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Geochemical correlation of Ordovician flow tuffs in North Wales
Author(s) -
Orton Geoff
Publication year - 1992
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350270403
Subject(s) - geology , rhyolite , pyroclastic rock , subaerial , ordovician , volcano , tephrochronology , geochemistry , basalt , volcanic rock , diagenesis , horizon , stratigraphy , volcanism , caldera , paleontology , tectonics , tephra , physics , astronomy
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.