Heavy Minerals in the Subsurface Tracking Sediment Sources in Three Dimensions

The former Dutch Geological Survey from 1950 to 2000 used heavy-mineral compositions to systematically distinguish lithostratigraphic units in single boreholes or borehole transects. This large-scale work has produced an extensive database, with 25,000 heavy-mineral analyses from ∼800 boreholes to a depth of 100 m, together with ∼15,000 from quarries, giving a total number of around 40,000 analyses across the Netherlands, providing a unique opportunity to visualise the 3-D build-up of the subsurface. The greatest challenge in this kind of approach is the visualisation of 3-D variation in 2-D figures. The construction of multiple contour maps of heavy-mineral concentrations, with stacked threshold maps and colouring codes for depths gives an effective overview of the temporal and spatial distributions of sediments derived from various sources to the area. The spatial variation of single minerals, as opposed to mineral assemblages, is used here to illustrate the geometry of sediments on a countrywide scale. The zircon distribution maps reveal the pre-Pleistocene margins of the major Caenozoic depositional basins and reflect tectonic influences. The geometry of the distribution of hornblende- and augite-rich sediments shows not only two depositional phases of Pleistocene Rhine sediments, but also reveals the ancient floodplain of the proto-Weser. Moreover, main channel directions, depositional basins, and delta-like features can thus be identified. By tracking the occurrence of Vosges-derived hornblende and Meuse-specific garnet, the domain of the Meuse sediments in different periods can be documented. It has also been shown that the Scheldt system appears to contain minerals from a source similar to that of the Meuse. The distribution of topaz reflects the influence and extent of sediments delivered by the Baltic system and reveals that the most likely transport mechanism was glacial and not fluvial. The examples are intended to exploit the opportunities provided by the extensive databases in visualising the distribution of sediments with differing provenance in the Netherlands. Re-analysis of old datasets and employing relatively simple methods provides new insights into the geometry of sediment bodies and the sedimentation history of subsurface deposits. © 2007 Elsevier B.V. All rights reserved.