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Tectonic Control of Middle Ordovician Volcaniclastic Sedimentation, North Wales

Geoff Orton

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 chan es all occurred within a short time span of a few million years.

1. Sediments beneath the GGT, a tectonically quiet period, were deposited in fluvial-deltaic environments. Fluvial braided-stream paleocurrents are strongly unimodal and reflect uplift to the east where earlier volcanic activity is known.

2. Facies analysis of sediments subjacent to the first member of the CCVF (about 25 km3) indicates a northeastern basin margin with alluvial fans in the north that yield unimodal southwest paleocurrents, and in central areas braided trunk streams that flowed southward, terminating as wave-influenced deltas which passed into shallow marine seas.

3. An opposite paleogeographic situation existed beneath the PHT outflow sheet (about 25 km3). Western basin-margin alluvial fans provided easterly paleocurrents, and a central fluvial facies yielded paleocurrents toward the north, which passed into shallow marine environments to the north. The PHT is correlated with the Llwyd Mawr ignimbrite (about 70 km3) to the west, which was ponded within a northeast-southwest volcanotectonic trough.

4. Between the PHT and the LRT (about 60 km3), nearshore storm-influenced fossiliferous sandstones pass to the northeast into more offshore environments with 15 to 20-m units of siliceous siltstone or swaley cross-stratified sandstones interbedded with cleaved mudstones. The paleoslope partly reflected minor resurgent doming associated with a second PHT and/or tumescence before LRT eruption, which was ponded within a trapdoor-type volcanotectonic depression.

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 in basins where preexisting structural zones of weakness facilitate crustal movement (e.g., Timber Mount caldera).

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.