2.3.2.2 Geology and hydrogeology


2.3.2.2.1 Three-dimensional geological model of the Clarence-Moreton bioregion

A three-dimensional geological model of the Clarence-Moreton bioregion and regional models were developed as part of this BA (Figure 7), as described in companion product 2.1-2.2 for the Clarence-Moreton bioregion (Raiber et al., 2016). In the Clarence-Moreton bioregion, geology has a major influence on many hydrological and ecological processes, and geological processes such as tectonic uplift and erosion shape terrain development and are major landscape-forming drivers. Geology also controls groundwater recharge and aquifer interaction and it also has a major influence on surface watergroundwater interactions.

The three-dimensional geological models have many applications to the Assessment. They highlight the spatial extent of alluvial, volcanic and sedimentary bedrock aquifers, and their structural and stratigraphic relationships. The models also include important topographic features such as Lamington National Park in southern Queensland, the Border Ranges National Park in northern NSW and the Main Range Volcanics in south-east Queensland. These are of great significance for hydrological processes and ecosystems as they are preferential groundwater recharge areas and also generate large volumes of surface water runoff. The bioregion-wide three-dimensional geological model also shows the five major regional alluvial aquifer systems that cover a substantial part of the Clarence-Moreton bioregion. These major alluvial systems are the Lockyer Valley alluvium, Bremer River/Warrill Creek alluvium and Logan-Albert River alluvium in Queensland, and the Richmond River and Clarence River alluvia in NSW (Figure 7). In the Richmond and Clarence river basins, estuarine and coastal sediments were deposited near the coast during the Quaternary. These can be identified in some bore logs, but their representation as separate layers in a three-dimensional geological model requires development of finer-scale models that focus exclusively on these areas, a task beyond the scope of this Assessment. Consequently, no differentiation was made in the three-dimensional geological model among different types of Quaternary unconsolidated sediments (i.e. alluvial, coastal or estuarine sediments).

Four higher resolution three-dimensional geological models were developed for the following river basins within the Clarence-Moreton bioregion:

  • Brisbane river basin (i.e. Lockyer Valley, Bremer river basin and Warrill creek basin)
  • Richmond river basin
  • Logan-Albert river basin
  • Clarence river basin.

These allow closer examination of local hydrological processes and regional groundwater dynamics, and provide the model structure for the numerical groundwater model of the Richmond river basin (discussed in companion product 2.6.2 for the Clarence-Moreton bioregion (Cui et al., 2016b)). In this product, the focus is on hydrological processes limited to the area covered by the Richmond river basin three-dimensional geological model domain (Figure 8) to provide the structure that underpins the numerical groundwater model.

Figure 7

Figure 7 Three-dimensional geological model of the Clarence-Moreton bioregion

Viewed from the south-east; the vertical extent is from –2500 to +1400 m Australian Height Datum (AHD); the north-south extent is 320 km; the maximum east-west extent is 140 km; the vertical exaggeration is 10. The dashed lines E to F and G to H show the orientation of cross-sections Figure 14 and Figure 15.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3); NSW Trade and Investment (Dataset 4)

2.3.2.2.2 Three-dimensional geological model of the Richmond river basin

The three-dimensional geological model of the Richmond river basin (Figure 8 and Figure 9) highlights geological and topographic features. It shows the spatial distribution and geometry of alluvia, volcanic and sedimentary bedrock stratigraphic units. Major structural features were projected onto the three-dimensional geological model, and major permanent or near-permanent streams are shown. Two major regional fault systems with potentially significant influence on groundwater flow are the East Richmond and Coraki faults. Another important structural feature is the mid-basin high, which separates the Casino Trough (where most CSG exploration has occurred and where the West Casino Gas Project is located) from the Lismore Trough. A further depositional centre is the Grafton Trough. Fault displacements are not modelled in this version of the three-dimensional geological model, but faults will be incorporated in a future model realisation as part of a CSIRO strategic funding project ‘Next generation methods and capability for multi-scale cumulative impact assessment and management’ (sub-theme ‘Tracer-based improvement of groundwater model conceptualisation and predictability’). Thick sequences of basalts, which are part of the Lamington Volcanics, occur in the northern part of the Richmond river basin (discussed in more detail in Section 2.3.2.2.6). The headwaters of the major rivers in the Richmond river basin (the Richmond and Wilsons rivers) are located in the steeper upper parts of the volcanics. Further downstream, these rivers are tidal-influenced. The Walloon Coal Measures, the major target for CSG exploration in the Clarence-Moreton bioregion, outcrop in the western part of the Richmond river basin. In the centre of the Richmond river basin and in the West Casino Gas Project area, they are covered by several hundred metres of younger sedimentary bedrock units and alluvial aquifers (discussed in more detail in subsequent sections).

