Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland
Simon P. Todd, 2000. "Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland", New Perspectives on the Old Red Sandstone, P. F. Friend, B. P. J. Williams
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The Late Silurian to Mid-Devonian Dingle Basin occupies a central position within the Iapetus Suture Zone of SW Ireland. The basin is believed to have formed from late Silurian times onwards as a product of sinistral transpression along several major faults within this suture zone. Conglomeratic sediments were deposited by moderately large gravelly fans shed into the basin from the NW (Glashabeg Formation) and the SE (Trabeg Conglomerate Formation). The systems fed into a large apparently through-going sandy, axial river (Slea Head Formation) that flowed towards the NE. The lateral, basin-margin systems were sourced from two disparate source terranes. To the NW lay a basic volcanic hinterland with some intermediate volcanic rocks and limestones, mudstones, sandstones and chert. During the Early Devonian time the SE drainage basin was underlain by sandstones, quartzites, phyllites and limestones probably intruded by a granite. Some of these lithologies can be found in outcrop in the pre-Dingle Group of the peninsula. Others need to be correlated with rocks of the oceanic terranes in the northern part of the suture zone. The southerly derived clasts have corollaries in the rocks of the Avalonian Leinster Terrane south of the suture. Following partial inversion of the Dingle Basin, the southerly hinterland was apparently further unroofed during mid-Devonian time, when the Inch Conglomerate Formation was deposited by alluvial fans shed northwards from a source area formed along the Dingle Bay Lineament. The Inch conglomerates are characterized by distinctive clasts of schist, gneiss, mylonite, tourmalinite and granite. The general picture of the Early Devonian deformation, intrusion by granite and unroofing of terranes currently partially exposed in central and southern Ireland within the Iapetus Suture Zone is largely consistent with clast lithotypes. However, some exploratory isotopic data indicate at least two possible vagaries in the interpretation. First, model Nd TDM ages of Trabeg sedimentary clasts yield several results older than typical southern Iapetean or Avalonian crustal material. This suggests a complex history of sedimentary mixing of material across the developing Iapetus Ocean. Second, two of three Rb–Sr muscovite–whole-rock dates of Inch metamorphic clasts indicate Silurian ages. These data are similar to Rb–Sr dates derived from the Carnsore and Saltees granites in the Rosslare Terrane, perhaps extending the geographical spread of this Silurian deformation.
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New Perspectives on the Old Red Sandstone
From the 1960s onwards, the Old Red Sandstone of both borders of the Atlantic Ocean has acted as a test-bed for the development of new ideas on the interpretation of fluvial, lacustrine and aeolian sedimentary rocks, and the investigation of tectonically-active basins. Much of the earlier reconnaissance work is now being reviewed in the light of further detailed field study, along with new developments in the understanding of the biostratigraphy, palaeobiology, geochronology, pedogenesis and tectonics.
Three general papers review recent work on the stratigraphical and chronological analysis of the Late Silurian, Devonian and Early Carboniferous strata, and summarize present understanding of the tectonics of the basins. These are then followed by twenty-seven contributions covering new work in Eastern USA, Canada, Ireland, Britain, Norway, Greenland and Spitsbergen.