Semeniuk & Semeniuk: Wetland sedimentary fill - particles, sediments, classification 
aeolian influx 
'A--. 
clay/silt released from yellow 
coating to sand by the acidic 
groundwater under wetlands 
' is bioturbated into the base of 
the sedimentary fill of the basin'TV: t^V/BASEMENT - 
.. * ... - 
A V- 
yellow coating to sand 
disaggregated by acidic 
waters, and clay/silt 
transported into basin 
by groundwater flow 
and seepage 
Figure 17. Idealised diagram illustrating the origin of mud-sized phyllosilicate mineral particles in wetland sediments: influx and input 
from fluvial, sheet-wash, aeolian, groundwater seepage, and (basal) bioturbation processes. 
leached of its iron oxide content, depleted of its 
interstitial yellow fine-grained component, and 
stripped of its clay/silt pellicular envelope, underlies 
the wetland where the sand is permanently or 
seasonally saturated with water. 
In this stratigraphic and hydrochemical setting, yellow 
sand, proximal to wetlands and water tables, under the 
influence of acidic groundwater associated with the 
wetland, or upslope humus-rich soils adjoining wetlands, 
is bleached white. The clay/silt and iron oxide skins that 
impart the yellow coating to the quartz grains, and the 
yellow interstitial fine-grained material are translocated 
vertically downwards to the water table or laterally to 
the wetland, or are bioturbated into the base of the 
wetland. During the wet season, in areas with steep 
hydraulic gradients, groundwater has been observed 
seeping into wetlands from the yellow sand margins. 
Such lateral seepage can carry iron in solution and clay/ 
silt in suspension. In this context, a Pleistocene yellow 
quartz sand terrain that is host to the wetlands, under 
conditions of acidic water, in the process of becoming 
white quartz sand, yields minute amounts of day/silt 
and iron oxides. The clay is transported and deposited 
into the wetland basin. Mud-sized phyllosilicate mineral 
particles thus can be deposited, albeit in minor amounts, 
into wetland basins where there is a steep hydraulic 
gradient into the basin from the adjoining uplands, and 
this clay/silt can occur in Bassendean Dune and 
Spearwood Dune settings, since both these Pleistocene 
dune systems are underlain by yellow quartz sand. 
Apart from wetland soils developed directly on 
Pleistocene quartz sand basement material, the quartz 
sand that forms sedimentary fill along the margins of 
wetlands clearly derives from the adjoining uplands (Fig. 
4G-H). This is demonstrated by the similarity of the 
petrography of the quartz grains of wetland fill to those 
of Pleistocene margins and basement, and by the tracing 
of the sand sheets and wedges from the wetland basin to 
the margin. Quartz sand is transported into the wetland 
margins from the sandy uplands by sheet wash, and the 
wave reworking of sandy wetland margins during stands 
of high water. Quartz sand also underlies the beachridge 
systems that locally form lunettes marginal to some 
wetlands. However, some of the quartz sand 
stratigraphically low in the sedimentary profile, near the 
contact of the accretionary sedimentary fill and the 
underlying basement sand, derives from bioturbation of 
the underlying basement sand upwards into the base of 
the accretionary deposit. 
Particles of quartz silt and quartz very fine sand are 
commonly found scattered in low abundance in central 
basin deposits of peat, diatomite, and calcilutite. Since 
there is no fluvial delivery to such basins, and there is no 
stratigraphic unit that can be traced to the centre of these 
basins, these fine-grained quartz particles provide 
evidence of aeolian delivery to wetland basins. The 
deposits thus consist of accumulating intrabasinal 
autochthonous wetland sediments with a low volume 
aeolian air-fall contribution. 
A summary of the common wetland sediments 
encountered in this study, as listed in Table 6, categorised 
as to primary end-member sediments, mixtures, and 
secondary (derivative) sediments, and as to how they 
formed, is provided in Table 7. 
Within a given basin, sediments also partition into 
central deposits, marginal deposits, and basal deposits, 
or central facies, marginal facies, and basal facies (Fig. 
18). The centres of wetlands are more consistently 
inundated or water saturated, and accumulate peat, 
diatomite, and calcilutite. The margins of wetlands, with 
desiccation, cementation, rising and falling of the water 
table generating extreme wetting and drying, wave 
action, reworking, sheet wash, and burrowing by 
vertebrate and invertebrate fauna, preferentially develop 
169 
