Journal of the Royal Society of Western Australia, 87:139-186, 2004 
Sedimentary fill of basin wetlands, central Swan Coastal Plain, 
southwestern Australia. Part 1: sediment particles, typical sediments, 
and classification of depositional systems 
V Semeniuk & CA Semeniuk 
V & C Semeniuk Research Group 
21 Glenmere Road, 
Warwick, W.A., 6024 
(Manuscript received July 2004; accepted February 2005) 
Abstract 
The common sediments in wetland basins occurring in the Spearwood Dunes, Bassendean 
Dunes and Pinjarra Plain of the central Swan Coastal Plain, southwestern Australia are peat, 
diatomite, calcilutite (carbonate mud), their intermediates such as diatomaceous peat, organic 
enriched diatomite or calcilutite, and carbonate skeletal gravel and sand (all formed by biogenic 
processes), as well as quartz sand and kaolinite-dominated mud (as terrigenous sediments), and 
various types of muddy sand formed as mixtures between biogenic mud-sized sediments and 
quartz sand, or between kaolinitic mud and quartz sand. Synsedimentary processes and diagenesis 
result in local cementation, desiccation, and reworking of these deposits, particularly along the 
margins of wetland basins, and hence the development of carbonate intraclast gravel and sand, 
diatomite intraclast gravel and sand, and peat intraclast gravel and sand. A clear distinction is 
made between wetland sediments and wetland soils, and between primary wetland sediments and 
the products of diagenesis within and around wetlands. Gravel-sized particles in the wetland 
sediments include plant remains, invertebrate faunal skeletons, and intraclasts. Sand-sized particles 
include plant remains, quartz, invertebrate faunal skeletons, and intraclasts. Petrographic 
microscopy and scanning electron microscopy show the < 63 pm particles that comprise the 
sediments are plant remains, diatom frustuies and fragments, sponge spicules and fragments, 
aggregates of calcite crystals derived from charophytes and the disintegrated clay-sized derivatives 
of these charophytic aggregates, comminuted invertebrate skeletons, quartz silt, and mud-sized 
phyllosilicate minerals. 
Wetland sediments can be categorised as to origin, i.e., biogenic versus terrigenous, as to whether 
they are end-member primary sediments, or derivatives, or mixtures, as to their location of 
formation relative to the basin, i.e., intrabasinal, autochthonous versus extrabasinal, allochthonous, 
as to their infiltrational and accretionary nature in relation to the basement material, and as to their 
location within a basin, viz., central basin versus marginal versus basal deposits. 
Keywords: wetland sediments, peat, diatomite, calcilutite. Swan Coastal Plain wetlands 
Introduction 
Wetlands, as natural topographic depressions in the 
landscape, fill with sediment, and as a result, globally, 
they are known to be important reservoirs of information 
on geohistory, geochemical history, vegetational 
succession and palaeoclimate (Barber 1981; Druckman el 
al ., 1987, Gasse et al r 1987; Talbot 1990; Baltzer 1991; 
Odgaard 1992; Leaney et al, 1995; Wright and Platt 1995; 
Fritz et al, 1999). The wetlands on the Swan Coastal Plain 
of the Perth Basin are prime examples of such archives as 
they contain vast and variable information in their 
sedimentary sequences. They have the potential to 
encode Holocene geologic, hydrologic, and 
hydrochemical history, vegetation changes, and climate 
changes, as preserved in their stratigraphic, diagenetic, 
isotopic, faunal, and floral records. 
While there is mention and some description in the 
literature on the central Swan Coastal Plain of types of 
© Royal Society of Western Australia 2004 
wetland sediments, their stratigraphic expression, and 
their local thickness (Simpson 1903; Teakle & Southern 
1937; Allen 1979; Megirian 1982; Hall 1983; Muir 1983; 
Hall 1985; Semeniuk 1988; Newsome & Pickett 1993; 
Coshell & Rosen 1994; Pickett 1998), to date, these 
sediments have not been subject to a systematic or 
comprehensive description. This paper, the first in a 
series on wetland sediments of the Swan Coastal Plain, 
describes the particle types that comprise the wetland 
sediments in this region, describes the main sediment 
types, and provides an interpretation of their origin. It 
also provides a preliminary outline of wetland sediment 
diagenesis in order to clearly separate primary sediments 
from diagenetic products. The stratigraphy and the age 
structure of sedimentary fills in wetland basins will be 
the subject of later papers (Semeniuk & Semeniuk 2005 
a). The scope of this paper is limited to those isolated 
wetland basins across the central Swan Coastal Plain, 
between Moore River and the Mandurah/Pinjarra region, 
mainly within the settings of the Spearwood Dunes, 
Bassendean Dunes and those local parts of the Pinjarra 
Plain that are not directly connected to fluvial systems 
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