Stratigraphy, optical dating chronology (IRSL) and depositional model of pre-LGM glacial deposits in the Hope Valley, New Zealand

Abstract

A 110 m thick succession of glacial valley fill is described from Poplars Gully, central South Island, New Zealand. The section consists of eight lithofacies assemblages that represent different stages of ice occupation in the valley. Basal sediments record an ice retreat phase followed by a glacial re-advance which deposited mass flow diamictons and till. A subsequent ice retreat from the site is indicated by the stratigraphic transition from till to thick glacio-fluvial gravels. This is followed by a probably short-lived glacier re-advance that caused folding and thrusting of proglacial sediments. Final glacial retreat from the valley led to the formation of a large proglacial lake. In total, Poplars Gully holds evidence for two major ice advances, separated by a glacial retreat that resulted in complete ice evacuation from the lower Hope Valley. Infrared stimulated luminescence (IRSL) dating on ice-proximal sediments from Poplars Gully yielded six ages between 181 and 115 ka BP. Our stratigraphic logging and dating results show that the fill sequence was not, as previously thought, deposited in association with ice advances during the Last Glacial Maximum (LGM) nor indeed during the last glacial cycle. LGM glaciers later overran the fill but we find that the older glacial sequences are considerably more voluminous than those deposited during the last glacial cycle. We also show that the mid-Pleistocene glaciers carved a much deeper valley trough than did glaciers during the LGM. Taken together these features are likely to reflect a significant difference in the magnitude of successive Pleistocene glaciations in the valley, with the mid-Pleistocene ice advances having been considerably larger than those of the last glacial cycle. The recognition of the in-situ survival of extensive pre-MIS 5 (Marine Isotope Stage) deposits in valley troughs that were later occupied by LGM glaciers represents a new feature in the Quaternary stratigraphy of the Southern Alps. The results demonstrate that New Zealand's commonly very large soft-sedimentary valley fills provide a valuable, yet largely unexploited, terrestrial sedimentary archive of successive glaciations in the region.