Volcanic breccia and the Lower Old Red Sandstone unconformity, Glen Coe, Argyll

Abstract

SynopsisAccording to existing accounts Lower Old Red Sandstone volcanic rocks, together with basal pockets of sediments, rest unconformably upon the quartzitic Moinian and Dalradian inside the north-eastern part of the cauldron-subsidence. Remapping of the ground has shown that although an unconformity is almost certainly present in the north, this does not apply to the southern part, where exposed contacts are vertical and strongly transgressive to the sheet-dip of the volcanic rocks. Here the junction is believed to mark the north-eastern margin of a linear vent, the south-western margin of which is covered by volcanic rocks.Most of the rocks previously regarded as sedimentary are now known to be volcanic. They consist of dykes of quartzitic explosion-breccia (formed underground) together with larger amounts of extruded explosion-breccia (vent-breccia) occupying the postulated linear vent, as well as a minor volcanic neck. The little undoubted sediment that exists is not in situ, but is present as inclusions, up to 100 ft across, in the volcanic rocks.