Geochemistry of Estuarine Sediments: The Kaikorai Estury.
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The Kaikorai estuary is a drowned river valley formed approximately 6000 years ago. Recently (about 100 years ago) a sand bar has formed, where the mouth of the estuary is today. The effect of the sand bar is to trap sediment within the estuary. The present day sediment is bimodal, a medium to fine grained sand and a medium silt. The sand grains have been derived from several source regions (Foveaux-Fiordland, Clutha, Catlins, local Tertiary sediments and Dunedin volcanics), and is, texturally and mineralogically, almost identical to nearby beach sands. Periods of heavy rain have washed loess, from the neighbouring paddocks, directly into the estuary. This Sediment combined with Haast Schist detritus form the medium silt. The estuary is exposed to climatic conditions, combined with the a small tidal variation (micro-tidal) produce a well dispersed sediment.
The presence of industries within the Kaikorai Catchment, along with atmospheric pollution have produced high concentrations of trace elements in the estuarine sediments. These elements show preferred enrichment in specific grain-size fractions. MORB, Chondrite, Primative Mantle, and NASC normalized plots of geochemical data froom estuarine sediments all exhibit distributions indicative of continental derived material. The NASC plot exhibits enrichment of chromium and zinc and indicates that chromium and zinc may be anthropogenic. The distribution of elements on a variation diagram exhibit either similar element ratios to those of the source rock, or an increased element ratio attributed to another source (possibly anthropogenic). The concentration of vanadium observed in the estuarine sedimenls is the result of lithogenic components. Vanadium is therefore a good trace element to use with dete1minative variation diagrams.
A conceptual model for an estuarine system is proposed. This model relates biological, chemical and physical parameters to the behaviour of sedimentary material and trace elements.
The presence of industries within the Kaikorai Catchment, along with atmospheric pollution have produced high concentrations of trace elements in the estuarine sediments. These elements show preferred enrichment in specific grain-size fractions. MORB, Chondrite, Primative Mantle, and NASC normalized plots of geochemical data froom estuarine sediments all exhibit distributions indicative of continental derived material. The NASC plot exhibits enrichment of chromium and zinc and indicates that chromium and zinc may be anthropogenic. The distribution of elements on a variation diagram exhibit either similar element ratios to those of the source rock, or an increased element ratio attributed to another source (possibly anthropogenic). The concentration of vanadium observed in the estuarine sedimenls is the result of lithogenic components. Vanadium is therefore a good trace element to use with dete1minative variation diagrams.
A conceptual model for an estuarine system is proposed. This model relates biological, chemical and physical parameters to the behaviour of sedimentary material and trace elements.
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vii. 57 p. ill, diagms, photos, 30 cm.
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1989Wilson
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Wilson, D, “Geochemistry of Estuarine Sediments: The Kaikorai Estury.,” Otago Geology Theses, accessed May 19, 2025, https://theses.otagogeology.org.nz/items/show/238.