de Broekert: Stratigraphy and origin of regolith, SW Yilgarn Craton 
In the course of petrographic analysis, attention was 
paid to differences in the textural and compositional 
properties of quartz (Folk 1974), this being the only 
primary mineral present in abundance within all 
lithofacies. A similar study of the varietal properties of 
zircon, including sensitive high-resolution ion 
j-picroprobe (SHRIMP) U-Pb age determinations, is to 
form the subject of a separate paper. 
Grain-size distributions of quartz in representative 
samples of each lithofacies were obtained by dry sieving 
V 2 $ intervals over the granule to fine sand-size range 
(4-0.063 mm, or -2-4 cf>). Sample preparation of loose 
piaterials involved dispersion in a dilute solution of 
£algon and NaOH using an overhead stirrer, followed 
by wet sieving through a 45 pm screen. Indurated 
samples, such as pisolitic duricrust, were first coarse 
crushed and then repeatedly boiled in concentrated 
fsJaOH and/or HC1, depending on the cementing agent, 
following dissolution of the cementing minerals, the 
samples were disaggregated using an ultrasonic probe 
a nd then wet sieved as above. Statistical measures of the 
grain-size distributions (mean, mode, sorting, skewness 
a nd kurtosis) were derived by the graphic methods 
described by Folk (1974). Terminology of overall grain- 
size distribution (mud, sandy clay, gravelly sand etc) is 
based on textural triangles developed by Folk et al. (1970). 
In this classification, the term "mud" refers to a sediment 
composed of subequal proportions of clay-sized (<4 pm) 
a nd silt-sized (4-63 pm) particles and has no implication 
of moisture content. Although the textural terminology 
of Folk et al. (1970) was developed for detrital sediments, 
its application is here extended to some highly weathered 
bedrock lithofacies which are very similar in texture and 
mineralogical composition to certain, generally clay-rich, 
detrital sediments. 
Stratigraphy 
Weathered Granite 
Distribution, geometry & dimensions. Weathered 
granite forms the bulk of the regolith at East Yornaning 
and is developed in all landscape positions except where 
fresh bedrock is exposed or where the Precambrian 
basement is composed of mafic dykes. It has a highly 
irregular basal surface, commonly varying greatly in 
thickness over small lateral distances. Nevertheless, the 
greatest thicknesses of weathered granite (40-50 m) tend 
to be concentrated along major valley floors, and along 
interfluve zones capped by VVestonia Formation nodular 
sandstone or Mulline Formation pisolitic duricrust (Fig 
3A). 
Lithic characteristics. Based primarily on differences in 
overall grain-size distribution, mineralogicai composition 
(essentially the degree to which the "weatherable" 
primary minerals in the parent granite have been altered) 
and stratigraphic position, weathered granite can be 
subdivided into the following three lithofacies. 
1. Saprock lithofacies. This material is very similar 
to granite (Table 1) except for minor alteration 
of the chemically unstable primary minerals (e.g. 
plagioclase, biotite, hornblende, apatite) along 
grain boundaries, cleavage planes, cracks 
and other macro- and micro-structural 
discontinuities. 
2. Lower saprolite lithofacies. This typically 
comprises white to yellow to light red, poorly 
indurated, matrix supported, coarse sandy clay 
(Fig 4B). Sand-sized particles are composed of 
muscovite, variably kaolinized microcline, and 
quartz, which together form a conspicuous 
palimpsest or remnant granitic fabric. Kaolinite 
pseudomorphs of biotite and plagioclase form 
much of the matrix and exhibit a moderate degree 
of crystal structure disorder. 
3. Upper saprolite lithofacies. This material is 
typically a white, poorly indurated, coarse quartz 
sandy kaolinite clay, similarly exhibiting a strong 
remnant granitic fabric (Fig 4C). Kaolinite 
pseudomorphs are more prominent than in the 
lower saprolite facies and exhibit a moderate to 
low degree of disorder. Framework quartz grains 
are mostly strongly unimodal, coarse, poorly 
sorted, mesokurtic and characteristically fine to 
very fine skewed (Fig 5). The presence of abundant 
highly spherical quartz grains in the fine to very 
fine sand-sized sieve fractions is also characteristic 
(Fig 6A). Petrographic examinations of fresh 
granite in the catchment indicate that these occur 
as inclusions within alkali feldspar (Fig 6B,C) and 
were released from the rock following weathering 
of the feldspar to kaolinite (Fig 6D). Conversely, 
the coarse sand-sized quartz grains are all 
subequant, very angular, and commonly contain 
holes left by the weathering of inclusions, such as 
biotite. Polycrystalline quartz, which accounts for 
about half of the coarse sand-sized grains, typically 
comprises 3-4 roughly equally-sized crystals with 
strongly undulose extinction (Fig 7A,B). 
A zone of unconsolidated, coarse-grained material 
with a highly porosity and permeability commonly 
occurs immediately above fresh granite and gneiss in the 
south-western Yilgarn Craton ("saprolite grit" aquifer of 
George 1990). This was not, however, observed at East 
Yornaning, possibly owing to poor surface exposure and 
the limited penetration depth of the cored boreholes. 
Structure. The weathered granite lithofacies are 
structureless, except for where cut by dolerite dykes, 
faults, shear zones or quartz veins. 
Stratigraphic relationships. The saprock lithofacies 
overlies fresh granite with a gradational contact over a 
vertical interval of <1 m and is overlain by the lower 
saprolite lithofacies with a gradational contact over a 
vertical interval of 1-3 m. A similarly gradational contact 
occurs between the lower and upper saprolite Lithofacies. 
All three contacts are highly irregular and rapidly 
converge approaching outcrops of fresh granite. 
Origin. Remnant granitic fabric, a progressive upward 
reduction in the proportion of primary minerals, 
gradational contacts, and the preservation of primary 
igneous structures, all indicate that the three lithofacies 
were formed by the weathering of granite. Similar 
materials have been described elsewhere in the south¬ 
western Yilgarn Craton by Gilkes et al. (1973) and McCrea 
et al. (1990), which these authors also regard as having 
been formed by the weathering of granite. 
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