Download Geology of King Island Part 2

Part 2: Excursions
Contents
Excursion 1: City of Melbourne Bay to Cottons Beach: ....................................... 1
Excursion 2: City of Melbourne Bay to ‘the Gut’ and beyond ............................ 12
Excursion 3: Stokes Point .................................................................................... 17
Excursion 4: Netherby Point - Currie Harbour .................................................... 25
Excursion 5: Cape Wickham ............................................................................... 31
Excursion 6: Naracoopa-Fraser Bluff .................................................................. 37
Excursion 7: Grassy Open-Cut Mine ................................................................... 42
Appendix 1: Glossary .......................................................................................... 46
Appendix 2: Table of GDA co-ordinates of excursion stops ............................... 50
Note: Most of the excursions involve visits to coastal outcrops. Times of high tide
should be avoided, and a wary eye kept on wind and wave conditions. Permission for
access may need to be gained from landowners. Maps for most of the excursions are
included. GDA metric co-ordinates for the excursion stops are shown in Appendix 2.
Excursion 1: City of Melbourne Bay to Cottons Beach:
Glacial tillite, shale, volcanic rocks and of the Grassy Group, of CryogenianEdiacaran age (~640 – 580 m.y.)
Park in the car park at the end of Skipworths Road, at City of Melbourne Bay (Fig.
26). You will see outcrops of diamictite belonging to the Cottons Breccia on the
grassy slope immediately west of the carpark, but save your energy for the better
outcrops on the coast. Go through the gates to the beach and then to your right along
the beach.
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Fig. 26: Geological map of City of Melbourne Bay area, showing stops for Excursions 1 and 2.
Stop 1: About 200 m along the beach, you will reach a promontory on S side of City
of Melbourne Bay consisting of a large outcrop of red shale belonging to the Yarra
Creek Shale (Fig. 27). Bedding (the original sedimentary layering) dips east here and
throughout this area, so this formation overlies (is younger than) the diamictite
(Cottons Breccia) next to the car park, and is older than the volcanic rocks on the next
point to the east. This shale (or fissile mudstone) originated as mud that slowly
accumulated on the sea floor some time after melting of ‘Snowball Earth’. The
seabed was probably quite deep, as there is no evidence in the shale of currents caused
by waves, storms or tides. The red colour (Fig. 28) is due to a small amount of
oxidised iron, in the form of very tiny, disseminated grains of hematite. It is thought
that oxygen levels in the atmosphere reached near-modern levels for the first time
about when this rock was deposited (in the Ediacaran Period). Oxygen dissolved in
the sea would also have been high and hence, sea-floor sediments more commonly
reddish and oxidised. The rock splits along parallel steeply-dipping planes, at an
angle to bedding (Fig. 28). This ‘cleavage’ results from a mild phase of deformation
(east-west compression) in probably Cambrian times. This deformation is probably
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also the cause of the eastward dip (tilt) of the bedding along the east coast of King
Island. There are three intrusions of basalt in the shale here. One is on the west side
of the point, next to the beach. It is pale green in colour, and it is hard to distinguish
from the shale, because the shale for 1-2 m on either side of it is also a pale greenish
colour. This colour change in the shale appears to be a contact metamorphic effect,
caused by the heat from the intrusion. This intrusion, only 1-2 metres wide, is parallel
to bedding here, but follow it north for 40 m and you will find it curves to the east,
cutting across the bedding in the shale. The other two intrusions, on the east side of
the point, are sills (i.e. parallel to bedding), much wider (10 - 20 m) and partly
covered by the sandy beach. The red shale changes to pale green adjacent to these
sills too. These intrusions are similar in their chemical composition to the City of
Melbourne Volcanics which overlie the Yarra Creek Shale (see Stop 2 & 3), and must
have been emplaced at the same time as those volcanics were being erupted.
Fig. 27: Looking south across City of Melbourne Bay, showing locations of Stops 1, 2 and 8.
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Fig. 28: Red shale of the Yarra Creek Shale at Stop 1, with bedding dipping at about 45 deg east, and
cleavage dipping about 70 deg east. The cleavage was caused by a phase of mild compression in
probably Cambrian times.
