Appendix 2 - Vegetation Communities and Regional Ecosystems

Literature often refers to the string of continental islands extending northwards to PNG as the
northern extension of Australia’s Great Dividing Range (Torres Strait NRM Reference Group
2005). As the Great Dividing Range is a geographical rather than geological feature, dividing the
watershed on Australia’s east coast into west flowing and east flowing streams (in a broad sense),
the term is considered irrelevant to the islands.
A range of lithologies are represented in this group with acid welded tuff, agglomerate, rhyolite
and andesite of the Torres Strait Volcanics most extensive in the southern group of islands
although extending through Moa, Mabuiag, and Gebar Islands (Willmott and Powell 1977). The
volcanics have been variably intruded by the Badu Granite, a coarse biotite granite which forms
the highest peaks on many of the islands including Banks Peak on Moa Island, the highest Peak
in the island group at 399m, and Mount Cornwallis on Dauan at 242m. These later intrusive
events have resulted in extensive horfeldsing of the rhyolite basement manifest in a number of
extremely resistant knolls and ridgelines, most prominent on Moa Island.
The larger islands in this geological group generally have well-developed coastal flats, often in
broad embayments separated by coastal headlands. The most extensive of these is located on
Moa Island where an extensive area of residual sand forms a thin cover over the granite and
volcanic basement rock. Relict alluvial fans, generally deeply dissected, often fringe these
residual areas and were once probably considerably more extensive. Similar features are noted
on Muralug, and to a lesser extent on Horn Island.
Coastal dunes are a common feature on the coastline of the acid volcanic islands, with younger
prograding beach ridge systems pronounced in a number of locations. The best development of
these features is on the south-west facing coastline of Muralug Island, as well as Friday Island
(see Swan 1981) where the dunes have developed aeolian (wind blown) features including
blowouts and some broad deflation hollows. An extensive relict dune system is mapped on Badu
Island, the extent of which has not been recognised in the previous studies of Willmott and
Powell (1977). The system is developed behind exposed embayments on the islands southeastern
side, and is represented by a gently sloping broad sand ridge which tapers to a near
uniform sand sheet on the dunes landward margins where it terminates against a series of low
granite escarpments. Field evidence indicates that dune sand has pushed over the top of low
granite saddles and headlands in some locations, being strong evidence that the broad system is a
relict feature from a period of transgressive dune building. The majority of the system is
stabilised with vegetation ranging from sedgeland to eucalypt woodland, which has resulted in a
suppressed and evenly graded morphology. In the area to the south-west of the Badu township
(in the vicinity of the current borefield), a large portion of the dunefield is in unstable form with
the dominant surface area of the dune formed by exposed coarse silica sand demonstrating
undulating morphologies which include shallow deflation basins, blowouts and low depositional
mounds (see Photograph 1).
The initiation of transgressive dune development is often linked to sea level rise (Hesp and Thom,
1990) and it is feasible to conclude that the transgressive event on Badu can be linked to a sea
level highstand inferred by Burne et al. (1995) as being 6 000 yrs B.P. It is also feasible to
conclude that the dune has degraded to the suppressed and stable morphology manifest in todays
landscape in the period lapsed since this sea level high stand. The timing of the event responsible
for dune destabilisation is unknown, although it is clear that it is relatively recent as the unstable
area forms a broad deflational basin which truncates well-wooded and stable dunes to the
immediate south. Orme (1990) suggests that periods of dune instability may be initiated by
anthropogenic disturbance such as burning. It is considered possible that dune instability is coincident
with the advent of permanent human occupation, prompted by indigenous land
management practices.
3d Environmental – Torres Strait Regional Ecosystem Mapping Project – August 2008
4