Regolith–landform mapping
in the Gawler Craton
— an alternative approach
Mike A. Craig1, John R. Wilford1, Ian J. Tapley2
(Cooperative Research Centre for Landscape Evolution and Mineral Exploration)
Introduction
Gold explorationists in the Gawler Craton face the
difficult task of exploration in a complex and deeply
weathered terrain masked by a discontinuous blanket of
calcrete, colluvium and aeolian sand. The regolith can either
help mineral exploration by providing broader targets or
hinder it by concealing the bedrock targets, preventing easy
sampling, mapping and photointerpretation of structure.
Mapping of landforms, and an understanding of the nature
and distribution of regolith materials, are critical to the
choice of effective geological and geochemical exploration
methods. Exploration in the craton must now include
assessments of regolith data integrated with geochemistry,
geophysics and remote sensing to ensure that the greatest
competitive edge is maintained.
Two regolith–landform mapping projects were
established at Half Moon Lake and Jumbuck, ~160 km west
of Kingoonya, where only sparse information on regolith and
landscape evolution was available. Geological mapping,
which also includes aspects of the geomorphology, has been
described in several Explanatory Notes to the 1:250 000
maps. Benbow (1983 — Coober Pedy 1:250 000 map area) is
of particular relevance to the Half Moon Lake landscapes.
The area has a semi-arid climate with low annual rainfall of
approximately 150 mm per year, with hot summer months
yielding mean maximum temperatures in January of 35°C
(Laut et al., 1977).
The regolith maps were compiled using a two-step
approach that allows the complexity of the regolith to be
assessed, as well as providing a rapid regional regolith
framework in which to interpret geochemical survey data
over a larger area. The Half Moon Lake map is based on ~10
days fieldwork. The first step consisted of regolith–
landform mapping based on detailed field-site observations
over an area measuring 26 by 24 km. During fieldwork, the
regolith characteristics of over 100 sites were recorded and
referenced using a Garmin GPS with a ±70 m locational
accuracy. The second step used the knowledge developed
during production of these maps to provide a succinct
description and evaluation of data sets, image processing
and enhancement techniques considered appropriate to
regolith and landform mapping and exploration in the
northwestern Gawler Craton. A portion of the final Half
Moon Lake map is shown on Figure 1.
The first area mapped included the Challenger prospect
and is called the Jumbuck Regolith–Landforms Map. The
area provided a ‘calibration pad’ to aid interpreting the three
1
2
c/- AGSO, PO Box 378, Canberra ACT 2601
c/- CSIRO Floreat Park WA 6014
MESA Journal 12
January 1999
98-0013
Fig. 1 Extract from the southeast portion of the Half Moon Lake
(1:100 000) Regolith–Landforms Map.
remotely sensed data sets used (e.g. Landsat TM, airborne
magnetics, gamma-ray spectrometry and a digital elevation
model DEM)) in areas to the south and west of the greater
Half Moon Lake region. Extrapolation and, to a lesser extent
interpolation, formed the basis for the second stage 64 by 81
km Half Moon Lake Regolith–Landform map. Although the
Half Moon Lake map is based largely on extrapolation of the
remotely sensed data sets, field observations were also
incorporated as ‘ground truthing’ in the interpretation
process.
Two DEMs with different resolutions were used in this
study. A high-resolution DEM of the Challenger area with
20 m pixels was generated by deMs Pty Ltd using
1:81 000-scale air photos with a scanning resolution of 15
mm (1.2 m photographic pixel). The 9-second DEM with a
pixel size of ~250 m was made available by AUSLIG,
AGSO and the Australian National University. Both DEMs
were used in generating the regolith–landform map of Half
Moon Lake.
Regional geology and landscape
features
Most of the rocks of the Half Moon Lake region are
poorly exposed, with either aeolian sand covering outcrops
or deep weathering altering bedrock composition and
structure at the surface. Silicification, ferruginisation and
kaolinisation are common. The oldest rocks are associated
with the Archaean Mulgathing Complex, which includes
metasedimentary gneissic sequences, granites, banded iron
17
formation and interlayered basic bodies (Daly and Fanning,
1993). Tertiary sediments include sandstone, clay and silt,
and various siliceous and ferruginous duricrusts. Quaternary sediments consist mostly of aeolian sand and lacustrine
clay. The aeolian sand may well be older than the 39.2±3.6
ka indicated by one, shallow (-0.9 m) thermoluminescence
sample.
