CRC LEME
Open File Report 177
ABSTRACT
Regolith-landforms and salt stores in the Angas-Bremer Hills
Wilford J
The combined interpretation of airborne gamma-ray spectrometry
and terrain indices derived from a digital elevation model along
with ground data has provided new insights into the distribution
of regolith materials and related salinity in the Angas Bremer Hills
of South Australia.
Modeling of the gamma-ray data has been able to separate highly
weathered landforms from areas characterised by thin soil and slightly
weathered bedrock.Highly weathered materials include leached ferruginous
soils,mottled colluvial and alluvial sediments and highly weathered
kaolinised bedrock. Weathered bedrock or saprolite is typically
mottled with iron fragments and nodules common in the upper part
of the profile.Iron induration is mostly associated with segregations
of iron in the mottled zone of the weathering profile.In places
these weathering profiles are covered by alluvial and colluvial
sediments that in places are also highly weathered (mottled and
ferruginised).
The gamma-ray data has the potential to improve existing soil-landscape
and geological mapping. Variations in the concentration of the radioelements
can be used to modify soil-landscape and geological unit boundaries
and assist in describing variations of specific attributes within
individual units.As similar gamma-ray responses can relate to different
materials at the surface it is recommended that the imagery be interpreted
within different geological formations and landform units (e.g.erosional
vs depositional landscapes).
In the Angas Bremer Hills,a long weathering history,dating back
to the Middle Mesozoic,combined with more recent tectonic activity
resulting in faulting,uplift and associated erosion has led to complex
landscapes where highly weathered landforms are juxtaposed with
youthful landforms with little regolith development.Analysis of
the gamma-ray imagery and digital elevation model has provided improved
clarity on the weathering and geomorphological evolution of the
region.
Regolith thickness and composition governs the capacity of the
landscape to store cyclic salts (e.g. those salts derived from rainfall).Weathering
is important when considering salts derived from bedrock minerals.In
both cases (e.g.salt derived from rainfall and bedrock)delineating
areas of deep weathering is important in predicting likely salt
stores.For a local region the highest salt stores are associated
with catchments that have thick soils and deeply weathered bedrock.The
stores are associated with valley alluvium,colluvial fans and highly
weathered bedrock.Stream ECs tend to be higher from these catchments
compared to those catchments with relatively shallow regolith.
Across the whole region the main driver for controlling the amount
of salt in the landscape is rainfall. The thickness and composition
of the regolith might determine the capacity of the landscape to
store salts,but rainfall largely determines the relative abundance
of salts in the profile.When comparing similar regolith profiles
across the study area,those profiles in the high rainfall zone (western
side of the Hills)store considerably less salt than those profiles
on the drier eastern side.
The highest salt fluxes are associated with Western Flat Creek
in the Mt Barker catchment.The average annual rainfall is the highest
in the study area.Modelling of the gamma-ray imagery indicates that
a high proportion of this catchment is deeply weathered,suggesting
that the highest salt exports are associated with deeply weathered
landscapes in high rainfall areas.Conversely,fresh water runoff
is commonly associated with less weathered catchments in areas of
high rainfall.
These relationships are summarised in a series of 3Dconceptual
models which detail relationships between the distribution and thickness
of regolith materials,salinity and hydrogeomorphic processes. These
models provide a basis for understanding processes controlling the
distribution of soil and river salinity across the Hills region.
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