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Open File Report 141

Landscape evolution and regolith development over the Mt Coolon Area, Central East Queensland

Li Shu

The Mount Coolon area has a complicated history of regolith development and landscape evolution. The dominance of a southerly flowing river system in the early Tertiary, the formation of a large lake system in the middle Tertiary, and the reversal of the river system in the late Tertiary make up the main episodes.

The Eastern Highlands of Australia in central Queensland divide the coastal plains to the east and the inland lowlands and hills to the west. To the west of the Highlands, a southerly drainage once existed as shown by the Burdekin, Cape, and Campaspe rivers as well as the upper reaches of the Suttor River and its tributaries such as Police Creek and Rosetta Creek. Tertiary sediments are found on the ancestral southerly flowing rivers.

Eruption of a large volume of basalt around Clermont in the south disrupted the southerly drainage. Tertiary basalts, extruded from fissures and circular vents over a large area during the Tertiary, filled in valleys and blanketed sub-basaltic topography. A palaeovalley under the basalt is evident from bore hole data from various companies. As a consequence of basalt eruption in the south, the southerly flowing Burdekin River system was choked, and a large lake was subsequently formed. With ongoing deposition in the Tertiary lake, flood waters in the Burdekin River rose to a level high enough to find a new course along a gap through the Eastern Highlands and divert the river to the east. With a large catchment and high erosional energy, the Burdekin River incised the Highlands and formed a conspicuous gorge and, as a result, a northerly flowing drainage was developed.

Deposition and erosion in the north Drummond Basin has been dictated by drainage changes. During the period when the southerly drainage was blocked, sedimentation took place at a rapid rate, giving rise to the Suttor Formation in the south and the Southern Cross Formation in the north. With the formation of the Burdekin Gorge, the base level of erosion for the Burdekin River was lowered rapidly, renewing erosion energy within the river system. As a result of rapid erosion, the Southern Cross and Suttor Formations were dissected and largely removed.

Once the easterly drainage system was established, sediments were deposited on the Suttor Formation and its equivalent where less affected by the major change from the southerly to the easterly drainage. New floodplains were built up again where intensive erosion of the Suttor Formations occurred. These late sediments are termed the Campaspe Formation in the Charters Towers region.

Along with landscape evolution, both Palaeozoic rocks of the Anakie Inlier and Tertiary sediments of the Suttor Formation have been deeply weathered in the area. Saprolites are common in the Anakie Inlier and the Drummond Basin sequence, and are in most cases are mottled, bleached or silicified. Late Carboniferous volcanics, mainly rhyolites and ignimbrites, are less weathered and crop out as saprock. Sediments of the Suttor Formation are ferruginised and silicified, giving rise to the formation of ferruginous duricrusts and silcretes. Lateritic duricrust forms mesa-cappings predominantly on Tertiary sediments, but it is also found on Palaeozoic rocks. Based on field observations, a regolith-landform map of the area (25x49 km) is prepared to a scale of 1:50,000.

This study suggests that ferruginous duricrust, nodules and pisoliths on saprolite of basement, are likely to be indicators of geochemistry at depth. Stream sediment sampling may be an alternative approach to explore erosional terrains. Alluviual material, including the Suttor Formation and soils on alluvial plains, is not suitable for geochemical exploration. However, where the cover is less than five metre thick, soil sampling, including specific sampling of mottles, would be effective (Scott, 1995; Scott, 1997; Anand et al., 1997). The probability of hydromorphic dispersion is better in sediments that have been weathered since deposition.


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