The Lower Palaeozoic Rocks of South-West Murrisk, Ireland

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.

Oroclinal bending in the Caledonides of western Ireland: a mid-Palaeozoic feature controlled by a pre-existing structural grain

“…The Mweelrea Formation occupies the core of the Mweelrea syncline, a gently plunging E-W synclinorium. Six major ignimbrite tuff bands occur within the Mweelrea Formation (Dewey, 1963), which vary in thickness up to 10 m. However, they are not all laterally continuous (Stanton, 1960;Dewey, 1963). Sampling was therefore restricted to four of the more laterally continuous bands.…”

Late Ordovician (Sandbian) brachiopods from the Mweelrea Formation, South Mayo, western Ireland: stratigraphic and tectonic implications

“…The formation links the underlying and disparate successions of the northern and southern margins of the basin, now exposed in the opposing limbs of the Mweelrea Syncline (Figure 1; Graham et al, 1985). The sandstones and conglomerates are punctuated by five or six ignimbrites, which allow for detailed correlation along strike and across the syncline (Stanton, 1960;Riggs and McConnell, unpublished data).…”

A tectonic profile across the caledonides of south Mayo

“…This cordillera persisted throughout Ordovician and Silurian times and it is considered likely that the Connemara antiform, and probably also the WNW antiform south of Clifden, were initially developed at this time. Stanton (1960) has demonstrated that the NW-SE Maam Faults (Fig. 1) were moving during the deposition of the Partry Series with a moderate downthrow to the northeast.…”