Download Rocks and Landscapes of the Boonah District

Rocks and Landscapes of the
Boonah District
Warwick Willmott
The Fassifern - the valley of the plugs
For those who know the Fassifern Valley its scenery is
unforgettable. Numerous dramatic peaks stud a broad fertile
valley, with steep forested ranges to the west, south and east.
How did these peaks come to be here, and what are
they made of? How does their geology and that of their
surroundings influence human activities today?
The peaks were once plugs of hard volcanic magma that
filled subsidiary vents or fractures beneath the flanks of two
ancient volcanoes. The ranges to the west and south are
remnants of these volcanoes. The plugs were only exposed
by the later erosion of surrounding rocks. In contrast, the
floor of the valley is composed of much older, easily eroded
sedimentary rocks.
This ancient history is best visualised by going back to the
origins of the oldest rocks of the valley, and then working
forward in time.
1. EARLY HISTORY – SEDIMENTS DEPOSITED ON
RIVER PLAINS
The oldest rocks exposed in the valley once were sediments
laid down in a broad subsiding area – the Clarence-Moreton
Basin - in the early Jurassic period (~205 -145 Million years
ago). This was part of a much larger subsiding area, the
Great Australian Superbasin, or the Great Artesian Basin.
This remarkable subsidence was related to the previous
history of the eastern edge of the continent. Before about
210 million years ago the eastern edge was in compression,
being affected, and heated, by ‘subduction’ (or pushing) of
an oceanic crustal plate to the east beneath the continent.
Sinking of the Great Australian Superbasin behind the
continental edge
After about 210 million years ago the subduction ceased,
the heavy oceanic plate slab beneath the continent sank,
the continental edge relaxed, the continental crust cooled
and hence contracted, and as a result the continent behind
gradually sagged.
A. The first sediments washed in were gravels and coarse
sands, deposited by very active rivers on broad plains – these
have been compacted and hardened to conglomerate and
sandstone of the Woogaroo Subgroup. In this district they are
exposed only along part of Boonah-Beaudesert Road where
they are ‘folded’ up from beneath younger rocks.
B. The next sediments were also sands, but of softer
types, and they were mixed with silts and muds that were
deposited on less active parts of the river plains. These are
now the Marburg Subgroup, and they are well exposed in
new cuttings along the Boonah-Beaudesert Road. Both
sandstone units are relatively resistant to erosion, and form
the hilly range defining the eastern side of valley. They give
sandy, relatively infertile soils, and much of their outcrop
has remained under forest cover until recently.
C. Thirdly, as the rate of subsidence of the Basin slowed,
streams on the river plains became sluggish and swamps
developed. Impure sands, silts, muds and coal were
deposited to form the Walloon Coal Measures. These rocks
are soft, weather deeply, and are poorly exposed, only
clearly seen in road cuttings at Maroon and west of Mount
Alford. They underlie much of the valley and give subdued
topography with clayey, moderately fertile soils.
Some time late in the history of the basin, rocks of the
Clarence-Moreton Basin were gently warped, or folded. The
older sandstones were warped up beneath the range between
Boonah and Beaudesert, and the Walloon Coal Measures
were warped down on either side in the Fassifern and
Beaudesert valleys (they were once on top of the intervening
range also, but have been eroded off there). As a result the
strata in the Fassifern Valley slope at gentle or moderate
angles to the west.
Cross- section of the sedimentary strata of the Fassifern and Beaudesert valleys
2. A LATER SHORT INTERLUDE
About 60 to 50 million years ago, in the early Tertiary
period, several narrow basins subsided down the length of
southern Queensland. This was possibly related to tension
in the crust as the Tasman and Coral Seas began to open up
to the east. They were filled by eroded silts and muds, and
some basalt lavas
A small remnant of one basin outcrops south of Peak
Crossing. Basalt lavas were erupted initially, and then a
small lake flooded the basalt terrain. Impure limestone
rich in magnesium (derived from weathering of basalt)
was deposited, and this has been mined for agricultural
dolomite (calcium magnesium carbonate). The limestone
resists erosion, and forms a ridge south of Peak Crossing.
3. THE TRANSFORMING EVENT FOR THE VALLEY
A. Eruption of two major volcanoes
Between 34 and 6 million years ago in the later Tertiary
period the eastern part of the Australian continent drifted
northwards over a ‘hot-spot’ in the Earth’s mantle, deep
below the crust. There basalt lavas were periodically
generated, and erupted through weaknesses in the crust to
build up many, sometimes overlapping, broad volcanoes.
