Chalk Group Lithostratigraphy: Northern Ireland - Creggan Chalk Member

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The Creggan Chalk Member (stratotype: Gortin Quarry, Creggan [IGR D 277 187]), 3.45 m thick at the type locality, is distinguished by the presence of abundant inoceramid bivalve shell fragments, and is the highest of three flinty, shell-rich units at the base of the Ulster White Limestone (Fletcher, 1977). The base of the unit is marked by the sudden increase in inoceramid shell, which is associated with a 'wavy' bedded unit, and the top is a hardground and glauconitised pebble bed, named the Bendoo Pebble Bed (Fletcher, 1977; Wilson & Manning, 1978). However, in some areas Fletcher (1977) recorded that the top of the Creggan Chalk was marked by a stromatolitic biostrome.

The fauna of the Creggan Chalk includes the echinoids Offaster pilula, Echinocorys tectiformis and E. sp. trans. truncata, and the oyster Pseudoperna boucheroni (Wilson & Manning, 1978).

Macrofossil Biozonation: O. pilula Zone (lower part only) (most of zone omitted at Bendoo Pebble Bed)

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.