The nature and field relations of the Ordovician Maumtrasna Formation,County Mayo,Ireland

The nature and field relationships of the Maumtrasna Formation of the Partry Mountains, Co. Mayo, show that the formation underlies the regionally mappable Glenummera Formation, which is of Llanvirn age, and it is laterally equivalent to the Rosroe and Derrylea Formations, also of Llanvirn age. The formation rests unconformably on two different volcanic domains, the basic volcanics of the Bohaun Volcanic Formation to the northwest (of unknown age) and the largely acidic volcanics and sediments of the Tourmakeady Volcanic succession to the southeast (Arenig). It is not the youngest formation in the Ordovician succession of the South Mayo Trough, as was previously thought. This largely conglomeratic unit forms a coarsening upward sequence deposited on alluvial fans which were inundated by a marine transgression at the base of the Glenummera Formation. Clasts in the conglomerates are mainly of granite and porphyry with a minor metamorphic component. The source is interpreted as an arc rooted on a metamorphic basement which lay to the south and east. The suggestion that this area was present day Connemara finds little support. This area may expose some of the ‘hidden’ geology of the Midland Valley of Scotland.