Processes controlling the composition of heavy mineral assemblages in sandstones

Abstract

Sandstone compositions result from a complex interplay between provenance and factors that operate during the sedimentation cycle. Accurate identification and discrimination of provenance depends on isolating provenance-sensitive features, and avoiding parameters that are influenced by other factors. Heavy mineral analysis offers a high-resolution approach to determination of sandstone provenance, because of the diversity of mineral species found in sandstones and because the factors affecting assemblages have been comprehensively evaluated. This paper presents the current understanding of the effects of processes operative during the sedimentation cycle. The original provenance signal may be overprinted by weathering at source prior to incorporation in the transport system; by mechanical breakdown during transport; by weathering during periods of alluvial storage on the floodplain; by hydraulic processes during transport and final deposition; by diagenesis during deep burial; and by weathering at outcrop. The most influential of these processes are hydraulics, which fractionates the relative abundance of minerals with different hydraulic behaviour, and burial diagenesis, which reduces mineral diversity through progressive dissolution of unstable mineral species. There is also evidence that weathering during alluvial storage plays a significant role. Two alternative, complementary approaches are recommended to identify provenance from heavy mineral data. The relative abundances of minerals with similar hydraulic and diagenetic behaviour are largely unaffected by processes operative during the sedimentation cycle, and utilize information gained from the entire heavy mineral suite. Determination of such ratios can be augmented by acquisition of varietal data, concentrating on the varieties shown by mineral types within the assemblage. A number of different varietal techniques are recommended, including optical differentiation of types based on colour, habit and internal structure, single-grain geochemical analysis, and single-grain geochronology.