MINOR SEDIMENTARY CYCLICITY IN LATE ASBIAN (UPPER D1) LIMESTONES IN THE LLANGOLLEN DISTRICT OF NORTH WALES

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Abstract
SUMMARY: A detailed lithostratigraphical study of the upper Dinantian (Asbian–Brigantian) limestones in the Llangollen district of North Wales has been made. The identification of minor cyclicity of sedimentation has been used to establish a new lithostratigraphical framework to supersede the former lithological divisions of Morton (1870; 1878). The following new formations are recognised: Ty-nant Limestone (early Asbian), Eglwyseg Limestone (late Asbian) and the overlying Trefor Limestone (Brigantian). In the Eglwyseg Limestone the sequence is divided into 9 to 10 minor cycles by prominent master bedding planes. Each minor cycle suggests alternation of transgressive and regressive periods which led eventually to emergence. Repeated subaerial exposure of limestone is recognised by features including palaeokarstic surfaces, calcareous laminated crusts and K-bentonite clays (palaeosols). The palaeokarstic surfaces were formed by contemporaneous chemical dissolution and the laminated crusts developed by in situ modification of the limestone possibly beneath a soil.