subdivisions

Divided into four members as in BGS memoir (e.g. Griffith and Wilson, 1982). It is proposed herein that the group be divided at the formation level in ascending order, the Belfast Marls Formation, the Island Magee Siltstones Formation, the Collinwell Sands Formation and the Kilcoan Sands Formation.

Lithology

Glauconitic ‘marls’, argillaceous siltstones and calcareous sandstones, and quartzose sands. Sequence divided into four formations noted above.

Definition of upper boundary

Unconformable at a major regional erosional break. Overlain by various members of the Ulster White Limestone Group.

Definition of lower boundary

Unconformable on a variety of older formations at a major regional erosinal break at the base of the Belfast Marls Member.

Thickness

About 21m to 22m in the Carrickfergus, Larne, Antrim and Belfast sheet areas.

Distribution

Throughout Northern Ireland. As a thin basal Cretaceous unit over structural highs and in the North Antrim Basin, where the group is undivided. Fullest development in the Midland Valley (East Antrim Basin) and the Southern Uplands as defined in Fletcher (1977) where group is divided.

Previous names

‘Mulatto’ a collective term for the greensands, conglomerates and marls beneath the White Limestone (Whitehurst, 1786); the ‘Greensands’ of Portlock (1843); Hibernian Greensand of Tate (1865); part of Hibernian Greensands as defined by Hancock (1961); the Upper and Lower Hibernian Greensands of Reid (1971). Hibernian Greensand Formation in BGS publications and Fletcher (1967).

Parent

None

Age and biostratigraphy

Upper Cretaceous, Cenomanian to Santonian. Various basal Upper Cretaceous zones up to Micraster coranguinum Zone.

References

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

The following entries, in stratigraphical order from the base, for the formations of the Hibernian Greensands 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 Hibernian Greensand Group as the formal notation for this succession of beds thus raising the rank of each of the following from member status).