CRC LEME
Open File Report 178
ABSTRACT
3D regolith architecture of the Jamestown area - implications
for salinity
Wilford J
The combined analysis of a digital elevation model, gamma-ray spectrometry,
magnetics, airborne electromagnetics (AEM) and drill holes has provided
new insights into the regolith-landform evolution of the Jamestown
region, located in the Mid North of South Australia. In particular,
the 3D architecture (geometry and composition) of the valley fill
deposits is better understood. Three dimensional models generated
from the integration of these datasets show the distribution of
regolith materials, salt stores and possible preferential flow paths
for groundwater in the region.
The Jamestown region is characterised by a range of landforms from
low relief colluvial and alluvial fans, floodplains and pediments
through to rises, low hills and hills. Depositional materials occur
in three main valleys: the Belalie, Bundaleer and Caltowie. Coalescing
colluvial and alluvial fans of Quaternary age have filled these
valleys to depths of up to 40 metres. The thickest sediments consisting
of silt, clay, fine sand and minor gravels occur in the Belalie
and Bundaleer valleys. The Caltowie valley has a thinner sediment
cover which appears to have a lower electrical conductivity when
compared with the other two catchments. Low angle pediments characterise
the upper parts of the Caltowie catchment.
Airborne gamma-ray spectrometry was effective in separating a variety
of bedrock types and regolith materials based on their radioelement
characteristics. A relationship between the surface texture of soils
on depositional plains and airborne K concentrations was used to
predict the surface silt content in each of the main valleys. The
Caltowie valley has less silt and a higher percentage of medium
to fine sand than the other two catchments. Whether these sandier
soils are representative of materials at depth is not known. Most
of the sediments in these valley systems are dominated by poorly
sorted debris or mudflow deposits that consist mostly of silt and
clay with minor gravels. Available drilling suggests that scattered
throughout the valley fill, but more common in the basal part of
the alluvial sequence, are traction or bed load deposits associated
with river channels. Many of these buried channel sediments contain
maghemite that are clearly delineated in high frequency component
of the airborne magnetic imagery. Magnetic channels show rill, dendritic
and braided patterns. In areas of thicker sediment cover, inter-woven
and possibly stacked, buried palaeo-channel networks are observed
in the magnetic imagery. These buried palaeo-channels define a palaeo-darainge
system that is not reflected in the contemporary drainage, which
is widely spaced and discontinuous. As a result of the Belalie valley
being partly filled with sediment there is evidence of superimposed
drainage diverting palaeo-drainage lines east of Jamestown. With
the exception of some local discrete conductive basement rocks,
the valley fill sediments and some areas of highly weathered saprolite
appear, in the airborne EM, as the most conductive materials in
the landscape. An isosurface (3D surface of equal value) of the
high, near-surface conductivity in most cases delineates the geometry
of the valley prior to sedimentation and infilling. Both the fine
grained sediments of the valley fill and highly weathered bedrock
are likely to contain the highest salt stores as implied by an interpretation
of the AEM data. Overlaying the salt stores (defined using the AEM)
on the magnetic channels highlights possible groundwater flow directions.
The surface expression of dryland salinity within the valleys occurs
where salt stores and associated regolith materials thin and where
basement barriers or valley constrictions impede groundwater flow.
Valley constrictions and associated dry land salinity occur in the
Bundaleer and Caltowie valleys but not the Belalie valley, despite
the fact that the Belalie valley has the highest salt store. This
might be explained by the fact that the Belalie system has a relatively
thick transported cover with numerous buried, and more transmissive
and inter-connected channel networks. It is proposed that this more
effective sub-surface drainage may prevent the build up of shallow
water tables and salts in the upper soil layer.
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