4.0 Overview of the Regional Surface and Subsurface Geology of the Duaringa Basin
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Duaringa Basin Report on Hydrological Investigations 4.0 Overview of the Regional Surface and Subsurface Geology of the Duaringa Basin 4.1 Introduction The surface geology of the Duaringa Basin project site and surrounding environment is characterised by the surface exposures of the Tertiary age Duaringa Formation sediments and surrounding exposures of the Permian age sediments (Figure 4.1). The Duaringa Formation is composed of interbedded mudstones, shale, oil shale siltstone and lignite beds and rare sandstone, conglomerate and basalt beds (Day et al., 1983). The Permian age sediments are part of the Bowen Basin stratigraphic sequence. The overlying Tertiary age Duaringa Formation does not form part of the formal Bowen Basin stratigraphic sequence. The surface exposures of the Duaringa Formation can be differentiated into recent exposures and older lateritic tablelands. There are also some small outcrops of Tertiary age volcanics exposed through the Tertiary age and Permian age sediments present in and near the Duaringa Basin. The significant rivers, such as the Mackenzie River and Dawson River, which traverse the Duaringa Basin, have deposited large volumes of alluvial sediment in broad braided plains on both the Duaringa Formation and Permian age sediments. The Duaringa Basin along with the Biloela Basin and Hillsborough Basin formed during the opening of the Coral Sea during the Eocene between 52 and 34 million years ago (SRK, 2008). These basins are bounded by NE to SW trending bounding faults along their western margins. Movement along these faults is largely sinistal (left lateral). However, there has been significant downward movement on the eastern side of the fault blocks, which has opened deep asymmetrical grabens (Veevers and Powell, 1994). These Tertiary age basins are filled with up to 1,200 m of fluvial and lacustrine sediments. The Basin filling appears to have been syndepositional with the downward movement of the western fault blocks. Therefore, it is believed that these basins were shallow lacustrine environments throughout their depositional history. The dominantly fine-grained sediments found within the rocks of the Duaringa Basin is evidence the shallow lacustrine environment existed throughout the Basin history. The sandier beds and the sandstones at or near the surface suggest that some subaerial fluvial deposition has occurred more recently. Shell (Smith, 2010) has suggested that the low energy lacustrine deposition in the Duaringa Basin was punctuated with periods of higher energy fluvial or subaerial deposition. The available evidence suggests that higher energy fluvial or subaerial deposition was very rare at depth within the Duaringa Formation stratigraphic sequence and may be confined to the upper Duaringa Formation. PR106887-REP-001; Rev 0 / 14 March 2011 Page 34 Duaringa Basin Report on Hydrological Investigations 4.2 Surficial Geology The surficial geology of the Duaringa Basins was originally mapped by Malone et al. (1970a) who completed the Duaringa 1:250,000 scale the Saint Lawrence 1:250,000 scale sheets in 1970 (Malone et al. 1970a and 1970b) (Figure 4.1). The Duaringa Basin is one of several Tertiary age basins indentified in the in north central Queensland (Figure 4.2). More recent mapping at 1:100,000 scale was been completed by DERM for the area surrounding the Duaringa Basin. Unfortunately, the 1:100,000 scale mapping focused on the Permian age rock exposures east and west of the Duaringa Basin proper (Figure 4.3). The field observation insets included on the Duaringa, Mount Bluffkin and Rookwood 1:100,000 scale geological sheets suggest that very few new observations were made in the Duaringa Basin (Figure 4.3). The Duaringa Basin surficial geology assessment, and to a large extent the subsurface geology, is based on work of Malone et al. (1970a and 1970b). The surficial geology of the Duaringa Basin consists of Quaternary age alluvium that has been deposited along the rivers and streams that cross and drain the Basin. A significant percentage of the Duaringa Basin is covered by a wide braided river floodplain associated with the Mackenzie and Dawson Rivers. The braided alluvium extends nearly the full length of the Duaringa Basin and is over five kilometres wide north of the Mackenzie / Dawson River confluence. The remaining surface of the Duaringa Basin is dominated by outcrop of the Duaringa Formation. Much of the exposed Duaringa Formation is laterised. This lateritised material forms the Duaringa Basin tablelands (Figure 4.1). The available published geological mapping does not indicate any surface faulting within these surface exposures. There are minor occurrences of Tertiary age and Tertiary-Cretaceous age volcanics exposed through the alluvial, Tertiary age and Permian age sediments (Figure 4.1). There are Tertiary age basalts exposed near May Downs, at the northern basin limit, and southwest of the town of Duaringa, at Bone Creek. There are some additional small exposures Tertiary-Cretaceous age volcanics to the east of the Isaac River, just east of the Duaringa Basin boundary. These volcanic rock occur in isolated outcrops between Lotus Creek in the north to Clarke Creek in the south. The volcanic rock outcrops are discussed further in Section 5.2.1. 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