Old Red Sandstone sedimentation in the northeast of Ireland

Alluvial sediments of Continental Old Red Sandstone aspect have been examined in northeast Ireland. These are of both Lower and Upper Old Red Sandstone age, separated by a major unconformity. Thick, fining upwards sequences of Lower Old Red Sandstone sediments were deposited within basins which lie along the projection of the Midland Valley of Scotland. The northern margins of these basins (in County Antrim, Tyrone and Fermanagh) were bounded by an active fault system that has the characteristics of the Highland Boundary Fault of Scotland. Although these basins have a similar palaeogeographical setting, and contain sediments of similar provenance to the Lower Old Red Sandstone of the Midland Valley of Scotland, deposition was not within one elongate basin. Within the Midland Valley of Scotland, sediment dispersal was directed by a regional southwesterly palaeoslope. In County Antrim, Lower Old Red Sandstone sediments (the Cross Slieve Group) were deposited in an embayment that opened to the northeast, suggesting a confluence of drainage from Scotland and Antrim. In County Tyrone and Fermanagh, sediments of the adjacent Dromore and Pomeroy Groups were deposited in separate basins. The pattern of sediment dispersal within these basins appears to have been predominantly to the south and west. At Ballymastocker, County Donegal, coarse grained sediments were deposited in a small fault controlled basin unrelated to the major development of Lower Old Red Sandstone basins further north. Sediments of Upper Old Red Sandstone age in the north of Ireland comprise thin sandstones and conglomerates of local derivation which conformably underlie the Carboniferous. The distribution of these rocks is not related to the Lower Old Red Sandstone basins and deposition was not in tectonically active regimes except in northeast Antrim, where alluvial fan sediments (the Red Arch Formation) overlap the Lower Old Red Sandstone. These sediments, previously considered to be Triassic in age, have a close resemblance to the Upper Old Red Sandstone of the western Midland Valley of Scotland