Succession and structure in the Llwyd Mawr Syncline, Caernarvonshire north Wales

Abstract

AbstractA succession of strata ranging in age from Upper Cambrian to Valentian is described. They occupy ground in, and adjacent to, the Llwyd Mawr syncline. Most of the rocks are marine but during mid‐Caradocian (Soudleyan) times a rhyolitic ignimbrite sheet was emplaced and rests disconformably on Llanvirn slates. It has a remanant thickness of 2300 ft and is a single cooling unit. A second volcanic episode (Lower Longvillian) produced 600 ft of bedded pyroclastic rocks with only subordinate ignimbrites. Mid‐Caradocian minor intrusions of rhyolite occur as domes and sills in association with the volcanic rocks. Minor intrusions of dolerite and micro‐tonalite are probably of Caledonian age.Three phases of movement are recognized within the main end‐Silurian Caledonian movements. The first movement phase, F1, produced the main fold architecture and the dominant associated axial‐planar cleavage, S1. The folds, which possess a Caledonoid trend, usually stand vertically, but rarely they verge slightly to the southeast. They are periclinal in style. The second movement phase, F2, was weak and produced a sporadically distributed set of open folds and associated axial‐planar crenulation cleavage, S2, of Caledonoid strike. The axial‐planes dip gently to the southeast quadrant. Folds of the third movement phase, F3, plunge northwest and the associated axial‐planar crenulation cleavage, S3, dips steeply to the northeast. The F3 folds produce a pronounced arcuation in the trace of the Fl structures from an azimuth of 60° in the northwest to 10° in the south of the area.