Chalk Group Lithostratigraphy: Northern Ireland - Portrush Chalk Member

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The Portrush Chalk Member (stratotype: Ballymagarry Quarry [C 890 408]), 14.28 m thick at the type locality, comprises much less massively bedded chalk than the majority of the White Limestone Formation, and can easily be recognised in the field as the interval of chalk separating two adjacent intervals containing giant-sized flints (Fletcher, 1977). Concentrations of inoceramid bivalve shell fragments occur at several levels, and there are locally developed hardgrounds associated with glauconitised pebbles, named the South Antrim Hardgrounds (Wilson & Manning, 1978). Persistent bedding planes delineate the member (Fletcher, 1977), which has a rich fauna dominated by echinoids (Echinocorys, Galerites & Cardiaster) and brachiopods (Carneithyris, Kingenella & Cretirhynchia arcuata) that suggests a correlation with the Beeston Chalk of the East Anglian Chalk Group succession (Wilson & Manning, 1978).

Macrofossil Biozonation: B. mucronata Zone s.l.

Correlation: see Correlation with other UK Chalk Group successions

References[edit]

FLETCHER, T P. 1977. Lithostratigraphy of the Chalk (Ulster White Limestone Formation) in Northern Ireland. Report of the Institute of Geological Sciences, No. 77/24.

WILSON, H E & MANNING, P I. Geology of the Causeway Coast, Vol. 2. Memoir of the British Geological Survey of Northern Ireland.

See: Beeston Chalk