A resurgent cauldron in the early Paleozoic of Wales, U.K.

Abstract

Abstract The paper presents a controversial interpretation of a mid-Ordovician volcano-sedimentary complex. It deals with the cyclic interdependence of intrusive, volcanic, and sedimentary processes, due to the development of a nearshore resurgent cauldron in the Caledonian fold belt of North Wales. Deformed volcanotectonic features include a resurgent dome and apical graben, surrounded by a moat and peripheral crescentic ring-fault, constituting a caldera 20 km in diameter. The resurgent Snowdon caldera developed through three cycles of ash-flow volcanism resulting from the continuous supply of magma into a shallow magma chamber emplaced beneath the floor of a marine basin. Each ash-flow cycle was preceded by the emergence, above sea level, of a geotumour that subsequently collapsed following eruption and evacuation of the magma chamber. Localized unconformities at the base of individual ash-flow cycles are compared with caldera margin and associated collapse features. Deeper-seated effects of caldera collapse are expressed as gaps in the Ordovician sequence due to normal faulting along the structural boundary of the caldera. Major ash-flow fissure vents were located at points of maximum unloading of the magma chamber by distention faults in its roof. Explosive mechanisms were triggered by rapid pressure release due to tectonic erosion. The presence of a resurgent cauldron implies that the Ordovician succession of North Wales is more complete than recorded in the literature, and that Caledonian structures were largely predetermined by Caradocian volcano-tectonics.