The ordovician (caradoc) volcanic rocks of montgomery, Powys, N. Wales

Abstract

The Ordovician (Caradoc, Soudleyan) rocks of Montgomery, Powys are shales interbedded with locally conglomeratic volcaniclastic sediments composed of andesitic detritus. New formal lithostratigraphic units are proposed: Montgomery Volcanic Group comprising in ascending order: Castle Hill Shale Formation, Castle Hill Conglomerate Formation and Quarry Sandstone and Shale Formation. The volcaniclastic strata are reinterpreted as deposits of a submarine volcaniclastic fan system sourced by contemporaneous andesitic island volcanism. The observed diagenetic sequence is typical of marine volcanic sandstones and was dominated by hydration reactions related to the degradation of abundant unstable volcanic detritus. Diagenesis has resulted in the virtual destruction of original porosity in the volcaniclastic rocks.

Age of the ordovician andesitic conglomerate of castle hill, Montgomery, Powys, Wales

“…The term conglomerate is thought to be more applicable than agglomerate because the rock, although related genetically to a volcanic event, is probably not pyroclastic; the clasts are commonly rounded, enclosed by a granular matrix and were probably lithified before incorporation into the flows. The stratigraphy and depositional environments have been described by Dixon (1988).…”