The volcanoes erupted progressively as the crust moved
northwards over the stationary ‘hot-spot’, and as a result
the oldest volcanoes are in the north, and the youngest in
the south. Southeast Queensland moved over the hot-spot
between 30 and 23 million years ago, when it appears to
have been the most active. In the Fassifern district the Main
Range Volcano erupted probably from a line of craters in
the centre of the valley around 24 million years ago. It was
built up by numerous basalt lava flows, but there were also
some flows of trachyte that are more resistant to erosion
and cliff-forming. The lavas originally extended right across
the valley to the eastern sandstone range, where some small
remnants remain. The Focal Peak Volcano erupted slightly
later from a centre just to the west of Mount Barney, and
also contributed lavas to the north and west (the junction
between the two volcanoes is not clear).
B. Last gasps of the Main Range Volcano – plugs, dykes,
sills, intrusions etc
Towards the end of many volcanoes, magma can be
intruded within, and beneath the lava pile, in bodies of
various shapes and sizes, and these were especially common
beneath the Main Range Volcano. Probably most did not
reach the surface, but a few may have filled subsidiary vents
on the volcano’s flanks. Flinders Peak may have reached the
surface, as its summit is composed of broken rock which
may be the result of volcanic explosions.
These intrusions are composed of a great variety of rock
types, which are�������������������������������������������
thought to have been ‘distilled off’ from
the basalt in a deep magma chamber, although some may
have come from contamination by surrounding rocks.
Unfortunately deep weathering and poor
�����������������
exposure of
many of the less resistant rock types makes identifying and
naming them in the field problematic.
Most obvious are vertical plugs of hard rhyolite and trachyte,
which were the end products of such ‘distillation’. They
are resistant to erosion, and remain as peaks after the
surrounding soft rocks are eroded away (such as the peaks
around Flinders Peak). Sub-horizontal sheets or ‘sills’ of
these rocks were also intruded between the strata of the
Walloon Coal Measures, and these give flat resistant peaks
such as Mount French. A circular fracture filled with
rhyolite has formed the ‘ring-dyke’ of Minto Crags. Some
bodies of trachyandesite are also semi- resistant, forming
high rounded hills.
Digital elevation model of three eroded ‘hot-spot’ volcanoes of
southeast Queensland
by Ben Cohen from NASA SRTM data
More significant in volume were intrusive bodies of other
rock types, such as micro-syenite, micro-diorite, basalt and
dolerite. These usually are more easily eroded, outcrop
poorly, and form lower, rounded hills, although they do
vary in prominence. Such intrusions underlie a large part
of the valley - some are large rounded bodies, whilst others
are thin sills and dykes. These rocks contain many dark
minerals rich in calcium, magnesium, and iron, and weather
to deeper, reddish brown soils of reasonable fertility. They
have been poorly recognised previously, but they contribute
significantly to the fertility of the valley.
C. Last gasps of the Focal Peak Volcano
The history of the Focal Peak Volcano is complex and
beyond the scope of this leaflet (see the book Rocks and
Landscapes of the National Parks of Southern Queensland)
However, to summarise, the volcano initially erupted basalt
lavas from a centre over the present Focal Peak, just west of
Mount Barney, and then the magma changed to rhyolite,
with some rhyolite lavas erupted from Mount Gillies to the
east. Towards its end, a large body of rhyolite magma was
thrust upwards to cool slowly to the resistant granophyre
of the present Mount Barney. Large thick ‘sills’ of rhyolite
magma were also intruded in a circular pattern around the
central mountain beneath Mounts Ernest, Maroon, May,
Phillip and Minnages Mountain.
4. Forming THE LANDSCAPE
The lavas of the two volcanoes originally extended right
across the valley to the eastern sandstone ranges (where
some small remnants remain), but since then large parts of
the volcanoes have been eroded away.
The eastern and central parts of the Main Range Volcano
have been almost entirely removed, leaving only the western
side, which now forms the present Main Range. Mount
Walker to the north is the only vestige of the central part of
this ancient mountain. This process has happened because
the streams draining the eastern flanks of the volcano were
steeper and more active than those on the west. Once
these eroded down through the lavas to expose the soft
Walloon Coal Measures beneath, they undercut the basalt
lavas above by development of a steep escarpment, which
retreated to the west. This escarpment can still be seen today
along the Main Range, particularly in The Ramparts north
of Cunninghams Gap, and its continuing retreat can be
clearly seen at Teviot Falls, where the basalt lavas are being
undercut and collapsing in the cliff face.
