The structure and stratigraphical relations of the Lough Nafooey Group, South Mayo

Abstract

Abstract:South Mayo occupies a unique position in the British and Irish Caledonides in preserving little deformed Ordovician sedimentary and volcanic rocks between outcrops of a Laurentian cover sequence (Dalradian Supergroup) that shows evidence for the Grampian Orogeny and now lie both to the north and the south. The regionally significant arc volcanic rocks of the Lough Nafooey Group (LNG) display a simple northward dipping and younging succession that shows overall change from more primitive to more evolved magmatic compositions with time. New field data are used to reject the previous interpretation of an isoclinal fold pair within the group. The age range of the LNG remains poorly constrained. The upper parts are early Ordovician in age, but the lower parts are likely to be Cambrian. The LNG can be shown to be unconformably overlain by patches of Floian-aged limestone (Currarevagh Limestone Formation) which in turn is unconformably overlain by clastic sediments of the Darriwilian-aged Rosroe Formation. The Rosroe Formation has a muddy basal subunit (Bencraff Member) in the west that appears to die out eastwards. The clear unconformity bounded stratigraphic sequence of the southern limb of the South Mayo Trough allows rejection of the hypothesis that it represents a dismembered faulted succession. Although there are clearly episodes of early Ordovician tilting, the first major compressional deformation that produced folds affecting the South Mayo Trough occurred sometime between the Late Ordovician (Sandbian) and Early Silurian (Telychian).