The five major landform types within the Half Moon
Lake map area are rises, dunefields, sheetflood fans
(including aeolian (erosional) plains as minor components),
floodplains and lacustrine plains (including alluvial
depressions and lunettes as minor components). Sheetflood
fans, which are grouped with aeolian plains because of their
extensive and subtle relief, are most common and account
for 94% of the area. They are often very difficult to separate
even with the aid of remotely sensed imagery and detailed
aerial photographic interpretation. The Half Moon Lake
area is characterised by very low relief on extensive
sheetflood fans and dunefields. Moderately well-developed
longitudinal dunes are common over the southern and
western part of the area, whereas thinner sheetflood colluvial
sediments and lag-covered rises are common to the northeast
and central east.
The Jumbuck area has more erosional units than
elsewhere on the Half Moon Lake map. Drainage is a
moderately to widely spaced mostly dendritic pattern;
streams are mostly intermittent and discontinuous or flow
into salt lakes. Streams over the southern part of the map are
sparse or absent due to the high porosity and permeability of
aeolian sand in that area. Minor erosional scarps are shown
on the Jumbuck regolith–landform map. These very subtle
scarps separate relatively more weathered regolith materials
above and younger, generally less weathered materials
below the scarp. The regolith along the upper edge of the
erosional scarp is cemented by silica in many places.
Regolith mapping
Regolith–landform units (RLU) are areas within which
similar landform and regolith characteristics can be isolated
at the scale of mapping (Pain et al., 1991). Due to the spatial
and compositional variability of weathered materials, it is
often difficult to map regolith directly or, more importantly,
consistently across a project area. In most cases, mapping
units are defined on the basis of landform (i.e. floodplain,
mesa, etc.). Landforms are used as a surrogate for mapping
regolith because landforms and regolith are usually related
spatially and genetically. RLUs do not necessarily show
uniform or pure regolith materials, but more typically show
associations where landform and regolith attributes are
linked. Purity of regolith shown on a map is largely
scale-dependent. With decreasing mapping scale (i.e.
1:250 000 to 1:100 000), the purity or uniformity of regolith
is likely to increase within each RLU. Regolith and
landform types for each RLU are indicated on the Half Moon
Lake and Jumbuck maps as a series of standard mapping
symbols and are expressed in the manner shown on Figure 2.
For example, alluvial sediments might be carbonate-rich and
elsewhere carbonate poor. This difference can be shown on
the map using the suffix 1 and 2 (i.e. ACaf1 and ACaf2).
Examples of field observation sites showing regolith
characteristics are included in the accompanying
photographs.
18
Regolith type
Modifier
ACaf l
Landform type
Regolith codes
Landform codes
IS
L
SC
ed
ul
ep
fs
un
er
af
pl
SH
RL
AC
AO
CH
Aeolian sand
Lacustrine sediments
Completely weathered
bedrock
Highly weathered bedrock
Lag
Channel deposits
Overbank deposits
Sheet flow sediments
Drainage depression
Longitudinal dunefield
Erosional plain (<9 m relief)
Sheet-flood fan
Lunette
Rises (9-30 m relief)
Floodplain
Lacustrine plain
99-0014
Fig. 2 Explanation of the regolith–landform codes, using channel
sediments deposited on a river floodplain as an example.
The Landsat TM data examined were recorded on 14
March 1995 following an extended dry period such that
ground conditions were dry with minimal green biomass and
an absence of annual grasses. Ratios of Landsat TM bands
are useful for separating and mapping different weathered
materials in the Gawler Craton. These include:
• 3/1 and 5/4 for mapping ferruginous saprolite and lags
• 5/7 for identifying residual and transported clays
• 4/2 for separating ferruginous from non-ferruginous
regolith.
