Journal of the Royal Society of Western Australia, 87(4), December 2004 
intraclasts form in wetland basins that have calcilutite 
and/or carbonate sand deposits. Induration of calcilutite 
by drying and desiccation (Fig. 4A), followed by 
fragmentation and reworking, yields simple intraclasts of 
calcilutite fragments. Cementation by carbonate, and 
fragmentation of the indurated sheets result in another 
type of intraformational clast. The host sediment may be 
calcilutite, skeletal gravel and sand, or earlier-formed 
intraclast gravel and sand. Cementation during periods 
of waterlogging and inundation, reworking into 
fragments during periods of desiccation, transport and 
rounding during periods of inundation, and re¬ 
cementation of clasts and mud deposits during further 
periods of waterlogging and inundation, generates the 
intraclast. Repeated cementation and re-working 
generates complex and polycyclic intraclasts. Reworking 
by wave action and accumulation of intraclasts into 
layers and laminae is most common along wetland 
margins. 
Diatomite mainly forms within wetland basins that are 
located mainly within quartz sand settings in deep water 
to shoreline environments. Diatomite accumulates as 
whole to fragmented frustules. At the stage where 
diatom frustules have become so fragmented that 
individual particles are smaller than the microstructure 
of the sieved wall, the sediment no longer is composed of 
particles that have intergranular porosity, nor do the 
diatom particles pack within the sediment as a series of 
randomly oriented plates. This type of sediment has the 
bulk properties of a quartz silt or a terrigenous clay 
deposit, and superficially can be mistaken for a 
terrigenous sediment, since it does not exhibit the 
properties of traditional porous "diatomaceous earth". 
This end product of diatom disintegration produces a 
sediment type similar to white clay or mud which is 
easily misidentified as a terrigenous deposit. If there are 
remnants of freshwater sponge spicules in such deposits 
(noting that normally in studies of stratigraphy and 
palaeo-sedimentology, the occurrence of sponge spicules 
implies marine or estuarine environments), these 
freshwater sediments can also be incorrectly interpreted 
as an estuarine sediment, i.e., a sponge spicule bearing 
estuarine clay deposit. 
During drying out periods in wetlands, the exposed 
floor of a diatom deposit in a wetland may be reworked 
by aeolian processes, transported shoreward, and 
trapped by peripheral vegetation, to accumulate as 
shoreline low relief ridges, essentially as shore-parallel 
supra-littoral to littoral deposits of "mud". Drying out 
and desiccation of diatomite also leads to the formation 
of diatomite intraclasts. Thus diatomite intraclast gravel 
and sand forms within wetland basins, and particularly 
along their margins, where desiccation has indurated, 
cracked and fragmented diatomite deposits, and high 
water conditions have reworked and rounded them to 
form thin shoreline deposits (Fig. 4B-G). These intraclasts 
may form clean washed gravel deposits, or may become 
embedded in a muddy matrix. Intraclasts are embedded 
in a diatomite matrix where diatom mud, reworked from 
the wetland margin, or reworked into suspension from 
basin centres by wave action during high water, is 
transported shorewards and consequently buries, or is 
mixed in with, the shoreline intraclasts. 
While dominantly marine organisms, sponges are 
known to be inhabitants of freshwater lakes (Williams 
1980). On the Swan Coastal Plain, while they make 
important contributions to wetland sediments as 
intrabasinal particles, sponge spicules and their 
fragments do not become abundant enough to form 
"spongolites". As noted earlier, generally they comprise 
< 10% of wetland sediments. 
Davis & Rolls (1987) documented freshwater sponges 
at North Lake, but considered them to be rare in the 
context of their sampling sites in the urban Perth area. 
During the surveys of sumplands and the marginal 
vegetated zone of lakes and sumplands during this 
study, encrusting sponges were commonly encountered 
on peripheral vegetation throughout the Swan Coastal 
Plain, though they were not abundant at a given site. 
However, the ubiquitous occurrence of sponge spicules 
and their fragments in the sediments, particularly in the 
stratigraphic column, suggests that they can make 
significant contributions to wetland sediments as silica 
particles beyond their seemingly apparent current rarity 
in many locations. The extent of their occurrence in the 
stratigraphic record may be a measure of their greater 
abundance in the past or an indication of their chemical 
durability as silica (and hence their increased 
preservation potential). While organic matter production 
may overwhelm the surface occurrence of sponge 
spicules in peat-generating wetlands at the sediment 
surface, in time, oxidation processes may result in the 
increase in concentration of spicules relative to organic 
matter. 
Mud-sized phyllosilicate mineral particles commonly 
are delivered to wetland basins by fluvial processes. 
However, where there clearly is no major fluvial channel 
source (as is the case for the isolated wetland depressions 
bordered by high dunes), or minor fluvial channel 
consequent drainage bordering a basin, and deriving 
from the adjoining dune high ground), phyllosilicate 
mud beds in wetlands, and phyllosilicate muddy sand at 
the base of wetlands derive from wetland margins, 
bioturbation into the base of the wetlands, and from 
aeolian sources. The processes whereby fine-grained 
clay/silt sediment is delivered to the wetland basin is 
summarised in Figure 17. 
In regard to mud-sized phyllosilicate mineral particles 
derived from the wetland margins or that occurring at 
the base of the wetlands, the mineralogy of these mud¬ 
sized wetland deposits is the same as that coating the 
yellow sand grains of the surrounding Pleistocene 
sediments. The stratigraphic array around and under 
wetlands residing in a yellow sand terrain is as follows: 
1. yellow quartz sand underlies the uplands adjoining 
the wetland basin; 
2. prior to the Holocene groundwater inundation yellow 
quartz sand also formed the floor of the ancestral 
wetland basin; 
3. white quartz sand, leached of its iron oxide content, 
depleted of its interstitial yellow fine-grained 
component, and stripped of its clay/silt pellicular 
envelope, immediately adjoins the wetland basin at 
levels where either high water tables or the capillary 
fringe of high water tables interact with the sand; 
4. white quartz sand, or grey quartz sand, similarly 
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