Sedimentological investigation of late glacial (Otiran) exposures along the southern margins of Lake Wanaka, South Island, New Zealand

Abstract

This thesis investigates the presence of possible episodes of glacial advance and retreat by a study of sedimentary exposures around the southern margin of the Wanaka basin in and around the Clutha area, and compares these records with other localities across South Island, New Zealand. The focus of this study is on the latter half of the last glaciation (Otiran), up to and including the Last Glacial Maximum (LGM).The advance and retreat cycles in this thesis are defined sedimentologically by two main environments: lacustrine and fluvial. The deposition of lacustrine sediment and associated deltaic sediments mark episodes of glacial retreat, where ice retreating up the Wanaka catchment created accommodation space for deposition. These lacustrine sediments are divided into ice proximal facies dominated by sub-aqueous mass flows, whereas ice distal facies are dominated by laminated lacustrine sediments and deltaic deposits. Glacial advances are defined by aggradation gravels (braid plains) where glacier ice is grounded on land. Glacier advances into lakes involve the deposition of sub-aqueous debris flows, but these are thinner than recessional units. Direct ice contact environments are marked by sediment deformation associated with ice overrun.Four major phases of ice oscillation are recognised in the study area. These are clearly separated on stratigraphic grounds. Age control is variable, but OSL data suggest the following sequence of events:Wanaka 1 oscillation occurred between 42 ka and38 ka. This advance is only represented at Beacon Point.The Wanaka 2 oscillation is not directly dated in this study, but there is incontrovertible sedimentological evidence for its existence. It is tentatively placed at around 38 -36 ka because it is constrained by the minimum age of the Wanaka 1 oscillation and the maximum age of the Wanaka 3 oscillationThe Wanaka 3 oscillation is constrained by the onset of gravel aggradation at about 32 ka and its termination occurred shortly after 29 ka. This advance is the Wanaka catchment NZ LGM advance.The Wanaka 4 oscillation is unconstrained, except that it post-dates the Wanaka 3 oscillation Based on its large extent, reaching the end of the Lake Wanaka basin, it is inferred to be the LGM advance in this system. The older advances in this system support the concept of more persistent extensive ice during the last glacial cycle (Otiran) than is generally considered.3