These ratio combinations can be displayed individually or
as various three-band false-colour combinations. However,
one of the most effective enhancements for discriminating a
range of different regolith materials is a technique called
Directed Principle Component Analysis (DPCA) developed
by Fraser and Green (1987). DPCA is used to separate clays
in the imagery by deriving principal components from ratios
of bands 4/3 and 5/7. Ratio 4/3 enhances green vegetation and
ratio 5/7 enhances a mixed response of vegetation and clay.
The DPCA operating on these band ratios is able to separate
the vegetation from the clay response. The ‘clay’ band
(derived from the second principal component) is then
Columnar to pod-shaped silcrete cobbles developed over
ferruginous sheetflow sands. (Photo 46674)
MESA Journal 12
January 1999
combined with a ratio of bands 5/4 and bands 7 + 1 in a colour
composite image. Ratio 5/4 highlights ferruginous materials
and bands 7 + 1 highlight silica-rich materials. The final
image is displayed as a three-band composite image with clay
in red, iron oxides in green and silica in blue (Fig. 3).
Ferruginous saprolite, iron duricrust and ferruginous gravel
lags appear in bright yellow hues. Silcrete (silicified
saprolite) appears in mottled green and yellow, and
ferruginous dune sands and sandplains appear in olive green
to apple green hues. Orange to yellowish orange hues
correspond to ferruginous sand and clay over depositional
plains. Floodplain and lacustrine sediments appear in red
hues. Highly calcareous soils containing calcrete lags and
granules appear in magenta.
The Landsat TM response is likely to be largely
reflecting bluebush (Marianna sedifolia) rather than directly
mapping surface carbonate. Bluebush prefers alkaline soils
and so is a useful indicator for the near-surface presence of
carbonate. Mulga and low trees along river channels and
water bodies generally appear in black or dark red hues. The
enhanced Landsat TM image (Fig. 3), once field-checked,
was the main mapping surrogate for extrapolating regolith
units from the Jumbuck calibration pad to the rest of the Half
Moon Lake map area. The patterns derived from the image
responses were used to either define regolith units or to
describe the variability of surface materials within units
defined by other mapping surrogates such as air photos and
gamma-ray spectrometry imagery.
Gamma-rays emitted from the surface are related to the
primary mineralogy and geochemistry of the bedrock,
and/or the nature of secondary weathering including regolith
materials (Fig. 4). Ninety percent of gamma-rays emanate
from the top 0.3–0.45 m of dry rock or soil (Gregory and
Horwood, 1961). Wilford et al. (1997) demonstrated the
value of gamma-ray spectrometric data for mapping regolith
and landforms, and the contribution these data can make to
understanding the processes responsible for development of
a landscape. Significantly, red–green–blue colour
composites of K, Th and U emissions recorded over the
Challenger and Half Moon Lake areas during the South
Australian Exploration Initiative (SAEI) program in 1995
have been most useful in developing the regolith landform
maps produced in this study. Because the Half Moon Lake
area is very poorly exposed, most of the gamma-ray
responses relate to distribution of different regolith
materials. High eTh (equivalent Thorium) values relate to
ferruginous silcrete, lags, saprolite and sand. The high
values are most likely to be due to scavenging of Th by Fe
oxides. Ferruginous lags and saprolite typically have higher
eTh concentrations than ferruginous silcrete and sand
Photos from top to bottom:
Blue bush is an excellent indicator plant for the presence of
carbonate in soils. (Photo 46675)
Erosional rise with goethitic fragments, vein quartz, angular and
very well-rounded highly spherical quartz pebbles and carbonate
nodules and granules as lag over highly ferruginised saprolitic
bedrock (BIFs). (Photo 46676)
Spheroidal, highly weathered, diffusely iron-stained granite
outcrop in a 2 m deep drainage depression. (Photo 46677)
Silcrete lag and pedogenic silcrete over partly silicified and highly
weathered kaolinised saprolite with quartz veining. Pedogenic
silcrete (1–1.5 m) appears as pods or as massive blocks with
brecciated fabrics. (Photo 46678)
MESA Journal 12
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landscape attributes and other data sets, for example
geochemistry, gamma-ray spectrometry and Landsat TM.
