Doctor of Philosophy
Department of Geology - Faculty of Science
Tadros, N.Z, Structure and tectonics of the Gunnedah Basin, N.S.W: implications for stratigraphy, sedimentation and coal resources, with emphasis on the Upper Black Jack group, PhD thesis, Department of Geology, University of Wollongong, 1995. http://ro.uow.edu.au/theses/840
The Gunnedah Basin has long been considered a foreland basin. This study found that the basin consists of several linearly arranged troughs defined by bounding longitudinal and transverse ridges and highs. These features, together with the distribution and type of the Early Permian sedimentary basin fill and the associated volcanic rocks, strongly indicate that the basin's origin is consistent with volcanic rift models. The Meandarra Gravity Ridge, which represents a significant zone of inhomogeneity in the upper crust caused by deep-seated dense mafic intrusions, provided a key tool for resolving the early history of basin development. The ridge is laterally displaced by low gravity transverse trends representing transfer structures which were active during rifting and thus bound rift compartments of varying size and subsequent thermal subsidence histories.
There is a striking correspondence between the high gravity anomalies and the trough areas which contain the thickest sedimentary pile (and hence maximum subsidence), and also between the low gravity transverse trends and the mapped transverse structural highs in the basin. These relationships, together with the linear arrangement of the troughs, enabled prediction of basement structure within areas of little or no borehole control, provided the basis for subdivision of the Mullaley and Gilgandra Suli-basins into structural subunits, and enabled prediction of basement faults which have little or no surface expression.
During the Early Permian, the volcanic rift-related transverse and longitudinal structures were the main source of sediment and effectively controlled and confined sedimentation to the trough areas (overlying the half-grabens). Thermal relaxation followed in the mid-Permian and caused basin-wide subsidence, widespread marine transgression and deposition of the Porcupine and Watermark Formations, but the rate of subsidence in the trough areas was still significantly higher.
The change to foreland tectonics in the mid-Permian provided a new source of sediment from the overthrusted New England Fold Belt. Tectonic loading in the Late Permian caused subsidence of the eastern half of the basin and establishment of lacustrine conditions. Structural readjustment along the eastern edge of the cratonic Lachlan Fold Belt caused uplift of a forebulge and shedding of quartzose detritus to the Westem Fluvial System which infilled the lake and expanded through the axial drainage via southwest-flowing tributaries. Fluvial incision of the underlying Westem Fluvial and Lacustrine Systems and sediment intermixing are evident in the main channel complex. Influx of coarse detritus from the New England Fold Belt and westward movement of the basin axis caused south-westward migration of the axial drainage complex.
Widespread silicic volcanism in the New England Fold Belt region contributed large amounts of pyroclastic detritus to the basin-fill. A major phase of lateral compression and thrusting of the New England Fold Belt onto the craton caused structural readjustment and uplift, particularly in the north, and ended Permian deposition in the basin.
Although foreland loading of the thrust belt was the dominant cause of subsidence during deposition of the Late Permian Black Jack Group and the Triassic Digby and Napperby Formations, the inherent volcanic rift-related basement structural elements had a significant effect by varying subsidence rates in the different basement compartments and consequently on development, distribution and geometry of depositional systems, their component fades and peat accumulation, as well as on emplacement and distribution of igneous intrusions and extrusions in the basin.
In the Late Permian and Triassic, longitudinal structures were periodically reactivated as thrust faults resulting in uplift and erosion of much of the upper Permian sequence particularly in the Maules Creek Sub-basin and the northern Mullaley Sub-basin.
The basement structural elements also controlled the distribution of igneous intrusions and volcanism. Late Carboniferous to Eariy Permian eruptions followed transfer faults and floored the developing Gunnedah Basin with basalt, while silicic volcanism appears to have developed parallel to the basin margins along longitudinal extension/detachment faults. Reactivated major transfer faults provided pathways for Jurassic and Tertiary phases of igneous intrusions and extrusions.