Figure 8

Figure 8 Three-dimensional geological model of the Richmond river basin; the extent of this model corresponds to the extent of the Richmond river basin groundwater domain

Viewed from the south; the vertical exaggeration is 12. Lines A to B and C to D show the orientation of the fence diagram (Figure 9) and cross-section (Figure 24). Faults and other structural elements are projected as vertical surfaces on the three-dimensional geological model.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 3, Dataset 5); NSW Trade and Investment (Dataset 4)

Figure 9

Figure 9 Fence diagram through the Richmond river basin showing geometric and thickness relationships between alluvial, volcanic and sedimentary bedrock hydrostratigraphic units

In brackets in the legend, the assignment of each unit to the four major hydrostratigraphic groups or geology types is shown. A–B and C–D refer to the orientation of the cross-section line in Figure 8.

Data: Bioregional Assessment Programme (Dataset 5)

For the purpose of landscape classification (described in detail in Section 2.3.3.1), all geological units within the Clarence-Moreton bioregion were assigned to four major geology types:

  • unconsolidated sediments – alluvium (surface alluvium up to approximately 40 m thick, representing an unconfined to semi-confined aquifer)
  • unconsolidated sediments – estuarine and coastal
  • fractured igneous rock – in the Clarence-Moreton bioregion, this category relates mostly to volcanic (extrusive) rocks. Fractured intrusive rocks also exist (e.g. Figure 10), but are only of minor significance for hydrological or ecological processes in the Richmond river basin due to their areally limited extent
  • consolidated sedimentary rock – this category includes all the Clarence-Moreton Basin sedimentary units (e.g. corresponding to Grafton Formation, Bungawalbin Member, Kangaroo Creek Sandstone, Walloon Coal Measures and Bundamba Group in Figure 10).

Figure 10

Figure 10 Three-dimensional geological model of Richmond River basin and four major simplified surface geology types

Data: Bioregional Assessment Programme (Dataset 1, Dataset 3, Dataset 5); NSW Trade and Investment (Dataset 4)

2.3.2.2.3 Groundwater flow directions, groundwater recharge and discharge

Groundwater recharge is one of the most important hydrogeological processes, as it has a major impact on the flow and composition of groundwater in aquifer systems. Having reliable information on the amount of recharge, as well as on its temporal variability and the spatial distribution of preferential recharge areas, is critical in water resource assessments and the development of groundwater models. Due to the spatial heterogeneity of geological materials, climatic variability and the scarcity of hydrogeological data in many regions, groundwater recharge is also one of the most difficult processes to estimate (e.g. Raiber et al., 2015).

For this BA, groundwater recharge to the aquifers of the Clarence-Moreton bioregion was estimated using chloride mass balance for the fractured volcanic rock and sedimentary bedrock aquifers for the entire bioregion, complemented by an assessment of the relationship between soils, vegetation and rainfall for alluvial aquifers (companion product 2.1-2.2 for the Clarence-Moreton bioregion (Raiber et al., 2016)). Groundwater recharge occurs via different mechanisms, including diffuse rainfall recharge and surface water recharge.

Diffuse rainfall recharge occurs where stratigraphic units are exposed at the surface. The three-dimensional representation of groundwater recharge in the Richmond river basin shows high spatial variability in recharge rates due to the variable hydraulic properties of the sediments and rocks.

The recharge assessment conducted as part of this BA suggests that there are several preferential recharge areas in the Clarence-Moreton bioregion (Figure 11 and Figure 12):

  • the entire extent of Main Range Volcanics and Lamington Volcanics
  • Woogaroo Subgroup outcrop in the Lockyer Valley (at the northern margin of the Clarence-Moreton bioregion)
  • the western margin of the Clarence-Moreton bioregion in NSW, where the sedimentary bedrock units are topographically elevated, is considered a major area of recharge to the sedimentary bedrock aquifers in NSW
  • Cenozoic intrusions – hydrochemical data and groundwater mounding near Cenozoic intrusions in the Bremer river basin and Warrill creek basin (Figure 11) suggests that these act as preferential recharge areas for the Walloon Coal Measures. Elsewhere within the Clarence-Moreton bioregion (e.g. Richmond river basin), there are relatively few Cenozoic intrusions and their influence on regional groundwater recharge processes is therefore likely to be very limited.

Although these areas are all considered as preferential recharge areas for specific aquifers, the recharge assessment indicates that recharge rates to the Main Range Volcanics and Lamington Volcanics are at least one order of magnitude larger than to most sedimentary bedrock units. The three-dimensional geological model shows that the Walloon Coal Measures, the primary CSG target, is exposed over extensive areas in both Queensland and NSW. However, the recharge assessment highlights that recharge rates to this unit are comparatively small.

Inferred groundwater flow directions of the sedimentary bedrock units are shown in Figure 11. In the Richmond river basin in NSW, the bedrock topographic gradients suggest that groundwater in the sedimentary bedrock likely flows from the elevated western margin towards the lower-lying eastern basin margin.

The three-dimensional representation of formation bases (Figure 13), where each formation base corresponds to the formation top of the underlying stratigraphic unit, highlights that there is no hydraulic connection between the Casino Trough and the Warrill Creek Syncline (which underlies the Bremer river basin, where the only operating coal mine in the Clarence-Moreton bioregion is located) due to the presence of two basement highs (Mount Barney basement high and South Moreton Anticline).

