REGIONAL GEOLOGY
From a regional standpoint, Boyne Island/Tannum Sands is located on the Coastal
Block; which is a major geological unit of central Queensland, extending from the Boyne
River Fault eastward to the coast and including the Narrows and Curtis Island. The
Boyne Island Fault has a north-westerly trend, and separates the Coastal Block from the
Rockhampton Block.
Historically, the sedimentary rocks, mainly Greywacke, of the Coastal Block were
deposited in Pre-Devonian time, and have undergone regional burial metamorphism
during late Devonian and Permian time.
During the late Permian, the block was uplifted along the Boyne River Fault in a near
vertical reverse faulting movement of about 12,000m. The extent of horizontal movement
along the fault is not known. Cross faulting, of short displacement and overturning of beds,
also took place, but, the area is now tectonically stable.
The Curtis Island Group, which comprises the near coastal block of sediments, have
undergone repeated tectonism and metamorphism; and, were uplifted in the Late
Permian to Early Carboniferous times.
LOCAL GEOLOGY
The Boyne Island and Tannum Sands (BITS) area includes the Shoalwater
Formation, which occurs as undulating to low hilly lands in the South and South
East. The Wandilla Formation is present as higher steeper hilly lands in the
Central and Southwest.
The Shoalwater Formation consists of Greywacke and subordinate amounts of
Mudstone; having a total thickness in excess of 10,000m .
Beds within the Greywacke are massive, very persistent along the strike and up to 3m
thick. In contrast, the Muds tone is very thinly bedded to laminate.
Gradationmetamorphism has occurred, so that both rock types have developed a
schistose mineralogy and texture.
Relief associated with the Formation is generally less than 60m a.s.l., on which a
closely spaced dendritic drainage pattern is developed.
During Quaternary times, marine and fluvial processes have given rise to the
present day landforms.
In the inland reaches of the Boyne River, there are coarse to fine-textured terrestrial
alluvial deposits in low stream terraces and higher level near flat or very gently
sloping floodplains and alluvial terraces around the confluence of South Trees Inlet
and Boyne River.
The geological formations include:
•Quaternary Aluviumcomprising
creek and flood-plain deposits in lower, intermediate and higher stream
terraces and flood plains.
•Quaternary Coastal Sand Depositswhich
occur mainly as a low beach ridges and swales.
•Quaternary Marine Depositswhich
comprise tidal mangrove area and saline flats.
•Late Carboniferous Shoalwater Formation-which forms part of the Curtis Island
Group, and comprises quartz arenite, mudstone and schist
•Early Carboniferous Wandilla Formation-which also forms part of the Curtis
Island Group, and comprises mudstone, arenite and chert lithologies.
The landforms can be directly related to the lithology of the parent rocks, or, the mode
of deposition of the parent material. Each of the geomorphic landforms developed
has its own characteristic soil types.
TOPOGRAPHY
On Boyne Island, an undulating system of low beach (sand) ridges with
intervening, poorly drained, depressional swales occurs as a relatively narrow
system (up to about 1 km in width) along the east coastal fringes.
These low ridges have developed on the Formation to the east, and appear to
represent former strand lines exposed as a result of a drop in sea level of about 3m
during recent times.
Extensive areas of tidal mangrove flats, mud flats and saltpans, consisting of deltaic
sand and mud deposits, occur along the tidal reaches of South Trees Inlet and in the
mouth of the Boyne River.
Cainozoic alluvial deposits including clay, sand and gravel, increase in thickness
towards the northern end of Boyne Island, and at South Trees Island, reach a
thickness of about 30m.
At the regional level, the area embraces low offshore islands and flat to moderately
undulating terrain near the coast, and extends westward into a series of relatively low, but
rugged, ranges which form part of the Great Dividing Range. The area is drained by the
Boyne and Calliope rivers and their tributaries.
Boyne Island lies between the two major branches of the bifurcated mouth of the Boyne
River. In the north-east, distributaries dissect the island to isolate South Trees Island, with
the land here rising only slightly above the level of the tidal flats.
Boyne Island is highest in the south, reaching 90 m (approximately) with moderate to
steep slopes.
SOILS
The soils in BITS are closely related to the geology and geomorphological landscape
development processes. Whilst the variation of soil types, in terms of major soil groupings,
is not great throughout the area, there is considerable local variation in colour, texture,
drainage status, stoniness and depth to rock. This is due to lithological differences, and
the thin