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Abstract

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 • within the 1300m. of the Chudleigh
Limestone Formation. • 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.
•The 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€,mic species Phragmodus
tasmaniensis sp. nov. and a probable member of the "Australian" fauna .
Rhipidognathus? careyi sp. nov.. The •assemblage 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..
Correlatil,n 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.

Item Type:	Thesis - PhD
Authors/Creators:	Burrett, Clive Francis
Keywords:	Geology, Stratigraphic, Conodonts, Limestone
Copyright Holders:	The Author
Copyright Information:	Copyright 1978 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1979. Bibliography: l. 287-306
Item Statistics:	View statistics for this item

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