Lake George revisited: New evidence for the origin and evolution of a large closed lake, Southern Tablelands, NSW, Australia

Abstract

Lake George contains the longest continuous sedimentary record of any Australian lake basin, but previous age models are equivocal, particularly for the oldest (pre-Quaternary) part of the record. We have applied a combination of cosmogenic nuclide burial dating, magnetostratigraphy and biostratigraphy to determine the age of the basal (fluvial) unit in the basin, the Gearys Gap Formation. Within the differing resolutions achievable by the three dating techniques, our results show that (i) the Gearys Gap Formation, began accumulating at ca 4 Ma, in the early Pliocene (Zanclean), and (ii) deposition had ceased by ca 3 Ma, in the mid late Pliocene (Piacenzian). Whether the same age control provides an early Pliocene (Zanclean) age for the formation of the lake basin is uncertain. During the Piacenzian, the vegetation at the core site was a wetland community dominated by members of the coral fern family Gleicheniaceae, while the surrounding dryland vegetation was a mix of sclerophyll and temperate rainforest communities, with the latter including trees and shrubs now endemic to New Guinea–New Caledonia and Tasmania. Mean annual rainfall and temperatures are inferred to have been ∼2000–3000 mm, although probably not uniformly distributed throughout the year, and within the mesotherm range (>14°C <20°C), respectively. Unresolved issues are: (1) Does the basal gravel unit predate uplift of the Lake George Range and therefore provide evidence that one of the proposed paleo-spillways of Lake George, that above Geary's Gap, has been elevated up to 100–200 m by neotectonic activity over the past 4 million years? (2) Did a shallow to deepwater lake exist elsewhere in the lake basin during the Pliocene?

KEY WORDS:

• Australia
• Lake George basin
• Lake George Range
• Gearys Gap Formation
• Neogene
• Zanclean
• Piacenzian
• cosmogenic nuclide burial age
• paleomagnetism
• palynostratigraphy
• Nothofagus temperate rainforest

ACKNOWLEDGEMENTS

Jenny McEwen-Mason (formerly Monash University) and present and former staff members at Geoscience Australia are gratefully acknowledged, in particular Elizabeth (Liz) Truswell and Bob Abell for their pioneer studies on Lake George, Eddie Resiak and Jamie Lankford in retrieving and facilitating re-sampling of core from BMR Core hole C354, Andrew McPherson and Dan Clark for comment on the neo-tectonic implications of the palynostratigraphic and paleomagnetic age determinations and Dan Clark and an anonymous referee for their thorough and perceptive comments on earlier drafts of this paper. We also gratefully acknowledge: (1) members of the ANU AMS group, Toni Wallner and Steve Timms, as well as John Stone (University of Washington) for access to his laboratory facilities and for his guidance on sample preparation for cosmogenic nuclide analyses; (2) Tara Macphail for assistance in drafting; and (3) Geoscience Australia for permission to use Figures 1–4. Permission © Commonwealth of Australia (Geoscience Australia) 2015 to use this material is released under the Creative Commons Attribution 4.0 International Licence. http://creativecommons.org/licenses/by/4.0/legalcode.