The CWM Clwyd Tuff, North Wales: A Palaeogeographical Interpretation of Some Ordovician Ash-Shower Deposits

Summary The Cwm Clwyd Tuff of Ordovician age comprises a sequence (0 to 180 metres thick) of ash-shower tuffs rich in pumice and reworked tuffs, with a single unit which is possibly an ash flow deposit. Grain size and thickness variations indicate a volcanic source to the east of the present outcrop. Compositional and grain size changes within graded beds suggest sub-aerial ash-shower deposition of these beds, while the abundance of accretionary lapilli, the poor sorting, absence of rounding, and lack of epiclastic material suggest that much of the ash sedimentation in the eastern part of the area was sub-aerial and probably occurred low on the flanks of a volcanic cone; contemporaneous, sharply incised channels support this conclusion. In the south and west of the area, the ash appears to have been deposited under marine conditions.