Once erosion removed the basalt lavas and carved into the
soft Walloon Coal Measures, the bodies of magma intruded
beneath the two volcanoes were exposed. Those resistant
to erosion such as rhyolite and trachyte remained standing
as sharp peaks or flatter mountains (depending on their
shape), while other types eroded more deeply to gently
rolling terrain amongst the sedimentary rocks.
Progressive erosion of the Main Range Volcano
b)after erosion
a) on intrusion
Erosion uncovers plugs, sills & dykes intruded beneath the
flank of the Main Range Volcano
The escarpment on the western side of the Main Range Volcano continuing to retreat. These are The Ramparts on the
northern side of Cunninghams Gap, where landslides occured in 2011 and 2013
Ben Cohen
5. INDIVIDUAL PEAKS
The prominent peaks of the valley with few exceptions are
composed of resistant rhyolite and trachyte.
Mount Greville and Mount Moon were probably domes of
rhyolite magma intruded beneath the surface rather than
vent fillings. Mount Edwards was a similar body of trachyte.
During its erosion it has been carved into two parts by
Reynolds Creek gradully cutting down from a higher level.
The group of peaks around Flinders Peak were intrusive
bodies, dykes and sills of trachytes of slightly different
colour, grain-size and mineral content. Flinders Peak itself
is a compound intrusion of various trachytes, cut by a thick
dyke on the northen side. Towards the top fragmental debris
of a volcanic breccia may be the products of a volcanic
explosion when the magma reached the surface. The nearby
Ivory’s Rock is a small plug of rhyolite, where some outcrops
of glassy obsidian represent sections that have been chilled
rapidly. Minto Crags at Croftby is a circular ‘ring’ dyke of
rhyolite, while the flat toped Mount French is developed on
several gently inclined sills of rhyolite.
dolerite weather to well developed red soils of good fertility.
The basalt lavas of the range to the west weather to deep
red and black fertile soils which support thick forests.
In contrast rhyolite and trachyte have minerals with few
nutrients and display only poor shallow soils.
Mount Alford is a complex intrusion of micro-diorite and
granophyre with a broken-up andesite capping, encirled by
several ring-dykes of trachy-rhyolite, including Glennies
Pulpit.The Sugarloaf southeast of Boonah is a plug of
basalt,and the flat-topped Cunninghams Lookout west of
Warrill View is formed by several flat sills of basalt and
coarser dolerite.
Several plugs and intrusions of basalt and dolerite have been
worked for local road pavement gravels and the regional
supply of crushed aggregate. One major quarry is active
in a large inclined sill of basalt between Peak Crossing
and Purga, and there is potential for additional quarries in
the future. The limestone at Peak Crossing is worked for
agricultural dolomite.
6. INFLUENCE OF THE GEOLOGY
Potential for coal deposits and coal-seam gas
The influence of the geology on land use in the valley is
quite marked.
The Walloon Coal Measures in other districts are major
sources of steaming coal for electricity generation and
export, and there are coal mines just to the north at
Amberley, Jeebroopilly and Rosewood. There is potential for
additional deposits farther south, and some small ones have
been outlined in years past, but here the coal seams appear
thin and discontinuous, and the very numerous intrusions
within the valley have disrupted the seams to such an extent
that significant workable resources will be difficult to locate.
Fertility of the soils
Only poor sandy soils are developed on the sandstone
ranges to the east and much of this country has remained
under forest or is used only for broad-scale grazing. The
Walloon Coal Measures in the centre of the valley weather
to heavy clay soils of moderate fertility. The extensive
intrusions of micro-syenite, micro-diorite, basalt and
Slope instability
Extensive landslides have occurred on cleared country on
Walloon Coal Measures to the east of Boonah. Removal of
the forest and its transpiration has resulted in higher water
pressures in the strata during high rainfall events, allowing
the soft weathered rock to fail and move down hill in debris
slides and earth flows. This is pronounced on slopes that fall
in the same direction as the rock strata, which in this valley
are mainly westerly facing. Landslides also occur on cleared
hill slopes underlain by weathered and transported basalt
debris at the foot of the Main Range, such as just northeast
of Cunninghams Gap.
Sources of quarry materials
Multiple intrusive sheets or sills of rhyolite magma forming
the peaks of Mount French after erosion of covering rocks
Google Earth image of the ring-dyke of Minto Crags,
originally a circular fracture filled by rhyolite magma. Revealed
by erosion preferentially removing the surrounding softer
sedimentary rocks
Based on a talk to the Fassifern Field Naturalists Club June 2013
© Geological Society of Australia Qld Div 2013