2.1.2 Geology

Summary

This section describes the datasets that have been obtained and compiled for the development of the three-dimensional geological model of the Clarence-Moreton bioregion. This section also describes the workflow used to build the three-dimensional geological models. The geological data described in this product inform the development of multiple three-dimensional geological models of surface water basins within the Clarence-Moreton bioregion.

Prior to this bioregional assessment, no three-dimensional geological models existed for most areas within the Clarence-Moreton bioregion (for example, no models existed for the Clarence-Moreton bioregion in Queensland). Furthermore, although pre-existing grids were available for selected sedimentary bedrock hydrostratigraphic units such as the Walloon Coal Measures (the primary target of coal seam gas (CSG) exploration) in NSW, no three-dimensional representations of alluvial or volcanic aquifers and shallower bedrock units existed anywhere in the Clarence-Moreton bioregion. The three-dimensional geological models developed during this bioregional assessment therefore substantially contribute to the understanding of the geometry and architecture of sedimentary bedrock units, volcanic aquifers and alluvial aquifers in the Clarence-Moreton bioregion.

Section 2.1.2.1 introduces the data types that underpinned the three-dimensional geological model. They include lithological and stratigraphic logs from groundwater bores and exploration and deep stratigraphic wells, airborne geophysical data, remotely sensed data, seismic data and geological structure information. ArcGIS and GoCAD were used to determine the extent of major stratigraphic units at the surface and in the subsurface by integrating and visualising all the associated data.

The major geological data sources in the Clarence-Moreton bioregion are geological maps, lithological and stratigraphic data from the Queensland and NSW groundwater databases, stratigraphic records from exploration wells and geophysical data. In addition, interpolated surfaces of selected geological contacts (i.e. interfaces between different sedimentary bedrock stratigraphic units) provided by Metgasco Limited and the NSW Department of Trade and Investment helped to inform the development of the three-dimensional geological models.

The Clarence-Moreton bioregion includes six major alluvial aquifer systems, which are likely to directly support many of the groundwater-dependent assets. These alluvial systems are the Lockyer Valley, the Bremer river basin and Warrill creek basin, the Logan-Albert river basin, the Richmond river basin, and the Clarence river basin. These alluvial aquifers overlie the bedrock stratigraphic units of the geological Clarence-Moreton Basin and the basalts of the Main Range Volcanics and Lamington Volcanics.

Data from groundwater bores underpin the characterisation of the interface between the alluvial aquifer systems and the underlying sedimentary and volcanic bedrock, which is crucial in determining how shallow and deep aquifer systems interact hydraulically. There are more than 12,000 registered groundwater bores with lithological information throughout the different river basins of the bioregion. However, only approximately 40% of these bores have stratigraphic information, and the stratigraphic information is often incomplete (i.e. not continuous for the entire bore length) or incorrect. When possible, the lithological logs were converted into stratigraphic logs, and these were subsequently used to define the contacts between the alluvium and the underlying sedimentary or volcanic bedrock. The spatial interpolation of the groundwater stratigraphic data shows that these different alluvial aquifer systems within the bioregion are very different with regards to their sediment thickness, the width of the alluvial plains and the shape of the valleys. However, there are also some common characteristics. For example, most alluvial systems in the bioregion are relatively narrow and deeply incised into the bedrock in their headwaters, with a typical thickness of approximately 10 to 15 m, and although the thickness distribution within each alluvial system is highly variable, all systems have a maximum thickness of approximately 30 to 35 m.

The subsurface geometry of the bedrock stratigraphic units of the Clarence-Moreton Basin within the Clarence-Moreton bioregion is generally very complex. The Clarence-Moreton Basin consists of several sub-basins, and within each sub-basin, multiple depositional centres exist where sediment thicknesses of more than 2500 m have been intersected or are inferred. In addition, as demonstrated by seismic data, tectonic activity has resulted in significant vertical displacements of bedrock units along faults.

The definition of the boundaries between the deeper volcanic and sedimentary bedrock units in the bioregion is based on the following principal sources: groundwater bore data; deep stratigraphic wells; CSG, petroleum and coal exploration wells; and seismic interpretation provided by Metgasco Limited and the NSW Department of Trade and Investment.

Overall, the spatial coverage with deep stratigraphic and exploration wells in the Clarence-Moreton bioregion is comparatively poor in relation to the structural complexity and compared to other sedimentary basins such as the neighbouring Surat Basin. However, the spatial coverage has considerably improved during the last 10 to 15 years due to the drilling of new exploration wells. Nevertheless, in some parts of the bioregion, such as where the Clarence-Moreton Basin underlies the Bremer river basin in south-east Queensland, there is still a substantial lack of knowledge on the depth of the sedimentary basin as most wells do not intersect the deeper stratigraphic units below the Walloon Coal Measures (the primary target for CSG) here.

In addition, the spatial resolution of reliable well log and seismic data is not sufficient everywhere to model how faults vertically displace bedrock units.

The three-dimensional geological models developed using the data sources and workflows described in this product form the basis for the development of a conceptual hydrogeological model that describes how geology, hydrogeology and hydrology are linked (described in companion product 2.3 for the Clarence-Moreton bioregion (Raiber et al., 2016)). It also forms the basis for the development of a groundwater model (described in companion product 2.6.2 for the Clarence-Moreton bioregion (Cui et al., 2016)).

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

Product Finalisation date

6 October 2016
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