Summary
99-0015
Fig. 3 Three-band Landsat TM image of second principal
component of ratios 4/3 and 5/7 in red, ratio 5/4 in green and the
addition of bands 7+1 in blue, over the lower left corner of the
Jumbuck regolith map.
because of their higher iron content. Elevated eTh in silcrete
may also relate to heavy mineral grains (e.g. zircons and
monazite). High K concentrations correspond to residual,
transported and lacustrine sediments. Quartzose sand
greater than ~0.4 m thick, with little or no associated
indurated materials, appears black in the imagery (i.e. where
all three radioelements are low). Yellow to yellowish green
hues commonly relate to sheet flow fans containing
ferruginous sand, calcrete nodules, silcrete lags and clay.
The DEMs were enhanced to provide shaded relief images
(Fig. 5) and then combined with other data sets to provide 3-D
perspective views of the map areas. The 3-D displays
facilitate the visualisation of complex relationships between
Regolith mapping and the enhancement of associated
remotely sensed data sets form an all important first step in
assembling data to address major exploration decisions. It
provides a regional regolith-landform framework and
identifies the potential sampling media of the wider area and
hence a context in which to plan geochemical sampling
strategies. Regolith-landform maps and their thematic
derivatives are critical in appreciating these regional regolith
characteristics and the associated landscape settings in
which geochemical exploration is taking place. Exploration
decisions are best made in the light of this knowledge,
preferably at the start of an exploration program rather than
at the end. This reduces the need for costly modifications by
repeat or infill surveys.
There is also an important post-survey role for
regolith–landform maps and thematic derivative maps when
interpreting results of regional and district scale geochemical surveys. The regional framework provides a
necessary context for the regolith materials and geomorphic
processes, in which the geochemical results should be
considered. The initial maps can be used to derive additional
maps by incorporating gridded analytical data from the
geochemical surveys results, i.e. Au, gold pathfinder
elements, chalcophile indices, etc. Links between the
regolith materials, landscapes and geomorphic processes
can be identified and provide further information about the
significance of any identified anomalies. Any links between
bedrock structure, sampling media, elemental values and
regolith materials need to be fully investigated to ensure the
N
99-0017
99-0016
Fig. 4 Three band gamma-ray spectrometry image with potassium
in red, thorium in green and uranium in blue, over the lower left
corner of the Jumbuck regolith map. High thorium correlates with
areas of iron-rich gravel lag and highly ferruginous sand.
20
Fig. 5 High resolution east-west sun-angle illuminated digital
elevation model of the Jumbuck regolith–landform map area.
Areas of higher elevation are shown in red hues and correspond to
erosional plains, rises and dunefields; west–southwest
longitudinal sand dunes are clearly visible to the southeast of
Jumbuck camp. Areas of low elevation in blue hues have typically
thicker alluvial and colluvial sediments.
MESA Journal 12
January 1999
Photos from top to bottom:
Pedogenic silcrete with well-defined pods and dentate structures.
(Photo 46679)
Lag of Fe silcrete, Fe nodules and minor quartz over very highly
weathered and mottled granitic saprolite developed as an
erosional rise. (Photo 46680)
Swale of dune field consisting of 0.7 m of fine aeolian red–brown
quartz sand over fine, slightly clayey calcareous sand. (Photo 46681)
Ferruginous lag (goethite 30% and haematite 70%) and fine quartz
sand over mottled granitic saprolite. (Photo 46682)
widest possible context is considered in assessing the
significance of survey results.
More detailed accounts of regolith mapping and remote
sensing techniques used in this study are contained within a
restricted unpublished report by Wilford et al. (1998), a
version of which is soon to be released.
Acknowledgments
This research was funded by the Cooperative Research
Centre for Landscape Evolution and Mineral Exploration
(CRC LEME), PIRSA Mineral Resources Group and the
Gawler Joint Venture. The research was part of a much
larger project involving regolith characterisation,
multi-element geochemistry and regolith landform
mapping. Thanks are extended to our colleagues who
commented on the manuscript.
For further information contact Mike Craig (ph. 02 6249
9453), John Wilford (ph. 02 6249 9455) or Ian Tapley (ph.
08 9333 6263).
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