Abstract
The purpose of this study was to investigate the mineralogy and geochemistry of the silt and sand fractions of the bedload sediment within the Taieri River, East Otago, New Zealand. The lithology of the catchment is dominated by metamorphic schist, with minor igneous and sedimentary rocks. The bedload sediment (<4mm) was sampled at eight locations and split into six different grain size fractions. These were analysed using XRD, XRF and LA-ICP-MS. In addition to the main work, 12 locations were sampled for water and suspended sediment and analysed by XRD (suspended sediment) and solution ICP-MS (dissolved load) to compare to the bedload sediment.
The sediments are primarily derived from the Otago Schist and recycled schist material, with all size fractions containing quartz, albite, illite/ muscovite, chlorite ± apatite, epidote, titanite and clay minerals. Quantitative mineralogy proved a useful tool for bedload sediment which has variable proportions of the same minerals within differing grain-size fractions. Tectonic discrimination diagrams classify the sediments as sourced from an active continental margin, ranging from shale and greywacke for the fine grain size fractions, while the coarser quartz rich fractions were primarily litharenite. These classifications are consistent with a schist source rock and the influence of quartz dilution. Physical weathering dominates in the Taieri River bedload system, however chemical weathering is important prior to the sediment entering the river system. Changes in source rock in the Kyeburn Tributary (lower grade basement rock, and tertiary sediments) are notable after the confluence of the Kyeburn river, but consequently this influence is overwhelmed by the mainstream. Clay mineralogy is related to CIA values and kaolinite derived from the schist weathering surface has been removed and transported via the suspended load rather than remaining within the bedload. There was no evidence of downstream fining with local geomorphology controlling grain size distribution. Hydrodynamic sorting results in heavy and clay minerals concentrating in the finer grain size fractions, which are the primary controlling minerals for REE distribution. The finest fraction of the bedload sediment (<63 μm) is closest to UCC and source schist, agreeing with previous literature that fine fractions are better source indicators. The suspended load is enriched in clay minerals with reduced quartz content compared to the bedload sediment. The elemental composition of the dissolved load is severely depleted compared to the bedload, with REE likely related to particulate matter.