Characterisation of the modern sand wedge on the South Otago Shelf

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Fleming, Betina

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The narrow continental shelf off southern Otago is swept by the swift-flowing (23.8 cm/s) Southland Current. Since sea level stabilised approximately 6500 cal yr BP, a large (60 x ~15 km) sediment wedge has developed on the South Otago shelf. Calculated sediment input to the shelf from all sources, including rivers and the Southland shelf, is estimated to be 4.48 Mt/yr, though over 50% of this is mud that bypasses the inner shelf to accumulate on the outer shelf and continental slope. The Clutha River dominates the total sediment input, contributing an estimated 3.14 Mt/yr to the shelf, of which 1.23 Mt/yr is stored in the modern sand wedge. Recent land use changes in the Clutha River catchment have altered sediment supply to the shelf by first introducing a large volume of sediment due to extensive alluvial gold mining and sluicing in Central Otago during the late 1800’s, and then reducing overall sediment flux after the construction of the Roxburgh hydroelectric dam in 1961. The Roxburgh Dam has almost halved the sediment load being supplied to the South Otago shelf by the Clutha River with likely consequences for the sediment budget of the shelf and the morphology of the sediment wedge. However, the response time of sediment accumulation and erosion on the shelf is unknown, nor the temporal variation in facies distributions with respect to sediment supply and other environmental variables. A survey of the wedge conducted in the late 1970s provides a record of the state of the sand wedge at this time and a potential benchmark to gauge relatively recent changes over the last 30 years.
This study aims to characterise the current state of the sand wedge at the mouth of the Clutha River by combining surface sediment samples and seismic stratigraphy. Sediment samples were collected from 11 locations along a shore-perpendicular transect across the wedge from the 100 m depth contour to 9 m water depth at the mouth of the Clutha River. A high-resolution single channel seismic profile was collected along the same line to compare wedge morphology to the distribution of surface sedimentary facies. The shore-perpendicular transect was oriented parallel to a previous seismic survey on the South Otago Shelf conducted in 1989 so that direct comparison between existing and new seismic profiles could potentially be made.
A slight shoreward migration of sedimentary facies has been observed compared to existing facies maps of the shelf. This could be attributed to changes in the currents around the shelf, particularly the depth of storm wave-base, or possibly to reduction in sediment supply from the Clutha River, causing gradual erosion of the sand wedge from the seaward edge and more efficient sediment transport away from the Clutha River mouth. The lack of a coarse-grained lag deposits on the seaward edge of the sand wedge indicates little, if any, erosion has occurred here
and is not likely to be a major contributor to this facies migration. Instead, a close relationship of facies distribution to the depth of storm wave-base is hypothesised. Results from the seismic survey indicate there has been no significant change in the thickness of the sand wedge on the South Otago shelf in the last 20 years. Large errors in positions recorded for the 1989 seismic profile of the wedge have made direct comparison between the existing survey and new data difficult. Visually aligning the lower erosion surface of the sand wedge in both profiles does not reveal any significant change in sediment thickness, but the poorly constrained position of the existing profile means that quantitative measurements of the differences between the profiles will be difficult.

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v. 70 p. Ill (some col). Digrms. 30 cm + 1 CD-ROM (4 3/4 in.).

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2012Fleming

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http://download.otagogeology.org.nz/temp/Abstracts/2012Fleming.pdf

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Fleming, Betina, “Characterisation of the modern sand wedge on the South Otago Shelf ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/543.

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