The stratigraphy of the arenig series in the Aberdaron to Sarn Area, Western Llŷcn, North Wales

Abstract

AbstractA new interpretation of the Arenig succession in the Aberdaron area is presented based on biostratigraphical correlation. A lithostratigraphy based on section correlation is presented, and three subareas are identified, each with a different stratigraphy.The Sarn Formation comprises the basal ‘flaggy’ sandstone unit in all three areas but is diachronous: in the south it underlies a Moridunian fauna whilst north of the Bryncroes fault it comprises the majority of the Arenig succession and is probably Fennian in age. The Aberdaron Formation is restricted to the south and is dominated by laminated siltstone. The Wǐg Member, a mudstone unit at the base with a Mordunian fauna is found only to the east of the Wǐg–Nefyn fault, whilst the breccio‐conglomerate Porth Meudway Member, at the top of the formation, is restricted to the west of the fault and is probably Fennian in age. The Arenig–Llanvirn boundary is approximately marked throughout the area by a tufaceous unit termed the Carw Formation.The new correlations require the repetition of the succession by a previously unrecognized NNE–SSW trending fault. The junction with the Mona Complex west of Aberdaron is considered to be predominantly faulted.The two subareas south of Bryncroes are sufficiently similar to suggest both belonged to the same deepening basin, probably initiated in the Moridunian. The absence of the Aberdaron Formation and the development of contemporary shallower water facies to the north is taken to indicate this area lay on the footwall of a normal fault bounding the basin to the northwest.