Optical dating of a scroll-bar sequence on the Klip River, South Africa, to derive the lateral migration rate of a meander bend

Abstract

Problems are often encountered in accurately establishing the rates of processes associated with river channel change. In this study, optical dating was used to obtain the lateral migration rate of an abandoned meander bend on the Klip River, eastern Free State, South Africa. Five samples were collected by augering along a transect across an approximately 200-m-wide scroll-bar sequence on the inner meander bend, and an additional sample was collected from the channel fill of the related oxbow lake. Optically stimulated luminescence (OSL) ages were derived for each sample using the single-aliquot regenerative-dose protocol on small aliquots of quartz. Incomplete bleaching was evident in some of the samples and thus the appropriate statistical analysis was performed using the ‘finite mixture model’ for the scroll-bar samples and the ‘minimum age model’ for the channel-fill sample. The ages calculated were in the correct chronological order for the depositional sequence, which was formed over the last ∼1000 years. The average rate of lateral migration across the scroll-bar sequence was found to be ∼0.16 m/yr but with an apparent increase in the rate occurring during the early sixteenth century. Where quartz-rich alluvium exists, the methods used in this study can potentially be applied to other river systems, enabling rates of channel change processes to be determined.

Realizing the potential of fluvial archives using robust OSL chronologies

“…Rittenour et al, 2005;Rodnight et al, 2005;Busschers et al, 2007), the degree of partial bleaching in the data is minimal. The appearance of partial bleaching in a dataset necessitates some statistical processing, although the selection and application of 'age models' is a frequent source of discussion (Bailey and Arnold, 2006;Rodnight et al, 2006;Thrasher et al, 2009).…”

A dating intercomparison study on Late Stone Age coastal midden deposits, South Africa

“…In both cases these display the fewest sensitivity changes and lacked strong trends through the measurement cycle ( Figure 5). This is consistent with the application of low-temperature preheats to young (Holocene) samples (e.g., Murray & Olley, 2002;Rodnight et al, 2005). A final check on the SAR protocol was a limited (n ϭ 12) dose recovery experiment (Murray & Wintle, 2003) for samples Shfd05036 and Shfd05037.…”

“…The lowest D e component was 2.59 Gy (43% of grains) and the second was 3.76 Gy (45%). Skewing with a high D e tail is commonly associated with partial bleaching (e.g., Bateman et al, 2003: Figure 3; Rodnight et al, 2005), although it is less common in eolian depositional contexts. It is possible that the D e distribution observed within both samples arises from the use of 5-cm-diameter sampling tubes, which may have led to the collection of sand of different ages if sedimentation rates were low or if samples were unintentionally taken across a temporal hiatus.…”

“…The results for sample Shfd05036 (from sand deposited above the midden) revealed that at the single-grain level this sample too had a skewed D e distribution (Skew ϭ 1.378, OD ϭ 36.3%), and the skewing of sample Shfd05037 became more pronounced (Skew ϭ 3.468, OD ϭ 68.6%). In order to attempt to exclude clear outlier D e values and isolate components within the single-grain data, both samples underwent finite mixture modelling (Roberts et al, 2000) with the over-dispersion parameter set as 12% as per Jacobs, Duller, and Wintle (2006a) and Rodnight et al (2005). For sample Shfd05036, finite mixture modeling identified two D e components, one of 1.36 Gy (22% of grains) and a second of 2.26 Gy (78% of grains).…”

“…Whatever the process, the effects on the samples appear more profound on the sediment directly beneath the midden. Final ages for both samples are based on the lowest D e component, as this is considered most likely to include the highest proportion of fully bleached grains (e.g., Rodnight et al, 2005) and therefore be closest to the true burial age.…”