Uppermost Triassic to Lower Jurassic stratigraphy in the Lough Foyle Basin of County Londonderry, Northern Ireland

Abstract

The Lough Foyle Basin is a half-graben that straddles the border between Northern Ireland and the Republic of Ireland and contains sediments that range in age from Lower Carboniferous to Holocene. The basin’s post-rift succession is represented by sediments of the Penarth Group and Lias Group. The lithostratigraphy and biostratigraphy of that interval are revised using new borehole material and existing outcrop. Palaeontological data provide a chronostratigraphic framework and aid palaeoenvironmental interpretations. Foraminifera, ostracods, palynomorphs indicate a nearshore, marginal marine depositional setting throughout much of the Rhaetian with a more marine, shelf and nearshore depositional setting for the Lias Group. The Penarth Group succession is similar to that elsewhere in Northern Ireland but the Lias Group (Waterloo Mudstone Formation) differs in that five distinct members can be recognised (Clooney Mudstone, Drummans Siltstone, Gortmore Mudstone, Tircreven Sandstone and Ballyleighery Mudstone), four of which are newly described. The deltaic and shelf sandstones of the Tircreven Sandstone Member are the only such examples preserved in the Jurassic strata of Northern Ireland and are some of the oldest in the Jurassic of the UK and Ireland, providing evidence of the proximity of the nearby Irish Landmass and representing a useful comparison for Early Jurassic sandstone reservoirs in offshore basins.