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

Abstract

The Silurian succession of North Galway is relatively well constrained in terms of environmental analysis and, in its lower half, palaeontologically. The initiation of a late Llandovery marine transgression can be demonstrated over fluviatile red sandstones. The deposition of shallow-water conglomerates at the base of a turbidite sequence within the succession indicates the long-lived presence of a channel system that was probably fault controlled. The back-stripping method allows a subsidence curve to be constructed for this succession. It demonstrates an initial period of rapid rift-related subsidence followed by a short-lived hiatus that may be due to the cessation of subduction in this part of the Caledonides. A comparison of the Galway subsidence curve with that of the Silurian succession at Girvan in Scotland shows strong, but diachronous, similarities. This tends to support an earlier suggestion that they formed part of a single but partitioned basin throughout most of the Silurian period. Although the eustatic fall in sea level at the Llandovery–Wenlock boundary can be recognized, the influence of tectonic regime and sedimentation rates were the controlling factors in determining relative sea levels.

Structural control of the deposition and subsequent deformation of the Killary Habour-Joyce Country Silurian succession

“…The concave-up backstripped tectonic subsidence curve for the basin indicates rifting during deposition of the Lettergesh Formation, with subsequent waning subsidence through Salrock Formation deposition (O'Connor and Williams 1999). The indicated c.3km across-fault elevation between the sSu local regional elevation at Currarevagh (Corrib) and the projected depth of the sSu west of the MVFZ, (prior to 'late Caledonian' shortening deformation), matches the observed >2.8km thick Silurian section preserved in northwest Connemara.…”

“…The Tonalee Formation is largely absent in the west and the overlying Lettergesh Formation represents a significant change in provenance and in depositional processes that has been interpreted as a response to an increased rate of faulting (O'Connor and Williams 1999). This led to subsidence outstripping sediment supply, with sediment gravity flows, mainly high-density turbidity currents, carrying sediments in the deeper water.…”

Scottish Silurian shorelines

“…for the active volcanism adjacent to the area (O'Connor & Williams 1999). Progressive shallowing occurred in the upper part of the succession, during deposition of the Lough Muck and Salrock foimations.…”

“…Stormgenerated sediments are developed throughout the marine sequence and volcaniclastic horizons apparently had little effect on the long-term evolution of the benthos (Harper et al 1995). Nevertheless, the contrasting Silurian histories of western Ireland, Girvan and the Pentlands can be accommodated in a fragmented basin, where the relative positions of blocks was controlled and partitioned by oblique-slip faulting (O'Connor & Williams 1999).…”

Caledonian strike-slip terrane accretion in W. Ireland: insights from very low-grade metamorphism (illite–chlorite crystallinity and b0parameter)

“…Northwards, it unconformably overlies the Rosroe Formation and Lough Nafooey Group. The succession represents alluvial deposits overlain by marine sediments (O'Connor & Williams, 1999). Phillips (1974) correlated the North Galway, Croagh Patrick and Louisburg–Clare Island groups, while Williams & Harper (1991) suggested that they were at least parautochthonous with respect to each other.…”