The Lough Nafooey Group is divided into 4 formations. The lowermost Bencorragh Formation consists of spilitic pillowed tholeiites, passing up into the blocky andesite flows, breccias, cherts and basic pillow lavas of the Finny Formation. This is overlain by the Knock Kilbride Formation comprised of spilitized calc-alkali basic to intermediate lavas, often pillowed, that are interbedded with breccias and graptolitic shales. Proximal turbidites containing rhyolite, granite and spilite clasts represent the highest Derry Bay Formation. New palaeontological evidence shows the Knock Kilbride Formation to be of upper Tremadoc (Lancefield 3) age. Petrological and structural studies show that the Lough Nafooey Group was folded and subjected to burial metamorphism prior to the onset of upper Llandovery sedimentation. Assemblages typical of the zeolite, pumpellyite and greenschist facies are recorded. Geochemical studies indicate a similar tectonic setting for the two main phases of eruption of basic lava (Bencorragh and Knock Kilbride formations) and it is suggested that these represent the early development of a Tremadocian island arc possibly related to the Grampian orogeny.

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