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We applied the partial-bleach (R-beta) thermoluminescence (TL) dating technique to two samples from massive post-glacial loess overlying the last major aggradation gravel (Burnham Formation) near Barrhill on the south bank of the Rakaia River. This site is roughly midway between the coast and the front range of Canterbury. Previously, the time of initiation of deposition of such loess in the region was only poorly constrained, to somewhere between the interval 16–32 ka and ca. 11 ka. Loess deposition has continued until today along margins of major braided rivers. Sample BHL-5, 30 cm above the gravel, gave a TL age of 11.7±1.9 (1σ) ka, and sample BHL-1, 1.35 m above BHL-5 and 1.9 m below the surface, gave an age of 5.49±0.70 ka. Thus age-depth extrapolation at the Barrhill site implies that loess deposition began at 13±2 ka and continued through the Holocene to the present. The average rate of deposition between the two TL samples was 0.22±0.07 mm/year (analytical error only), and possibly greater thereafter. The presence of the loess wedge dating from 13±2 ka indicates that the Rakaia River began to entrench at about this time in this middle segment of the piedmont plain. |
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