Chapter 14 The Effects of Burial Diagenesis on Detrital Heavy Mineral Grain Surface Textures

Abstract

A wide-ranging study of heavy mineral grain surface textures using the scanning electron microscope has demonstrated that heavy mineral dissolution is widespread in sedimentary basins worldwide. Many heavy mineral grains, including andalusite, amphibole, chloritoid, epidote, garnet, kyanite, olivine, pyroxene, sillimanite, staurolite and titanite show dissolution textures, which range from pits, mamillated surfaces, facets and deep parallel grooves and furrows, to hacksaw terminations and skeletal forms. Dissolution has been observed in sedimentary basins from the North Sea, Faeroe-Shetland Basin, Rockall Trough, Norwegian Sea, Germany, France, Ivory Coast, Nigeria, south Caspian Basin, Bengal Basin, India, Borneo, US Gulf Coast and Trinidad. Heavy mineral grain surface texture studies therefore testify to the fact that intrastratal dissolution is a global, rather than a local, phenomenon. Some heavy minerals (apatite, monazite, rutile, spinel, tourmaline and zircon) rarely display surface corrosion during deep burial, and are considered to be essentially stable. However, tourmalines have been observed to develop etch facets under some deep burial circumstances, and both monazite and chrome spinel show signs of incipient corrosion in extreme burial situations. Apatite shows both overgrowth and corrosion textures in the subsurface, with overgrowths tending to develop as burial proceeds. Etched apatite generally coexists with unetched grains irrespective of burial depth, and the corrosion is therefore not related to in-situ diagenetic processes. Dissolution of apatite is considered to occur through weathering in the source area and during periods of alluvial storage during transport, rather than in-situ corrosion in the subsurface. Evaluation of the extent to which heavy mineral assemblages have undergone post-depositional modification by dissolution processes is crucial when evaluating provenance, and the scanning electron microscope has an important role to play in this regard.