The sedimentology and palaeogeography of some Devonian sedimentary rocks in Southwest Ireland

Abstract

The area investigated for this thesis is the western end of the Iveragh Peninsula, Co. Kerry, southwest Ireland. The rocks in the area are predominantly of Old Red Sandstone facies, totalling over 6600m in thickness, and were deposited near the northern fault-bounded margin of the extensive half-graben feature known as the Munster Basin. Towards the end of the basin's history, continental conditions were replaced by marginal marine as the sea transgressed into the area from the south. The objectives of the study are to provide a description of the poorly known sedimentary facies from this part of the basin, to interpret these in terms of processes and environments of deposition, and to review and clarify long-standing issues arising from the poorly known age and correlative significance of the lower parts of the Iveragh succession. Mapping and detailed logging of selected sections have been carried out. A number of facies types are identified, and interpretation of these in the light of palaeocurrent and other evidence suggests a terminal-fan model for sediment deposition. Sediment transport was largely by unconfined episodic sheet floods , with transport consistently from the north. Calcrete formation, although limited, suggests a semi-arid climatic setting. Fish fossils and plant miospores date the basin-fill as Upper Devonian in age, possibly just extending down into latest Middle Devonian. Miospore assemblages allow regional correlation with other areas in the Munster Basin. Minor igneous intrusive and extrusive rocks in the area have been mapped and described, and consist of contemporaneous acid pyroclastics as well as late or early post-Devonian intrusives. One dyke in the area is considerably younger, probably Tertiary in age. A thick tuff bed, the Keel Tuff Bed, is traced over part of the area, and may have regional correlative significance.