Open File Report 141
Landscape evolution and regolith development over the Mt Coolon
Area, Central East Queensland
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
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.