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TRIASSIC PLANTS OF EASTERN AUSTRALIA

Locality Bacchus Marsh Council Trench

The Triassic flora of the Council Trench Formation, Victoria, Australia

Triassic sedimentary rocks occur at only two localities in Victoria (the Bald Hill area at Bacchus Marsh, and at Yandoit in the central highlands), most of the state experiencing erosional conditions at that time. The “Council Trench" was excavated in the 1870s on the southern flank of Bald Hill in a search for building stone (Robertson 1934). The very small Triassic deposit exposed in the Council Trench consists of interbed-ded quartzose sandstones, polymict conglomerates and siltstones laid down in fluvial settings. Its distribution was mapped by Roberts (1984) who first used the name Council Trench Formation. The beds contain very fragmentary plant remains that have been described by McCoy (1892, 1894, 1898), Chapman (1927), Douglas (1969, 1973), and most recently by Webb & Mitchell (2006). The bulk of the plant fossils appear to derive from a siltstone bed about 4 m above the base of the exposed trench section. A maximum age constraint of about 235 Ma (mid-Triassic) was placed on these strata by Duddy (2003) on the basis of fission-track dating of apatite and zircon crystals. The fossil flora contains no strongly age-diagnostic taxa but is consistent with a Middle or Late Triassic age.
 
The fossil flora of the Council Trench Formation contains at least 20 species of  sphenophytes, liverworts, ferns, pteridosperms, cycadophytes, ginkgoaleans and conifers that likely derive from a mix of swamp and elevated sub-environments of an alluvial plain experiencing a cool, humid climate (Webb & Mitchell 2006). The NRM collections contains a very small assemblage (only two specimens) of fossil plants from the Council Trench Formation collected by N.O. Holst in 1897.

 

Reference

Chapman, F., 1927. Monograph on the Triassic Flora of Bald Hill, Bacchus Marsh, Victoria. Memoirs of the National Museum of Victoria 7, 121-156.

Douglas, J.G., 1969. The Mesozoic floras of Victoria. Parts 1 and 2. Memoirs of the Geological Survey of Victoria 28, 1-310.

Douglas, J.G., 1973. The Mesozoic floras of Victoria. Part 3. Memoirs of the Geological Sur-vey of Victoria 29, 1-185.

Duddy, I. R., 2003. Chapter 9 - Mesozoic. In Geology of Victoria, W.D. Birch, ed., Geological Society of Australia Special Publication 23, 239-286.

McCoy, F., 1892. Report on palaeontology of the Geological Survey for the year 1891. Annual Report, Secretary of Mines, Victoria for 1891 (1892), 30.

McCoy, F., 1894. in Officer, G. & Balfour, L., Further notes on the glacial deposits of Bacchus Marsh. Proceedings of the Royal Society of Victoria, new series 6, 139-143.

McCoy, F., 1898. Note on an additional genus of fossil plants found in the Bacchus Marsh sandstone. Proceedings of the Royal Society of Victoria, new series 10, 285-286.

Robertson, W.P., 1934. Local Freestone. Bacchus Marsh Express 6th October 1934.

Webb, J.A. & Mitchell, M.M., 2006. Stratigraphy and palaeoflora of the Triassic Council Trench Formation, central Victoria. Proceedings of the Royal Society of Victoria 118, 113-127.

Map of Australia showing the distribution of Triassic sedimentary basins (yellow) and the location of the Council Trench, Bald Hill, near Bacchus Marsh, Victoria, Australia.

Fluvial sandstones and conglomerates of the Council Trench Formation, Bald Hill, near Bacchus Marsh, Victoria, Australia. Photo: S. McLoughlin 2000.