Stuart Creek Paleoflora

"The past is a Foreign Country" (Hartley, 1967)

General

Anna Creek Station (one of the Kidman properties) which encompasses the area known as the Stuart Creek Palaeochannel is the largest cattle station (ranch) in the world, covering an area of 34,000 sq. km (6 million acres). It is still 8,000 sq. km larger than its nearest rival in the Northern Territory of Australia, Alexandria Station. The largest American cattle station “ranch” is 3,000 sq. km. The Stuart Creek Palaeochannel is located in the Lake Eyre region of central South Australia. A map has not been shown on purpose as collecting is NOT allowed and the area is only open for scientific study.

The area is semi-desert to desert and is very remote. You are literally the only person within 4,000 sq. km other than station staff (2 to 3 people). The roads are very poor with frequent wash outs and breakaways and you require a four wheel drive vehicle. Medical assistance if required is via the Royal Australia Flying Doctor Service.

Geology

The geology of the area encompasses several stratigraphic groups with interfingering lithologies and is is complex. Cretaceous Bulldog shale of marine origin has been gently deformed and duricrusted to form gibber plain uplands of the Stony desert area, and the weathering of fluvial and lacustrine sediments, of Eocene and Miocene age, have formed breakaways. The area is cut by several major ephemeral streams, including Coopers Creek, and the desert is flanked by the dunefields of the Strezleki desert.

There is considerable geological interest in the area because of the record of surface environments in the Eyre basin over the past 40 million years, and the famous silcrete floras and mammalian megafauna. Despite this the area is poorly studied and almost nothing has been published on the regolith and geomorphological history of the Stony desert itself.

Recently the area has been of interest because of the complex interrelationship of lacustrine, fluvial, and aeolian sediments and variable development of duricrusts, offer good scientific analogues for Mars, with the exception of carbonate and sulphate soils and sediments.

Palaeontology

Watchie Sandstone and Willalinchina Sandstone Sedimentology and Palaeoenvironments

The Watchie Sandstone is a lacustrine sequence comprising, in upwards succession, a local channel-sand facies and widespread lag deposits, a transgressive fine-grained lacustrine facies and regressive strandline deposits with a wavebase lag. Lags indicate a low-gradient surface and strandlines still incorporate pedogenic silcrete clasts.

The palaeocurrent evidence is consistent with longshore processes and bar accretion onto a foreshore and larger bedforms indicate storm events. Ridges have formed only along the eastern shoreline, indicating a strong westerly airstream and silicification and ferruginisation alternated during lacustrine regression. Insects (including ants and termites) lived in developing soil profiles and rhizonodules indicate that plants grew on these soils.

The Willalinchina Sandstone was deposited in a fluvial channel environment and is interpreted as a broad, shallow meandering to braided channel system which abuts a floodplain to lacustrine palaeoenvironment. Storm and associated flood events have also deposited bar, levee and floodplain deposits which interfinger with fluvial derived sediments.

Field observations support this interpretation: A thin lens of basal conglomerate has been overlain by several meters of cross-bedded fine to medium-grained sandstone containing minor lenses of course-grained sandstone. Silicification has occurred in several places.

The silcrete is defined by several closely spaced vertical structures which has been referred to in past literature as “ants nest or reed mould silcrete ”. The vertical structures have been interpreted as reed casts, however, there is no evidence of organic matter found within the Willalinchina Sandstone Unit. A possible alterative is soft sediment deformation caused by gas escaping from rotting vegetation. This scenario would explain why the gas escape structures are not evident throughout the sandstone unit, but are preferentially preserved.

Stratigraphic Analysis and Age

Sediments from the Stuart Creek Palaeochannel have been placed in the Watchie and Willalinchina Sandstone units although most of the Palaeochannel area is located within the later. Preliminary palaeobotanical work suggests that the Willalinchina Sandstone correlates with the youngest phase of the Eyre Formation in the Lake Eyre Basin and is Eocene in age whilst the Watchie Sandstone has been interpreted as Miocene and correlates with the Billa Kalina Basin. However, recent stratigraphic analysis may indicate that the Willalinchina Sandstone could be Miocene to Pliocene in age. Further investigation needs to be made in this area before a definite date can be attributed to the Stuart Creek Palaeochannel.

