Construction of a 1:250,000 scale regolith-landform map of the Cobar area of central New
South Wales (NSW) Australia, demonstrates the presence of a wide range of previously undescribed
regolith materials, landforms and landscape features in the region. The map covers the east-west extent
of the Cobar Basin, extends to the west onto the Darling River Floodplain, and east onto rocks of the
Girilambone Group. The mapping area is centred on the Cobar township and covers -14,730 krn2
between 303113 and 446113 E and 6483184 and 6586183 N (AGO 66, MGA Zone 55). 48 regolithlandform
units have been identified, including both transported (alluvial, colluvial, aeolian, lacustrine)
and in situ materials. A range of siliceous, ferruginous and calcareous indurated materials are also
present.
Four major drainage types have been identified based on lithological, sedimentological and
topographic differences in alluvial materials. The 4 drainage types include: 1) modern drainage; 2)
maghemite and quartzose gravels elevated 1-2 m relative to the modern drainage; 3) higher
topographically inverted, and at least partly silicified, gravels; and, 4) sediments of Cretaceous origin.
Multiple phases of drainage stability and instability from the Cretaceous to the present are indicated
within the sediments. Breaching of drainage divides and increased dissection of the modern drainage,
especially to the south of Cobar, indicate possible tectonic movement across a major regolith-landform
boundary in the southern map area. Colluvial materials are more widespread to the north of Cobar
reflecting the increased landscape dissection to the south. Colluvial fans are preserved adjacent to
major rangefronts. Aeolian and lacustrine materials include longitudinal dunefields of the Darling
River floodplain, source bordering dunes, and small lunettes associated with the Barnato Lakes system.
Regolith-landform mapping at Cobar has been used to assess the applicability of previously
developed landscape evolution models of the Cobar Block and surrounding region, and to develop a
new landscape evolution model for the region. The new landscape evolution model of Cobar indicates
minimal deposition of Cretaceous sediments, succeeded by high-energy early Tertiary fluvial regimes
across the Cobar landscape. Weathering and sediment deposition continued into the Miocene, coupled
with deep valley incision on the Cobar Block associated with early Oligocene regression. By the close
of the Miocene, the Cobar Block had eroded to predominantly bedrock terrain and widespread filling of
previously incised valleys occurred. A decrease in erosion and fluvial activity led to the formation of
the modern drainage during the Pliocene-early Quaternary, followed by the formation of alluvial,
aeolian and lacustrine deposits in the later Quaternary. Regionally, Eromanga Basin sediments were not
extensive over the Cobar Block, and low rates of erosion are recorded at Cobar from the Cretaceous to
the present. Former northerly drainage did exist in this area in the Cretaceous, but was limited in
distribution. By at least the Early Tertiary the Cobar area was a structural high and drainage systems of
the region had assumed their current configuration. These findings do not support interpretations of
AFTT data of significant cover and subsequent stripping over the Cobar Block in the Early Tertiary.
Evidence of landscape evolution from the Cretaceous to the present suggests that the Cobar
landscape has been responding to changes in the primary landscape forming factors of lithology,
climate and to a lesser degree, tectonics. Variations in the these three primary landscape forming
factors have contributed to ongoing weathering, relatively continuous deposition, and periods of
relative stability and instability, particularly in response to climatic and baselevel fluctuations, within a
dynamically evolving landscape throughout the entire Tertiary. Former landscape evolution models of
peneplanation and pediplanation, based on correlation of palaeosurfaces including duricrusts, a deep
weathering profile developed during extended planation in the Early Tertiary, and tectonism during the
late Tertiary in the Cobar area, are not supported by evidence preserved in regolith-landform features at
Cobar.
| Identifer | oai:union.ndltd.org:ADTP/218639 |
| Date | January 2003 |
| Creators | Spry, Melissa J., n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Management |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Melissa J. Spry |
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