Quaternary deposits and landscape evolution in northeast Southland, New Zealand

Abstract

Three kinds of Quaternary sediments are present in northeast Southland: aeolian deposits, terrace gravels, and hill slope colluvium. The aim of this paper is to review and describe the Quaternary sediments and erosion pattern of northeast Southland and reconstruct the likely climate under which sedimentation and erosion occurred. Aeolian deposition occurred intermittently from 250,000 yr B.P. but stratigraphic studies indicate widespread erosion of loess in the period 80,000–25,000 yr B.P. before a final period of loess accumulation in the late Otiran (δ 18O stage 2). Terrace gravels are attributable to two sources. A younger suite of three terraces is attributed to aggradation phases of the Mataura River. An older suite of three more weathered terraces is attributed to aggradation by a river flowing eastwards through the Waimea Plains from Lumsden to Gore. The river is named the Lumsden River. At about 180,000 yr B.P. this river ceased flowing through the Waimea Plains. Colluvium on hill slopes is regionally stratified into two layers. The lower deposits are identified as weathered sand and gravel sized colluvium derived from argillite and siltstone screes, dated as being formed before 29,000 yr B.P., and attributed to δ 18O stage 4. The upper, stonier deposits are attributed to δ 18O stage 2. The erosion and deposition pattern of aeolian deposits, terrace gravels and slope colluvium cannot be explained by a temperature drop of c. 5°C calculated for the last glacial period. We conclude that on lowlands vegetation cover was periodically limited by extreme droughts and cold windy conditions, causing erosion of previously-deposited loess and encouraging scree formation on slopes. During these periods the severe climate of northeast Southland approximated to that of a cold desert.