The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man

SUMMARY A revised lithostratigraphic subdivision of the Dinantian rocks at the southern end of the Isle of Man is presented. Resting on the Cambro-Ordovician Manx Slate Group is the Langness Conglomerate, representing a locally-derived alluvial fan deposit. The succeeding Derbyhaven and Castletown Formations are both thinly-bedded, fine-grained dark limestones with variable amounts of shale partings; the Derbyhaven Formation has yielded fossils suggesting an early Arundian to Holkerian age, whilst the Castletown Formation appears to be mostly of early Asbian age. The latter is diachronously overlain by the thick Balladoole Formation, composed of lime-mud mounds, of Asbian age. The Poyllvaaish Formation is of similar mudmounds and beds, with original depositional dips of up to 20° and yielding late Asbian to early Brigantian faunas. The penecontemporary Close-ny-Chollagh Formation consists of black limestones and shales enclosing an unusual complex of slipped and foundered blocks from the upper Poyllvaaish limestones, with considerable distortion of the black beds around the blocks. The Scarlett Volcanic Formation consists of subaqueous volcanic breccias, vesicular and pillow basaltic lavas and fine-grained tuffs, with local intercalations of black limestones of early to mid-Brigantian age. Disruption surfaces and contortions, previously thought to be due to thrust-faulting, are now regarded as contemporary mass-flow and eruptive disturbances.