A new map and interpretation of the geology of part of Joyces Country, Counties Galway and Mayo

Abstract

A new detailed (1:25,000) geological map, cross-sections and descriptions are presented of a large part of Joyces Country, consisting of Connemara Dalradian Schists unconformably overlain by both Silurian (Upper Llandovery to Wenlock) and Carboniferous (Tournaisian to Lower Visean) rocks; only the edge of the last has been mapped. The Dalradian rocks contain D2, D3 and D4 folds and are of staurolite to sillimanite grade with late andalusite and cordierite, constrained to between 470 and 463 Ma. Major D3 folds are folded by the D4 Connemara Synform and the complementary D4 Joyces Antiform, which predate the intrusion of the 462 Ma Oughterard Granite. The folding of the Silurian rocks, termed Scandian as it was intra-Silurian, is identified precisely as a short late Wenlock to early Ludlow episode, (430 to 424 Ma; Gradstein et al. 2012), probably Gorstian Stage (427 ± 3 Ma), and hence not end-Silurian (419 Ma). The mechanism whereby the Scandian open folding of the Silurian rocks took place above the relatively high-level rigid Dalradian basement in the northern part of the area is shown to be by closely-spaced nearly vertical E–W faulting and shearing along the strike of the sub-vertical foliation in the Dalradian Schists. This controlled the trend of the folds in the Silurian rocks. At deeper levels in the basement in the southern part of the area, far fewer, and more widely spaced faults involving larger movements are probably related to syn-Scandian folding. At least five major folds in the exposed Silurian are identified. There would have been more folds (now eroded away) in the south of the area above the Dalradian rocks. The area is crucial in exposing part of the brittle-ductile transition in the Dalradian during the Scandian folding and the solution of the reciprocal problems of how the basement controlled the folding of the Silurian rocks and how their folding affected the Connemara basement.