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

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 50¬¨¬µ 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 ˜í¬•\\(^{18}\\)O values of dedolomites (-4.0 to -5.0‚ÄövÑ‚àû\\(_{PDB}\\)) are within the limits given by Keith and Weber (1964) for marine carbonates. The samples show a tendency towards a depletion in \\(^{18}\\)O 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 ˜í¬•\\(^{18}\\)O values in the dolomites (-2.1 to -5.6‚ÄövÑ‚àû\\(_{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 ˜í¬•\\(^{18}\\)O values in limestones (-5.4 to -8.6‚ÄövÑ‚àû\\(_{PDB}\\)) reflect re-equilibration. Comparison with recent rain and cave waters shows that this re-equilibration most probably took place during early diagenesis. The ˜í¬•\\(^{13}\\)C values in dedolomites (+0.2 to -0.6‚ÄövÑ‚àû\\(_{PDB}\\)), dolomites (+1.8 to -0.7~\\(_{PDB}\\)) and limestones (+1.6 to -l.5‚ÄövÑ‚àû\\(_{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.