ABSTRACT

Petrographic studies of fine grain (<125 µ) shallow water carbonate sediment have been limited by the relatively poor resolution of the light microscope and the tedious, time-consuming procedures associated with transmission electron microscopy. Scanning electron microscopy offers an attractive alternative to the traditional methods of analysis, and possible applications of the instrument to the study of Recent carbonate sediment have been explored using artificially prepared samples. Skeletal material from known invertebrate orgainisms was pulverized in mortar and pestle, separated into standard size fractions, and examined at magnifications between 20 and 5000 X. Ultrastructural patterns characteristic of particular organisms are easily recognized on isolated grains of fine sand size material and in some silt size grains as small as 4 µ in diameter. Biogenic silt size particles only a few microns thick can be distinguished from inorganic calcite or aragonite. The smallest particle size in which specific skeletal ultrastructures can be identified is a function of the absolute size of the repitious units on which criteria for recognition are based, and is not limited by the resolving power of the scanning electron microscope. Experiments suggest that if ultrastructural patterns are not visible on the surfaces of sediment particles, grains can be embedded in epoxy resin, sectioned, polished, and etched to display relic structure. It is expected that criteria for recognizing skeletal ultrastructures in disaggregated material will be refined as more known taxa are described by scanning electron microscope techniques. The recognition of specific relic structures in fine-grained sedimentary particles should then allow more complete histories to be drawn for sedimentary regimes by skeletal carbonates.