Methodology and microprobe assays as tools for more reliable analysis of detrital heavy minerals data (with some preliminary minerological results from Belgian Tertiary marine sediments)
1998
Bustamante-Santa Cruz, L. (Geological Survey of Belgium, Brussels (Belgium). Dept. of Quaternary Geology and Continental Shelf)
The study of complex detrital heavy-mineral associations, the nature of which seems controlled by tectonic and environmental climatic conditions affecting the parent rocks and the sedimentary basins, provides important information related to the distribution patterns of sediments, the nature and positioning of petrographic sources, the lithostratigraphical subdivisions, the palaeogeographical evolution and the genesis and contamination of magmatic rocks. The reliability of contrasting quantitative data from heavy-mineral analyses, generally considered as disparate and unreliable for statistical treatment, is a function of the quality of the mineralogical determinations, and of the knowledge of grain size and morphological features of detrital particles, which show differential hydrodynamic behaviour depending on their individual form and density. Experience shows that simple, rapid grain counts are inadequate for understanding the complex nature of heavy-mineral associations, which may also be generally modified at the surface of clastic deposits by post-depositional chemical weathering processes, the effects of which are more pronounced where detrital deposits rest on a calcareous substrate. The use of modern physical and chemical analytical techniques (X-ray, microprobe, cathodoluminescence analyses) permits reliable identification of mineralogical species in order to eliminate from sedimentary petrological research and from the scientific language the sack term "alterite". The use of this term implies an important loss of geological information and leads to erroneous interpretation. Several recent studies have allowed remarkable progress in the sedimentary-petrology field and seem to indicate renewed interest in the heavy minerals, which present a relevant geological method, the oversimplification of which formerly led to its disuse.
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