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	[[Category:Northern Ireland]]		[[Category:The geology of Northern Ireland]]

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W I Mitchell

== Waterloo Mudstone Formation ==
[[File:P947940.jpg|thumbnail|Detailed lithostratigraphy of the Late Triassic (Rhaetian) and Jurassic rocks. (P947940)]]

The succession of conformable strata at the Triassic-Jurassic boundary (Rhaetian-Hettangian stages) is exposed at Waterloo, near Larne [[Media:P947848.jpg| (P947848)]]<ref>Ivimey-Cook, H C. 1975. The stratigraphy of the Rhaetic and Lower Jurassic in East Antrim. ''Bulletin of the Geological Survey of Great Britain'', 50, 51–69.</ref>. Between the top of the Penarth Group (Rhaetian) and the lowest bed in the Waterloo Mudstone Formation with the ammonite ''Psiloceras planorbis'' [[Media:P948079.jpg| (P948079) Fossil 3]], which defines the base of the Jurassic System (Hettangian Stage), are strata with no ammonites that are termed the ‘Pre''-planorbis'' beds’ [[Media:P947940.jpg| (P947940)]]. At Waterloo the ‘Pre-planorbis beds’ of the Waterloo Mudstone Formation are up to 10 m thick.

At the base of the Waterloo Mudstone Formation the Triassic (Rhaetian) age of the ‘Pre-planorbis beds’ is defined by the composition of miospore assemblages recovered from the Larne No. 1 and Langford Lodge boreholes <ref>Warrington, G, and Harland, R. 1975. Palynology of the Trias and Lower Lias of the Larne Borehole. ''Bulletin of the Geological Survey of Great Britain'', 50, 37–50.</ref>, <ref>Manning, P I, Robbie, J A, and Wilson, H E. 1970. Geology of Belfast and the Lagan Valley. ''Memoir of the Geological Survey of Northern Ireland'', Sheet 36 (Northern Ireland).</ref>. Between the base and top of these strata the effects of marine transgression cause an overall increase in the numbers of organic-walled marine microplankton relative to land-derived miospores. The presence of the dinoflagellate cyst ''Rhaetogonyaulax rhaetica'', the index fossil of the latest Triassic Rr Biozone, in the Westbury and Lilstock formations and in the ‘Pre-''planorbis'' beds’ confirms their Rhaetian age [[Media:P947940.jpg| (P947940)]]. The Pre-planorbis beds in Northern Ireland predate the Psiloceras planorbis Subzone of the Hettangian Stage (Early Jurassic), and are therefore Late Triassic (Rhaetian).
==References==
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[[Category:Northern Ireland]]

Waterloo Mudstone Formation, Triassic, Northern Ireland - Revision history

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