PGK YP lecture

Prof. Brian Williams (Geological Sciences, Universities of Aberdeen and Dublin)

Abstract
The Namurian sediments of the Shannon (Clare) Basin preserve a range of depositional environments from turbidite fans, through slope deposits to deltaic systems. This Upper Palaeozoic clastic suite was accommodated in an intraplate location which had developed over the Iapetus Suture; the Basin’s downwarp history clearly reflecting re-activation of this Caledonian feature. Thickness and facies variations in the Visean/Namurian sediments indicate that the Basin was centered on an ENE – WSW lineament [the Silvermines – Navan “line”] which is believed to be an extension of the Solway Firth Lineament. During the Namurian the Basin was filled by over 1.5 km of sediment. The clastic suite is preserved in the broadly synclinal W. Clare Basin and comprises a fine to medium grained sediment supply derived mainly from the W and SW via fluvio-deltaic. Sedimentation rates were generally high except during periods of rising relative sea level when slowly-deposited marine bands were developed.

In total contrast, the Siluro-Devonian sediments of the Dingle Basin preserves the most complete marine Silurian to ORS magnafacies (+ 4 km thickness) in W. Ireland which is structurally constrained between two fundamental (Caledonian) ENE-trending structural lineaments lying south of the Iapetus Suture zone. These bounding faults, of sinistral strikeslip style, exerted major control on the sedimentary architecture of this Basin fill. The broadly upward-coarsening sediment infill initially reflects the final phases of Iapetan subduction through the accumulation of lavas and volcaniclastics interbedded with shallow marine Silurian sediments; the overlying, continental ORS sediments exhibit a UC motif from lacustrine through ephemeral-fluvial to perennial-fluvial and alluvial fan sediments to complete the Basin fill prior to the late Lower Devonian [Emsian] Acadian Orogeny.

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