Sedimentation studies in the Silurian rocks of north-west Galway, Eire
The Silurian rocks of north-west Galway are a clastic sequence at least 2700 m thick resting unconformably on Connemara Schist (Dalradian) in the south and on Ordovlcian rocks in the north-east. The full thickness of Silurian is not seen, as in the west it is aver thrust by Ordoviclan strata, and in the east the younger beds have been removed by erosion. The earliest Silurian rocks (Upper Llandoverian) were probably deposited under terrestrial and deltaic conditions, and were followed by a transgression during which fossiliferous shallow marine sediments accumulated. The overlying Wenlockian succession consists of basal boulder beds and a great thickness (approximately 1500 m) of graded sandstones, passing up into shallow marine fossiliferous siltstone and sandstone. The youngest beds are sparsely-fossiliferous red siltstones and mudstones. The present study is limited to the Wenlockian rocks of the Killary Harbour area, i.e., those above the lowermost terrestrial and shallow marine deposits. The stratigraphy is revised, and new subdivisions proposed. The name Upper Owenduff Group (McKerrow and Campbell, 1960) is retained, but the upper limit of the group is redefined to exclude the upper fossiliferous shallow marine sediments and is divided into two new formations, the Lettergesh and the Glencraff, consisting dominantly of graded sandstones. A new group name, the Killary Harbour Group, is proposed for the uppermost shallow marine portions of the Wenlockian. It comprises the newly-defined fossiliferous Lough Muck Formation, and the Sal rock Formation, with its characteristic red beds.