Appendix 2 - Vegetation Communities and Regional Ecosystems

5.1.3 Land Zone 3
Difficulties with the distinction between Land Zone 1 and Land Zone 3 have been indicated
previously in Section 5.1.1. Land Zone 3 in this study includes all transported alluvial material
except those affected by tidal inundation; older sediment and colluvial profiles affected by deep
weathering, laterisation and clay alteration, and; residual sediments. The major landforms
associated with Land Zone 3 include piedmont fans and outwash plains; fluvial sediments and
terraces associated with drainage channels, and; alluvial sediments associated with freshwater
swamps. Problematic areas which concern the distinction between Land Zone 3 and Land Zone 5
on Moa and Muralug Islands are described in Section 5.1.4.
The most extensive occurrences of Land Zone 3 are associated with gently overlapping alluvial
fans and outwash plains on Muralug and Moa Islands. This is not surprising considering that due
to their considerable size, these islands have a well-developed drainage network which is lacking
on many of the smaller islands in the study area. The composition of the alluvial plains depends
largely on provenance, with alluvium derived from finer grained acid volcanic rocks generally
being silty and poorly drained. Coarser granitoid rocks produce sandier alluvial types which are
relatively well-drained, often supporting luxuriant riparian vine-forest vegetation. On the southeastern
side of Muralug Island where the dominant geology is diorite, relatively fertile clay rich
alluvial plains have developed. The soil associated with these plains is mildly vertic (shrink and
swell) with a well-drained upper soil profile and gentle gilgai micro-topography. The geological
code TQH characterises the majority of the fringing coastal plains on the larger islands, indicating
an undifferentiated unit comprising Tertiary, Quaternary and Holocene age sediments. In the
majority of cases (particularly on Horn and Muralug Island), coastal plains are formed by low
angle, evenly graded alluvial fans which slope to tidal margins without any abrupt variation in
topography. Local sea level provides a control on the base level of erosion, influencing the angle
of the fringing alluvial fan, the rate of sedimentation, and the depth of fan incision. It is evident
that the dominant control on the distribution of surficial sediments on these fans is sheetwash and
alluvial process rather than being residual sediment weathered insitu. On this account, the
majority of the more extensive coastal plains are classified as LZ3 features rather than LZ5 as
might be otherwise inferred. A relative sea level fall would reactivate alluvial process, initiating a
new episode of fan incision and erosion, forming a transient sequence of relict alluvial surfaces.
As discussed in Section 5.1.4, relict alluvial surfaces are present in some locations on the larger
islands, possible testament to a recent sea level fall as inferred by Burne et al (1995). The
possibility that the alluvial covering on these broad coastal plains is surficial, overlying deeply
weathered sediments from previous depositional cycles, or bedrock weathered insitu should be
considered. Until in-depth soil profiling and geomorphic studies are undertaken on these coastal
plains, it is not possible to confirm their true nature and land zone classification can only be based
on visible geomorphic character.
As previously mentioned in Section 1.1.3, remnants of the Fly Platform, manifest as sandy clay
rises on Saibai and Boigu Islands are described as Quaternary age by Wilmott et al. (1972),
although broad physiographic groupings of Paijmans et al. (1971) classify the islands as part of
the Tertiary age Oriomo Plateau. Evidence produced from sediment coring undertaken by
Barham (1999) support early Pleistocene age for these features, based both on observation of
ferruginous pisoliths within the soil profile and experimental results. It is therefore not
inappropriate to classify these broad clay plains as Land Zone 5 features, although this
reclassification has not been implemented in this study, due largely to the poorly constrained
definition of what constitutes a LZ5 feature. For the purpose of this study, the majority of the
alluvial landform on Saibai (and all of Boigu) has been classified as Land Zone 3, corresponding
to the coastal plain of Bleaker (1983) on the adjacent PNG coast, which shares similarities in both
landform and vegetation, and is classified as a Holocene feature (Blake and Ollier, 1970). The
lateritised sediments on the coastal margins of Saibai are classified as Land Zone 7 features and
are described in the following section.
3d Environmental – Torres Strait Regional Ecosystem Mapping Project – August 2008
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