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

Abstract

The nature and lield relationships o f the Maumtrasna Formation o f the Partry Mountains. Co. Mayo, show that the formation underlies the rcgionally mappable Glenummera Formation. which is of Llanvirn age, and it is laterally equivalent to the Rosroc and Derrylea Formations. also o f Llanvirn age. The formation rests unconformably on two different volcanic domains, the hasic volcanics o f the Bohaun Volcanic Formation to the northwest (of unknown age) and the largely acidic v~lciinics and sediments of the Tourmakeady Volcanic succession t o the southeast (Arcnig). It is not the youngest formation in the Ordovician succcssion of the South Mayo Trough. as was previously thought.This largely conglomeratic unit forms ii coarsening upward sequence deposited on ;illuvinl fans which were inundated by a marine transgression at the base o f the Glcnummcrn Formation. Clasts in thc conglomerates are mainly o f 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.

A revision of the Ordovician stratigraphy for the western Ireland Caledonides and implications for tectonic models

“…Pudsey (1984) and Williams (1984) both ascribed an alluvial fan-fan delta environment to the Mweelrea Formation. Graham (1987) suggested a similar environment prevailing during deposition of the Maumtrasna Formation despite the fact that the supposedly equivalent nearby Rosroe Formation had been interpreted as a submarine fan sequence (Archer, 1977). The implications of this revised stratigraphy for the tectonic model are detailed in Section 6 below.…”

“…The 2 km 2 of green mudstones, informally termed the Lough Shee shales (Figure 1), conformably overlie the conglomerates of the Maumtrasna Formation and were originally assigned to the Glenummera Formation by Graham (1987). However, Harper et al (1988) correlated a trilobite fauna from the shales to trilobite species reported by Pudsey (1984), from the base of the midLlanvirn Glendavock Shale Member in the Mweelrea Formation.…”

“…Graham, 1987;Dewey and Mange, 1999;Clift et al, 2003) have maintained that the Maumtrasna Formation is the lateral equivalent of the Derrylea Formation on the northern limb and the Rosroe Formation on the southern limb of the Mweelrea-Partry Syncline. The Maumtrasna Formation was first placed stratigraphically below the Glenummera Formation based on a study by Graham (1987). This sequence was primarily based on the upper contact of the Maumtrasna Formation with the Lough Shee mudrocks, which was interpreted as a gradational coarsening-upwards sequence (Graham, 1987).…”

“…The Maumtrasna Formation was first placed stratigraphically below the Glenummera Formation based on a study by Graham (1987). This sequence was primarily based on the upper contact of the Maumtrasna Formation with the Lough Shee mudrocks, which was interpreted as a gradational coarsening-upwards sequence (Graham, 1987). However, this stratigraphical suggestion can only be accepted if the Lough Shee mudrock inlier was interpreted as being the Glenummera Formation.…”

An ordovician fauna from Lough Shee, Partry Mountains, Co. Mayo, Ireland

“…In the area of the Maumtrasna plateau north of Lough Nafooey, Williams (1984) has convincingly shown that the distinctive unit, ascribed by Dewey (1963) to the Maumtrasna Formation, is simply part of the Mweelrea Formation. But in the Partry Mountains area Graham (1987) maintains that the conglomerates of the Maumtrasna Formation referred to by previous workers are laterally equivalent to the Rosroe Formation of the south limb succession. It is the purpose of this paper to describe an hitherto undocumented fauna from green mudrocks in the Partry Mountains and to assess its regional significance.…”