Australia: The Land Where Time Began

A biography of the Australian continent 

The Nelly Creek Flora, Lake Eyre Sub-Basin, South Australia

Nelly Creek, the easternmost branch of Welcome Creek, crosses the very dry, desolate landscape of Muloorina Station and flows into Lake Eyre South. Nelly Creek flows only after rain, but quickly dries to a series of saline pools that soon evaporate. At one point along its course a ridge of salt-whitened rock, an outcrop of the silicified Eyre Formation, dams the creek. Water is held for longer than in other pools along the course of the creek and the creek bed remains damp for longer after the water has all gone from the other saline pools.

There is evidence that in the Middle Eocene, 45 Ma, closed canopy forests grew here, in the early stages of Australia's existence as an independent landmass. The vegetation covering the land was of the Gondwanan type it had before separation, already diversified to many types that were adapted to the soil types of the time. There are no examples at the present of these forests, palaeoflora mixta, that contained elements that today would be separated out as temperate, tropical and sclerophyll forests. And the communities of broadleaf and sclerophyll communities would not be found together today.

The fossils in this site occur in the black carbonaceous mud in the bed of the creek next to the bar of rock that dams the creek, where a deposit of mummified leaves has been found. The fossils are 45 million years old. The plant material hasn't been mineralised. The leaves, with outer cuticle, have veins with some preserved tissue between them. The woody stems are not permineralised, being like carbonised wood. Some of the wood has been identified as Ackama (Caldduvia), a member of the CUNONIACEAE family. Corkwoods are present-day species of this family that are found in tropical rainforests in eastern Queensland.

In the recovered wood it has been possible to find the hyphae of brown rot fungi by microscopic examination. It seems amazing that such delicate structures can survive 45 million years of mummification. Presumably the mud didn't stay damp over such a long period in such a barren landscape with extremely high evaporation rates, it probably resulted from the watertable being close to the surface as has been found at Lake Frome.

Because of the nature of the mud, plant material can be separated from mud and mounted on slides for examination. Because of this, plant remains can be identified to the family, and occasionally to the genus level. Also in the mud are pollen and spores. These allowed the dating of the deposit, showing them to be from 45 Ma. This means they are approximately contemporaneous with other plant fossil sites from the Middle Eocene forests at Maslin Bay and Golden Grove near Adelaide, South Australia, and the fossil floras, that are believed to be slightly younger, at Anglesea in Victoria and at Narriga in New South Wales. They also contain similar mummified material.

Plant Groups Identified in the Nelly Creek Deposit

All the above sites appear to have been covered by forests that were very warm temperate or possibly sub-tropical or even tropical. Groups represented by the mummified plant remains include PROTEACEAE, MYRTACEAE, LAURACEAE, araucarian conifers (such as Agnathis), podocarp conifers, CASUARINACEAE (Gymnostoma), and flame trees or kurrajongs (Brachychiton) STERCULIACEAE. 269 leaves were preserved sufficiently to make an identification. Of these, 64 % were from 1 taxon which has not been associated with a known plant family, so presumably is now extinct. Later excavations has uncovered more plant material that has yet to be fully studied.

Groups Identified by Pollen & Spores

Also found in the Nelly Creek deposit were pollen and spores that allowed the identification of a number of plant groups. Among groups identified were Anacolosa (the olive family, OLACACEAE), a number of members of the PROTACEAE - Banksia, Xylomelum (wood pear), Genuina/Hicksbeachia, Greviliea and Beauprea, Cupania (tuckeroo - SAPINDACEAE), Casuarina & Gymnostoms, grasses, RESTIONACEAE, MYRTACEAE - Eucalyptus, Baeckea-Backhousia, Tristania (brushbox, water gum), Ilex AQUIFOLIACEAE - holly family), SAPOTACEAE, CUNONIACEAE, ERICACEAE - lilies, MALVACEAE (Hibiscus family), palms (ARECACEAE), Santalum (quandong or native pear SANTALACEAE), misteltoes, Austrobextus and Dissiliaria EUPHORBIACEAE, araucarian and cypress conifers, many ferns and mosses, Sphagnum. Nothofagus - with fusca and brassii pollen types - and Podocarpus are only minor constituents of the pollen in the deposits. It is thought the beech pollen may have been blown from long distances to the site, possibly from as far as the Flinders Ranges and Willoran Ranges. It is well known that Nothofagus pollen travels for large distances.

The nature of the Nelly Creek sediment indicates that in the Middle Eocene rivers meandered through a flat landscape, just as they do today. The big difference being that it is now a desolate landscape, unlike the forested area of the past. In drier areas there may have been Casuarina woodland and sclerophyllous vegetation. The contact between the base of the Tertiary Eyre Formation and the Cretaceous Winton Formation outcrops in the eroded river bank, about a kilometre up stream from the Nelly Creek site. The basal layer of the Eyre Formation is a zone of rounded, shiny quartz pebbles, that is underlain by the Winton Shale. Large amounts of carbonised wood have been found in the carbonaceous lenses in the Winton Shale. The wood is not permineralised, appearing more like charcoal. Microscopic examination found that compression has prevented the preservation of structure.

The spores and pollen of the sediments allow the dating of the deposit, and show that the area was covered in the Cretaceous by conifer forests with an understorey of ferns. Only 2 angiosperm taxa are included. The pollen of Microcachrys conifers comprises 60 % of the pollen found at the site. Less abundant are other podocarps, araucarians and ginkgoes. The ferns were mainly ground ferns, not tree ferns. This Cretaceous flora suggests the area had a cool temperate climate during this period. The sedimentation implies the region was already a flat area of low relief, much like it is at the present.

Sources & Further reading

  • Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
  • Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
  • Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993

 

 

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading