Journal of the Royal Society of Western Australia, 87(4), December 2004 
autochthonous deposits in wetland basins, and as 
signalling intrabasinal sedimentation. On the Swan 
Coastal Plain, the occurrence of the end-member 
sediments and their mixed intermediates reflects 
geomorphic setting as well as hydrological and 
hydrochemical setting (see Semeniuk & Semeniuk 2005a), 
and can be used as environmental indicators when 
preserved in the stratigraphy. All three of the key end- 
member sediments are fundamentally biogenic in origin, 
viz., plant remains generate peat, diatoms generate 
diatomite, and charophytes and invertebrate fauna 
disintegrate to form calcilutite. Secondary derivatives of 
the key primary wetland sediment types, or their 
intermediates, include the intraclast gravels and sands 
derived from peat, diatomite, and calcilutite. They are 
developed most commonly as marginal deposits, as a 
result of processes of desiccation, cementation, 
induration, extreme wetting and drying from rising and 
falling water tables, wave action, or reworking. The 
effects and alterations of these various processes, 
although operating in the overall wetland, are most 
pronounced in the marginal sediments. Extrabasinal, 
allochthonous terrigenous sedimentary material, within 
wetlands, such as quartz sand and mud-sized 
phyllosilicate mineral particles, are delivered to the 
basins by sheet wash, fluvial transport, and aeolian 
transport. 
Sediments composed of mixed particle types reflect 
one or more of several processes. They may represent the 
concomitant production of biogenic material ( e.g ., 
diatoms within a peat-generating basin to develop 
diatomaceous peat; or epiphytic sponges together with 
plant detritus generating a spongolitic peat), or the 
biogenic mixing of various sedimentary layers by 
bioturbation. In lower parts of the stratigraphic section, 
mixes of sand and muddy wetland sediment may 
represent infiltrational sedimentation, mediated by 
bioturbation or gravitational water. On the wetland 
margins, they reflect the various contributions to the 
sedimentary accumulation, e.g., extrabasinal 
allochthonous sediment brought to the wetland by sheet 
wash, or wave reworking of sandy wetland margins, and 
mixed with the feather edge of autochthonous sediment 
deposits generated from within the basin, or aeolian 
transport of fine-grained sediment from basin centres to 
basin margins, and the mixing of the fine-grained 
material with quartz sand adjoining the basin. Some 
mixes of sediments reflect the bioturbation of formerly 
interlayered environmentally distinct deposits. 
In the light of the classification presented in this paper 
in Figure 12, and the proposed compositional boundaries 
within the sediments of the "peat family" between peat 
sensu stricto and diatomaceous or calcilutaceous peat, the 
nomenclature of peat is discussed further here. In the 
literature, the terms "peat" and "peaty" is applied to a 
large range of accreted organic matter, varying in organic 
content. Some authors define peat quantitatively in terms 
of organic matter content. For example, Dachnowski 
(1920) restricts the term to accumulations of plant matter 
> 20 cm thick and with > 60 % organic matter; in a review 
of peat, Clymo (1983) notes that the term is generally 
applied to materials with > 80% organic matter; and 
Leeper & Uren (1993) attach the term "peaty" to material 
with organic matter content > 20 %. Many other authors, 
on the other hand, do not define peat in relation to a 
quantitative content of organic matter but use the terms 
"peat" or "peaty" for any organic rich sediment or soils 
implicitly conveying the notion of rich organic content 
without quantification or even reference to works that 
define peat quantitatively. As Clymo (1983) points out, 
however, "peat" is not a single homogenous substance: it 
varies in vegetative composition, colour, structure, fabric 
attributes and proportion of fibre, bulk density and water 
content, organic matter content, concentration of 
inorganic solutes, occurrence of mineral precipitates, and 
extent of decomposition. As such it will have a variable 
appearance and variable properties dependent on source 
material, microbial, diagenetic, and pedogenic processes, 
hydrological setting, hydrochemistry, depth of burial, 
and age. In this paper, we assign the term "peat" to 
accreted sedimentary material with > 75% organic matter, 
varying in structure and fabric from fibrous (partly 
decomposed plants and detritus) to fine-grained (fully 
decomposed plants). We place the boundary for peat 
sensu stricto at 75% organic matter for practical reasons in 
relation to the other subdivisions of the ternary 
classification (noting though that the 75% threshold is 
close to the 80% boundary for peat of peatland scientists 
cf. Clymo 1983), and assign organic rich sediments with 
50-75% organic matter to the "peat family", but with 
adjectival descriptors to denote the subdominant 
constituents in the material. 
We also note that iron-sulphide-rich fine-grained 
sediments may be dark grey or black but not necessarily 
peat. Iron sulphide rich diatomaceous peat, or iron 
sulphide and organic matter enriched diatomite, for 
examples, superficially may resemble peat, and in this 
context it is necessary either to have determined the 
organic matter content of the sediment by laboratory 
analyses, or to have examined the material as mud 
particle mounts under a petrographic microscope in 
order to assign the sediment the correct identification. 
Generally within wetland basins, carbonate sediments 
form the most complex suite of products, reflecting 
diverse biogenic, biomediated, hydrochemical and 
diagenetic origins, reflecting diverse mineralogic type 
(viz., calcite, Mg-calcite, aragonite, and dolomite), 
reflecting the propensity for carbonate mineral species to 
readily undergo diagenesis, and reflecting hydrochemical 
and stratigraphic history (von der Borch 1976; Wright 
2000; Roberts et aL, 2004). In this context, the 
environmental significance of calcilutites is discussed 
further here. 
Calcilutites, variably known in the literature as lime 
muds, calcitic muds, aragonitic muds, or carbonate muds, 
have been documented from a range of aquatic 
environments in Western Australia and globally. They 
have been recorded in freshwater to saline 
hydrochemical systems, from lacustrine to marine 
environments (Bathurst 1971; Hakanson & Jansson 1983; 
Wetzel 1983; Hammer 1986; Semeniuk 1988; Backhouse 
1993; Newsome & Pickett 1993; Coshell & Rosen 1994). 
Their mineral composition has been noted as calcitic, Mg- 
calcitic, and aragonitic, or mixtures of these, and with 
local dolomite. The origin of the different mineralogic 
particle types has been determined or interpreted to be 
biogenic, inorganic precipitations, or diagenetic (Muller 
1971; Muller et al, 1972; Kelts & Hsu 1978; Hakanson & 
Jansson 1983; Tucker & Wright 1990). For lacustrine 
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