Abstract
Glacial lakes that formed during deglaciation following the Last Glacial Maximum contain long continuous sedimentary records of the Late Pleistocene and Holocene history of the surrounding catchment. In the Northern Hemisphere and South America, seismic reflection surveys have been used to determine the paleoenvironmental change that has occurred in the catchment following the retreat of glaciers, and to identify tectonic signals within the sedimentary infill. This type of research has been underutilised in New Zealand, particularly in the large glacial lakes in Central Otago. This project aims to integrate 2D seismic data and densely spaced almost pseudo 3D seismic data to identify seismic facies that can be used to make stratigraphic seismic interpretations of the tectonic and hydrological activity and paleoenvironmental change recorded in the sedimentary infill.
Two 2D seismic reflection surveys were collected in 2021 and 2022, which loosely covered the entire lake and provided a baseline understanding of the sedimentary features present in the lake. A densely spaced almost pseudo-3D seismic survey was subsequently collected in 2025 in the southern part of the Glenorchy Arm, where 500 m of sedimentary infill had been imaged in the 2021 survey. The seismic data were processed in GLOBE Claritas using a typical ‘marine’ workflow to produce high resolution seismic reflection images of Lake Whakatipu. The seismic images for the lines in the Glenorchy Arm and main east-west basin of Lake Whakatipu were interpreted using S&P Kingdom software to identify seismic facies within the sedimentary infill. 3D surfaces of the lake floor, combined with seismic facies in the 2025 survey, and the basement were interpreted to build a 3D model of the deposition of the sedimentary infill.
The infill of Lake Whakatipu was imaged from the surface of the lake down to the acoustic basement at a maximum depth of ~900 m in the main east-west channel. The sedimentary infill is up to ~500 m thick and has a vertical seismic resolution of ~2 m throughout the sedimentary infill, and a horizontal seismic resolution of ~27 – ~37 m in the shallow sediment and ~55 – ~66 m in the deep sediment. The sedimentary infill is composed of subparallel continuous horizons that are disrupted by several sedimentary features. Two glacial features are identified directly above the basement horizon and correspond to the glacial environment before the retreat of the Whakatipu Glacier. Landslide deposits from the sides of the lake could relate to earthquakes in the Otago Fault System or storm events. The appearance of paleochannels in the shallowest 150 m of sediment indicates the transition from a paraglacial environment with high rates of sedimentation to an interglacial environment with increased fluvial influence on sediment transport and deposition. The preservation of the paleochannels is hypothesised to be linked to increased sedimentation into Lake Whakatipu following an Alpine Fault earthquake. Gas pockets in the shallowest sediment represent a more recent interglacial transition to a more modern alpine environment with increased vegetation and increased concentration of organic material being deposited into the lake.