Contrasting mineralogy of Upper Jurassic sandstones in the Outer Moray Firth, North Sea: implications for the evolution of sediment dispersal patterns

Upper Jurassic sandstones in and adjacent to UK North Sea Block 15/21 (Outer Moray Firth) contain heavy mineral assemblages that provide evidence for differences in provenance, sediment dispersal patterns and depositional histories between the Ivanhoe/Rob Roy Terrace, North Halibut Graben, South Halibut Graben and Tartan Ridge. Variations are demonstrated using provenance-sensitive parameters, including apatite/tourmaline ratios, monazite/zircon ratios and garnet compositions. Seven different sandstone types have been identified on this basis. Scott and Piper sandstones (late Oxfordian-early Kimmeridgian) have contrasting mineralogies, indicating that they were derived from different provenances. However, the two sandstone packages lack internal stratigraphic variations, indicating that each is relatively homogeneous in terms of sediment supply. The succeeding Claymore sandstones (late Kimmeridgian-Volgian) display much greater variation, both regionally and stratigraphically. Heavy minerals suggest that the Main Claymore depositional system was more laterally extensive than that of the Upper Claymore, which was more locally developed and coalesced to a smaller degree. The most significant change in provenance took place with the onset of Main Claymore deposition, when the first major recycling of older Upper Jurassic sand took place. This indicates shedding of sediment from the Halibut Horst area, due either to uplift through intrabasinal tectonism or to a fall in sea level. Sediments in the South Halibut Graben contain minerals indicating exposure of granitic skarns on the Halibut Horst at this time. The area was compartmentalised in Upper Claymore times, with different dispersal systems operating north and south of the Halibut Horst, indicating that the horst continued to be an important control on sediment distribution.