Abstract
The general geology of an area of Ordovician and Silurian rocks in North Connemara is described. The oldest strata are the Arenigian Sheefry Grits, a thick marine series of slates and greywackes derived from the south. Many of the beds exhibit perfect grading; they are thought to be the deposits of turbidity currents. The Glenummera Slates follow conformably, and are succeeded by the deltaic Mweelrea Grit series, of Middle Ordovician age. Marked facies changes show that the source of sediment lay to the south, and many of the conglomerate pebbles can be referred to the Connemara Schists. In the lower part of the Mweelrea Grits are thick persistent beds of welded tuff, possibly the products of eruptions of Peléan type. Massive graded tuff-beds in the Sheefry Grits are thought to have had a similar origin. Silurian (Llandovery?) quartzites and slates, with a basal conglomerate, rest un-conformably on the Ordovician rocks. A small late Caledonian granite pluton was emplaced by stoping, and associated with it are acidic and appinitic dykes. Minor disturbances occurred during the Upper Ordovician, but the main folding was Caledonian. Right-handed stresses involving the formation of two large tear-faults swung the Caledonian strike round into a west-north-westerly direction.
Published Version
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