Abstract Ordovician rhyolitic tuffs in North Wales are correlated on the basis of their whole‐rock trace element composition. Multivariate statistics verify that major ignimbrite sheets are chemically unique, with Zr, Y and Nb being the most useful elements to characterize individual deposits. Fractionation processes during transport and emplacement of the pyroclastic flows results in little lateral variation in the whole‐rock geochemistry of deposits; different diagenetic histories in subaerial and subaqueous environments did not mask recognition of primary ignimbrite units. A partially welded vitric ash flow tuff is correlated with the Pitts Head Tuff ash flow sheet, and a tuff horizon previously assumed to be reworked Pitts Head Tuff is shown to be chemically most similar to the fourth member of the Capel Curig Volcanic Formation. Validity of these correlations is upheld by biostratigraphic and sedimentological evidence. The new correlations necessitate a revision of the stratigraphy in North Wales and indicate a significant lapse of time (perhaps one million years) between emplacement of the Pitts Head Tuff and eruption of the Lower Rhyolitic Tuff of the overlying Snowdon Volcanic Group.
Abstract The Pitts Head Tuff Formation, of Ordovician (Caradoc) age, was emplaced as a thick ( c. 700 m) intracaldera sequence and two outflow units comprising welded acidic ash-flow tuff. The Pitts Head pyroclastic flows were erupted subaerially but the lower and most extensive of the outflows crossed a shoreface, and continued for several kilometres offshore. The flow entered the sea without disruption and, following deflation and tuff emplacement, displaced the shoreface several kilometres to the east and northeast. Post-eruption subsidence in the northeast resulted here in the rapid establishment of environments deeper than had previously existed. The lower outflow tuff is parataxitically to eutaxitically welded in both the subaerial and marine environments. The extremely regular plane-parallel welding foliation of the subaerial tuffs, however, contrasts with the locally highly deformed foliation of the tuff deposited beyond the shoreface. The deformed foliation, associated with irregular zones of intense siliceous nodule development, is ascribed to the upward streaming of water vapour generated at the tuff/sediment boundary. Elsewhere rheomorphism within the tuff was caused by instability resulting from emplacement on slopes related to faulting. Continued movement initiated extensive brecciation, detachment, and local gravity sliding of large rafts of tuff.
Middle Ordovician volcaniclastic sedimentation in North Wales was strongly controlled by volcanotectonic uplift and subsidence. In the Middle Ordovician (Caradoc Series = Trenton Group), North Wales lay within an ensialic back-arc basin on immature continental crust. Both a dominant northeast to southwest structural grain and a lesser northwest trend were repeatedly reactivated as faults, which affected the style of volcanicity and sedimentation. Within over 1 km of siliciclastic sediments, 4 major ash flows exist: the Gwern Gof Tuff (GGT) and Capel Curig Volcanic Formation (CCVF) of Soudleyan age, the Pitts Head Tuff (PHT), and the Lower Rhyolitic Tuff (LRT) of Longvillian age. Following each volcanic episode, paleogeographic configurations completely changed. These changes all occurred within a short time span of a few million years. In each sequence, marine transgressions occurred above tuff units. The complex depositional sequences indicate that extensional normal faulting, tilting of fault blocks, and uplift due to minor strike-slip movements were contributing factors in basin evolution. However, the agreement between paleogeography and known sources of ash-flow tuffs, the general preeruption uplift, and the postemplacement subsidence indicate magmatic processes were even more important in influencing crustal uplift and regional detumescence. Therefore, these processes may be most effective inmore » basins where preexisting structural zones of weakness facilitate crustal movement (e.g., Timber Mount caldera).« less
ABSTRACT Short term variability in delta form and process can be partly explained by the relative strength of hydraulic parameters such as river discharge, discharge variability, wave energy flux and tidal range. However, the calibre or grain size is also important. The amount, mode of transport and grain size of the sediment load delivered to a delta front have a considerable effect on the facies, formative physical processes, related depositional environments and morphology of the deltaic depositional system. The available grain size influences (1) the gradient and channel pattern of the fluvial system on the delta plain; (2) the mixing behaviour of sediment as it discharges into the ambient basin waters at the river mouth; (3) the type of shoreline, whether reflective or dissipative, and its response to both wave energy and tidal regime; and (4) the deformation and resedimentation processes on the subaqueous delta front. Long term aspects of deltaic sedimentation, including a few generalized relationships between sediment supply and physiographic setting, are briefly introduced. The need for further detailed research on modern and ancient deltaic dispersal systems is emphasized, and specific suggestions are given for future research.
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