As outlined in companion products 2.1-2.2 and 1.3 for the Clarence-Moreton bioregion (Raiber et al., 2016; Murray et al., 2015, respectively), the Lockyer Valley was not included in the preliminary assessment extent (PAE). This is because the Walloon Coal Measures are not present in approximately 80% of the Lockyer Valley (Figure 14). Furthermore, where the Walloon Coal Measures are present in the Lockyer Valley, they are only very thin (0 to 100 m) and covered by thick sequences of the Main Range Volcanics (Figure 15). Together with the location at the basin margin where coal thickness and gas saturation are likely to be very small, there is sufficient evidence to indicate that there is unlikely to be any potential for CSG development in the Lockyer Valley.

The assessment of geology and hydrogeology conducted during this BA suggests that throughout the Clarence-Moreton bioregion in Queensland and NSW, there are several areas where upward leakage occurs from the sedimentary bedrock to the shallow aquifer and to the surface at the down-gradient end of the flow paths. An example of a sedimentary bedrock discharge area in the Clarence-Moreton bioregion that is best constrained by monitoring data is located at the eastern boundary of the Bremer river basin (Figure 11). The potentiometric surface of the Walloon Coal Measures in this discharge area suggests that groundwater flows towards a relatively small area at the eastern margin of the basin south-west of Ipswich, where it is likely to discharge into the alluvium and to the surface (Figure 11). In other areas such as the Richmond river basin where less groundwater monitoring data exist, the bedrock topographic gradient is derived from the three-dimensional geological model; this suggests that, similar to the Bremer river basin, groundwater in the sedimentary bedrock flows towards the lowest point in the eastern part of the Richmond river basin near Coraki (e.g. Figure 12 and Figure 13), where it is likely to discharge into the alluvial aquifers.

Figure 11

Figure 11 Three-dimensional geological model of the Bremer river basin, Warrill creek catchment and Lockyer Valley

Superimposed are the potentiometric surface map, inferred groundwater flow directions and discharge areas of Walloon Coal Measures in the Bremer river basin and Warrill creek catchment.

The vertical extent is from –2000 to +1200 m AHD; the north-south extent is 100 km; the maximum east-west extent is 95 km; the vertical exaggeration is 10.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3)

Figure 12

Figure 12 Three-dimensional representation of groundwater recharge distribution in the Clarence-Moreton bioregion

The vertical extent is from –2500 to +1400 m AHD; the north-south extent is 320 km; the maximum east-west extent is 140 km; the vertical exaggeration is 10.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2, Dataset 3, Dataset 6); NSW Trade and Investment (Dataset 4)

Figure 13

Figure 13 Three-dimensional representation of formation bases of Walloon Coal Measures (shown as colour-coded elevation), Koukandowie Formation, Gatton Sandstone and Woogaroo Subgroup

Geological structures and Richmond River groundwater model domain are also shown.

The north-south extent is 320 km; the maximum east-west extent is 140 km; the vertical exaggeration is 12.

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2)

Figure 14

Figure 14 Cross-section G–H through the Bremer river basin, Warrill creek catchment and the Lockyer Valley, highlighting the absence of the Walloon Coal Measures in the Lockyer Valley

For orientation of cross-section, see Figure 7.

Data: Bioregional Assessment Programme (Dataset 2)

Figure 15

Figure 15 Cross-section E–F through the Bremer river basin, Warrill creek catchment and the Lockyer Valley, highlighting the spatial relationship of the Walloon Coal Measures and the Main Range Volcanics in the Lockyer Valley

For orientation of cross-section line, see Figure 7.

The dashed line at the interface of the intrusions and the sedimentary bedrock indicates that the subsurface geometry of these intrusions is unknown. Consequently, they have not been modelled in the three-dimensional geological model and were instead added subsequently to the cross-section.

Data: Bioregional Assessment Programme (Dataset 2)

2.3.2.2.4 Unconsolidated sediments – alluvium

2.3.2.2.4.1 Alluvia overview

Alluvial aquifer systems host many important water-dependent assets in the Clarence-Moreton bioregion. Throughout the Clarence-Moreton bioregion, alluvial depositional and aquifer systems follow a generalised pattern of upper, mid and lower catchment alluvial development (Figure 16).

Figure 16

Figure 16 Different alluvial systems recognised in upper, mid and lower zones for a generalised catchment in the Clarence-Moreton bioregion in south-east Queensland

See Queensland Government (2015a) for more information.

