Sedimentary response to ignimbrite emplacement across a fluvial–shallow-marine transition: Ordovician Mweelrea Formation, South Mayo Trough, Western Ireland

The Ordovician Mweelrea Formation is part of the basin fill of the South Mayo Trough in western Ireland and records progradation of alluvial facies northwestward across shallow- and marginal-marine Mweelrea ‘passage beds’ to marine Glenummera Formation facies. Early in Mweelrea Formation sedimentation, the first of a series of ignimbrites was emplaced, causing a disruption in this progradation. The preserved thickness of the ignimbrite is up to ca. 12 m, and the sedimentary response varied depending on the environment that was affected. Eastern exposures of the Mweelrea Formation are coarse grained and represent fluvial facies. Emplacement of the ignimbrite caused overlying sediment to become richer in feldspar as well as coarser grained; heavy-mineral laminations are common here and in many supra-ignimbrite exposures, but the depositional style was unchanged. The ignimbrite overlies decreasing thickness of fluvial sandstone towards the west to where it directly overlies shoreline and shallow-marine passage beds. Here, sandstone that overlies the ignimbrite is fluvial, indicating that emplacement of the ignimbrite brought about an abrupt progradation. At the westernmost outcrops of Mweelrea Formation, the ignimbrite lies at the contact with the underlying marine Glenummera Formation. Marine sedimentation continued after emplacement of the ignimbrite, although a coarse conglomerate horizon immediately overlying the ignimbrite together with development of a delta suggests that sediment input was vastly increased.