The Dinantian rocks of the Sellafield area, west Cumbria

Abstract

SUMMARY This paper presents a summary of the present state of knowledge of the Dinantian (Carboniferous Limestone) rocks of the Sellafield area as a result of drilling investigations for UK NIREX Ltd. An easterly thinning wedge, formed as a result of pre-Permian erosion has a maximum onshore thickness of 300 m north of Sellafield and a feather edge about 1 km east of the Sellafield site. The Dinantian rocks rest unconformably on the Ordovician Borrowdale Volcanic Group and are overlain unconformably by Permian breccias and conglomerates, known locally as ‘Brockram’. The Basal Beds comprise a thin, impersistent succession of siliciclastic beds overlying the sub-Carboniferous unconformity. A maximum thickness of 149 m of Dinantian strata was proved in Sellafield Borehole 3. A thin development of beds of late Chadian age, the Martin Limestone, is overlain by 100m of peritidal to shallow inner ramp limestones of the Frizington Limestone (new name), of Holkerian age, and 42 m of cyclic, platform carbonates of the Urswick Limestone of late Asbian age.