Data: Adapted from DSITI (Dataset 7), © The State of Queensland (Department of Science, Information Technology and Innovation), 2015

2.3.2.2.4.2 Upper catchment

Alluvial sediments in the topographically higher upper catchment (headwaters) of the Richmond river basin (and in other similar catchments within the Clarence-Moreton bioregion) are relatively thin (typically less than 15 m thick) and are composed mostly of unconsolidated coarse-grained sediments such as boulders, gravel and sand (Figure 17). These have been derived from erosion of the Lamington Volcanics and deposited in a high-energy fluvial environment. Streams in the upper catchment erode deeply into the surrounding basaltic landscape, thus forming narrow (typically less than 500 m wide) and steep v-shaped valleys. Due to the coarse composition of the alluvial sediments of the upper catchment, groundwater in these unconfined aquifers is stored and transmitted rapidly through relatively large intergranular voids between boulders, gravel and sand. In the alluvial channels in the upper catchment, only thin clay and soil profiles overlie the alluvial sequences as sediments are frequently re-worked during high-energy rainfall and flow events. Due to the absence of an overlying confining layer, the alluvial aquifers in the upper catchment are unconfined, thus resulting in high infiltration and recharge rates (Figure 12), and consequently lower groundwater salinity of less than 1000 µS/cm (Raiber et al., 2015; Figure 18). As a result of the limited alluvial development and the predominance of coarse-grained alluvial sediments in these upper reaches, rapid discharge from the basalts to the thin and areally restricted alluvial deposits occurs due to a close hydraulic connection with short flow paths (commonly only hundreds of metres) and short residence times (likely to range from days to months). The response of water levels to rainfall is near-instantaneous, suggesting that most of the alluvial aquifer is saturated during wet conditions particularly following episodic high rainfall events (Figure 17(a)). Conversely, groundwater levels in the upper alluvium rapidly decline during drier climate periods or persistent droughts, when much of the alluvial aquifer becomes unsaturated (Figure 17(b)).

Figure 17

Figure 17 Hydrological conceptual model of alluvia in the upper reaches during (a) wet and (b) dry periods

See Queensland Government (2015b) for more information.

Source: Adapted from DSITI (Dataset 7), © The State of Queensland (Department of Science, Information Technology and Innovation), 2015

2.3.2.2.4.3 Mid-catchment

The mid-catchment of the Richmond river basin and other similar areas within the Clarence-Moreton bioregion are characterised by wider floodplains (approximately 0.5 to 2 km wide) and thicker alluvial sequences (approximately 15 to 30 m thick) compared to the upper alluvium (Figure 19). The relative proportion of finer-grained sediments such as clay and silt increases, and there is greater likelihood of soils forming above the alluvial sequences. The greater abundance of fine-grained sediments and the presence of clays or soils on top of the alluvium potentially result in lower recharge rates when compared to the upper catchment, though recharge rates are still considered to be high.

Figure 18

Figure 18 Groundwater chemistry clusters, median electrical conductivities and hydrochemical water type in the Richmond river basin

Additional information on groundwater chemistry in the Richmond river basin is available in companion products 1.5 (McJannet et al., 2015) and 2.1-2.2 (Raiber et al., 2016) for the Clarence-Moreton bioregion.

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 8)

The change from a sedimentary composition dominated by coarse-grained sediments in the upper alluvium to a dominance of finer sediments in the lower catchment is associated with the change to the lower-energy meandering fluvial system for the latter. Unlike the upper catchment where the alluvium is exclusively underlain by highly fractured and hydraulically transmissive basalts of the Lamington Volcanics, the alluvium in the mid-catchment is more likely to be underlain by sedimentary bedrock units, which are hydrostratigraphic units with significantly lower hydraulic conductivities than the Lamington Volcanics. Due to a larger thickness and a different sediment composition, the alluvial aquifers in the mid-catchment respond differently to climatic extremes. During wet periods (Figure 19(a)), they have shallow water levels and are close to full saturation. However, during drier months or periods of severe drought, the larger aquifer thickness and the higher degree of aquifer confinement means that the alluvial aquifers have a larger ‘buffering’ capacity compared to the alluvial aquifers in the upper catchment (Figure 19(b)). As a result, they have more consistent groundwater levels even during extensive drought periods, and support a wider range of ecosystems and ecological processes. Furthermore, due to the more complex recharge processes, the groundwater chemical composition becomes more variable in comparison to the upper catchment, as represented by a wider range of electrical conductivities and variable groundwater chemistry (Figure 18).

Figure 19

Figure 19 Hydrological conceptual model of alluvia in the mid-catchment reaches during (a) wet and (b) dry periods

See Queensland Government (2015c) for more information.

Data: Adapted from DSITI (Dataset 7), © The State of Queensland (Department of Science, Information Technology and Innovation), 2015

Figure 20

Figure 20 Example of temporal variability of groundwater levels in an alluvial aquifer overlying sedimentary bedrock in a mid-catchment location in the Lockyer Valley in the Clarence-Moreton bioregion in Queensland

For orientation of cross-section line, see Figure 11.

2.3.2.2.4.4 Lower catchment

Alluvial aquifers in the lower catchment of the Richmond river basin (i.e. down-gradient of Casino) and other similar areas within the Clarence-Moreton bioregion tend to be significantly wider (up to approximately 15 km) and deeper (ranging mostly between approximately 25 to 50 m) than those in the upper and mid-catchment (Figure 21). Typically, the alluvial sediments in the lower catchment are fining-upward sequences of unconsolidated and, less commonly, semi-consolidated sediments. Within these alluvial sequences, gravel- and sand-rich layers occur near the base, overlain by thick deposits of finer-grained floodplain silts and clays that have been deposited in a low-energy floodplain environment. Throughout the Quaternary, the Richmond River frequently changed its course across the floodplain of the lower catchment. Abandoned stream channels are now preserved in the landscape as paleochannels. As these paleochannels consist mainly of coarse-grained sediments (sand and gravel) relative to surrounding finer-grained material, they can transmit groundwater faster and can form perched aquifers.

