de Broekert: Stratigraphy and origin of regolith, SW Yilgarn Craton 
NW trend of these suggests a close genetic relationship 
with dolerite dykes or the baked margins in granite 
alongside the dolerite dykes. The small hills capped by 
Mulline Formation are, on the other hand, mostly 
composed of variably weathered granite, whereas the 
interfluve plateaux are mostly composed of the 
Westonia Formation. Although rarely fully exposed, it 
appears that the Mulline Formation has an undulating 
sheet-like geometry with an average thickness of about 
one metre. 
Lithic characteristics. The Mulline Formation is 
typically a reddish brown, indurated to well indurated, 
strongly nodular to pisolitic, quartz sandy aluminous 
duricrust (Plate IB). Pisoliths range up to about 1.5 cm 
in diameter and comprise nuclei of framework- to 
matrix-supported fine to coarse quartz sand, enveloped 
by laminae of framework-supported fine to very fine 
quartz sand. Contacts between laminae are commonly 
disconformable suggesting multiple phases of growth 
by accretion. The inter-pisolith domain typically 
comprises pisolith fragments and quartz sand thickly 
coated by gibbsite, which also commonly occurs as 
ooids. Matrix material of the pisolith nuclei, their 
surrounding laminae and the inter-pisolith domain is 
fine-grained gibbsite, quartz, X-ray amorphous 
minerals, highly disordered kaolinite, hematite, 
maghemite and boehmite, in approximately that order 
of decreasing abundance. 
Two less common, but highly genetically significant, 
variations occur with respect to the composition of the 
pisolith nuclei. The first relates to Mulline Formation 
duricrust overlying dolerite, wherein the pisolith nuclei 
tend to be very iron-rich and contain only a few small 
highly corroded quartz grains. The second relates to 
where Mulline Formation duricrust directly overlies 
weathered granite, wherein the pisolith nuclei exhibit a 
well-developed remnant granitic fabric in which the 
remnants of quartz-feldspar-mica domains can be 
recognised. 
Differentiation of the three types of Mulline Formation 
is generally difficult to achieve in the field, hence 
separate lithofacies have not been established. 
Nevertheless, there is a tendency for those pisoliths with 
remnant granitic nuclei to be loosely packed, large and 
irregularly shaped (more nodular than pisolitic). Pisoliths 
with highly ferruginous nuclei situated over dolerite tend 
to be equally large and poorly formed, but in this 
instance much more dense, magnetic and dark red. 
Structure. The Mulline Formation is typically massive, 
but sporadically contains cavities and vertical pipes that 
are empty to partly filled with detritus. As with the 
nodular sandstone facies of the Westonia Formation, the 
pipes are probably related to the growth and decay of 
ancient plant roots. 
Stratigraphic relationships. The basal surface of the 
Mulline Formation is poorly exposed, but is inferred to 
be gradational with weathered granite, weathered 
dolerite and thick sequences of Westonia Formation. 
Correlation. Except for a lack of kaolin spherites and a 
greater abundance of secondary aluminium minerals 
(principally gibbsite), the Mulline Formation at East 
Yornaning is very similar to the Mulline Formation at its 
type section (Glassford 1987). It also correlates in terms 
of lithology and stratigraphic position with most other 
pisolitic iron- and aluminium-rich duricrusts in the 
south-western Yilgarn Craton (e.g. Mulcahy 1967; Finkl & 
Churchward 1973; Hickman et al. 1992), although these 
may not all have been formed by the same process or at 
the same time. 
Origin. Mulline Formation pisolitic duricrust is 
essentially a secondary or "overprint" lithofacies that 
cuts across primary lithological contacts. In some cases it 
has developed entirely within sediment (Westonia 
Formation), whereas in others it has developed within 
weathered Precambrian granite or dolerite. The nature of 
the original or "host" material is most clearly reflected by 
the composition of the pisolith nuclei, rich in quartz sand 
rich for Westonia Formation, hematite-rich for weathered 
dolerite, and gibbsite-rich (with remnant granitic fabric) 
for weathered granite. In the case of weathered granite, it 
is necessary for the host material to have been either 
saprock or lower saprolite in order to account for the 
presence of gibbsite pseudomorphs after feldspar within 
the pisolith nuclei. 
The alteration (pedogenetic, diagenetic, weathering) 
processes responsible for ferruginous or aluminous 
pisolitic duricrust formation are complex, varied and 
beyond the scope of this paper (e.g. Nahon 1991; Anand 
& Paine 2002). An essential ingredient in the formation of 
Mulline Formation pisolitic duricrust, however, appears 
to have been the addition of fine to very fine quartz sand 
(Glassford & Semeniuk 1995), in the same manner 
inferred to be responsible for development of the nodular 
sandstone facies of the Westonia Formation. Indeed, the 
nodular sandstone facies of the Westonia Formation can 
be regarded as an immature form of, or transition to, the 
sand-rich facies of the Mulline Formation (compare Plates 
1A and IB). Incipiently developed pisoliths within some 
samples of otherwise strongly pisolitic sand-rich Mulline 
Formation, provide further evidence of this paragenetic 
sequence. 
Another condition necessary for the development of 
Mulline Formation pisolitic duricrust appears to have 
been a fairly impermeable substrate, preventing excessive 
downward movement and dilution of the added fine 
sand grains (Glassford & Semeniuk 1995). This 
requirement would have been readily satisfied in the case 
of outcropping weathered basement and thin successions 
of Westonia Formation overlying weathered basement, 
but would not have been achieved where the Westonia 
Formation was thick. Thus, depending on its thickness, 
the Westonia Formation may either have been completely 
converted to Mulline Formation, or incompletely 
transformed to Mulline Formation over a small depth 
beneath the ancient landsurface (Table 2). 
Age. The age of Mulline Formation pisolitic duricrust has 
not been directly determined. However, a late Tertiary 
(?Miocene) age seems probable based on an inferred 
middle to late Tertiary age for the Westonia Formation, 
which it overprints, and an inferred Quaternary age for 
the Nuendah Formation, into which the Mulline 
Formation has been reworked. Using palaeomagnetic 
evidence, a late Tertiary age for "lateritization" has also 
been determined by Schmidt Embleton (1976) for 
outcrops of Paleozoic and Mesozoic sediments in the 
northern Perth Basin (data reinterpreted by Pillans, in 
Anand & Paine 2002). 
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