Subdivision of Holocene glacial deposits, Ben Ohau Range, New Zealand, using relative-dating methods

Abstract

Relative-dating methods measure post-depositional alteration of surficial deposits. Measured in this study are the size of the yellow-green Rhizocarpon lichen, percent lichen cover, weathering-rind thickness, quartz-vein height, surface oxidation, and pitting of clasts. The data are used to subdivide Holocene deposits in the Ben Ohau Range and to suggest correlation with moraines in other parts of the Southern Alps. Glacial and/or rock-glacier deposits of five different ages are identified in the Ben Ohau Range. Ages can be approximated mainly from the rind-development curve of Chinn (1981). From youngest to oldest the deposits are: Dun Fiunary (∼ 100 yr); Whale Stream (∼ 250 yr); Jacks Stream (∼ 3,000 yr); Ferintosh (∼ 4,000 yr); and Birch Hill (∼ 9,000 yr). All but the Whale Stream have been previously named, but data presented here result in improved definitions of mapping units and more accurate age assignment. The present climate of the Ben Ohau Range is increasingly continental to the south. A similar climatic gradient existed for much of the post-Birch Hill Holocene, as shown by the glacial depositional facies: moraines are common to the north, whereas rock glaciers, especially of Jacks Stream and Ferintosh ages, are common to the south. In some areas, rock glaciers that have long been inactive have recently been reactivated. Data from moraines in the Waimakariri Drainage and the Mount Cook, Cameron Valley, and Rakaia River areas improve previous correlations, as relative-dating methods provide age information where 14C ages are either lacking or limiting, or when a single relative-dating method is subject to erroneous interpretation.