Geology and geochronology of the Nyl River floodplain sediments, Limpopo province, South Africa

[摘要] The Nyl River floodplain, located in the Limpopo Province, is one of the few activesedimentary basins that exist within the South African interior, providing a uniqueopportunity to study the effect of climate change on fluvial systems. Progradation oftributary fans into the Nyl/Mogalakwena River has raised the surface by 30 m and forced thecourse of the river westwards towards the Waterberg. Periods of progradation depositedthick sequences of coarse-grained sediments with sand- to gravel-sized mean grain sizes andcoarsely-skewed populations in the distal reaches of the tributary fans. These periods wereinterspersed with periods of relative non-deposition, when active sedimentation on the fanceased and shallow lakes (or vleis) developed in the trunk river, resulting in deposition offine-grained, organic-rich, floodplain sediment layers with silt-sized mean grain sizes andfinely-skewed distributions in the extreme outer reaches of the tributary fan. The alternatingprogradational sequences and non-deposition events produced interlayered floodplain andfan deposits in the furthest reaches of the tributary fans along the banks of theNyl/Mogalakwena River.Incised river cuts within the Rooisloot tributary fan were dated using OSL and 14Ctechniques. For OSL samples, the SAR protocol was used to measure the equivalent doseand 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. TheOSL ages ranged from 99 years to 3884 years, constraining the sampled deposits within thelate Holocene. Although the 14C ages agreed with this range, carbon contamination of thesamples resulted in inverted and overestimated ages. Based on stratigraphic relationships thenon-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 ~ 800–1000years ago and ~ 500–700 years ago. The rate of aggradation (0.29 cm/year) calculatedimplies that the entire 30 m deposit could have been deposited in 9 000 years. However, anindependent study by McCarthy et al. (2011) proved that tributary sedimentation began priorto 220 ka. Therefore, in order to deposit 30 m of sediment over 220 ka, either the mid – lateQuaternary sedimentation rate was lower than the recent past (Late Holocene) or the systemperiodically undergoes extensive erosion in order to flush the accumulated sediment fromthe tributary fan system.

[发布机构] University of the Witwatersrand