Estimating ages of late Quaternary stream terraces from analysis of weathering rinds and soils

Abstract

Rates of growth of rock weathering rinds and rates of changes of soil morphologic and chemical properties that are calibrated at sites of known ages produce powerful tools for estimating ages of geomorphic surfaces, particularly when materials datable by conventional methods are not widespread. Sequences of latest Quaternary stream terraces from the South Island of New Zealand are ideally suited for application of these calibrated-age techniques because climate and source materials for terrace deposits are internally consistent within two regions, West-land and Marlborough. Modal and mean thicknesses of weathering rinds formed in surface cobbles of Torlesse graywacke provide age estimates for latest Pleistocene and Holocene stream terraces in Marlborough with uncertainties estimated at ±5% to ±40%. Modal rind thickness of subsurface cobbles provides age estimates for latest Quaternary Westland sites with uncertainties of ±10% to more than ±50%. Few dated soils are available to constrain rates of soil development in Westland and Marlborough. Furthermore, soils were sampled from only one pedon on each surface. Morphologic data, however, provide least-squares age estimates for Westland sites with uncertainties typically greater than ±100%, and the calibration data can further bracket likely soil ages to less than 50% uncertainty. The regression data from Marlborough are less reliable, but profile-index ratios help bracket ages to at least ±50%. Distributions of oxalate-extractable Fe and AI and total major-element concentrations allow age estimates for Westland and Marlborough soils with uncertainties estimated at ±15% to ±50%. Despite these uncertainties, the three techniques combine to provide age estimates with the high resolution needed to examine details of fault-slip rates or stream downcutting. The calibration data and weathering rates used herein are applicable only to the New Zealand region studied, although similar calibrated-age techniques should provide high-resolution dating in many other regions. Such techniques are particularly useful when multiple approaches are available to provide independent cross-checks.