Heavy mineral analysis of Nirex borehole 13B from Sellafield, west Cumbria

Heavy mineral analysis of Nirex borehole 13B from Sellafield, west Cumbria provides the first characterisation of the heavy mineral content of the Ormskirk Sandstone and the upper ~ 100 m of the underlying Calder Sandstone Formation. These sandstones are the two upper units of the Triassic Sherwood Sandstone Group. The non-opaque heavy mineral assemblage is dominated by an assemblage of predominantly rounded to well-rounded grains of tourmaline + zircon + rutile ± apatite. Variations in ATi (apatite-tourmaline index value) define a ~60 m thick zone of apatite-poor sandstone in the uppermost part of the Ormskirk Sandstone. Comparison with previous work on the Triassic sandstones of Cheshire suggests that the upper limit of these apatite-poor sandstones may mark the site of the Hardegsen Disconformity in west Cumbria. The implications of these data for the provenance of these rocks is limited, except that there is no evidence of a change in provenance from the underlying St Bees Sandstone Formation, the lowermost unit of the Sherwood Sandstone Group. The heavy mineral data may be useful for the purposes of stratigraphical correlation of disputed sections of Triasssic rocks in this region (see Section 6, Further work for details).