Alluvial sedimentation patterns in the Munster Basin, Ireland

Abstract

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 genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

“…Terrestrial Old Red Sandstone (ORS) facies dominate with placer and fluvial sequences, whilst marine facies began to develop by the late Upper Devonian (Clayton et al, 1980;MacCarthy, 1990). Syn-to post-orogenic intrusions (for example, the Donegal, Leinster, Newry and Corvock granites) associated with the final closure of the Iapetus Ocean and the late stages of the Caledonian Orogeny (also known as the Acadian Orogeny) were emplaced during the Silurian and Early Devonian, c.430-380 Ma (Chew and Stillman, 2009;Halliday et al, 1980;Murphy, 1987;O'Connor, 1975O'Connor, , 1989O'Connor and Brück, 1978;).…”

Fluvial Systems in Desiccating Endorheic Basins

“…Nichols (1987) and Hirst & Nichols (1986) documented the Luna and Huesca Systems in Oligo-Miocene fluvial deposits in the northern part of the Ebro Basin, Spain, and considered that hundreds of metres of channel and overbank deposits were the products of fluvial distributary systems that were 40 km and 60 km in radius, respectively. Deposits of similar character and dimensions have also been recognized from the Devonian of Britain and Ireland, for example the Munster Basin (Graham, 1983;Williams et al, 1989;MacCarthy, 1990;Sadler & Kelly, 1993;Williams, 2000).…”

“…The second main group of examples are from Devonian strata in the North Atlantic area (Fig. 2), where fluvial, lacustrine and aeolian facies have been deposited in endorheic basins under different climatic conditions: southern Ireland (Graham, 1983;Williams et al, 1989;MacCarthy, 1990;Sadler & Kelly, 1993;Richmond & Willliams, 2000;Williams, 2000), east and northeast Greenland (Friend et al, 1983;Kelly & Olsen, 1993) and Spitsbergen (Friend & Moody-Stuart, 1972). A further example of an endorheic basin is taken from the Lower Jurassic of the Hartford Basin, New England (Demicco & Kordesch, 1986).…”

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