Surface water in the Richmond river basin is generally fresh (Raiber et al., 2016). The observed higher groundwater salinities in the alluvium of the lower Richmond river basin down-gradient of Grafton suggest that river recharge to the alluvium is likely to be less significant (Raiber et al., 2015; Figure 18) than in the upper catchment. Diffuse recharge from precipitation seems to be the more dominant recharge process in the lower catchment. The presence of low permeability silt and clay at the top of the alluvia means that the aquifer is commonly semi-confined, and recharge rates are substantially lower than those in the alluvium of the upper catchment. However, near the coastal eastern part of the Richmond river basin, recharge rates to the alluvium are higher than those in the central part of the catchment due to the predominance of sandy soils that facilitate rapid recharge, as supported by the prevalence of lower groundwater salinities in these coastal areas.

Unlike the upper and mid-catchments, the lower Richmond river alluvia are underlain mostly by sedimentary bedrock units such as the Grafton Formation (Figure 10). In comparison to the Lamington Volcanics, sedimentary bedrock units such as the Grafton Formation are less permeable, and there is likely to be a smaller degree of hydraulic connection across the interface between the sedimentary bedrock and the alluvium. This suggests that the relative contribution of the sedimentary bedrock discharge to the overall alluvial water balance is small during normal climatic conditions. However, examples from other parts of the Clarence-Moreton bioregion show that during prolonged drought periods, the relative water level difference (head gradient) between the alluvial and sedimentary bedrock aquifers can increase (Figure 20) due to a more rapid and pronounced drop in the alluvium water level compared to that of the sedimentary bedrock aquifers (Raiber et al., 2015). As a result of the increasing head gradient, there is a higher potential for discharge from the sedimentary bedrock to the alluvium in some areas during dry periods. Strongly elevated groundwater salinities of more than 20,000 µS/cm observed in the alluvial aquifer in some areas in the Clarence-Moreton bioregion (e.g. Lockyer Valley) suggest that locally, and particularly at the edge of the alluvium and at the down-gradient end or regional flow paths of the sedimentary bedrock, a considerable proportion of the alluvial groundwater in these zones is sourced from the underlying bedrock (which often contains highly saline groundwater; McJannet et al., 2015) during drought conditions.

An example from the Bremer river basin (Figure 22) where no or only small watertable rises were recorded two years after high rainfall and flooding highlights that unlike in the upper or mid-reaches, there is a considerable time lag between rainfall or flooding and the water level response in the lower alluvia.

Figure 21

Figure 21 Hydrological conceptual model of alluvia in the lower catchment reaches

See Queensland Government (2015d) for more information.

Data: Adapted from DSITI (Dataset 7), © The State of Queensland (Department of Science, Information Technology and Innovation), 2015

Figure 22

Figure 22 Temporal variability of groundwater levels in the lower catchment (example from Bremer river basin)

2.3.2.2.5 Unconsolidated sediments – estuarine and coastal

Estuarine zones are the interface between fresh water and marine environments. The Richmond River estuary includes the tidal reaches of the Richmond River from the coast to Casino, Wilsons River to Boatharbour, Bungwalbin Creek and North Creek (Aquatic Biogeochemical & Ecological Research, 2008; Hydrosphere Consulting, 2011).

Estuarine sediments consist of unconsolidated sediments such as clay and sand. In lithological logs from groundwater bores, marine influence is commonly recognised by the presence of ‘shells’. In the three-dimensional geological model, the estuarine and coastal sediments are not differentiated from alluvial sediments. A subdivision of the unconsolidated sediments in the three-dimensional geological model is possible, but it would require a finer-scale three-dimensional model of the coastal area within the Richmond river basin which was beyond the scope of the Assessment. However, the extent of the estuarine and coastal sediments superimposed onto the three-dimensional geological model of the Richmond river basin is shown in Figure 10.

Interaction between seawater and freshwater can occur far inland in the Richmond river basin due to the small topographic gradient. Sampling by Hydrosphere Consulting (2011) suggested that an oligohaline salinity environment (corresponding to a brackish surface water salinity of up to approximately 9 mS/cm) within the Richmond River estuary exists beyond Coraki (Figure 18).

