Australia: The Land Where Time Began

A biography of the Australian continent 

The Stuart Creek Silcrete Flora

Silcrete ridges on Stuart Creek Station contain the Stuart Creek fossil flora. The dating of these silcrete deposits is disputed, but it seems probable it is of Middle Eocene age. This age would mean the Stuart Creek fossils were of about the same age as the Golden Grove and Nelly Creek floras, so also about the same age as the Anglesea and Nerriga floras. The silcrete is the form of all sizes of slabs and boulders, and the scree that covers the flanks and merges into the adjacent gibber plains. There are leaf impressions on many of the scree rocks. Many of the fossils are coloured orange, red, pink or yellow. Some have a patina from the surface oxidation in the desert sun.

There is a very large diversity among these ancient fossils from the ridges near Stuart Creek. The present site of this one time forest has mainly Acacia tetragonophylla which grows among the silcrete blocks. A. tetragonophylla is called 'dead finish', because it is often the last plant standing by the end of a drought, and often looks dead until the rain comes and comes back to life. Gumnut-type fruits and conifer twigs and branches have been found. One of the large silcrete blocks has a 9 cm wide hole through it that is 1 m deep. The trunk had rotted away but left an impression of its surface. This cast is in the form of adventitious rootlets covering the stem. Tree ferns are the only plants to have this mass of rootlets covering their stems.

Among the flora of the silcrete deposits so far identified are conifer shoots of PODOCARPACEAE (Dacrycarpus and Dacrydium), ARAUCARIACEA and CUPRESSACEAE (Papuacedrus). Eycalyptus and other capsular-fruited MYRTACEAE. There are many leaves of MYRTACEAE - Eucalyptus and/or Melaleuca. Brachychiton leaves - flame trees, kurrajongs, lace barks, STERCULIACEAE, PROTEACEAE (Banksieaeformis and Orites), CASUARINEACEAE (Gymnostoma and Casuarina).

The silcrete flora seems to represent a Palaeoflora Mixta forest type. There were possibly paperbark communities in adjacent swamps. The conditions under which the sediments were deposited can be seen from the fossilised ripple marks on some of the rocks, suggesting a quiet water body, as would be present in a swamp. As with the other fossil sites described, this also seems to have been a flat area with low relief and meandering rivers, among gallery forests and with oxbow lakes, and probably swamps. It is believed the climate here was warm and humid, but because the site is so far from the coast, it could well have had a seasonal dry period.

The silcrete is silicified Willalinchina Sandstone at Stuart Creek. It dates to the same period as the Eyre Formation at Nelly Creek. At the base of the sandstone formation is a layer of conglomerate containing rough pebbles and silcrete pieces from an earlier period of silcretisation that are cemented together.

'Reed mould' silcrete tops the unit, the formation mechanism of which is unknown. This silcrete has vertical channels or hollow tubes through it. Some blocks of pudding stone silcrete have surface patterns that look like Aztec carvings.

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