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CSPG Special Publications

Abstract


Devonian of the World: Proceedings of the 2nd International Symposium on the Devonian System — Memoir 14, Volume I: Regional Syntheses, 1988
Pages 325-340
Europe and North Africa

Devonian Rocks in Ireland and Their Relation to Adjacent Regions

J. R. Graham, G. Clayton

Abstract

Most Devonian rocks in Ireland are of Old Red Sandstone facies. Nowhere is a conformable upwards transition from Silurian strata proven although one may occur in the 2.9 km thick Dingle Group in the southwest. In contrast, Devonian rocks pass conformably upwards into Carboniferous strata in many areas. In the south, this boundary occurs in marine facies but farther north it is in terrestrial facies. Biostratigraphical control for all but the latest Devonian strata is poor; relying mainly on palynology, although fish faunas are important locally.

There are major differences in facies, ?basin size and thickness of succession between Upper Devonian and older sequences. Lower and Middle Devonian rocks occur in scattered outcrops in the northern part of Ireland. Volcanic rocks are commonly associated with these successions which are dominated by conglomerates. They appear to have accumulated in small, localised basins of variable provenance. Many, if not all, of these basins are associated with major NE trending (Caledonian) faults that are inferred to have contemporaneous strike slip motion. There are close comparisons with the Devonian of Scotland which lies along strike to the northeast.

Thick Upper Devonian sequences are confined to a much larger (12,500 km2) basin in the southern part of Ireland, the Munster Basin. This is an extensional basin with a known maximum sediment thickness of 6.5 km. Volcanism was local. Marginal successions are locally complex with conglomerates and unconformities. They are mainly undated and in places could be older than Upper Devonian. Both palynological and fish data constrain the major part of the basin fill as Upper Devonian. Sedimentation rates were high (0.4 m/1000yr), but the fill consists mainly of fine sand and mud transported in a fluvial distributary system. In latest Devonian time, during major northward transgression, new hinge lines developed and the basin began to invert. In many respects the Munster Basin differs from areas along strike to the east in Western Europe, not least in the high maturation levels of its organic material. In latest Devonian (Strunian) time, widespread alluviation occurred to the north and east of the Munster Basin as the area responded to a rising base level.


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