A Dissertation submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the Degree of Master of Science
in Geology.
Johannesburg 2013 / The Nyl River floodplain, located in the Limpopo Province, is one of the few active
sedimentary basins that exist within the South African interior, providing a unique
opportunity to study the effect of climate change on fluvial systems. Progradation of
tributary fans into the Nyl/Mogalakwena River has raised the surface by 30 m and forced the
course of the river westwards towards the Waterberg. Periods of progradation deposited
thick sequences of coarse-grained sediments with sand- to gravel-sized mean grain sizes and
coarsely-skewed populations in the distal reaches of the tributary fans. These periods were
interspersed with periods of relative non-deposition, when active sedimentation on the fan
ceased and shallow lakes (or vleis) developed in the trunk river, resulting in deposition of
fine-grained, organic-rich, floodplain sediment layers with silt-sized mean grain sizes and
finely-skewed distributions in the extreme outer reaches of the tributary fan. The alternating
progradational sequences and non-deposition events produced interlayered floodplain and
fan deposits in the furthest reaches of the tributary fans along the banks of the
Nyl/Mogalakwena River.
Incised river cuts within the Rooisloot tributary fan were dated using OSL and 14C
techniques. For OSL samples, the SAR protocol was used to measure the equivalent dose
and the burial dose was determined using the CAM and MAM. Emission counting methods,
including TSAC, GM-beta counting and HRGS were used to determine the dose-rate. The
OSL ages ranged from 99 years to 3884 years, constraining the sampled deposits within the
late Holocene. Although the 14C ages agreed with this range, carbon contamination of the
samples resulted in inverted and overestimated ages. Based on stratigraphic relationships the
non-deposition events have been dated at approximately 750–800 years ago, 600 years ago,
475 years BP and 100–150 years ago and two major periods of aggradation at ~ 800–1000
years ago and ~ 500–700 years ago. The rate of aggradation (0.29 cm/year) calculated
implies that the entire 30 m deposit could have been deposited in 9 000 years. However, an
independent study by McCarthy et al. (2011) proved that tributary sedimentation began prior
to 220 ka. Therefore, in order to deposit 30 m of sediment over 220 ka, either the mid – late
Quaternary sedimentation rate was lower than the recent past (Late Holocene) or the system
periodically undergoes extensive erosion in order to flush the accumulated sediment from
the tributary fan system.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/12951 |
| Date | 01 August 2013 |
| Creators | Colarossi, Debra |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | image/jpeg, application/pdf |
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