Australia: The Land Where Time Began
Fossil Site Walsh River and Elizabeth Creek
Mixed Marine Invertebrate and Vertebrate Sites Dating to the Early Cretaceous are Present in the Walsh River Cliffs and Elizabeth Creek. Both of these localities are located in Wrotham Park, northwest of Chillagoe, northern Queensland.
The Wrotham Park area, which is on a highly seasonal, high-flow tropical river, has produced many marine faunas dating to the Early Cretaceous, a time when the Great Artesian Basin (GAB) was flooded, over the last 134 years. The faunal record at these sites that is intensely abundant and diverse, includes ammonites, belemnites, crustaceans, bivalves, gastropods, fish, the remains of plesiosaurs, echinoderms (comatulids), ichthyosaurs and stromatolites, and a microfauna and microflora. Terrestrial taxa have also been found in these sites such as coniferales, fragments of pterosaurs and many large fossil logs. A younger sequence stratigraphy is represented by the Elizabeth Creek section than the Walsh Cliffs, with a slightly different ammonite fauna.
The Doncaster Formation is the stratigraphic unit that is represented. Originally defined as the Blackdown Formation on the Walsh River, this unit was correlated with the Doncaster Formation within the rest of the Great Artesian Basin. The unit represents maximum flooding of the Australian Craton in the period 115-110 Ma, and is essentially mud-dominant, with minor grey-green limestones, intraclastic conglomeratic units and rarer greensand and coarse labile sandstones. There are a number of sedimentological features such as hardgrounds, cool-water stromatolites, intraclastic horizons, as well as burrowed and bored nodule horizons contained in the sequence.
Scientific importance and research Potential
This is the best exposed marine sequence in Australia dating to the Early Cretaceous (Late Aptian). Key index taxa are contained which document the marine flooding event in the Late Aptian, which covered ⅓ of the continent. Over the last century the site has been moderately well studied, including recent stratigraphic studies and detailed palaeontology (e.g. Etheridge, 1920; Jack & Etheridge, 1892; McHenry et al., 2005). Recent collections and excavations of marine reptiles have been made in this site, in particular a specimen of a plesiosaur that is 85% complete.
There is good research potential at Walsh River and Elizabeth Creek, as new material is constantly being uncovered as continued renewal of outcrop by severe monsoonal erosion events occurs, and this will continue into the foreseeable future. Planned research work includes documentation of stromatolitic horizons (Cook, unpublished), marine reptile faunas (McHenry, Cook, Kear and Morell, pers. comm.) and other opportunistic discoveries such as pterosaur material (Salisbury, pers. comm.).
The robustness of the scientific work that has been undertaken so far is modest. Revision in detail is required for ammonite faunas, as well as other mollusc faunas.
Excellent nektic and benthic assemblages dating to the Early Cretaceous (Late Aptian), are contained in the Walsh River and Elizabeth Creek, without parallel anywhere in the Great Artesian Basin or elsewhere in Australia. Some rare taxa have been found at the site. There are several endemic taxa, though most are represented by specimens of poorer quality throughout the Great Artesian Basin. Extremely high potential for palaeoenvironmental reconstruction is offered by this site, including the use of stable isotope geothermometry and other geochemical tools for environmental assessment.
Taphonomy and the condition of the site/fossils
Vertebrate fossils that are almost complete have been recovered from within nodules. Ammonites from this site are of high commercial value and have been collected commercially, together with crustaceans, and have been for a number of years. As a result of this the site has been placed under some stress from exploitation, though the property managers have implemented a restricted access regime which has ameliorated the problem.
Comparison with similar Australian sites
The best example of marine faunas from the Cretaceous in Australia have been recovered from this site which far exceeds the diffuse Cretaceous sites from the northern Eromanga Basin and Surat Basin. The Miria Marl from Giralia Range of Western Australia is unparalleled, though it is of a much younger Cretaceous assemblage, and a much more extensive vertebrate record than that of the Miria Marl is contained in the Wrotham Park sites.
This site contains a complete nektic and benthic ecosystem record of its time, and therefore is of use in reconstructing the cool-water Aptian faunas and ecosystem. It gives a dioramic insight of the marine fauna inhabiting the largest inland sea that has flooded the Australian continent. Rapid fluctuation in sea level engendered the speciation of the Mesozoic. This is true of the Late Aptian stage of northern Australia, an area in which a maximum sea level induced speciation in the shallow continental basin (Oosting, 2004). A vast diversity of benthic and nektic fossil forms is a reflection of this. The diverse heteromorphy ammonites belonging to the Australuceras-Tropaeum clade is notable, which produced at this time bizarre and somewhat plastic morphologies. This is indicative of a genus-level group that was in flux due to the rapid change in niche and broad ecospace availability. Occurrences elsewhere of the remains of plesiosaurs are complemented by the presence in modest numbers at this site and are a key time of speciation for the group. From this horizon are known large pliosaurids, especially Kronosaurus, as well as a number of unnamed plesiosaurs, which includes specimens with significant gut contents demonstrating diverse feeding strategies (McHenry et al., 2005).
Author: M. H. Monroe
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|