The Carboniferous Langness Conglomerate Formation, Isle of Man; an alluvial fan rift-phase deposit

Abstract

Re-examination of the Langness Conglomerate Formation (LCF) on the coast near Dreswick Harbour, Isle of Man, has established that it is over 65 m thick, twice previous estimates. It formed as a small stream-dominated fan deposit during syndepositional faulting which controlled thickness and facies variations. The sub-Carboniferous surface shows considerable relief, perhaps as much as 10 m and is intricately brecciated and channelled just beneath the base of the conglomerate indicating a pluvial climate even prior to deposition of the LCF. Palaeocurrent data shows that sediment was derived from the NNW. Low in the LCF the sediment is all local in origin, but more exotic material is present higher up. The succession matures texturally (sorting and rounding improve) and mineralogically (lithic sand decreases) upwards and the succession fines upwards. The finer, upper part onlaps northwards across the older Manx Group extending for over 4 km inland thinning to 8 metres. This is consistent with the view that the Isle of Man formed a regional high during the Lower Carboniferous. The LCF resulted from the initial fault movements that led to inundation of the Isle of Man High in the late Chadian to early Arundian.