Chalk Group Lithostratigraphy: Northern Ireland - Glenarm Chalk Member

From Earthwise
Jump to navigation Jump to search

The Glenarm Chalk Member (stratotype: Parishagh Quarry, near Glenarm [D 304 156]), c. 8 m thick at the type locality, comprises flinty, locally inoceramid shell-rich chalk, with a locally developed hardground complex in its upper part, named the North Antrim Hardground (Fletcher, 1977). The base of the member is the weathered out bedding plane immediately overlying the Altachuile Breccia, and the top is a similar weathered-out surface. 'Wavy' bedding planes and nodular burrow-form flints occur in the lower and middle parts of the succession, locally associated with glauconitised pebbles, that in some areas become associated with hardgrounds. Fletcher (1977) tentatively suggested that these hardgrounds, were stratigraphically lower than the North Antrim Hardground. Wilson & Manning (1978) designated the Glenarm Chalk below the North Antrim Hardground 'α' and that above 'β', and noted that the chalk above the hardground contained a higher proportion of bioclastc debris, although this subdivision is really recognised on faunal changes. Fletcher (1977) noted that irrespective of the presence of the North Antrim Hardground, the higher part of the succession was characterised by massive nodular flints and the appearance of conspicuous inoceramid shell fragments (the first record of conspicuous inoceramid shell in the post Larry Bane succession). The North Antrim Hardground, typically comprising an upper, strongly developed, mineralised hardground and pebble bed, and a weaker surface c. 0.5 m below, equates with the Catton Sponge Bed of East Anglian Chalk Group successions, at the junction of the Weybourne Chalk (below) and Beeston Chalk (above) (Fletcher, 1977; Wilson & Manning, 1978).

The fauna of the Glenarm Chalk is dominated by echinoids and belemnites (Fletcher, 1977). Below the North Antrim Hardground the brachiopod Cretirhynchia woodwardi and the echinoids Echinocorys and Cardiotaxis are common; the matrix of the hardground contains common Belemnitella and less frequent C. woodwardi, and above the hardground there is a rich meso-fauna of bryozoans associated with the brachiopod Cretirhynchia arcuata and the echinoid Cardiaster cordiformis (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: hardground, Catton Sponge Bed, Weybourne Chalk, Beeston Chalk