CRC LEME
Open File Report 130
ABSTRACT
Alluvial landscapes of the Northern Kennedy Gap Area, Mt Isa
District, Queensland
M. R. Jones
Quaternary landscape evolution has been investigated in an area
located mid-way between Mt Isa and Camooweal, on the Kennedy Gap
1:100 000 sheet. Here, Proterozoic bedrock forms prominent ridges
having a general north-south orientation and rising up to 180 m
above the surrounding plains. Drainage is provided by streams, whose
upper catchments are confined by bedrock ridges. The area extends
only marginally onto the plains that continue from the base of the
Mt Isa Inlier to the south and west. Despite the arid environment,
the principal agents of landscape evolution are the rivers and creeks
which carry away the products of weathering in the high country
and distribute these sediments across the lowlands. The study area
lies across the divide separating northward drainage to the sea
and southward drainage to central Australia.
There are six major streams present, each with distinctive drainage
patterns associated with different levels of erosional activity.
The most active of the streams is the northward-flowing Judenan
Creek, which has a highly channelled catchment. The stream is continuing
a long-term trend for expansion to the west and south along erosion
scarps in bedrock. Weathered Eastern Creek Volcanics in the Judenan
Creek catchment are susceptible to erosion, and the numerous channels
ensure that the weathered mantle is readily removed. The sediments
produced are derived from a wide area of bedrock, and are predominantly
"new" sediments. In contrast, the adjoining Cattle Creek
catchment to the west contains a mixture of older alluvial deposits
now being reworked, and only minor "new" sediments derived
from low lying bedrock outcrops.
Other catchments in the area include the bedrock-confined and aligned
Gidya Creek, which drains to the north, and Buckley River, which
flows westwards across the strike of the bedrock. In the south,
Johnson and Wilfred Creeks are conduits for westward moving sediments
produced in their upper catchments. Johnson and Cattle Creeks join
Buckley River, and contribute to the drainage towards central Australia.
The sediments are derived from the area within 2-5 km of the drainage
divides between the upper catchments. For the most part, the surficial
sediments are thin and young. There is no evidence of widespread
thick sequences of transported deposits overlying an incised and
back filled bedrock surface. Rather, the alluvial deposits are mainly
confined to narrow corridors along the valley floors. The alluvial
deposits form a blanket which protects the shallow underlying bedrock
from significant erosion.
The alluvial deposits are only a proportion of the total sediment
throughput during the evolution of the landscape to its present
form. The blanketing unconsolidated sediments are in transit to
distant depositional areas. It is unlikely that these deposits would
remain in place long enough for geochemical haloes to develop from
underlying mineral deposits. Overall, the processes of erosion,
transport and deposition are diffusive for indicator minerals. However,
there is the potential for stream bed concentrations of heavy minerals
which could be related to up-catchment sources.
The differences between the catchments are related to the confining
geology that determines the lateral limits of catchment development,
and the underlying geology that determines the erodability of the
catchment substrate. Judenan Creek is the main catchment where erosional
processes are most active. In part, this can be related to Judenan
Creek having a steeper slope to its base level (Gulf of Carpentaria)
in comparison with the other streams draining to the west and south
towards Lake Eyre. The Judenan Creek catchment may also contain
more erodable bedrock than in neighbouring streams. The erodability
could be related to deep weathering and or fracturing of the bedrock
allowing surface or near-surface chemical concentrations indicating
underlying mineral deposits. The Judenan Creek catchment contains
known occurrences of copper and uranium mineralisation. This catchment
may need further evaluation of its potential to contain economic
mineral deposits.
The Eastern Creek Volcanics also underlie parts of the Gidya Creek
catchment, which may warrant investigation for anomalies in the
regolith. Stream sediment geochemistry could assist exploration
in the Judenan and Gidya Creek catchments.
Iron-rich pisoliths and nodules have proved to be useful sampling
media for geochemical exploration in Western Australia. In the Kennedy
Gap area, iron pisoliths and nodules are abundant. Haematitic and
goethitic pisoliths and nodules occur in-situ in weathering profiles,
and also in transported deposits where secondary goethitic cementation
has occurred. Geochemical analyses of goethitic cutans and haematitic
centres of iron pisoliths are expected to give different results.
Careful interpretation of the environment of deposition is required
before selecting samples for analysis. This is essential if geochemical
anomalies are to be successfully related to source areas.
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