Category:Chalk Group Lithostratigraphy: Northern Ireland

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In Northern Ireland, the Chalk Group is represented by the Ulster White Limestone Formation, comprising intensely hard, flinty chalk, with regionally developed hardgrounds and pebble beds, representing major erosional scour events (Fletcher, 1977; Wilson & Manning, 1978). The hardness of the formation is due to compaction and hydrothermally induced secondary cementation of pore spaces caused by the overlying succession of thick Tertiary basalts (Wilson & Manning, 1978). The base of the Chalk Group is much younger (Santonian) than elsewhere in the UK, and the youngest chalk includes strata belonging to the Lower Maastrichtian, and therefore partly coeval with the top of the Chalk Group in East Anglia (Wilson & Manning, 1978).

The Ulster White Limestone, 120 - 150 m thick, is subdivided into a maximum of 14 members, mainly delimited by erosion surfaces and distinguished primarily by flint type (Fletcher, 1977). At no single locality is the full succession exposed, and the higher members are only locally preserved (Fletcher, 1977). However, it is clear from the descriptions of these units, and detailed lithological logs of the type sections (Fletcher, 1977), that the status of some of these members is questionable. There are few conspicuous lithological contrasts compared, for example, with the Chalk Group lithostratigraphy of Bristow et al. (1997) for southern England. This is acknowledged in part by Fletcher's (1977) description of the Ulster White Limestone as 'possessing a considerable degree of lithological homogeneity', and by the emphasis placed by Fletcher (1977) and Wilson & Manning (1978) on the importance of recognising one particularly distinctive member (Larry Bane Chalk Member), and then subdividing the succession into strata that pre-date that member (Pre- Larry Bane Chalk succession) and strata that post-date that member (Post- Larry Bane Chalk succession). In some cases, the only criteria for subdividing the succession appears to be the presence of laterally persistent erosion surfaces, particularly where these coincide with faunal changes. Overall, the Ulster White Limestone can be seen as an interval of fairly uniform, hard, flinty chalk, locally enriched in inoceramid shell near the base, and at local intervals higher in the succession. There are some conspicuous hardgrounds and flints, that in some cases have been used to subdivide the succession (Fletcher, 1977), but they do not appear to signal any fundamental change in the character of the chalk. Locally, however, there are a couple of intervals in the higher part of the Ulster White Limestone characterised by huge, morphologically distinct flints, for which separate member names seem better justified.

The subdivisions of the Chalk Group of Northern Ireland (and key marker beds) recognised by Fletcher (1977) and applied by Wilson & Manning (1978), are as follows:

DIAGRAM HERE

Macrofossil Biozonation: U. socialis Zone, M. testudinarius Zone, O. pilula Zone, G. quadrata Zone, B. mucronata s.l. Zone, B. lanceolata Zone, B. occidentalis Zone

Correlation: see Correlation with other UK Chalk Group successions

References[edit]

BRISTOW, C. R., MORTIMORE, R. N. & WOOD, C. J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 108, 293-315.

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: Larry Bane Chalk Member