The Chinaman Creek Limestone is a major component of the Broken River Group that formed a broad shelfal succession in the northern part of the Graveyard Creek Subprovince, southwest of Greenvale in north Queensland. The formation is up to 790 m thick and consists largely of clastic limestone intervals up to 250 m thick of very thick to thin-bedded calcirudite and calcarenite and sporadic, thin interbeds of calcareous mudstone, siltstone and sandstone. More substantial siliciclastic intervals of lithic sandstone, siltstone and mudstone, and rare conglomerate up to 100 m thick are interlayered.
Reefal limestone intervals up to 30 m thick and 2 km long are locally represented in the unit, mainly in its lower parts. These are mainly stromatoporoid–heliolitid–rugose coral framestone, with prominent heads of the rugosans Endophyllum and Phillipsastraea in the upper and lower parts of the succession, respectively.
Bioclastic components are largely of stromatoporoid or coralline debris (with Amphipora characteristics), but fragments of crinoids, brachiopods, bivalves, gastropods, ostracods and calcareous algae are also represented. Corals and stromatoporoids are locally found in growth positions and large valves of Stringocephalus and megalodont bivalves are prominent in the upper part (Withnall, 2012).
Megalodont bivalves (superfamily Megalodontoidea) are unusual and very distinctive fossils of the middle Paleozoic to early Mesozoic. Many species are large and thick-shelled, and have great preservation potential. So, it is surprising that, although these clams had a largely cosmopolitan distribution in the Silurian and Devonian and were locally very abundant, their taxonomy and paleoecology are poorly known. They were inclined mud resters, oriented with sagittal plane vertical, and grew partly submerged in, or on top of, generally firm lime mud substrates, in tropical shallow water (de Freitas et al, 1993). Megalodont bivalves are predominant in the spectacular slab of limestone donated to the NRG, as seen in the shown in the photograph above.
Most of the specimens in the slab represent slices through the ‘beak’ part of articulated shells, many still in growth position. Others show different sections of the shell—still the ‘beak’ area but sliced more obliquely so that a small part of the valves distal to the ‘beak’ are also shown (they look like spines). The shells were projecting at an angle from the sea-floor (hence the oblique sections) as the slab has been cut at an angle to bedding. Bits of sliced valve away from the thick beak region show as much thinner curved white lines (Henderson, 2012).
de Freitas, Brunton and Bernecker, 1993. Silurian Megalodont Bivalves of the Canadian Arctic and Australia: Paleoecology and Evolutionary Significance. Palaios vol. 8, p. 450–464. https://doi.org/10.2307/3515019
Henderson, Bob, James Cook University, pers. comm. to Rob Cameron May 2012.
Withnall, Ian, Geological Survey of Queensland, pers. comm. July 2012.
John Bain, Director National Rock Garden