Middle-upper Ordovician conodonts and stratigraphy of the Gordon limestone sub-group, Tasmania.
Creators: name: Burrett, Clive Francis
Publisher: name: University Of Tasmania
Year: 2023

description: The Gordon Subgroup attains a thickness of 2100m of carbonates and minor siltstones in its redefined type section in the Florentine Valley and ranges in age from Late Canadian-Llandoverian. Detailed studies of the conodonts from the Chazan-Edenian parts of the sub- group, throughout the western half of Tasmania, reveal a faunal sequence very similar to that found in the North American Midcontinent fauna] province. Lithostratigraphic studies in the subgroup at Mole Creek in northwestern Tasmania have led to the recognition of a conformable succession. 'of seven mappable members ‚Äö within the 1300m. of the Chudleigh Limestone Formation. ‚Äö The Standard Hill Member is 120m. thick, conformably overlies the Moina Sandstone, is oncolitic with Minor calcarenites and was probably deposited mainly, in a low, intertidal environment. This member is overlain by the Ugbrook Nodular Member which consists of 120m. of nodular agillaceous limestones, barren of macrofossils, and minor , trilobite-bearing micrites. It is probably -7 the same age as a cross-bedded sandstone south of Standard Hill. The nodular limestone -is interpreted as a lagoonal deposit formed behind a sand bar. The Sassafras Creek Member .is a .135m. thick sequence of micrites and dolomicrites with a 3m. thick biocalcarenite bed at the base and a thin oncolitic bed at the top. The 470m. thick Dog's Head Member is a sequence of micrites and dolomicrites containing chert nodules and several beds of silicified macrofossils. The Mole Creek Member consists of 75m. of reddish siltstones containing an.orthid/ stictoporellid/pliomerina fauna. 'The Overflow Creek Member consists of 45m. of mainly unfossiliferous dolomicrites and dolosiltites probably deposited on a tidal flat crossed by tidal Channels. The subtidal Den Coralline Member consists- E 45m. of highly fossiliferous . biomicrites and biocalcirudites overlain by 40m. of creamy textured, mainly macro-unfossiliferous,micrites. Brachiopodal/trilobitic siltstones overlie the creamy textured micrite and are the youngest beds of the Gordon .Subgrcup. These are overlain conformably by white unfossiliferous quartzites of the Eldon. Group. The Chudleigh Limestone was mainly deposited under peritidal conditions and only 20% of the sequence was deposited in a subtidal environment. Six major subtidal sequences may be identified. Four of these are in the Dog's Head Member, one is in the Mole Creek Member and one is in the Den Member and above. Very stable tectonic conditions and low amplitude eustatic oscillations are indicated for the Mole Creek area from the Chazyan-Edenian. The conodont collections from Mole Creek and the Florentine Valley may be used to erect a local succession of assemblages. All assemblages contain Panderodus gracilis (Branson and Mehl), Drepanoistods suberectus (Branson and Mehl) and Belodina eompressa (Branson and Mehl). Assemblage A is found in and just above the Standard Hill Member at Mole Creek and in and just above the Cashions Creek. Formation in the Florentine Valley. It is also found in oncolitic and nononcolitic limestones in many parts of the state. ‚ÄöThe assemblage consists of Phragmodus.flexucsus Moskalenko, Belodl.;na alobamensis Sweet and Bergstriim, Acontiodus cf. nevadensis Ethington and Schumacher, Belodella copenhagensis (Ethington and Schumacher), Panderodus serpaglii sp. nov., Appalachignathus? Bergstr3m.etal. and Drepanoistodus forceps-(Lindstr6m). Correlation to North.American faunas 5 and 0 (Chazyan) and to the Kirenskiy Sub-formation of Siberia is Suggested. This assemblage is overlain at Mold Creek and in the Florentine Valley by Assemblage B which contains the end‚Äövá¬®,mic species Phragmodus tasmaniensis sp. nov. and a probable member of the \Australian\" fauna . Rhipidognathus? careyi sp. nov.. The ‚Äöassemblage is correlated [with North] American Fauna 7.by means cf abundant specimens of Chirognathus ImonodactyZuS Branson and Mehl and by the earliest occurrence of Plectodina aobileata (Stauffer). Assemblage B-is found at the base of the limestone sequences in the Vale of Belvoir and at Zeehan in the Everlasting Hills and in the lower Limestone Member of the Benjamin Limestone and the Ugbrook Member at Mole Creek. Assemblage C contains P. aculeata Phragmodus undatus Branson and Mehl and Erismodus sp. This and 'succeeding assemblages are found in most areas and correlation with Fauna 8 is suggested. Assemblage contains the same species as Assemblage C but Bryantodina? abrupta (Branson and Mehl) and Mectodina florentinensis sp. nov. are added.. Correlatiln with Fauna 9 is probable. . Assemblage E. is similar to D. but P. aculeata is replaced by P. cf. furoata (Hinde) and correlation with Fauna 10 is suggested. The highest assemblage (F) contains. 0. robustus (Branson Mehl and Branson) Oulodus cf. oregonia Branson Mehl and Branson and P. cf. furcata and correlation with the Edenian is suggested. Parts of this assemblage are found at Bubs Hill Mole Creek Picton River Florentine Valley and Ida Bay and suggest that limestone deposition terminated at about the same time throughout the state. Limestone deposition at Flowery Gully terminated in the Whiterock and may have terminated in the Chazyan at Railton and in the BlackeriVeran at Melrose. Conodonts below Assemblage A have not been studied in detail though the presence of Periodon aculeatus Hadding and other conodonts studied by D.J. Kennedy suggests correlation to the Whiterock. Whiterock faunas are known at Railton at Flowery Gully and in the-Florentine Valley. The lowest Limestones in the Vale of Belvoir and near Zeehan are Blackriveran in age and overlie intertidal srliciclastics and suggest that a westwards transgression occurred from the early Whiterock onwards. The depositional area of the Gordon Subgroup may be compared to that of modern Shark Bay in Western Australia. Both may be considered as Shallow water embayments into large peneplained continents and both contain thick sequences of carbonates dominated by intertidal limestones As an embayment into continental crust no large scale relative movements are considered necessary to account for the differences between the Gordon Subgroup carbonate's and the deep water graptolite-bearing lutites of eastern Tasmania and Victoria.. In a small embayment flanked by extensive carbonate mud -flats hypersalinity would be expected and may account for the low conodont diversity. The very low conodont yields per kg. are ascribed to hypersalinity and other harsh and unstable environmental features and also to a high rate (32mm/1000 yrs.) of sedimentation. The environmental preferences of most conodont species was for subtidal conditions and only .Panderodus serpaglii sp. nov. Rhipidognathus? careyi sp. nov. C. monodactyla Erismodus sp. and to a lesser extant P. tasmaniensis were well adapted to tidal flax conditions. Mapping of the Conodont Alteration Index throughout the state necessitates the postulation of a high heat flow concentrated around the peripheries of the central Precambrian blocks during the Devonian."