Dinantian river systems and coastal zone sedimentation in northwest Ireland

Northwest Ireland either formed or lay close to the coastal zone and represented the northernmost extent of marine sedimentation for large parts of the late Dinantian. The succession deposited during the initial early Viséan transgression in Donegal starts with northerly-derived fluvial gravels which pass upwards into coarse-grained fluvial sandstones and associated siltstones. Channel cross-sections are relatively small, suggesting stream widths of a few tens of metres and depths of up to 4m. These data suggest small rivers with limited drainage basins and are inconsistent with a single drainage basin >95 000 km2, which had been proposed previously. Depositional style in the coastal zone was dominated by laterally migrating tidal creeks, initially containing siliciclastic detrita but with the proportion of carbonate increasing upwards. The succession in North Mayo is similar to that in Donegal but generally lacks the fluvial gravel unit. Widespread deposition of limestones and shales was interrupted in late Arundian times by a southerly progradation of first-cycle feldspathic sandstones, the Mullaghmore Sandstone Formation and its correlatives. In the more proximal environments this is represented by fluvial sediments cutting down into the pre-existing carbonate shelf. This has the basic character of a Type I sequence boundary and represents a major lowering of sea level in northwest Ireland. During this clastic influx, large tidally-influenced fluvial channels developed, with channel widths up to 200 m and depths >5 m. This indicates an increase in river size in this area during the Dinantian which may have been due to river capture during the evolution of a relatively low-relief landscape. The regression that led to the southerly progradation of elastics is of similar age to fault movements documented elsewhere in Ireland, and may have a regional tectonic cause.