Northern Ireland Chalk nomenclature (Ulster Cretaceous Province) - Hibernian Greensands Group: Belfast Marls Formation

Name The Belfast Marls Member was first used in Griffith and Wilson (1982). The term is upgraded to formation status herein

Type section Cloghfin Port area [J 483 938] where three exposures of the formation occur, Woodburn Glen South and 503 m north-west of Knock Lodge.

Primary Reference Section Carrs Glen near Belfast [J 311 793], in a borehole near Magheramorne [J 433 975]. Numerous localities on the Larne, Antrim, Carrickfergus and Belfast sheet areas (see Griffith and Wilson (1982), Page 48, Figure 16A).

Formal subdivisions

None

Lithology

Pale grey strongly bioturbated silty marls with a scattering of dark green glauconite grains (not exceeding 40%). Transitional ‘zone’ at the top contains relatively little glauconite. Fossiliferous with concentrations into discreet laterally continuous shell-beds.

Definition of upper boundary

Conformable, junction arbitrarily drawn at the base of the lowest persistent hard calcareous siltstone of the overlying Island Magee Siltstones Formation. This junction appears at the top of a transitional zone showing an upward decrease in glauconite content.

Definition of lower boundary

Unconformable, oversteps various older units.

Thickness

Maximum thickness of a little less than 3.0m known from a borehole near Magheramorne [J 433 975] in the Larne (sheet 21) area. Minimum thickness of 1.6m over the Knockagh Axis north of Belfast.

Distribution

East Antrim Basin and Southern Uplands area of Northern Ireland.

Previous names

Glauconitic Sands of Tate (1865) and subsequent workers.

Belfast Marls Member

Parent

Hibernian Greensand Group.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli to basal Acanthocerus rhotomagense Zone.

References

Griffith and Wilson (1982).