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Depositional and diagenetic environments of the Gordon subgroup (Ordovician) Gunns Plains, N.W. Tasmania.


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Khwaja, Azam Ali 1980 , 'Depositional and diagenetic environments of the Gordon subgroup (Ordovician) Gunns Plains, N.W. Tasmania.', PhD thesis, University of Tasmania.

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The Ordovician Gordon Subgroup at Gunns Plains is represented by
over 450 m of carbonates. Three major peritidal carbonate environments
(subtidal, intertidal and supratidal) are recognized. Microfacies
observed are (a) subtidal - sparse and packed biomicrites, biosparites,
pelbiomicrites, fossiliferous micrites, and micrites; (b) intertidal -
sparse biomicrites, biopelmicrites and pelbiomicrites, pelmicrites and
pelsparites, intrapelsparites, intrabiomicrites, intraoncosparites,
intrarnicrites and intrasparites, intraclast-bearing micrites, intraclast-bearing
fossiliferous micrites, other minor allochem-bearing micrites
and dismicrites; (c) supratidal - dismicrites, fossiliferous micrites,
intraclast-bearing fossiliferous micrites, intraclast-bearing micrites
and intramicrites. Abundant mudcracks, birdseyes, and vertical and
random burrows characterize the supratidal and intertidal facies, while
diverse fossil types and random bioturbation features are characteristic
of the subtidal facies.
Carbonate sedimentation took place dominantly during regressions
as shown by asymmetric cyclicity. These depositional regressions
represent progradation of the supratidal sequences. Microfacies
variation suggests supratidal conditions in the south and subtidal
conditions in the north of the study area. Ca, Mg, Sr, Na, Mn and Fe
concentrations reflect depositional and diagenetic environments, in
particular dolomitization and dedolomitization.
In most samples studied, an equigranular mosaic of finely
crystalline, euhedral to subhedral, ideal to ferroan dolomite commonly
ranging in size from 10 to 50µ occurs in burrows, in and around intraclasts,
and along mudcrack margins. Vertical distribution of the dolomite content
in the sections indicate different episodes of dolomitization, of varying intensity, at the end of each regressive cycle. This early diagenetic
dolomite is representative of tidal flats undergoing sabkha diagenesis.
Dedolomitization textures are easily recognizable in this limestone.
Both petrographic and chemical studies (Ca and Mg scans across dolomite
crystals) demonstrate dedolomitization. The degree to which individual
dolomite crystals are affected is highly variable ranging from a small
clot to a whole dedolomitized rhomb. Observation of these textures
suggests that most have formed as a result of the centripetal type of
dedolomitization. The complex nature of occurrence of the dedolomite
fabrics has been the main obstacle in making a quantitative petrographic
estimate of the dedolomite content.
A first attempt has been made to estimate the dedolomite content
by combining volumetric estimates of the dolomite content and their
Mg/Ca molar ratios. Curves representing different percentages are
constructed with the Mg/Ca = 1 curve corresponding to zero percent
dedolomitization. As most of the dolomites in the study area are ideal
dolomites, dedolomitization is implied for cases in which the dolomite
content and the Mg/Ca ratio deviate from the Mg/Ca = 1 curve. The degree
of dedolomitization in the studied sections ranges up to 69%.
Vertical variation of the dedolomite content appears to be related
to the regressive phases, generally being abundant in the supratidal
environment or where supratidal is absent, in the upper intertidal
environment of each cycle. Increase of dedolomitization towards the
supratidal suggests that dedolomitizing solutions were derived from a
landward source. The variable amount of dedolomite in the different
depositional cycles indicates that dedolomitization occurred in episodes
of various intensity at the end of each cycle.
Trace element relationships inferred for dedolomitization are
losses of Sr, Na and Mg, and a gain in Mn. The relationship or iron
to dedolomitization remains uncertain. Sr and Mn concentrations
obtained for the dedolomitizing solutions are similar to those of
aragonite, implying that the transformation of aragonite to calcite had
not been completed.
The δ180 values of dedolomites (-4.0 to -5.0%0 PDB) are within the
limits given by Keith and Weber (1964) for marine carbonates. The samples show a tendency towards a depletion in 18O with increasing
dedolomite content. Water to rock ratios (open and closed system) and
initial values of the dedolomitizing solutions show that sea-water
could not have been responsible for dedolomitization. The δ180 values
in the dolomites (-2.1 to -5.6%0 PDB) are heavier compared to calcites
analyzed from the same sample. It is believed that the initial enrichment
prior to dedolomitization in dolomites must have been significantly
different. The 8180 values in limestones (-5.4 to -8.6%aPDB) reflect
re-equilibration. Comparison with recent rain and cave waters shows
that this re-equilibration most probably took place during early
diagenesis. The δ13c values in dedolomites (+0.2 to -0.6%PDB), dolomites
(+1.8 to -0.7~PDB) and limestones (+1.6 to -l.5%o PDB) are also
believed to have undergone re-equilibration during early diagenesis.
It is proposed that, in cases where the dedolomite fabrics have
been recognized, the trace element concentrations for dolomitization may
not be valid unless the geochemical changes due to dedolomitization are
also taken into account.

Item Type: Thesis - PhD
Authors/Creators:Khwaja, Azam Ali
Keywords: Geology, Stratigraphic, Geology
Copyright Holders: The Author
Additional Information:

Thesis (Ph.D.) - University of Tasmania, 1981. Bibliography: l. 184-195

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