Volcaniclasic sedimentation in a caradocian marginal basin, North Wales

Thesis

A spectrum of volcaniclastic sequences have been recognised in theinfill of a tectonically-active Caradocian ensialic marginal basin inNorth Wales. Volcanism within the basin was bimodal with extrusion ofboth rhyolite and basalt. Pyroclastic eruptions, usually rhyolitic,were extremely energetic with long recurrence intervals. Large volcanicevents, in producing welded sheets of ash flow tuff obliterated preexistingsource terrains; any loose volcanic ash resulting from smallerpyroclastic eruptions was contained within localised volcanic cones andhad little effect on the surrounding sedimentary budget. Depositionalfacies of both eruption styles suggest little intrabasinal volcanicrelief. Correlation of the more laterally extensive units usinggeochemistry provides a useful stratigraphic framework for examinationof the intercalated epiclastic successions.

Several hundred metres of such sediment were deposited in a widevariety of environments. Lines of basement weakness were repeatedlyactivated as faults, controlling both basin configuration and alluvialcatchment areas. A variety of coarse-grained fan- and braidplain-deltatypes are distinguished, controlled by (1) the nature and geometry offluvial depositional systems, and (2) the orientation of coastlines withrespect to directional variation in the intensity of wave energy flux.

Sandstone petrography indicates most delta systems derived theirsediment load from erosion of uplifted granitic and sedimentary faultblocks with a subordinate contribution from coeval volcanism.Integration of volcanological and sedimentary data shows thatvolcanotectonic uplift and subsidence was the dominant control onsedimentation, with the most significant changes in basinpalaeogeogeography related to silicic eruptions. The magnitude and styleof both volcanic events and associated tectonic changes is related tovariation in crustal strength resulting from the emplacement, migrationand vesiculation of magma bodies.

Authors: Orton, G

• DOI: 10.5287/ora-kq0bnvnq9
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English