THERMOLUMINESCENCE AGES OF POSTGLACIAL LOESS, RAKAIA RIVER, SOUTH ISLAND, NEW-ZEALAND

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 th e south bank of the Rakaia River. This site is roughly midway between the coast and the front range of Canterbury. Previously, the time of i nitiation of deposition of such loess in the region was only poorly co nstrained, to somewhere between the interval 16-32 ka and ca. 11 ka. L oess deposition has continued until today along margins of major braid ed rivers. Sample BHL-5, 30 cm above the gravel, gave a TL age of 11.7 +/-1.9 (1 sigma) ka, and sample BHL-1, 1.35 m above BHL-5 and 1.9 m be low the surface, gave an age of 5.49+/-0.70 ka. Thus age-depth extrapo lation 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 Ri ver began to entrench at about this time in this middle segment of the piedmont plain. Copyright (C) 1996 INQUA/Elsevier Science Ltd