Floral Record

The Australian Tertiary plant fossil record is very poor. Studies indicate that tropical to temperate rainforests occupied south eastern and south western Australia for much of the early-Tertiary period with a contraction of these rainforest communities in the mid to late-Tertiary.

The discovery of the fossil floras at Stuart Creek confirm the Stuart Creek Palaeochannel is one of the richest, most extensive Tertiary plant fossil localities in Australia, if not globally.

Initial Analysis

Initial analysis of floral elements indicate a mosaic of plant communities with sclerophyllous woodlands (dominated by Eucalypt spp.) interspersed with riparian rainforests and deciduous, marginal monsoon forests existed along the Stuart Creek Palaeochannel during the mid-Tertiary. These plants grew along the watercourses where permanent water enabled them to survive seasonal dry periods.

Interpretation of fossil floras and palaeoclimate

Observational studies in tropical and temperate forests have demonstrated a direct relationship between leaf form (foliar physiognomy) and local climate. For example, leaf length to leaf width and stomata number and size correlate to rainfall and ambient temperature. Mean canopy leaf size is also strongly correlated with mean annual temperature.

Palaeobotanical investigations of the Stuart Creek Palaeochannel (Willalinchina Sandstone) provides valuable information on the mid-Tertiary distribution of Eucalyptus spp. and other plants. To date 245 leaf types, 47 fruit and seed types and 2 major wood types have been identified.

The sclerophyllous component (identified from linear to lanceolate leaf forms) dominated the flora and would have grown on the exposed drier plains in the more open forest areas of the floodplain. Many of the fossil leaves are indiscernible from extant Eucalyptus leaves and the minimal variation in leaf form suggests that Eucalyptus has existed in the Stuart Creek area for ~15 million years. The ecological niche of this species, based on this information, appears to be similar to that of extant Eucalyptus.

The rainforest component of the flora grew in areas that provided a permanent water resource and are not present in the Stuart Creek area today, their absence indicative of a changing climate and environment. As the climate became increasingly drier, these plants died out to eventually become confined to the present monsoonal and tropical rainforests of northern Australia.

Ancient Eucalyptus are not the only plant fossils that have been uncovered; numerous other plant species are likely to be extinct representatives of extant genera. As with the Eucalyptus, similarity in leaf forms between fossil leaves and extant leaves have made identification problematic and it’s highly likely that many of these species were intermediate rainforest / sclerophyllous species sandwiched between the effects of climate change.

One species that appears to have been positively identified from its deeply incised proteaceous leaf form is Banklsieaeformis praegrandi. This fossil plant has been found in low numbers within the Stuart Creek Palaeochannel and has been linked to the extant species Banksia chamaephyton which is restricted to a small area of heathland in Western Australia (Greenwood, 1997).

My Visit and Access

I was lucky to be granted permission to visit the area September 2003. The fossil flora are not immediately noticeable and considerable searching is required to find good locations that have been exposed by erosion; I spent three days dawn to dusk combing the low lying breakaways until I found fresh exposures.

Please be aware that access to this area is STRICTLY PROHIBITED without express permission from the land owner. Removal of fossils or collecting is NOT ALLOWED.

References

Krieg, G. W., Rogers, P. A., Callen, R. A., Freeman, P. J., Alley N. F. and Forbes, B. G., 1991. Explanatory Notes Curdimurka South Australia. 1:250,000 Geological Series Geological Survey of South Australia, Peacock Publications, pp 35 – 38.

Rowett, A., 1997. Earthwatch '96. MESA, Journal 5, pp 27-29.

Stuart Creek Photographic Gallery (click image)

Images taken with Canon G5. They are to be replaced soon with images taken on my recent visits.

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