Identifying provenance-specific features of detrital heavy mineral assemblages in sandstones

The composition of heavy mineral assemblages in sandstones may be heavily influenced by processes operating during transport, deposition and diagenesis. As a consequence, conventional heavy mineral data may not be a reliable guide to the nature of sediment source material. Certain features of heavy mineral suites, however, are inherited directly from the source area without significant modification, such as the varietal characteristics of individual mineral species. This paper describes an alternative approach to varietal studies that concentrates on relative abundances of minerals that are stable during diagenesis and have similar hydraulic behaviour. Ratios of apatite to tourmaline, TiO₂ minerals to zircon, monazite to zircon, and chrome spinel to zircon provide a good reflection of the source rock characteristics, because they are comparatively immune to alteration during the sedimentary cycle. These ratios are described as index values (ATi, RZi, MZi and CZi, respectively). This approach avoids some of the practical problems associated with varietal studies, such as the need to make subjective decisions about mineral properties or to use advanced analytical techniques that may not be accessible to the analyst. It also makes use of more components of the heavy mineral suite and thus provides a more balanced view of provenance characteristics. The use of these ratios is illustrated with examples from Upper Jurassic sandstones in the Outer Moray Firth area of the UK continental shelf and Triassic sandstones from onshore and offshore UK. Heavy mineral indices, notably ATi and MZi, show marked variations in Upper Jurassic Piper sandstones of the Outer Moray Firth. Main Piper sandstones have lower ATi and MZi values compared with Supra Piper sandstones, indicating significant stratigraphic evolution of provenance. The UK Triassic shows major regional variations in a number of index values, including ATi, MZi and CZi, demonstrating that sediment was supplied from several distinct source regions. This indicates a need for some modification of existing palaeogeographic models for the UK Triassic.