CRC LEME
Open File Report 51
ABSTRACT
Regolith-landform relationships in the Bottle Creek Orientation
Study, Western Australia
Churchward, H.M., Butler, I.K. and Smith, R.E.
Regolith-landform Relationships
A framework of regolith stratigraphy and landforms was established
for the area surrounding the Bottle Creek Au deposits, some 200
km north west of Kalgoorlie, as a basis for geochemical studies.
An early phase of broad reconnaissance provided several sites at
which the regolith was examined in detail and the landform-regolith
relationships were defined. A regolith-landform map (1:10 000 scale)
was produced for the upper Bottle Creek catchment. To test the findings
of the detailed investigation, a study of the regolith was carried
out over an area of approximately 450 km2 surrounding the detailed
study at a reconnaissance scale. A 1:25 000 map of this was produced.
Several well-defined regolith types were identified by these studies
which relate, directly or indirectly, to a deeply weathered mantle
and to its modification by landform processes. These regolith materials
were either horizons of a deep profile developed by in situ weathering
of basement rock or of transported debris, derived from this profile
by erosion. Generally they form extensive surface and subsurface
bodies so that the regolith, at any particular location, commonly
comprises several strata.
The nature of the regolith stratigraphy is strongly related to
the landforms with which they are associated so that a framework
of landform regimes provides a useful concept for considering the
regolith in this area. Thus, units of relatively stable, deeply
weathered tracts were recognized as relics of a once more extensive
landsurface that has been fragmented by fluvial action and replaced
by erosional and depositional regimes.
At Bottle Creek, regolith types associated with the ancient, deeply
weathered landsurface (the residual regimes) are mainly various
expressions of the (upper) ferruginous horizon of the laterite profile,
along with the mottled zone, the pallid saprolite and the saprock.
Several transported regolith types, of colluvial and alluvial origin,
were recognised in both erosional and depositional regimes.
Ferruginous Horizon(s)
The ferruginous horizon comprises various types of lateritic residuum
but the more common are duricrusts having abundant lateritic pisoliths,
generally with yellow-brown skins, set in a brown to red-brown clayey
matrix. At depth, this material merges with the more ductile clays
of the mottled zone. In addition, large irregular to lensoid bodies
occur in the upper part of the regolith. The rock-like nature of
these masses contrasts with the surrounding brittle, pisolitic,
lateritic residuum referred to above. Some of these masses are Fe-rich,
having a dusky red matrix and dark brown to black nodules that can
be magnetic. Such materials have been placed in the broad class
of Fe-rich duricrusts. Another rock-like mass is diffusely mottled
brown to pale brown and some have vermiform voids. These can be
variously classified as ferruginous saprolite, vermiform duricrust
or fragmentary duricrust. Other Fe-rich bodies in the upper regolith
are goethite-rich pods which generally have box-work textures. At
Bottle Creek, these gossan-like bodies occur close to a carbonaceous,
previously pyritic shale in the underlying parent greenstone sequence.
Residual Regimes
The residual regimes at Bottle Creek form gently undulating tracts
that are extensive on the divide between the Raeside and Ballard
drainages. The principal type location for the regolith stratigraphy
of this regime is at the Emu test pit, within this undulating tract.
Most crests in this terrain are slightly stripped with consequent
exposure of an array of ferruginous materials from the upper parts
of the regolith. These are predominantly ferruginous saprolite but
there are also some pisoliths and pieces of Fe-rich duricrust, as
well as clay and sand released by weathering. Such materials contribute
to the colluvial mantle that extend down-slope from the crest, covering
the pisolitic, lateritic residuum; the latter forming the more extensive
substratum of the residual regime. On the mid- and upper slopes,
colluvium is less than 1 m thick; whilst beneath the lower slopes
and local drainage floors, the colluvium is as much as 4 m thick.
The lag composition also varies with topographic position; coarse
fragments of ferruginous saprolite dominate the crests. Some yellow-brown
cutan-coated pisoliths also occur here and are generally indicative
of some subcropping pisolitic, lateritic residuum. On surfaces,
down-slope from these crests, lags of dark brown to black granules
are dominant; there is little quartz or lithic material. The soils
are acid and have developed in a fine, sandy loam colluvium which
has granules of similar composition to the lag. Hardpans appear
at a depth of 1 m and continue for depths of from 3-8 m.
Erosional Regimes
The landforms and the regolith types in the erosional regimes
present a more complex picture reflecting active geomorphic processes.
Deeper units of the weathered mantle, as well as country rock, are
exposed. This regolith is dominated by a shallow, generally calcareous
soil, and a lag of lithic fragments; there are outcrops of vein
quartz and goethitic Fe-segregations. Gentle slopes occur as pediments
below low breakaways. These slopes are mantled by acid red earths,
developed in a pedisediment, and have a lag dominated by coarse,
ferruginous saprolite, lithic fragments and quartz. Erosion is active
in such areas.
Depositional Regimes
The most extensive depositional tracts are mantled by a friable
clay, being an alluvium of sheet flood origin; acid red earths have
developed on this material. The lags are dark brown to black granules
of mixed origin, with medium sized (2-4 cm) lithic and ferruginous
saprolite fragments, and quartz clasts, as a minor, though characteristic
component. This alluvium overlies pallid saprolite and saprock at
a depth of 1-1.5 m but it may also be found mantling pockets of
pisolitic lateritic residuum or coarse deposits in palaeochannels.
Some of the depositional tracts are being further modified by erosion,
resulting in land-surfaces having regolith types comparable with
those in other erosional regimes.
Regolith Evolution and a Framework for Geochemical Dispersion
The topographic relationships and regolith stratigraphies revealed
by this study indicate a polyphase, multi-process history. Many
of the regolith types resulting from this complex array of processes,
have a distinctive pattern. The residual regimes at Bottle Creek
are dominated by a regolith that is the result of intense in situ
weathering, some of the uppermost regolith has been deposited by
local colluviation. These areas have had a relatively stable geomorphic
history. In contrast, depositional regimes here represent areas
that have received fluvial detritus from much further afield and
this material varies from highly weathered to relatively fresh and
is generally of diverse lithological origin. Prior to deposition,
these areas can have been subjected to widespread, though incomplete,
stripping of the more weathered regolith types. The regolith in
erosional regimes is, in detail, complex with exposure of a variety
of variably weathered lithologies. Understanding this general geomorphic
framework assists our appreciation of geochemical dispersion and
thus it provides a basis for the developing sampling strategies
for this weathered terrain.
Last updated: Thursday, January 06, 2000 08:57 AM
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