Melanie Oakes
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B. Sc. Honours Thesis
(PDF - 66 Mb)
The Early Carboniferous (Tournaisian) Grantmire Formation belongs to the Horton Group and is 800 m thick, based on exposures and drill core in the northern part of the Sydney Basin onshore. The 503 m measured section of the Grantmire Formation in drillcore PE 83-1 is dominantly pebble conglomerate with interbeds of siltstone and minor beds of sandstone. The conglomera~e (facies 1) is light to medium red, polymictic, poorly sorted, and clast supported with subangular to subrounded clasts. Conglomerate beds reach 15 m thickness with a maximum recorded clast size of 22 cm. They are divided into three subfacies: interbedded pebble conglomerate/sandstone, pebble to cobble conglomerate, and small boulder conglomerate. The other facies are sandstone (facies 2), siltstone with multiple sandy layers (facies 3), coarse siltstone (facies 4), and fine siltstone (facies 5). Siltstones are medium reddish brown and in two facies have calcareous nodules with green reduction patches and/or envelopes suggesting paleosol or shallow groundwater origin. Macroscale patterns suggest coarsening upward sequences on the 10-50 m scale and a rare 100 m scale are the result of fan progradation as indicated by thickening upward trends and increasing clast size. Siltstone-rich intervals suggest distal fan or interfan conditions. Mesoscale (<Sm) coarsening upward sequences may represent small lobe or levee progradation whereas large-scale fining upward sequences (5-10 m) are channel fills.
The Grantmire Formation has been interpreted as the clastic fill of fault-bounded basins within the region of the Sydney Basin. Currently, the Grantmire Formation is the only mapped unit in the Sydney Basin Horton Group. The presence of black shales in the Horton Group is important for hydrocarbon potential regionally; they are not presently identified in the Sydney Basin. The main clast types are chert, sedimentary lithoclasts, quartzite, volcanic clasts, and granitic clasts. Chert is derived form an older sedimentary source than the siltstone and sandstone clasts. Volcanic clasts are dominantly rhyolite with minor basalt that could have young or reworked origins. Acidic plutons are the origin of granitic clasts and likely provide a significant proportion of sand-sized quartz, feldspar, and mica.
Grantmire paragenesis begins with deposition ofsand- and gravel-sized clasts with iron-rich clay. The clays were oxidized at the surface or in the shallow subsurface early mt he depositional history forming hematite grain rims. Calcite nodules with fine mosaic textures in siltstone, are linked to shallow groundwaters. A locally pervasive poikilotopic calcite cement was emplaced prior to significant burial. Calcite commonly partially replaces potassium feldspar grains, possibly around the same time interval or subsequently. Dissolution of some grains, clays and calcite cement postdates consolidation and has generated secondary porosity.
Porosity of sandstone and conglomerates averages 9.6% and ranges from 4.2 to 15.7% and permeability averages 2.26 md and ranges from 0.06 to 7.72 md. Reservoir quality ranging from poor to good is likely controlled by variable amount of detrital clay, authigenic minerals, carbonate cement, paleosol development, and irregular laminac of finer material.
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Pages: 137
Supervisor: Martin Gibling
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