de Broekert: Stratigraphy and origin of regolith, SW Yilgarn Craton 
that acts as a protective cap for other less indurated 
regolith materials at the landsurface. Woolnough (1927), 
who coined the term duricrust, believed it to be the 
product of deep weathering under conditions of strongly 
seasonal rainfall and perfect peneplanation, but it is used 
here purely as a descriptive term without genetic 
connotation. Another term, frequently used 
synonymously with the ferruginous form of duricrust, is 
"laterite", but I avoid the use of this term owing to 
numerous conflicting opinions as to its meaning (Terrill 
1956; Bourman 1993; Eggleton & Taylor 1999). A similar 
problem exists with the term "ferricrete", which in South 
Australia is generally used to describe iron-rich surficial 
materials regardless of their origin (Milnes et al. 1985; 
Bourman 1993), but in Western Australia is commonly 
used for detrital sediments that have been impregnated 
and cemented by iron oxides and oxyhydroxides (Anand 
1998; Anand & Paine 2002). 
General setting 
The East Yornaning catchment comprises an area of 
about 14 000 ha within the western margin of the 
wheatbelt, bounded by latitudes 32°39' and 32°46’ S and 
longitudes 117°1T and 117°25' E (Fig 1). The catchment is 
elongate in an E-W direction and is drained by a fourth- 
order tributary (sensu Strahler 1952) of the Hotham River, 
in the upper reaches of the Murray drainage basin (Fig 1). 
Its eastern interfluve is shared with the Swan-Avon 
drainage basin (Beard 1999) and, more importantly, also 
forms part of the Meckering Line (Mulcahy 1967). 
Originally delineated by Jutson (1934), this NNW-SSE 
trending zone marks the transition from relatively steep¬ 
sided, narrow-floored and high-gradient valleys in the 
west, to much broader and flat-floored valleys locally 
occupied by chains of salt lakes (playas) to the east (Fig 1). 
Most workers (e.g. Bettenay & Mulcahy 1972; Mulcahy 
et al. 1972) accepted the view of Woolnough (1918) and 
Jutson (1934) that the Meckering Line represents the 
landward (eastward) limit of stream rejuvenation 
following late Tertiary (?Pliocene) epeirogenic uplift of 
the Darling Peneplain to form the Darling Plateau, part 
of the Great Plateau of Western Australia. A very 
different interpretation by Finkl & Fairbridge (1979) is 
that the valleys west of the Meckering Line were cut by 
marginal uplift of the Yilgarn Craton in the Late Jurassic 
associated with the onset of sea-floor spreading between 
Australia and Greater India. 
Drainage within the East Yornaning catchment is 
seasonal and follows a dendritic pattern in higher-order 
streams, which becomes rectangular in lower-order 
tributaries owing to the increased influence of basement 
structure (Fig 2). Being very close to the inland limit of 
incised drainage, the major valleys are still fairly flat- 
floored and of low gradient (Mortlock type valley form 
of Bettenay & Mulcahy 1972). The side-slopes of major 
valleys are gentle (-2.5%) and typically terminate in 
outcrops of fresh bedrock, or small cuestas of pisolitic 
duricrust, most of which dip gently to the north and are 
bounded by low breakaways to the south. Outcrops of 
pisolitic duricrust also occur as spurs extending down to 
the valley floors. Although the largest exposures of 
bedrock are in upland areas (Fig 2), fresh bedrock crops 
out in almost all landscape positions, including the valley 
floors, reflecting the highly irregular and unpredictable 
form of the basal weathering surface (Fig 3A). Elevation 
within the catchment ranges from -310 to -450 m AHD, 
with the majority of the landsurface lying between the 
320 and 370 m contour intervals. 
The basement rocks at East Yornaning comprise part 
of the Western Gneiss Terrane of the Archaean Yilgarn 
Craton (Myers 1990), and have been mapped by Chin 
(1986) as plutons of even-grained to seriate to 
porphyritic, biotite granites and adamellites (Fig 4A). 
These were emplaced after the last period of regional 
deformation and metamorphism, and are thus 
characterised by a lack of gneissic foliation or 
metamorphic recrystallisation. Also forming a large part 
of the basement in the catchment are Mesoproterozoic 
dolerite dykes of the Boyagin dyke swarm (Lewis 1994; 
Pidgeon & Nemchin 2001). The dolerite dykes are poorly 
exposed, but in aeromagnetic imagery can be traced for 
tens of kilometres across the catchment, mostly following 
a NW trend. In middle to upper landscape positions, the 
dolerite dykes are locally associated with prominent 
landsurface and basement highs (Fig 3A). This may be 
due to the relatively high resistance to weathering of the 
dolerite dyke itself (Bettenay et al. 1980), or more likely to 
the relatively high resistance to weathering and erosion 
of the baked margins in the adjoining contact 
metamorphosed granite (Prider 1948). 
A Mediterranean climate, with cool, wet winters and 
hot dry summers, characterises the region. The average 
annual rainfall for Narrogin is about 500 mm with most 
rain (70%) falling during mid-May to October. Annual 
potential evaporation is about 1 900 mm and exceeds 
rainfall for 9 months of the year (Bureau of Meteorology, 
personal communication 1996). Most of the catchment 
was cleared for agriculture in the 1950s and is now 
widely affected by secondary salinity. 
Methods 
Exposures of regolith in the form of breakaways, road 
cuts, stream banks, drainage ditches, farm dam spoil and 
three transects of cored boreholes drilled to refusal depth 
(Fig 2) were used to produce stratigraphic sections and 
obtain representative samples of all major regolith rock- 
types (lithofacies) within the catchment. An additional 26 
boreholes, drilled by the rotary air-blast method, 
provided more accurate information on the depth to fresh 
bedrock, but were not lithologically logged as part of this 
study. Surface exposures were mapped onto 1:25 000 
scale colour aerial photographs, which in conjunction 
with stereoscopic observation and interpretation of 
airborne electromagnetic imagery (de Broekert 1996) 
formed the basis for the construction of a surface geology 
map (Fig 2). 
Epoxy-impregnated thin sections were prepared of 
most samples to better characterise aspects of fabric, 
texture and composition. As a further check on 
mineralogical composition, samples that were thin- 
sectioned were also air dried, ground and X-rayed from 
5-65° 20 at l u min 1 using Cu Ka radiation. Semi- 
quantitative estimates of mineral abundance were 
obtained using the SIROQUANT software package 
(Taylor & Clapp 1992) and kaolinite crystallinity (order- 
disorder) was estimated using a peak height ratio and 
empirical index developed by Hughes & Brown (1979). 
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