Heavy mineral stratigraphy in the UK Trias: Western Approaches, onshore England and the Central North Sea

Abstract

Population studies on the detrital heavy minerals (64–250 μm, specific gravity >2.92) of the Trias in the North Sea Basin demonstrate that there are well-defined regional and stratigraphical variations which are of use in correlation and palaeogeographic reconstruction. Two provinces are recognized: a region comprising the Southern North Sea, onshore England and the English Channel; and a Central North Sea Area.The heavy mineral assemblages of the Triassic sandstones of the Southern North Sea, onshore England and the English Channel are dominated by tourmaline. Wide-ranging correlation between the Southern North Sea and the English Channel has been achieved by using an horizon at which the heavy mineral assemblage is particularly rich in tourmaline. More detailed and local correlation is possible onshore, where rapid changes in the abundance of garnet, zircon, sphene and staurolite indicate the presence of local sources supplying sand detritus to the Trias.The Triassic sands of the Central North Sea are characterized by heavy mineral assemblages dominated by zircon, garnet or apatite. The zircon-rich assemblage is associated with the Smith Bank Formation and is interpreted as having been derived from the reworking of the Permian Sandstones to the west. The apatite-rich assemblage is linked to the Skagerrak and Marnock formations and was probably derived from an eastern Scandinavian source. The garnet-rich assemblage is of more limited extent and is restricted to sediments of Early Triassic age: its origin could have been the metamorphic terrain exposed in the region of the Halibut Horst.Preliminary studies on the chemistry of apatite (%F), tourmaline (%TiO2) and garnet (%CaO) grains from selected samples suggest that there is useful variation for the purposes of regional stratigraphy.