Wales was part of a continental convergent plate margin for ∼50 m.y. during Ordovician time. The crustal stress regime was one of sinistral transtension, due largely to oblique subduction. Early uplift and subsequent growth of subaerial arc volcanoes were followed by development of a broad, fault-bound, marine basin (>150 km wide) in an infra-arc or back-arc setting. Within the basin, relatively narrow grabens became sites of pronounced subsidence and bimodal (basalt-rhyolite) volcanism. The grabens developed in fault-splays above steep, crust-penetrating discontinuities that focused extension and plumbing of magmas at these sites. At Snowdon, the Bedded Pyroclastic Formation records submarine and island basaltic volcanism with associated sedimentation at the site of an active rhyolitic caldera volcano. The volcanoes formed on the axis of the Snowdon Graben, and the basaltic succession, which contains several unconformities due to marine abrasion at sea level, shows a complex history of alternate uplift and subsidence. The uplift is attributable to resurgence of rhyolitic magma and was >336 m in total. Subsidence, due mainly to crustal extension, was >500 m at a rate of ≥500 m/m.y.
The Bedded Pyroclastic Formation shows clear evidence of tectonic influence in the timing, location, and style of basaltic magmatism. Repeated reactivation of magmatic plumbing systems demonstrates fault- or fault-intersection controlled channeling of magmas in the upper crust. Episodes of relatively vigorous subsidence preceded copious magmatism, and both intrusive and extrusive basalt magmas exploited contemporaneously active normal faults. Pronounced topographic ponding of extrusive products, and highly variable trends of fault-block subsidence and rotation evident from the volcanogenic sedimentary deposits, testify to chaotic (piecemeal) displacements on faults spaced commonly at < < 500 m. Diverse marine volcaniclastic systems developed in response to active volcanism and/or faulting with attendant littoral abrasion and current reworking. The systems defined include (1) progradational repose-slope volcanic piles and sedimentary fans with associated slope-foot aprons, (2) transgressive shoreline sedimentary aprons reflecting erosion of island volcanoes, and (3) a fault-controlled, emergent-shoreline, sedimentary apron complex. All of the aprons formed pre-dominantly by deposition from high-density turbidity currents; most show evidence of deposition above wave base. Catastrophic sediment supply, rapid aggradation, extreme variability in hydraulic properties of clasts, and considerable vertical displacements led to development of lithofacies and lithofacies associations that are unlike those of broadly analogous nonvolcanic systems.