Processes controlling the composition of heavy mineral assemblages in sandstones

Sandstone compositions result from a complex interplay between provenance a nd factors that operate during the sedimentation cycle. Accurate identifica tion and discrimination of provenance depends on isolating provenance-sensi tive 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 comp rehensively evaluated. This paper presents the current understanding of the effects of processes operative during the sedimentation cycle. The origina l provenance signal may be overprinted by weathering at source prior to inc orporation in the transport system; by mechanical breakdown during transpor t, by weathering during periods of alluvial storage on the floodplain; by h ydraulic processes during transport and final deposition; by diagenesis dur ing deep burial; and by weathering at outcrop. The most influential of thes e processes are hydraulics, which fractionates the relative abundance of mi nerals with different hydraulic behaviour, and burial diagenesis, which red uces mineral diversity through progressive dissolution of unstable mineral species. There is also evidence that weathering during alluvial storage pla ys a significant role. Two alternative, complementary approaches are recomm ended to identify provenance from heavy mineral data. The relative abundanc es of minerals with similar hydraulic and diagenetic behaviour are largely unaffected by processes operative during the sedimentation cycle, and utili ze information gained from the entire heavy mineral suite. Determination of such ratios can be augmented by acquisition of varietal data, concentratin g on the varieties shown by mineral types within the assemblage. A number o f different varietal techniques are recommended, including optical differen tiation of types based on colour, habit and internal structure, single-grai n geochemical analysis, and single-grain geochronology. (C) 1999 Elsevier S cience B.V. All rights reserved.