Tertiary Lake Bunyan, Northern Monaro, NSW, part II: Facies analysis and palaeoenvironmental implications

Abstract

The five Members of the Miocene Lake Bunyan which make up the Bunyan Formation are described in detail, their palaeoenvironmental implications considered, and possible correlations discussed. The Rose Valley Member comprises a range of lake edge and bottom sediments. The Cooma Creek Member consists of thin lignite seams interbedded with clay and probably developed in shallow lake margins containing heath and swamp species. The Cloyne Member consists of sediments containing volcanic and other lithic debris which is weathered to kaolinite and goethite; rapid deposition is indicated at a time of local volcanism. The Rosebrook Member represents a very quiet depositional environment, but the high crystallinity of the fine‐grained, kaolinitic clay may result from synthesis within the lake. The Bonnie Doon Member diatomites are cyclical deposits formed in response to both annual and longer term cycles. Considerable variation in lake water levels is indicated including dry lake phases when soils developed. Much of the clay in the sediments would have been derived from deep weathering profiles of the Monaro Surface. The source of the volcanic debris in the Cloyne Member is uncertain but this material could not have been associated with Monaro Basalts (54–36 Ma). Absence of rainforest taxa in pollen assemblages of the Cooma Creek Member indicates a cooler, drier climate for the Lake Bunyan Basin than occurred in other parts of the region during the Miocene. The Lake Bunyan sedimentary sequence is another example of continental sedimentation in the Eastern Highlands of Australia during the Miocene. There is a corresponding gap in the sedimentary record of marginal basins such as the Murray. There is also a general absence of substantial sedimentation in the Eastern Highlands during the Oligocene and earlier epochs of the Cainozoic.

Key words:

• lake
• diatomite
• clay
• volcanogenics
• lignite
• Nothofagus
• Miocene