Recognition and mapping of basement structural elements and understanding of the basin's tectonic history highlighted the interrelationship between basin origin, tectonics and sedimentation and provided a framework for sedimentological and stratigraphic analysis and coal resource evaluation.
An hierarchial approach to genetic stratigraphic analysis has been applied to the study of the upper Black Jack Group. The analysis included definition of the geometry and distribution of genetic stratigraphic units and mapping sediment dispersion pattems within the sequence in order to develop sedimentation models and to detenmine the palaeogeography and tectonism expressed as basin subsidence and uplift in the source regions.
Sand body geometry of the upper Black Jack sequence emphasises the structurally controlled fluvial character, with a major axial trunk channel complex fed by easterly and westerly contributory channels.
The Hoskissons Coal and the Breeza Coal Member are regionally extensive, have tectonic and time significance and serve as genetic sequence boundaries. These seams separate genetic sequences of distinctly different depositional settings, bedding architecture, and sediment composition. The basinwide accumulation of the Hoskissons peat represents a significant period characterised by almost total non-deposition of terrigenous elastics and marks a change in basin depositional history from predominantly deltaic and shallow marine sedimentation to fluvial and lacustrine conditions of the upper Black Jack depositional episodes. The Breeza Coal Member marks the change from the Westem Depositional Episode, during which the basin fill was dominantly quartzose, derived from the Lachlan Fold Belt in the west, to the Eastern Depositional Episode when the sediment supply was from the New England Fold Belt in the east and was mainly lithic.
Four major genetic elements: the Hoskissons Peat-swamp, the Lacustrine, the Western Fluvial and the Eastern Fluvial Systems, have been recognised by their different lithology, and depositional and tectonic setting and by their palaeogeographic relationships.
The depositional systems served as mapping units with bounding surfaces for the contained genetic facies and allowed establishment and or refinement of correlations of several subregional coal seams. These seams, although not representing sequence boundaries, have time significance locally and have been used in finer subdivision within the larger genetic stratigraphic packages. Detailed lithofacies mapping of the interseam sediments provided the means to reconstruct the evolution of the upper part of the Black Jack fluvial systems, enhanced recognition of depocentres, revealed areal and stratigraphic distribution of sand bodies within the genetic units and ultimately allowed recognition of the impact of basement structures on sand distribution.
The genetically defined units provided the basis for the establishment of a new formal lithostratigraphy for the entire Permo-Triassic Gunnedah Basin sedimentary fill. In all, three groups, three subgroups, 13 formations and six members have been formally named and described in detail for the Permian section. Three formations and two members have also been formally named and described in the Triassic section. The expanded knowledge of the Gunnedah Basin sequence also allowed correlation of the stratigraphy and depositional history with those of the Sydney Basin.
The Hoskissons Coal, is the product of the Hoskissons Peat-swamp System and consists predominantly of vitrinite-poor, inertinite-rich coal with high liptinite content. Vitrinite content decreases upward in the lower section of the coal and increases upward in the upper section with a con'esponding increase in the amount of disseminated and discrete mineral matter. Liptinite (mainly sporinite) content increases towards the top where alginite is also present.
The Lacustrine System displays cyclic alternation between upward-coarsening sediments of lake margin facies and organic-rich mudstone of lake basin facies.
Architectural element analysis of the Western Fluvial System indicated that the sandstone was deposited by discrete, broad, probably shallow, low sinuosity channels comparable with Models 9 and 10 of Miall (1985) for low sinuosity rivers, and the middle-upper Brownstones of Allen (1983). In vertical profile, the sequence combines features characteristic of low sinuosity rivers of the "Piatt type" and the "South Saskatchewan type", or its ancient analogue - the Battery Point Formation.
Axial and tributary channel fill deposits of the Eastern Fluvial System consist mainly of volcanic-lithic conglomerate. The flood plain facies consist of carbonaceous sediments, thick stony coals and tuff.
