The Carboniferous rocks of the Donegal syncline

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Abstract
Summary: The Donegal syncline in north-western Ireland is a shallow faulted downfold preserving a thickness of over 4000 feet of Lower Carboniferous rocks. Only the lower zones of the Viséan stage are represented : they rest upon the underlying metamorphic rocks with strong unconformity. Along the south-eastern flanks of the fold the sequence is closely comparable in facies with the Ballyshannon Limestone, the Bundoran Shale and the Mullaghmore Sandstone of the Sligo syncline to the south. Along the northern and north-western flanks of the fold, the Ballyshannon Limestone passes laterally into arenaceous rocks about Lough Eske and Banagher Hill. The overlying shales, though more persistent, carry tongues and lenses of sandstone and conglomerate that thicken northwards until along parts of the northernmost outcrops the whole Viséan succession is almost completely arenaceous. The western outcrops, between McSwyne's Bay and Slieve League, display thick basal boulder-beds succeeded by sandstones ; these pass upwards into highly fossili-ferous limestones and shales overlain by sandy oolitic limestones, together about 600 feet thick. The rocks display strong overlap westwards, 2000 feet of the lower beds at Bruckless not being represented in the Largymore syncline. The minerals of the sandstones and the facies changes indicate the proximity of a northern and north-western shoreline. The floor of the sedimentary basin suffered a near-shore downwarp to accommodate the thick deltaic sediments of the northern outcrops. The Ballyshannon Limestone and the overlying shales carry a fauna of Upper Caninian and probably early Seminulan age. The tectonic structure has a caledonoid grain and is partly inherited. Simple in essentials, it is complex in detail with several large oblique fractures that are in part tear-faults.

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