Lake George revisited: new evidence for the origin and evolution of a large closed lake, Southern Tablelands, NSW, Australia. 2: earliest Pleistocene (Gelasian) environments

ABSTRACT

The three sedimentary units infilling Lake George provide the longest quasi-continuous sedimentary record of any Australian lake basin. A combination of cosmogenic nuclide burial, magnetostratigraphy and biostratigraphic dating techniques previously has shown that the basal (fluvial) unit, 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). The same techniques confirm the middle unit, the (fluvio-lacustrine) Ondyong Point Formation began accumulating in the late Pliocene and deposition continued into the earliest Pleistocene (Gelasian) when a shallow but probably laterally extensive freshwater lake extended across the drillhole site. Our data provide a minimum Gelasian age for tectonic blockage of former spillway(s) and formation of paleo-Lake George. Whether this was the earliest lake to form within the basin is unknown, since the dated intervals are separated by a ferric hardpan, interpreted as representing a prolonged period of erosion or non-deposition. Temperate rainforest angiosperms including Nothofagus growing during the late Pliocene had been extirpated or become extinct during this interval, although a number of gymnosperms, now endemic to New Caledonia, New Guinea, New Zealand and Tasmania still survived in the otherwise sclerophyll-dominated vegetation. The succession of plant communities is considered to be due to effectively drier local conditions, which in turn reflect regional aridification during the Plio-Pleistocene transition, despite the formation of a freshwater lake across the basin. The sequence provides a reliable framework for recognising and correlating Plio-Pleistocene deposits elsewhere on the Southern Highlands.

KEYWORDS:

• Lake George basin
• Ondyong Point Formation
• earliest Pleistocene
• Gelasian
• cosmogenic nuclide burial age
• paleomagnetism
• palynostratigraphy
• Plio-Pleistocene aridification
• Nothofagus
• extinction–extirpation

Acknowledgements

The assistance of present and former staff members at Geoscience Australia, Canberra, is gratefully acknowledged, in particular Elizabeth (Liz) Truswell and Bob Abell for their pioneer studies on Lake George, and Jamie Lankford in retrieving and facilitating re-sampling of core from BMR Corehole C354. We thank Aziz Noor (Core Laboratories Australia Ltd), Perth, for the initial processing of three of our 10 samples, thereby confirming that plant microfossils were well preserved in some sections of the Ondyong Formation. Andrew McPherson and Dan Clark kindly funded, and Christian Thun and Jessica Byass skilfully processed, the remaining seven samples at Geoscience Australia. We thank the Australian Research Council for funding the Lake George Project (LP140100911). Permission @Commonwealth of Australia (Geoscience Australia) 2015 to use Figures 1–4 is released under the Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/legalcode.

Disclosure statement

No potential conflict of interest was reported by the authors.