Journal of the Royal Society of Western Australia, 86(2), June 2003 
outcrops of granitic saprolite and the Westonia and 
Mulline formations are also likely to have been important 
sources. Aeolian dust, infiltrated into the bimodal sand 
during or after its deposition, is likely to have been 
sourced from outside of the drainage basin. 
Age. On the basis of its inferred aeolian mode of 
deposition and uppermost stratigraphic position, the 
Gibson Formation is likely to have been deposited during 
arid phases coeval with high latitude glaciations during 
the Pleistocene (Glassford & Semeniuk 1990, 1995). 
Bleaching and fluvial reworking of the originally mainly 
yellow quartz sand is likely to have been accomplished 
during the Holocene, or interglacial phases during the 
Pleistocene when the climate would have been relatively 
humid and a relatively thick vegetation cover would 
have been supported. 
Nuendah Formation 
Distribution, geometry & dimensions. The Nuendah 
Formation forms most of the landsurface at East 
Yornaning (Fig 2) and comprises the bulk of the 
sedimentary regolith volume. It occupies narrow 
palaeovalleys cut into weathered basement beneath the 
modem valley floors and extends up the valley sides as 
irregular sheets that wedge out against outcrops of fresh 
granite, or less commonly, outcrops of the Westonia and 
Mulline formations (Fig 3A). A thickness of about 20 m 
for the Nuendah Formation appears to be typical where 
it fills palaeovalleys, decreasing to an average of about 2 
m where it mantles the valley sides. 
Lithic characteristics. The Nuendah Formation is very 
lithologically heterogeneous, but can be broadly 
subdivided into the following two major lithofacies based 
on differences in overall grain-size distribution and 
mineralogical composition. 
1. Muddy sand lithofacies. This facies dominates the 
valley floors throughout the catchment, and also 
the valley sides in the upper (eastern) part of the 
catchment where outcrops of granite are 
particularly widespread. Typically, it comprises 
light grey, poorly indurated, framework 
supported, muddy to clayey, medium to coarse 
sand (Fig 10B,C). Framework grains are weakly 
unimodal to polymodal, poorly sorted, mesokurtic 
to platykurtic, coarse to fine skewed (Fig 5), and 
composed of angular mono- and polycrystalline 
quartz, granitic rock fragments, and microcline and 
plagioclase feldspar (Fig 10B,C). Where filling 
palaeovalleys, there is an overall decrease in the 
abundance of feldspar and granitic rock fragments 
with depth. Matrix material typically comprises 
silt-sized quartz and feldspar, and moderately 
disordered kaolinite. 
2. Gravelly muddy sand lithofacies. This facies 
dominates interfluve zones and upper valley 
slopes in the lower (western) part of the catchment, 
where outcrops of Mulline Formation pisolitic 
duricrust and Westonia Formation nodular 
sandstone are most common (Fig 2). It interfingers 
downslope with the muddy sand facies, and 
typically comprises grey to brown, poorly 
indurated, framework supported, gravelly muddy 
sand (Fig 10A). Gravel particles range from 
granule to coarse pebble in size and are composed 
of feldspar, granitic rock fragments, and brown 
ferruginous nodules and pisoliths with chipped 
and polished external surfaces. Matrix muddy 
sand is of similar texture and composition to the 
muddy sand facies, described above. 
Structure. The muddy sand facies is crudely horizontally 
stratified along valley sides and normally-graded in 
ephemeral stream channels. Interbedding of the two 
Nuendah Formation lithofacies is widespread in lower 
slope and valley floor positions. 
Stratigraphic relationships. In valley floor and side 
positions, the Nuendah Formation typically 
unconformably overlies the upper saprolite facies of 
weathered granite with a sharp contact. However, where 
filling palaeovalleys, it unconformably overlies the 
saprock facies of weathered granite with a sharp contact 
marked by a thin angular quartz gravel lag (Table 1; Fig 
10). Less commonly, the Nuendah Formation 
unconformably overlies the Gibson Formation with a 
sharp contact. Lithostratigraphic investigations of 
regolith in the central and north-eastern Yilgam Craton 
indicate that the Nuendah Formation also interfingers 
with the Gibson Formation (Glassford 1987; Glassford & 
Semeniuk 1995), although this could not be confirmed at 
East Yornaning owing to a lack of suitably positioned 
boreholes or surface exposures. 
Correlation. In terms of lithology and stratigraphic 
position, the Nuendah Formation at East Yornaning 
correlates with the clayey sand facies of the Nuendah 
Formation in the north-eastern Yilgam Craton (Glassford 
1987). In the north-eastern Yilgarn Craton, however, this 
facies of the Nuendah Formation is typically confined to 
the base of breakaways, whereas at East Yornaning it 
occurs in most landscape positions. The more widespread 
distribution of the Nuendah Formation at East Yornaning 
probably relates to increased fluvial dissection attendant 
upon a much greater local relief and higher rainfall. 
The Quairading Sandstone of Salama (1997) correlates 
with the Nuendah Formation at East Yornaning in that it 
too is largely a clayey quartzo-feldspathic sand that infills 
bedrock-bounded palaeovalleys developed beneath major 
modern valley floors. The Nuendah Formation is 
unlikely, however, to correlate with other quartz sandy 
palaeovalley fills in the region that are overlain by a thick 
unit of clay (Waterhouse et al. 1994; De Silva et al. 2000). 
Palynological evidence suggests that these valley fills are 
of Eocene age, which is considerably older than the 
Nuendah Formation (see below) and more similar in age 
to the Westonia Formation. 
Origin. On valley sides the Nuendah Formation has 
been, and continues to be, principally deposited by 
unconfined sheet-wash following heavy rains and to all 
lesser extent by continuous down-slope creep. In valley- 
side gullies and along the major valley floors the 
Nuendah Formation is principally deposited within 
ephemeral fluvial channels and associated flood plains. 
Palaeovalleys filled with Nuendah Formation beneath the 
present valley floors may reflect phases of stream 
rejuvenation arising from tectonic uplift or changes in 
climate. In view of the catchment's proximity to the 
western margin of the Great Plateau and Darling Fault, 
the former seems more likely. 
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