2.3.2.2.6 Fractured igneous rocks

Fractured igneous rocks in the Clarence-Moreton bioregion consist mostly of extrusive volcanic rocks such as the basalts of the Lamington Volcanics, which cover large surface areas in the northern part of the Richmond river basin and the Main Range Volcanics in Queensland (Figure 8). Recharge and groundwater flow processes, as well as hydrological features associated with the basalts such as streams and springs are represented by the ‘permeable rock’ conceptual model (Figure 23). The median thickness of the Lamington Volcanics and Main Range Volcanics determined from the three-dimensional geological model for the Clarence-Moreton bioregion is 128 m, and its maximum thickness is 825 m near the crest of the volcanics in topographically elevated areas (e.g. close to the border between Queensland and NSW). Within the Richmond river basin, the median thickness of the Lamington Volcanics estimated from the three-dimensional geological model is 105 m. However, these volcanic rocks do not consist of a single homogeneous basalt flow or one single aquifer. Rather, the basalt sequence consists of many overlapping basalt flows with a maximum thickness of approximately 10 m each (e.g. Brodie and Green, 2002). These are stacked together and commonly separated by lower-permeability layers including the clay-rich weathering profiles developed during a depositional hiatus between periods of volcanic activity. In addition, groundwater bore logs show that fluvial paleodrainage systems and lacustrine sediments covered by subsequent basalt flows exist in some areas of the Lamington Volcanics, as also previously indicated by Brodie and Green (2002).

These different zones of varying rock permeability affect the capacity of different basalt flows to store and transmit groundwater. At the top and the base of the basalt flows, zones consisting of broken vesicles commonly occur, and these provide considerable primary pore space that can store and transmit groundwater. Vesicular zones of different basalt flows in the Lamington and Main Range volcanics are likely to be connected by well-developed fracture networks (representing secondary porosity) (Figure 23), resulting in high recharge rates and low salinity of groundwater (Brodie and Green, 2002; McJannet et al., 2015; Raiber et al., 2015). Rainwater infiltrates rapidly through the fractures of the highly elevated areas near the NSW–Queensland border where soil profiles are thin and aerially-extensive outcrops of fresh and unweathered basalt occur. These form preferential recharge areas of the Clarence-Moreton bioregion (Figure 12). At the edge of the basalt flows, at the interface of the higher-permeability basalts and the lower-permeability overlying alluvial aquifers, groundwater discharges to the surface as springs, and these may feed streams such as those on the Alstonville Plateau (Figure 12) (Brodie and Green, 2002).

Figure 23

Figure 23 Conceptual model of volcanic rocks and their interaction with underlying sedimentary bedrock in the Clarence-Moreton bioregion; hydrological features associated with the volcanic rocks are also shown

See Queensland Government (2015e) for more information.

Data: Adapted from DSITI (Dataset 7), © The State of Queensland (Department of Science, Information Technology and Innovation), 2015

2.3.2.2.7 Consolidated sedimentary rock

For the purposes of the landscape classification, all sedimentary bedrock units within the PAE of the Clarence-Moreton bioregion were grouped into a single main category ‘consolidated sedimentary rock’ (Figure 10).

However, there are distinctive hydraulic differences between these rocks, as described in more detail in companion product 2.1-2.2 for the Clarence-Moreton bioregion (Raiber et al., 2016). These differences are related to the variable composition of the different rocks, where some units are generally composed of coarser-grained rocks such as sandstone (e.g. the Woogaroo Subgroup, which in some areas is dominated by coarse-grained and well-sorted quartz sand), whereas other units (e.g. Walloon Coal Measures) are dominated by fine-grained material such as mudstone, shale, coal and siltstone, with only minor sandstone. These differences were taken into consideration when developing the numerical groundwater flow model as discussed in other companion products for the Clarence-Moreton bioregion: product 1.1 (Rassam et al., 2014); product 1.2 (Raiber et al., 2014); product 1.5 (McJannet et al., 2015); and product 2.1-2.2 (Raiber et al., 2016).

As a result of their variable composition, and as a broad generalisation, the sedimentary bedrock units in the Clarence-Moreton bioregion can be differentiated into aquifers and aquitards (aquitards are typically described as a ‘seal’ in petroleum systems studies). Given the spatial and vertical heterogeneity of these units, they can sometimes form an aquifer in a particular part of the bioregion (e.g. at the margin of the Clarence-Moreton Basin where the rocks tend to be composed of coarse-grained materials), but can also act as an aquitard elsewhere (e.g. further away from the basin margin and/or at greater depth where the permeability is typically lower). The spatial (horizontal and vertical) relationships and relative thicknesses of inferred aquifers and aquitards in the Richmond river basin are shown in Figure 8 and Figure 24.

Figure 24

Figure 24 Cross-section through unfaulted three-dimensional geological model of Richmond river basin, highlighting aquifer geometry, relative thicknesses of aquifers and aquitards

The orientation of this cross-section corresponds to line A–B in Figure 8.

Data: Bioregional Assessment Programme (Dataset 5)

2.3.2.2.7.1 Aquifers

In the Clarence-Moreton bioregion, the alluvium and the volcanic rocks are the major aquifers used for groundwater extraction. In addition, three sedimentary bedrock units are classed as aquifers, namely the Kangaroo Creek Sandstone (the lower member of the Orara Formation), the Piora Member (lower member of Grafton Formation) and the Woogaroo Subgroup. The Kangaroo Creek Sandstone and the Woogaroo Subgroup are the two that are currently utilised, or have the most future potential for groundwater extraction. Current groundwater usage in the Richmond river basin is estimated at approximately 11,618 ML/year based on entitlements (McJannet et al., 2015). More than 85% of these allocations are from the alluvial and volcanic aquifers (Figure 25).