Depositional setting had a significant influence on quality and continuity of the Hoskissons Coal. Therefore, in addition, the study included comprehensive analysis of the maceral composition of the Hoskissons Coal and applied coal facies analysis in order to understand the peat-forming environment and the factors that controlled the changes in that environment and consequently variations in coal quality and distribution. Subsidence was the major factor that controlled development of the Hoskissons peat swamp, influx of clastic sediments and consequently, coal quality and thickness. Rate of subsidence was uneven across the basin and was structurally controlled, and in part influenced by compaction of the peat or of the underlying platform of marine sediments.
Initially, shoreline sands (of the preceding upper Watermark-lower Black Jack/Arkarula Depositional Episode), in the south, protected the peat swamps from inundation by the sea and kept the water table high enough to produce vitrinite-rich plies, low in mineral matter and sulphur, at the base of the seam. The general trend of upward decrease then increase in vitrinite content of the Hoskissons Coal indicate a gradual fall and subsequent rise in the water table which ultimately "drowned" the peat and established lacustrine conditions in the eastern part of the basin. Raw and washed coal isoash trends show a dominant fluvial influence on coal quality. Geographic zonation of the SiO2/Al2O3 ratios of the coal ash corresponds closely with regional trends in the associated depositional systems. Si02/Al203 ratios are high in the north and south-west as a result of the influence of the quartz-rich western fluvial sediments. Tectonic stability and minor influx of clay-rich sediments from the New England Fold Belt region contributed to the low to medium Si02/Al203 ratios in the east.
Basement structural elements were active during sedimentation and influenced subsidence, sedimentation and peat accumulation. There is a strong relationship between the Hoskissons Coal lithotype profile, quality and thickness and basement structural elements. Organic-rich mudstone formed in rapidly subsiding areas in the centre of the trough areas where, in some places, the Hoskissons Coal has been replaced by clastic sediment. Lowest rates of subsidence on the highs and ridges produced a thin seam and exposed the coal to some degradation and partial erosion.
Sand content of the underlying platform had some influence on subsidence and consequently on coal thickness and quality. Localised compaction of the peat also contributed to subsidence in the north and central eastern areas.
Coal seams above the Hoskissons Coal, were also strongly influenced by their depositional setting. Peat accumulated in interchannel areas adjacent to the axial channel complex and principal tributaries. Location of the axial channel complex was largely structurally controlled and occupied the areas of maximum basin subsidence. Moderate subsidence rates in the north tended to entrench the axial channel complex which, as a result, isolated the peat swamps in the interchannel areas and produced fewer seams that are hard to correlate. Rapid subsidence rates in the south and south east allowed greater lateral shift through aggradation and avulsion and favoured thick widespread peat accumulation except where the swamp has been disrupted by sediment influx from nearby channels.
The comprehensive analysis of the structure and sedimentation, complemented with a study of the peat swamp environment and coal facies analysis, greatly improved understanding of the factors which controlled peat swamp development and peat formation, coal quality and distribution, seam thickness and splitting, and the nature of the mineral matter in the coal, and provided the basis for reliable seam correlations. All of these are important factors in the assessment of the basin's coal resources, which are estimated at 29 billion tonnes of potentially usable in situ coal. The Black Jack Group contains the vast majority of that resource, and nearly half of the total resource is contained in the Hoskissons seam.
The remaining coal resources are contained mainly in the six seams overlying the Hoskissons Coal, particularly in the Caroona area, except near the axial channel complex where quality deteriorates and in some cases the seam is split or replaced by fluvial channel deposits.
Exploration in the basin has concentrated on the Breeza, Caroona, West Gunnedah and Narrabri areas within the potentially economic zone of shallow (<500>metres), good quality coal in the east, where borehole spacing is between 4 and 8 kilometres or less and seam correlations are well established, allowing the coal resources to be calculated to Intended status. The bulk of the resources of Hoskissons and the overlying seams is amenable to underground mining methods only.