Waterloo Mudstone Formation, Triassic, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
W I Mitchell
Waterloo Mudstone Formation
The succession of conformable strata at the Triassic-Jurassic boundary (Rhaetian-Hettangian stages) is exposed at Waterloo, near Larne (P947848). Between the top of the Penarth Group (Rhaetian) and the lowest bed in the Waterloo Mudstone Formation with the ammonite Psiloceras planorbis (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’ (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 , . 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 (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).
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