The Woogaroo Subgroup, which is the deepest unit in the Clarence-Moreton Basin (Figure 26), contains the freshest groundwater of all sedimentary bedrock units (McJannet et al., 2015), with potentially high yields. It is extensively used for groundwater extraction in Queensland near the northern margin of the sedimentary basin in the Lockyer Valley (Figure 7), where the unit occurs at shallow depths (McJannet et al., 2015). In contrast, the Woogaroo Subgroup is not extensively used for groundwater extraction in the NSW part of the Clarence-Moreton bioregion due to its considerable depth (typically at least 1500 m below ground surface) and limited surface outcrop areas. Recharge takes place in the western part of the Woogaroo Subgroup in NSW whereas groundwater discharge mostly occurs towards the eastern side. However, the areal extent of recharge areas of the Woogaroo Subgroup in NSW is limited to narrow bands or small isolated areas compared to that of the Lockyer Valley. Due to the limited data availability for the Woogaroo Subgroup in the Clarence-Moreton bioregion in NSW in terms of both groundwater levels and water quality, it is not currently possible to assess the potential implications of the limited recharge area on groundwater flow and quality within this unit.

Figure 25

Figure 25 Current groundwater allocations in the Richmond river basin (based on McJannet et al., 2015)

Data: Bioregional Assessment Programme (Dataset 3, Dataset 4, Dataset 5, Dataset 10); Bureau of Meteorology (Dataset 9)

The Woogaroo Subgroup is vertically separated from the major coal-bearing unit (the Walloon Coal Measures) through the overlying Koukandowie Formation and Gatton Sandstone, which in most areas have a combined thickness of approximately 1000 m or more. The potential for groundwater extraction from the Woogaroo Subgroup in the Richmond river basin is limited by the very considerable depth and lack of understanding of bore yields and water quality.

The Kangaroo Creek Sandstone Member as defined by Doig and Stanmore (2012) is composed of quartzose sandstone and conglomerate. The median thickness of the Kangaroo Creek Sandstone as derived from the three-dimensional geological model is 175 m and its maximum thickness is 370 m. The groundwater salinity of the Kangaroo Creek Sandstone is typically low (Parsons Brinckerhoff, 2011; McJannet et al., 2015), but this assessment is based on limited data from the eastern part of the basin. Yields are mostly low (less than 1 L/second), except where bores intercept zones of enhanced permeability (McKibbin and New South Wales Department of Land and Water Conservation, 1995; Parsons Brinckerhoff, 2011). Doig and Stanmore (2012) indicated that the Kangaroo Creek Sandstone has poor aquifer properties (i.e. low yields) below a depth of 150 m. Recharge to the Kangaroo Creek Sandstone, which has in the past been explored as a conventional gas reservoir (Raiber et al., 2014), is likely to occur at its outcrop areas in the western part of the Clarence-Moreton bioregion in NSW. Although poorly constrained by current monitoring data, the aquifer architecture and the three-dimensional geological model suggest that groundwater flows from the highly-elevated western areas towards the east. In the stratigraphic sequence of the Clarence-Moreton Basin, it is located above the Walloon Coal Measures, and is vertically separated from the Richmond Seam (the shallowest coal seam of the Walloon Coal Measures) by the Maclean Sandstone (Figure 24), which is considered to have a low permeability.

Figure 26

Figure 26 Simplified stratigraphy and generalised hydraulic characteristics of the major stratigraphic units in the Clarence-Moreton bioregion

The lower-most units (Koukandowie Formation and below) are not included in the Richmond river basin groundwater model as they are not currently utilised for groundwater extraction due to their considerable depth, their inferred low yields and the high salinity of groundwater contained within Koukandowie Formation and the Gatton Sandstone (McJannet et al., 2015). Colours correspond to the colours of the three-dimensional geological models.

According to the three-dimensional geological model of the Clarence-Moreton Basin, the Grafton Formation (as defined by Doig and Stanmore (2012)) has a median thickness of approximately 150 m and a maximum thickness of approximately 500 m. It consists of an upper and a lower member (Rapville and Piora members, respectively) with very different hydraulic characteristics. The Piora Member, the lower member of the Grafton Formation, is composed of medium- to coarse-grained quartzose sandstone with an extensive clay matrix (Doig and Stanmore, 2012). While sometimes described as an aquifer, its reported low bore yields of about 0.3 L/second suggest that it is more likely an aquitard. The Rapville Member, the upper member of the Grafton Formation, is composed of interbedded sandstone, siltstone and mudstone, and is considered to be an aquitard or aquiclude (Doig and Stanmore, 2012). Overall, despite its considerable thickness and extensive surface outcrop area, few groundwater bores are screened within, and source water from, the Grafton Formation in the Richmond river basin (Figure 25). The lack of groundwater extraction from this unit may be due to the easier access from alluvial or volcanic groundwater resources. However, it probably also indicates that the Grafton Formation is likely to have low yields and/or poor groundwater quality, and represents a low permeability aquifer (Piora Member) or aquitard (Rapville Member) with limited potential for groundwater extraction.

