de Broekert: Stratigraphy and origin of regolith, SW Yilgarn Craton 
Gibson Formation 
Distribution, geometry & dimensions. At East Yomaning, 
the Gibson Formation occurs as small sheets and pods 
situated in middle to upper slope positions (Fig 2). A 
marked increase in abundance of the Gibson Formation east 
of the Meckering Line (Brewer & Bettenay 1973; Glassford 
1987), where the climate becomes drier and there has been 
less fluvial reworking of surficial material, suggests that 
the sheets of Gibson Formation at East Yornaning were 
once much more extensive. Limited borehole intersections 
and sand pit exposures (Tables 1, 2) indicate that the 
Gibson Formation has a thickness of 0.5-2 m. 
Lithic characteristics. The following two lithofacies of the 
Gibson Formation can be recognised, based primarily on 
differences in colour. 
1. Grey bimodal sand lithofacies. This facies forms 
the vast bulk of the Gibson Formation at East 
Yornaning and is composed of white to light grey, 
loose to poorly indurated, framework supported, 
poorly sorted, bimodal very coarse and fine quartz 
sand (Fig 5). Trace amounts of fresh microcline 
feldspar can be detected in X-ray diffractograms 
and thin sections of the medium to fine sand-sized 
sieve fractions. Framework quartz grains are 
virtually all monocrystalline, highly etched and 
very angular to angular, though a small proportion 
of coarse, well rounded to rounded quartz is also 
present (Fig 8C). Where overlying granitic 
saprolite, or some other impermeable material, 
basal sections of Gibson Formation grey sand 
contain large, irregularly-shaped, authigenic 
ferruginous nodules (Table 2), or less commonly a 
~0.5 m thick zone cemented by kaolin, gibbsite, 
goethite and silt-sized quartz (Fig 8). 
2. Yellow bimodal sand facies. This facies has a very 
restricted spatial distribution at East Yornaning, 
having only been found in the top of borehole 
YN41 (Table 1). The yellow sand facies of the 
Gibson Formation is very similar in composition 
and texture to the grey bimodal sand facies, except 
that it contains a high abundance of kaolin 
spherites (-10% in YN41) and has framework 
quartz grains with thin coats of yellow, goethite- 
impregnated, kaolinitic clay. The yellow bimodal 
sand facies of the Gibson Formation can be 
distinguished from the bimodal sand facies of the 
Westonia Formation, which is also yellow, on the 
basis of stratigraphic position and by the presence 
of trace quantities of fresh feldspar and a larger 
amount of kaolin spherites. 
Structure. Except for authigenic ferruginous nodules, 
and labyrinthoid structures developed within the basal 
cemented zone of the grey sand lithofacies (Fig 8), the 
Gibson Formation is massive. 
Stratigraphic relationships. Gibson Formation bimodal 
sand unconformably overlies the Westonia Formation 
and variably weathered Precambrian granite and dolerite 
with a sharp contact (Tables 1, 2). It also unconformably 
overlies Mulline Formation pisolitic duricrust, but in this 
case the contact is highly irregular owing to the infilling 
of cavities with the upper part of the duricrust. Where 
not exposed at the land surface, the Gibson Formation is 
unconformably overlain by the Nuendah Formation. An 
interfingering contact between the Gibson and Nuendah 
formations is, however, locally present in he central and 
north-eastern parts of the Yilgarn Craton (Glassford 1987; 
Glassford & Semeniuk 1995), indicating that the two 
formations are at least in part temporally equivalent. The 
presence of an interfingering contact could not be 
confirmed at East Yornaning due to the lack of suitably 
positioned boreholes and exposures. 
Correlation. Both grey and yellow bimodal sand 
lithofacies of the Gibson Formation at East Yornaning 
correlate in terms of geometry, lithology and 
stratigraphic position with the sand sheet faces of the 
Gibson Formation in the central and north-eastern 
Yilgarn Craton (Glassford 1987). A major difference, 
however, is that the Gibson Formation at East Yornaning 
is dominantly grey, whereas the Gibson Formation in 
central and eastern parts of the Yilgarn Craton is 
dominantly yellow or reddish yellow (Glassford 1987). 
Yellow and grey sands with or without coarse sand- 
to pebble-sized rounded quartz particles are also widely 
distributed in upland positions along the south-western 
margin of the Yilgarn Craton (Mulcahy 1960; Mulcahy et 
al. 1972; Finkl & Fairbridge 1979; Bettenay et ah 1980; 
Asumadu et al. 1991). In the Darling Range, these sands 
occupy shallow troughs along major interfluve zones 
(Goonaping valley form of Bettenay & Mulcahy 1972) 
and are locally cemented to form a thin "duricrust" 
(Bettenay et al. 1980; Asumadu et ah 1991). In both of 
these respects, this sediment is similar to the Gibson 
Formation at East Yomaning. 
Origin. Basal contacts with a variety of regolith strata, 
including weathered dolerite dykes, and the presence of 
a small but distinct quantity of well rounded quartz 
grains, indicate that the Gibson Formation is 
sedimentary, and not derived from the in situ weathering 
of Precambrian crystalline basement. Well-developed 
bimodal grain-size distributions, the presence of kaolin 
spherites in the yellow sand facies, and a widespread 
distribution throughout all landscape positions, 
including the interfluve zones between major drainage 
basins, further indicate that the Gibson Formation is 
likely to be aeolian (Glassford & Semeniuk 1995). 
The gradation between yellow sand in eastern (drier) 
areas and grey sand in western (wetter) areas indicates 
that the grey sand was formed from the yellow sand by 
the degradation of kaolin spherites and removal of 
goethitic clay coatings from quartz grains as a 
consequence of increased rainfall and water throughflow. 
Assisting in the removal of grain coatings and bleaching 
of the yellow quartz sand is likely to have been the 
complexing and mobilization of iron by organic exudates 
(Hingston 1963; Enright 1978). 
Basal cementation of the grey sand facies of the Gibson 
Formation probably resulted from the downward 
translocation of silt-clay sized material derived from the 
degradation of kaolin spherites and the removal of grain 
coatings from originally yellow sand, as noted above. 
Additional silt-clay sized particles would have been 
derived from matrix material deposited along with the 
framework grains or thereafter, such as from aeolian dust. 
Provenance. Trace quantities of fresh microcline feldspar 
within the Gibson Formation indicate that it was partly 
derived from exposures of fresh granite, although 
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