Semeniuk & Semeniuk: Wetland sedimentary fill - particles, sediments, classification 
and diatomite are intrabasinal biogenic deposits in that 
they are definitively restricted to the wetland basin and 
have formed in response to water chemistry. They are 
also the most common accretionary deposits. Quartz 
sand and mud-sized phyllosilicate mineral particles are 
extrabasinal, since their source can be unequivocally 
demonstrated to be the wetland margins. The transitional 
muddy sediments (with the mud component being 
diatoms, calcilutite, or phyllosilicate mineral particles) at 
the base of wetland fills commonly are infiltrational, in 
that they have formed by the introduction of intrabasinal 
sediment into the underlying basement sand (Fig. 16). 
Muddy sediments also form along wetland margins, and 
here they can be accretionary in that the fine-grained 
component is intrabasinal, and it is mixed in with 
extrabasinsal sand derived by sheet wash from the 
surrounding uplands. 
Particles and material are contributed to the sediments 
by the variety of biota that inhabit or visit wetlands. 
These particles and materials from extrabasinal sources 
include scats, skeletons, foreign plant material, and 
imported food remains from vertebrates such as reptiles 
and mammals, and invertebrates such as insects. 
Intrabasinal particles and materials include mollusc shell, 
other skeletons from vertebrates (e.g. f fish) and 
invertebrate fauna, tests, sponge spicules, crustacean 
exoskeletons, zooplankton tests, as well as plant remains 
from in situ trees and shrubs (trunks, branches, leaves, 
roots, flowers, fruit), herbs and sedges, algae, and 
phytoliths and diatoms. Biogenic silica (as diatoms, 
sponge spicules, and phytoliths) is conspicuous, 
ubiquitous, and important in wetland sediments. Diatom 
frustules, and fragments, for instance, can occur in a 
range of wetland sediment deposits, constituting a 
significant proportion of peaty sediments; similarly, 
sponge spicules contribute to peaty sediments, as well as 
to diatomites, and phytoliths also make contributions to 
wetland deposits as intrabasinal particles, especially in 
peats, though overall they remain in relatively low 
concentrations in the sediments. 
Peat in wetlands derives from vegetation detritus. 
Under acidic and anaerobic conditions within the 
substrate, and under water, aerobic bacterial decay of 
vegetation detritus is arrested or reduced, and plant 
detritus cumulatively accretes to form organic-rich beds, 
with anaerobic microbial and fungal breakdown of the 
material. Sites of high vegetation productivity, coupled 
with low rates of organic decay, combine to develop peat 
beds. Peat forms in two main environments: 1. deposits 
of plant material, either comprising layers of plant 
detritus, leaves, sheaves, stems, roots, twigs, branches, 
and trunks, representing plant material directly 
accumulated under plant cover, or comprising wholly in 
situ root-structured, (buried) vertical plant stems, mixed 
with or alternating with horizontally accumulated plant 
material; these types of peat deposits tend to form root- 
structured and fibrous peat; and 2. deposits of organic 
matter, leaves, and other detritus transported to and 
deposited in deeper water; these types of peat deposits 
are structureless, or weakly laminated, and have a 
variable content of diatoms; these latter peat deposits 
may grade into diatomaceous peat to organic matter rich 
diatomite. For these latter peats, in extant environments 
where organic matter is accumulating in deeper water, 
the surface deposits are water-rich 20-100 cm thick 
deposits of flocculated organic matter, which grades 
downwards into an organic matter gel some up to 100 
cm thick, which in turn grades into firm peat. 
In Western Australia, researchers have informally 
subdivided peat into fibrous types, and massive or non- 
fibrous types, essentially implicitly recognising the two 
depositional environments noted above. However, from 
the SEM results, it would appear that the category of 
massive or non-fibrous peat noted by previous workers, 
in fact, may be partly organic-carbon-rich diatomite or 
diatomaceous peat. In addition, there is the issue that not 
all fine-grained black or dark grey sediment are peats. 
The dark tone to fine-grained sediments may be partly 
due to organic matter and partly to finely disseminated 
metal sulphides, particularly iron sulphide. 
Peat intraclast gravel and sand form from processes of 
desiccation or fire. The surface of peats may dry out to 
form polygonal cracks, and prolonged summer drying 
and desiccation (particularly if regional water tables are 
falling during dry phases of climatic cycles), or drying by 
fire, leads to progressive fragmentation of the cracked 
surface layer and to the development of angular peat 
clasts that are incorporated as layers into the peat 
deposits. In general, peat is most desiccated along the 
margins of a basin, where drying out will occur first with 
shrinking of the water body across a basin, or where fires 
have resulted in the drying-out of these deposits. Once 
formed, peat clasts may interact with sand bodies that 
are peripheral to the basin, and during the following wet 
season, through wave action and sheet wash, they 
become interlayered with sand. 
Calcilutites are dominantly biogenic in origin. While 
studies elsewhere suggest that carbonate may precipitate 
inorganically within lakes as calcite, or rarely as Mg- 
calcite or aragonite (Muller 1971; Muller et ah, 1972; 
Hakanson & Jansson 1983; Tucker & Wright 1990), in this 
study, particle morphology, microstructure and 
ultrastructure, and EDS data indicate that the < 63 mm 
fraction of carbonate particles is dominantly of biogenic 
origin derived from disintegrated semi-calcareous 
charophytes and carbonate-impregnated filamentous 
algae, and comminuted skeletons of calcareous fauna. 
Also, the range of mineralogy of the tests of invertebrate 
fauna and charophytes determined by XRD and EDS 
adequately explains the range of mineralogy of the 
calcilutites. SEM studies show that while there is some 
diagenetic precipitation of carbonate within a pre¬ 
existing carbonate sediment host (radiating crystals of 
acicular aragonite embedded in calcilutite exemplify 
this), and field and laboratory observations show that 
some calcite is generated by the burning of wood, there 
has been no evidence for in situ chemical precipitation of 
mud-sized carbonate as intrabasinal sediment. The 
restriction of carbonate mud largely within limestone 
terrains, and the chemistry of the wetland waters ( i.e ., 
their alkalinity and salinity) indicates that such 
environments are favourable for charophytes and 
calcareous fauna which draw on carbonate, bicarbonate 
and calcium from the groundwaters. 
Carbonate skeletal gravel and sand form in wetland 
basins where there is accumulation of mollusc and other 
calcareous invertebrate tests. Wave action along wetland 
margins tends to concentrate accumulations of skeletons 
into shoreline or nearshore ribbon deposits. Carbonate 
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