2.3.2.2.7.2 Aquitards

Several units considered to be aquitards exist within the sedimentary bedrock sequence of the Clarence-Moreton bioregion (Figure 26).

The two stratigraphic units underlying the Walloon Coal Measures (the stratigraphic unit that contains the major coal resources) are the Koukandowie Formation and the Gatton Sandstone (both part of the Bundamba Group in Figure 26). It is poorly documented whether those two units act as aquifers or aquitards throughout the Clarence-Moreton bioregion. However, the assessment of water quality in companion product 1.5 for the Clarence-Moreton bioregion (McJannet et al., 2015) has shown that the Koukandowie Formation and the Gatton Sandstone have the highest groundwater salinity in the bioregion, with a median electrical conductivity of 4750 and 5000 µS/cm, respectively. In the linked Surat Basin (part of the Northern Inland Catchments Bioregional Assessment), the Evergreen Formation (the equivalent to the Gatton Sandstone) is commonly described as an aquitard. Recent sedimentological analyses and a geochemical baseline assessment of the Hutton Sandstone (the equivalent of the Koukandowie Formation) in the north-eastern Surat Basin have suggested that groundwater flow is restricted to a relatively small fraction of the unit’s total thickness (Guiton et al., 2015; Suckow et al., 2015). These analogues from the Surat Basin together with the mostly poor-quality groundwater observed in the Koukandowie Formation and Gatton Sandstone in the Clarence-Moreton bioregion (McJannet et al., 2015) suggest that those two units probably have low yields and represent low-permeability partial aquifers or aquitards. However, additional hydraulic and physical and chemical property data for these formations are required for a more reliable assessment of their hydraulic characteristics. These types of data are not currently available to inform the BAs.

At the regional scale, the Walloon Coal Measures are typically considered as an aquitard in the Clarence-Moreton and the linked Surat basins. However, due to their spatially variable composition, their hydraulic character is also likely to vary considerably. Towards the basin margin, the Walloon Coal Measures consist of coarser-grained sedimentary material due to the proximity to the original sediment source area. For example, near the margins of the Warrill Creek Syncline in the Bremer river basin (Figure 11) artesian flow occurs from groundwater bores screened in the Walloon Coal Measures. Towards the centre of the geological basin, fine-grained rocks such as mudstone, siltstone, fine-grained sandstone and shale inter-bedded with coal predominate. Groundwater recharge to the Walloon Coal Measures occurs in the western part of the Richmond river basin in NSW, where the Walloon Coal Measures outcrop over large areas. There are few groundwater bores screened in the Walloon Coal Measures in this outcrop area, and consequently, few groundwater chloride measurements were available to estimate recharge rates using chloride mass balance, hence, the recharge assessment was based mostly on data from Queensland (Raiber et al., 2016). The limited number of measurements in NSW combined with the large number of chloride measurements in the Clarence-Moreton bioregion in Queensland suggests that recharge rates to the Walloon Coal Measures are small (less than 10 mm/year corresponding to less than 1% of annual average rainfall; Figure 12). The lack of groundwater bores screened in this unit throughout the Clarence-Moreton bioregion and particularly in the Richmond river basin is an additional indication that the Walloon Coal Measures likely have low yields and/or poor groundwater quality.

The Maclean Sandstone forms the upper part of the Walloon Coal Measures in the Richmond river basin (Figure 10). It is composed of silty and lithic, coarse- to fine-grained feldspathic sandstone (Doig and Stanmore, 2012). It has a median thickness of 87 m based on estimates from the three-dimensional geological model, and is described by Doig and Stanmore (2012) as an effective low-permeability top seal (aquitard) that extends over much of the basin and limits the vertical leakage of gas from the coal seams to the surface. Although somewhat counter-intuitive that a stratigraphic unit described as sandstone is considered an aquitard, it is important to note that the type-section where the Maclean Sandstone was first described is located at the basin margin near the sediment source area, where units generally consist of coarser-grained material than in the centre of a sedimentary basin. Furthermore, when the Maclean Sandstone was first described by Flint et al. (1976), only limited down-hole petrophysical data existed in the Clarence-Moreton Basin. As more petrophysical data became available from exploration drilling, the understanding of the sedimentological composition and hydraulic character of different units has evolved, as also observed in the neighbouring Surat Basin. These newly acquired data show that the contact between the Maclean Sandstone and the overlying Kangaroo Creek Sandstone is marked by a distinct decrease in the rate of penetration (the drilling rate describing how many metres are drilled per minute) and a higher density displayed by the Maclean Sandstone in down-hole geophysical profiles.

According to the three-dimensional geological model, the Bungawalbin Member has a median thickness of about 94 m. It is composed of mudstone and carbonaceous mudstone interbedded with fine-grained sandstone (Doig and Stanmore, 2012). Doig and Stanmore (2012) have suggested that it is an aquitard that prevents vertical leakage of groundwater.

The Rapville Member (the upper member of the Grafton Formation) is discussed as part of the Grafton Formation.

Last updated:
11 July 2017
Thumbnail images of the Clarence-Moreton bioregion

Product Finalisation date

19 January 2017