Features in the field: Ignimbrites of the Yr Arddu syncline

Stephen K Donovan (The Netherlands) In July 1979, I was one of more than 20 undergraduate students at the Department of Geology, University of Manchester, to undertake their final year mapping project in the Snowdonia National Park in North Wales. My mapping area was the Yr Arddu Syncline, about 4km southeast of Beddgelert in Gwynedd. The rock succession is comprised of slates and sandstones, overlain by acid volcanic rocks, with a range of intrusions (mainly acidic), such as microgranite, but also including dolerite. A feature of this succession was the range of features beautifully exposed in the volcanics and intrusions (Figs. 1 to 4). Fig. 1. Features in acidic igneous rocks, Yr Arddu syncline, North Wales (Upper Ordovician).A: [NGR SH 6267 4554] Large acidic fragment (about 60cm maximum dimension) in Pitts Head Tuff Formation. The fragment shows lenticular lapilli. Such large fragments are the exception rather than the rule in the Pitts Head Tuff Formation.B: [NGR SH 6334 4594] Contact between the Pitts Head Tuff Formation (left) and the Composite Intrusion weathered out as a crack to the right of the hammer. Note that the cleavage of the ‘baked’ tuff has not been picked out by weathering, unlike the unbaked rock to the far left.C: [NGR SH 6267 4579] Bedding in Rhyolite Tuff, dipping steeply to the right. Finer grained tuff (left of centre) overlies tuff with small fragments. The finer grained tuff is overlain, in turn (right of centre), by rubbly tuff with numerous small rhyolitic fragments and then … Read More

To access this post, you must purchase Annual subscription, 12 Month Subscription or Monthly subscription.
%d bloggers like this: