de Broekert: Stratigraphy and origin of regolith, SW Yilgarn Craton 
Figure 6. A: Spherical quartz grains in the fine sand-sized 
fraction of the Westonia Formation. B,C: Inclusions of spherical, 
fine to very fine sand-sized quartz in alkali feldspar within fresh 
granite. D: Spherical, fine sand-sized quartz grains in granitic 
upper saprolite after having been released from alkali feldspar 
by weathering. Spherical, fine sand-sized quartz grains in the 
Westonia Formation and other sedimentary units therefore 
indicative of a granitic provenance. Note that spherical grains 
appear "rounded", but are actually very angular when viewed 
under high magnifications. A = sample YN41 5.6 m, B = sample 
YN11/1, C = sample EY45/1, D = sample YN41 26.1 m. A,D 
scanning electron photomicrographs; B,C optical photo¬ 
micrographs using cross-polarized light. 
earliest Tertiary weathering has been identified in the 
eastern Yilgarn Craton based on the palaeomagnetic 
dating of ferruginous mottles (Pillans, in Anand & Paine 
2002 ). 
Weathered Dolerite 
Distribution, geometry & dimensions. An aeromagnetic 
survey of the East Yornaning catchment indicates that 
weathered dolerite may form up to 15% of the weathered 
Precambrian basement (World Geoscience Corporation, 
unpublished data). Thicknesses of up to 40 m are 
common, although once again the depth of weathering is 
highly variable with outcrops of fresh dolerite occurring 
in all landscape positions. Important factors controlling 
the depth of weathering are likely to include dyke width 
and associated proximity to granitic baked margins, 
degree of fracturing, grain-size and mineralogical 
composition. Most weathered dolerite dykes are near¬ 
vertical and about 10-20 m wide (Fig 3A). 
Lithic characteristics. Based on limited analyses of 
samples from East Yornaning and descriptions of 
weathered dolerite from the Darling Range (Davy 1979; 
Anand & Gilkes 1984), it appears that the weathering of 
most primary minerals in dolerite is completed over a 
very small vertical distance from fresh rock. The 
saprock and lower saprolite lithofacies are therefore 
very thin (1-3 m) and will not be discussed further. 
Upper saprolite samples of dolerite at East Yornaning 
typically comprise white, poorly indurated to 
indurated, slightly very fine quartz sandy kaolin clay. 
Quartz typically makes up about 5% of the rock and 
occurs as small (50-250 pm), angular, embayed, 
monocrystalline grains with slightly undulose 
extinction. Remnant interstitial and granophyric fabric 
is clearly evident in thin section. 
Structure. Apart from the remnants of chilled margins 
and perhaps xenoliths of granitic country rock, doleritic 
saprolite lacks primary igneous structures. Secondary 
structures, in the form of thin closely-spaced ferruginous 
bands, very similar in appearance to Liesegang rings, are 
however common. 
Stratigraphic relationships. The contact between 
weathered dolerite and weathered or fresh granite was 
not observed but is inferred to be vertical to subvertical, 
planar and sharp. 
Origin and age. Remnant igneous fabric and sharp near¬ 
vertical contacts with fresh or weathered granite indicate 
formation by the chemical weathering of dolerite (Sadlier 
& Gilkes 1976; McCrea et al. 1990). The age of dolerite 
weathering would be similar to that of granite, discussed 
above. 
Westonia Formation 
Distribution, geometry & dimensions. The Westonia 
Formation typically occurs along major interfluve zones 
as tabular-shaped bodies, which are either flat-lying or 
gently dipping toward the valley floor (Figs 2, 3A). A 
maximum thickness of 9.5 m is reached in borehole YN41 
(Table 1), although breakaway exposures indicate that 
thicknesses of 2-4 m are more typical (Table 2). The unit 
has been extensively truncated by erosion and may once 
have extended for a greater distance toward the valley 
floor. 
69 
