The Ballyshannon Limestone and Basal Beds of the Carboniferous of north west Ireland

Abstract

A revised palaeogeographical reconstruction of the Visean sediments of N.W. Ireland is presented; it is based on the detailed analysis of the coast sections of Counties Sligo and Donegal previously ascribed to the Ballyshannon Limestone (C2S1) and Basal Beds, and reconnaissance of inland areas. The stratigraphy of these outcrops is reappraised in the light of further studies of the lithology, palaeontological assemblages, bore-hole core records and structural analysis. The effects of faulting are considerably greater than formerly appreciated; hence it is argued that previous descriptions of the Ballyshannon Limestone and Basal Beds have involved erroneous correlations of fault blocks. Thus in addition to the Basal Beds the Carrowmoran Sandstone, a western extension of the Mullaghmore - Kildoney - Mountcharles Sandstone outcrop, of approximately S2 age, was ascribed to the basal Carboniferous arenaceous facies. Similarly much of the Ballyshannon - Ballina Limestone succession can be correlated with beds of post C2S1 age. Accordingly the Carricknacusha Shale is regarded as the western extension of the Benbulben Shale (S2D1); while representatives equivalent to the Glencar and Dartry Limestones (D1) are also found in the beds previously attributed to the C2S1 carbonate transgression. The constancy of the various lithostratigraphical units facilitates correlation over a wider area than was previously appreciated and many of the facies variations previously inferred have proved unnecessary. Accordingly the overall palaeogeography has been simplified, though at individual horizons detailed facies patterns are described in both deltaic sediments, and coral-brachiopod associations developed marginally between the dysphotic tropical belt and shallow, warm waters of the open shelf. The influence of the positive element represented by the Ox Block is shown to have been over a wider area than was formerly thought, although its influence in time was restricted to. The extension of the correlation beyond this area, westwards into the Ballina syncline and southwards to Carrick-on-Shannon has necessitated the review of the Ballina succession. In that district the pre-S2 beds are demonstrably thinner than previously realised. From the three areas it is possible to reconstruct a north easterly transgression of the Visean sea in times from a more calcareous environment of Ballina, across the positive Ox Block, into the area of maximum deltaic sedimentation in Co. Donegal. This was followed by a northeasterly regression of deltaic sedimentation during the deposition of the Ballyshannon Limestone. A further deltaic phase, of age, is recorded; during which the area of maximum deltaic sedimentation was east of Carrowmoran, while its southwestern limits cannot have lain far from Carrowmoran and Ballymote, since Ballina and Carrick-on-Shannon occupied the off-shore mud-zone facies.