Tectonic control on sedimentation during transgression: a case study from Silurian successions in Ireland and Scotland

The Silurian succession of North Galway is relatively well constrained in t erms of environmental analysis and, in its lower half, palaeontologically. The initiation of a late Llandovery marine transgression can be demonstrate d over fluviatile red sandstones. The deposition of shallow-water conglomer ates at the base of a turbidite sequence within the succession indicates th e long-lived presence of a channel system that was probably fault controlle d. The back-stripping method allows a subsidence curve to be constructed fo r this succession. It demonstrates an initial period of rapid rift-related subsidence followed by a short-lived hiatus that may be due to the cessatio n of subduction in this part of the Caledonides. A comparison of the Galway subsidence curve with that of the Silurian succession at Girvan in Scotlan d shows strong, but diachronous, similarities. This tends to support an ear lier suggestion that they formed part of a single but partitioned basin thr oughout most of the Silurian period. Although the eustatic fall in sea leve l at the Llandovery-Wenlock boundary can be recognized, the influence of te ctonic regime and sedimentation rates were the controlling factors in deter mining relative sea levels.