HOW DO FLOODPLAIN AGES COMPARE IN ADJACENT ALLUVIAL AND BEDROCK REACHES IN SOUTH AFRICAN HIGHVELD RIVERS?

This work is a case study in using OSL dating techniques to compare floodplain stability in alluvial and bedrock river reaches in the South African Highveld. Many rivers are neither fully alluvial or fully bedrock but instead, have alternating bedrock and alluvial reaches in their long profiles. Floodplain stability is likely to vary in the two reach types due to differences in accommodation space for lateral migration. In bedrock reaches, channels tend to have narrower valleys than in alluvial reaches where channels have more space to meander over wide belts. In alluvial reaches, floodplain reworking occurs over longer time periods than in neighboring bedrock reaches.

In the Highveld region of South Africa, the Karoo Supergroup, a complex of sandstones and shales extensively intruded by dolerite, creates a spatially variable pattern of resistance to erosion. Where channels flow over sedimentary sandstones and shales, they have an alluvial morphology meandering over ~1.5 km wide belts. In longitudinally adjacent dolerite bedrock reaches, the gradient is steeper and inset meanders rework floodplain sediments that extend 30-50 m across the bedrock valley.

In order to establish differences between ages of floodplain sediments in bedrock and alluvial reaches, we use OSL dating techniques on floodplain sediments in both reach types on three rivers on the South African Highveld. Samples were collected from near channel features as well as from the margins of the floodplains in alluvial and bedrock reaches on the Klip and Mooi Rivers and Schoonspruit. The Single Aliquot Regenerative dose (SAR) approach was used on small aliquots of 180-212 µ quartz to determine ~50-100 equivalent doses (D_e) for each sample. Because D_e distributions and the fluvial depositional environment indicate a likelihood of partial bleaching, the Minimum Age Model was used to model the burial dose and associated error used in the age determination for most samples. Our results indicate that alluvial reaches have floodplains are stable over ~10⁴-10⁵ years while longitudinally adjacent bedrock floodplains are stable over ~10³ years.