Constraining rates of landscape denudation using geophysical models : a case study from Lake Rotoiti, Nelson Lakes, South Island
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Glacially overdeepened depressions are ubiquitous throughout the Southern Alps of New Zealand, but their sub-surface extent is poorly understood. They are significant sinks for both catchment drainage and catchment sediment flux, which makes them difficult to characterise, but enticing for scientific study. Their trapped sedimentary sequences contain information about the history of recent glaciations and the rates of landscape erosion under recent climatic conditions. This study has investigated one such depression under Lake Rotoiti, Nelson Lakes, situated in the Northern quarter of the South Island. The approach of this study was to use geophysical methods alongside geomorphic studies, in order to constrain the depth to bedrock, and characterise the sedimentary deposits that have infilled. The following investigations were undertaken: The distributions of surficial sedimentary deposits within the Lower Travers Valley and Black Valley were mapped; a gravity survey was undertaken over the lake-end moraines in order to model their thickness; a single channel seismic reflection survey was undertaken over Lake Rotoiti in order to image the basement surface and overlying sedimentary sequences; the shapes of the bedrock valley walls in the 'trough section' of the Lower Travers Valley were modelled with 2nd - 9th order polynomial equations in order to project their continuation into the subsurface - these were constrained by the geophysical data. The post-glacial deposits within the Lower Travers Valley comprise floodplain fluvial deposits and slope-bottom diamict deposits such as scree, alluvial outwash, landslide and valley-side debris flow deposits. Moraine deposits from 5 different glaciations mostly occur in the St Arnaud region North of Lake Rotoiti. The youngest moraine which bounds Lake Rotoiti has been dated at 14-17 ka (Suggate, 1988b). Gravity modelling of sediment-fill at the lake-ends indicates a substantial sediment thickness. Moraine deposits over some regions are at least 250 m thick. The gravity modelling results indicated that the basement has been glacially overdeepened. The lake is dammed entirely by the 14-17 ka moraine, but a bedrock sill rises to at most 40m beneath the surface 1 km NW of the lake, and is also inferred to rise at the head of Black Valley, 5 km NE of the lake. Seismic reflection data imaged a basement reflector at a minimum depth of 225 m beneath the lake level, and reflectors from the upper surface of the lake-damming moraine. This information was used alongside the gravity constraints and the polynomial models, to construct a coarse, simplistic model of the 14-17 ka glacial erosion surface. By taking the difference between this surface, and the present day topography, it was possible to derive an isopach map of the sediment package that has accumulated since deglaciation, and calculate a volume of 0.67 km3 for this sediment package. This volume estimate was used to calculate a minimum denudation rate of 449-545 tkm-2a-1 (225-273 mmka-1 ) for the Lake Rotoiti Catchment over the last 14-17 kyr. By making some further sediment budget assumptions an upper limit on sediment storage was estimated at 2850 tkm-2a-1 (1173 mmka-1 ). These constraints on denudation indicate that the Lake Roitoit Catchment has been eroding at a much slower rate than higher rainfall regions further south. The Lake Rotoiti Landscape is an example of an older West Coast alpine landscape that is likely to contain more relict glacial erosion features and early paraglacial depositional features than faster eroding catchments further to the South.
Thesis description:
1 v. (various pagings) : ill., maps ; 30 cm. + 5 sheets + 1 DVD
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2006Hill
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POLYGON ((172.80671540197298 -41.794420165215051,172.864672968775835 -41.794979677651362,172.85817978703318 -41.851212811696584,172.820471746967627 -41.885952918698258,172.809091794876821 -41.852094942343847,172.806078496049082 -41.794419091878709,172.80671540197298 -41.794420165215051))
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Citation
Hill, Matthew Galloway., “Constraining rates of landscape denudation using geophysical models : a case study from Lake Rotoiti, Nelson Lakes, South Island,” Otago Geology Theses, accessed February 8, 2025, https://theses.otagogeology.org.nz/items/show/462.