Silurian collision and sediment dispersal patterns in southern Britain

Abstract

The evidence is reviewed for the timing of collision between the microcontinent of Eastern Avalonia (southern Britain and adjacent areas) and the Laurentian continent. Recent palaeomagnetic results placing Eastern Avalonia in a high (50°) southern latitude in mid Ordovician time are now consistent with faunal evidence for the first time. The resulting apparent polar wander path is evaluated and suggests that Eastern Avalonia detached itself from a southern peri-Gondwanan latitude in the early Ordovician, moved northwards, and approached Laurentia by the late Ordovician. Its western corner probably impinged on Laurentia in the early Silurian and it docked against the Laurentian margin during Silurian and early Devonian time with a component of anticlockwise rotation.This kinematic history is supported by a compilation of sediment dispersal patterns on Eastern Avalonia. A low-volume Ordovician and earliest Silurian supply from within the microcontinent was overwhelmed in late Llandovery time by a large volume of southwest-derived turbidites, probably from the uplifting impact zone to the west. This source was later augmented by a high-volume clastic supply to the north margin of the microcontinent. Eastward migration of this source through Wenlock and Ludlow time reflects the progressive anticlockwise docking of Eastern Avalonia against the Laurentian margin. The earliest sign of a large-volume supply from Baltica is in the late Wenlock, arguing against any earlier hard collision.

Process regime change on a Silurian siliciclastic shelf: controlling influences on deposition of the Gray Sandstone Formation, Pembrokeshire, UK

“…This transgression was punctuated by an episode of local tectonic uplift during latest Aeronian through to earliest Telychian times (Walmsley and Bassett 1976;Clayton 1994). This event correlates with a switch from lateral to axial sediment influx into the basin (Wilson et al 1992;Woodcock et al 1996;Davies et al 1997), being possibly related to the initial docking of Eastern Avalonia with Laurentia (Soper and Woodcock 1990). Shelf exposure occurred in southwest Pembrokeshire at this time (Hillier 2002), with a lowsinuosity fluvial network feeding lowstand submarine fans in the northerly Welsh Basin.…”

“…Sediment was possibly locally sourced from the southern Avalonian hinterland Pretannia (Cope and Bassett 1987), or from uplift created by Laurentian/ Avalonian collision to the west (Soper and Woodcock 1990;Woodcock et al 1996). Sand-grade sediment entering the area had a distal, northern provenance from the Laurentian highlands by early Ludlow times (Sherlock et al 2002).…”

Depositional environment and sequence architecture of the Silurian Coralliferous Group, Southern Pembrokeshire, UK

“…This system, forming a thin interval of the basin fill, correlates basinward with significant lowstand fans within the deep-water Welsh Basin to the north. This event corresponds to an important switch from lateral to axial sediment transport in the basin (Cave 1979), which Soper and Woodcock (1991) suggest might be due to the initial collision of Eastern Avalonia with Laurentia. This collision probably led to transpressional reactivation of structures, and significant uplift within the westernmost region of Eastern Avalonia (Prettania), whilst other areas within the Welsh Basin experienced extension and subsidence (Woodcock and Strachan 2000).…”

“…Further afield, Llandovery volcanics were encountered in the Maesteg Borehole beneath the South Wales Coalfield (Figure 3; Penn 1987), and it seems likely that the area presently confined by the Variscan Front in South Wales, together with the Woolhope and Usk basins to the northeast, encompass a semi-regional rift event within the Eastern Avalonia microplate. It is unclear whether rifting was related to the development of the Rheic Ocean to the south (Pharaoh et al 1991), or to transtension associated with the collision of Eastern Avalonia and Laurentia (Soper and Woodcock 1991). Facies changes within the Skomer Volcanic Group document an easterly increase in shallow marine strata from Skomer Island to Marloes Sands (Bridges 1976), and it is plausible that the half-graben rifted topography defined cuvettes that were periodically flooded from the open marine Welsh Basin to the north and east.…”

Palaeotopography of the northern portion of the Telychian (Silurian) turbidite basin in central Wales

“…Clearly, there was a basin centre near the site of the future Plynlimon Dome during the utilis Sub-Biozone, but by the latest Telychian griestoniensis Biozone it was present no longer as transport within the Pysgotwr Grits was uniformly to the NNE from south of Plynlimon to Dinas Mawddwy (Smith, 1987). Similarly, no source from the northeast is seen in the Wenlock turbidites of the Welsh Basin (Cummins 1969) in contrast with the situation in NW England (Soper and Woodcock 1990). If the northeasterly source is indeed the area of the Berwyn Dome, a possible reason for its demise in the latest Telychian is its post-ravinement subsidence to deep shelf depths during the regional late Llandovery transgression of the Welsh Borderland (Zeigler et al 1968).…”