Optically stimulated luminescence age constraints for fluvial aggradation terraces and loess in the eastern North Island, New Zealand

Abstract

Eighteen optically stimulated luminescence (OSL) ages of fluvial aggradation terrace deposits and loess coverbeds are presented from four river catchments in the eastern North Island of New Zealand. Six samples date T1 aggradation deposits; two ages (19.6 ± 2.0 and 23.9 ±1.8 ka) are from silt beds within aggradation gravels; and four ages (16.3 ± 1.5, 17.0 ± 1.5, 20.9 ± 1.5, and 23.7 ± 1.7 ka) are from silt or sand units resting on aggradation gravels. These ages support correlation with the Ohakea Terrace in the Rangitikei catchment. Four older ages for aggradation deposits are 67.6 ± 6.8,70.0 ± 6.3, 75.3 ± 5.5, and 115.2 ± 11.2 ka. We tentatively reassign the first three to T3, which is correlated to the Porewa Terrace in the Rangitikei catchment. Loess coverbed ages of 18.0 ± 1.8, 18.5 ± 1.4, and 23.1 ± 1.8 ka for L1 imply it is Ohakean loess, and 39.7 ± 2.5 ka for L2 imply it is Ratan loess. The T1 and L1 ages confirm widespread aggradation and loess deposition during the last glacial period Marine Isotope Stage (MIS) 2. The ages for T3 suggest another such period during cool stadial MIS 4.

Keywords:

• OSL dating
• fluvial terrace
• loess
• tephra
• Gisborne
• Hawke's Bay