Early Tertiary basalts and tuffaceous sandstones from the Hebrides Shelf and Wyville-Thomson Ridge

A sequence of Palaeocene basalts and late Palaeocene-early Eocene tuffaceous sandstones has been recovered from three boreholes on the northern Hebrides Shelf and Wyville-Thomson Ridge. The basalts, cored in boreholes 85/5B and 85/7 on the northern Hebrides Shelf, can be seismically correlated with the extensive N Atlantic lava sequence. The basalt in 85/5B displays major element chemistry typical of alkali-basalts but trace and rare-earth element (REE) patterns are typical of olivine-tholeiites. In contrast, the basalt in 85/7 displays geochemical characteristics typical of a depleted tholeiite. Despite their different compositions, the basalts have similar ratios of the highly incompatible elements La and Ta which may reflect different degrees of partial melting of similar source material. The tuffaceous sandstones, cored in borehole 85/2B on the Wyville-Thomson Ridge, were deposited in a warm, shallow marine environment contemporaneous with the reworking of nearby basaltic terrains. Nummulites rockallensis Hinte & Wong occurs abundantly throughout the sandstones; its only previous known occurrence is from latest Palaeocene-earliest Eocene sediments at DSDP Site 117A on Rockall Bank. Regional mapping indicates that the tuffaceous sandstones overlie the basalts, thus the latter can be assigned to the Palaeocene (or older). This is consistent with K-Ar dates of 57.3-63.1 Ma obtained from the basalts.