Chapter 41 Correlation of Triassic Sandstones in the Strathmore Field, West of Shetland, Using Heavy Mineral Provenance Signatures

Integrated heavy mineral, mineral-chemical and zircon age data show that Triassic sandstones in the Strathmore Field result from the interplay of sediment derived from eastern and western sources. The Early Triassic Otter Bank Formation is interpreted as having a source on the British margin of the Faeroe-Shetland rift. Two main provenance components (recycled Devonian-Carboniferous Upper Clair Group in conjunction with Lewisian orthogneiss) were involved. The overlying Foula Formation (Middle-Late Triassic) was derived from high-grade metasedimentary/charnockitic basement rocks, interpreted as lying in the Nagssuqtoqidian belt of southern East Greenland on the opposite side of the rift. Zircon age data from the Foula Formation also provide evidence for an important Permian igneous event along the proto-northeast Atlantic rift. The switch in sediment supply from easterly-sourced to westerly-sourced detritus is the most clearly defined correlative event in the Triassic succession of the Strathmore Field. Variable supply from a subordinate zircon-rich component (probably of granitic origin) provides a basis for intra-Foula subdivision and correlation. The upper part of the Otter Bank Formation is characterised by a relatively high apatite/tourmaline ratio, believed to indicate the initial appearance of sediment from East Greenland. The construction of the correlation framework for the Triassic succession in the Strathmore Field depends crucially on identification and quantification of parameters that are sensitive to changes in provenance and insensitive to other processes that operate during the sedimentation cycle. This study demonstrates that ditch cuttings and core samples yield closely comparable heavy mineral data, indicating that construction of correlation frameworks can be readily achieved using ditch cuttings samples, although ideally cuttings data would benefit from calibration with core material.