Post‐Caledonian southern Ireland witnessed the development of a NE‐SW orientated half‐graben known as the Munster Basin. More than 7 km of non‐marine sediments accumulated in the basin during the late Middle and Late Devonian. Marine conditions became established in the southern part of the basin at the end of the Devonian. In this paper, a model for the evolving style of sedimentation in the basin and its periphery is constructed with the aim of identifying the major factors which controlled sedimentation patterns and the architecture of the basin fill. The depositional history of the basin is considered in terms of four successive episodes. During Episode I, gravelly alluvial fans flanked upland areas around the northeastern and northern basin perimeter. These graded southwestwards to a floodplain dominated by sheet‐floods. In the western part of the basin, the first of three major fluvial influxes into the basin commenced. During Episode II, the first influx developed into a large sandy braided complex. The sediment was derived from a distant source area located to the north and west of the basin and was transported diagonally across the basin towards the southeast. Episode III witnessed a second influx which drained into the basin from the northeast and north. River channels were of low sinuosity and graded distally to an ephemeral playa lake. Episode IV was marked by a third fluvial influx from the west and northwest. This was confined to the southern half of the basin and drainage was directed towards the east. The fluvial distributaries were flanked by permanently flooded overbank areas. This influx coincided with the first marine transgression which advanced westwards. The end of Episode IV coincided with the beginning of the Carboniferous and was marked by a major marine transgression. Sediment input to the basin was influenced by stable areas occupied by granitic plutons on either side of the basin and a southward downthrowing fault along its northern margin. The drainage direction was principally controlled by E‐W trending within‐basin faults and an E‐W trending stable area located to the south. The basin was fundamentally of the axial transport type, the main drainage having been directed towards the east though there was also a strong lateral influx from the north, northwest and northeast. Stable areas around the, basin periphery resulted in either no sediment preservation or sequences of multistorey channel deposits while thick sequences dominated by fine‐grained floodplain or overbank deposits characterized areas of higher subsidence rate within the basin. Movement on the northern basin‐margin fault was probably the major cause of the first fluvial influx, while regional subsidence of the basin and its northern periphery resulted in the second influx. The third influx was a response to local subsidence in the southern part of the Munster Basin. This also contributed to the simultaneous westward marine transgression in this area towards the end of the Devonian. Source area denudation and re...

The Munster Basin of southern Ireland contains a thick (7 km +) succession of Old Red Sandstone sediments interpreted as the product of various alluvial processes. The present study presents a preliminary sedimentological and palynological analysis of a grey succession informally known as the Foilcoagh Bay Beds, which is the lowest unit of the Sherkin Formation exposed in the southern part of the basin. Sedimentological analysis of the succession suggests that it is the product of sinuous distributary channels, flanked by permanently flooded overbank areas that endured occasional crevasse splay floods. These conditions evolved into a protected lagoon or lake that received periodic high energy floodings from an adjacent marine environment. Palynological study has refined the age of the Foilcoagh Bay Beds as mid-Frasnian time. Palynofacies analysis has provided direct evidence of marine influence as revealed by the presence of marine microfossils and abundant amorphous organic matter at some dark grey mudrock levels. This suggests deposition in a well-established lacustrine or lagoonal environment in which anoxic conditions prevailed at intervals and which was subjected to a period of marine incursion. The recognition of a marine influence in the Munster Basin at an early stage in its history has several important implications, including the following (1) previous models for the basin that suggested an enclosed centripetally draining entirely non-marine system have to be re-evaluated; (2) the drainage pattern and direction of the marine incursion were probably controlled by localized subsidence along an east-east direction; initiation of subsidence associated with the development of the east-west-trending South Munster Basin may have commenced much earlier than previously considered; (3) the occurrence of marine conditions has been tentatively correlated with the Rhinestreet mid-Frasnian sea-level highstand; (4) the recognition of marine conditions early in the known history of the basin provides a pathway for fish to have entered the basin; (5) the basin may have had a marine connection throughout much of Late Devonian time, opening the possibility for base-level control of the alluvial systems within the basin by external eustatic factors.

This paper describes the sedimentation style associated with the basal Carboniferous transgression in southern Ireland and the influence which this event had on the palaeogeography of the region. The transgression as marked by the base of the Carboniferous succession is shown to represent one of several genetically related transgressive pulses which commenced during the Late Devonian. At this time an east‐west trending graben, the South Munster Basin, developed in southern Ireland. This was initially a non‐marine depositional site in which sediment was derived from the north and west. Subsidence and eustatic sea‐level rise resulted in a marine transgression which proceeded in a rhythmic style resulting in a number of transgressive‐stillstand pulses. The first transgressive pulse (T1) advanced in a westerly direction along the basin axis resulting in the development of an epicontinental‐like sea. The shoreline remained essentially static along the northern basin margin initially until a second transgressive pulse (T2) resulted in expansion of the marine area. The latter proceeded by gradual northward erosive advance of a barred coastal area as far as the northern basin margin where the stability of the bounding platform halted its progress. Erosion of the barrier shoreface was insufficient to destroy all the backbarrier lagoonal deposits which are preserved as a thin transgressive diachronous unit which grades northwards to a coastal alluvial plain. Immediately preceding the basal Carboniferous transgression (T3), a shallow, wave‐dominated, storm‐influenced shelf sea occupied the basinal area. Two sublittoral sand bar complexes developed on the shelf under the influence of shore‐parallel current regimes, apparently derived from source areas located on either side of the epicontinental sea The basal Carboniferous transgression took place in two pulses. The first (T3a) resulted in a rapid reduction in sand supply to the shelf and deposition of clay. The barrier shoreline responded by erosively retreating across the lagoon, leaving a transgressive lag in its wake. Its northwards advance was, however, limited due to the relative stability of the northern platform. Sand supply to the shelf was completely terminated in the second pulse (T3b) and the barrier rapidly migrated erosively across the northern platform for a considerable distance such that the coastal plain is overlain by a thin transgressive lag. This transgressive phase was immediately followed by shoreline stillstand and progressive shallowing of the shelf. An open sandy shelf developed on which offshore sand bars accumulated under a storm and wave dominated regime. Clay deposition continued in the deeper part of the basinal area but was eventually terminated as the shelf sands prograded centripetally into the basin. The main factor that controlled the style of the overall transgression was an interplay between eustatic sea level rise and basin subsidence. The rate of relative sea‐level rise together with the effect of differential subsidence and f...

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