Notes on the Glen Coe Cauldron Subsidence, Argyllshire, Scotland

Abstract

General inward dip of the main boundary fault surrounding the Glen Coe cauldron subsidence indicates that the ring fault has the over-all form of an upward-opening cone. The cone shape is substantiated by peripheral upturning of down-dropped volcanic rocks against the encircling ring fault. Some rocks are even overturned. A peculiar zone of rhyolite and associated breccia that puzzled the pioneer geologists (Clough and others, 1909) contains rhyolitic dikes and intrusive breccias. Two rhyolitic dikes, composed of welded pyroclastic rocks, qualify as possible feeders for welded tuffs. Ignimbrites in the volcanic pile confirm repeated explosive activity during the evolution of the cauldron. Marked subsidence occurred after major episodes of explosive activity, as indicated by lake deposits and coarse sediments that overlie the ignimbrites. Flinty crush rock (Clough and others, 1909) is a fine-grained product of fluidization, as suggested by Reynolds (1956). Transported inclusions of volcanic rocks prove that the flinty crush rock is a fine-grained intrusive breccia. Most breccias near the main boundary fault are intrusive or explosion breccias, but the breccia (Hardie, 1963)south of Stob Mhic Mhartuin is probably of sedimentary origin. Fault intrusion (Clough and others, 1909) is a catchall name for a wide variety of plutonic and hypabyssal bodies. The common irregularity of many intrusions is a typical of ring dikes. The classification of igneous ring complexes is discussed. In Britain, there is no unequivocal example of an outward-dipping ring fracture at Ardnamurchan, Mull, or Slieve Gullion, but outward-dipping ring faults are present in the Mourne Mountains. The ring fracture that encircles the downdropped volcanic rocks on Ben Nevis is essentially vertical, as seen in exposures along the north and east sides of the annular fault. At Glen Coe, the writer did not recognize any evidence for resurgent doming.