Stop 2: About 100 m across a sandy beach brings us to the next headland. Rather
poorly exposed amongst the shoreline boulders, can be seen the upward (eastward)
transition from Yarra Creek Shale to the City of Melbourne Volcanics, the first of
three volcanic formations. Firstly, you see Yarra Creek Shale with small basaltic
intrusions with highly irregular margins, which shows that the sediment was still soft
when volcanism began. There are also volcanic breccias of angular basalt fragments
with ‘jigsaw-fit’ margins, set in mudstone – the result of explosive interaction
between hot magma and water-saturated mud on the seafloor (Fig. 29). Then, further
east, there is volcanic breccia at the base of the City of Melbourne Volcanics,
composed of angular fragments of basalt, set in finer (smaller) volcanic fragments
(ash); this is the result of continuing, explosive, probably submarine eruption. The
breccia becomes finer upward (i.e. fragments become smaller) over ~30 m, then
follows a layer of green laminated sandstone, about 1 m thick (Fig. 30), abruptly
overlain by pillow lava, which here forms the upper part of the City Of Melbourne
Volcanics. The pillow lava is an exceptionally well preserved example of its type
(Fig. 18 (Part 1)). It results from quieter (non-explosive) extrusion of lava under
water. Hot magma entering the seawater instantly cools to form a skin; continued
magma pressure expands the pillow much like a balloon, until the thickening skin
prevents further expansion and the magma breaks out to form another pillow. Note the
chilled margins - fine-grained, almost glassy rims, about 10 mm thick, of the pillows;
and the concentric zone of elongate vesicles (gas bubbles). Note that the vesicles are
radially elongated, i.e. in a direction at right angles to the pillow margins. Perhaps the
gas bubbles nucleated on a retreating solidification front. A similar phenomenon can
sometimes be seen in ice cubes.
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Fig. 29: Volcanic breccia with angular fragments of dark basalt in a pale yellow-brown mudstone
matrix, produced by rapid chilling and fracturing of lava encountering water-saturated mud, at Stop 2.
Fig. 30: Thinly bedded green sandstone overlain by pillow lava, in the City of Melbourne Volcanics,
near Stop 2.
Stop 3: To avoid an arduous clamber around the rocky headland, take a short cut
inland (600 m) to the southeast corner of the paddocks, where a rough track leads
through a belt of scrub southwards back to the coast. Once back amongst the coastal
exposures, you will walk about 400 m south through more of the City of Melbourne
Volcanics. There are some beautiful wave-washed exposures of the same volcanic
breccia seen east of Stop 2. There are some slight differences from Stop 2: here, the
basal few metres of the overlying lava is massive, though pillows appear higher up
(further east). There is good exposure of the top of the Yarra Creek Shale, with small
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irregular intrusions of basalt, in a landward embayment of the coastal cliff. A little
further, very nice examples of these irregular intrusions, of pale green basalt in red
mudstone, can be seen at the northern end of Cottons Beach (Fig. 31).
Fig. 31: Highly irregular intrusions of basalt (pale green) in red Yarra Creek Shale, at the northern end
of Cottons Beach. The mud must have been still soft at the time the basalt intruded.
Stop 4: Walk along the beach to the mouth of Cottons Creek, the beginning of
extensive coastal exposure of Cottons Breccia. This stop and the next one encompass
the most varied and interesting succession of rocktypes seen in the Cottons Breccia,
which formed during the ‘Snowball Earth’ glaciation at the end of the Cryogenian
period (see Part 1). The upper part of the Cottons Breccia here (the part closest to the
sea) is the prominent outcrops of bedded reddish sandy conglomerate and pebbly
sandstone on the left, looking south. This conglomerate is unusual for the Cottons
Breccia, and it may have been deposited by rivers, fed by melting glaciers (Fig. 32).
A narrow (~ 1 m) feldspar-porphyry dyke intrudes near the base (landward side) of
this conglomerate (Fig. 33). It probably intruded at the same time as the third of the
volcanic formations was being erupted (the Grahams Road Volcanics), as its
composition is similar. At the mouth of Cottons Creek, the conglomerate is underlain
by greenish sandstone (on the right, looking south: Fig. 34 388). Parts of the outcrop
show honeycomb weathering (Fig. 35 376), and there are a few thin ironstone beds.
Honeycomb weathering, common in coastal environments, is caused by cycles of
wetting and drying, and growth of salt crystals that prise apart the grains of the rock.
Once a depression starts to form, its floor will weather away faster than its drier,
upraised edges. A thin section of this green sandstone, under the microscope, shows it
to be made up of tiny pieces of dark green altered volcanic glass with ragged and
pointy edges, which indicate that originally they were hot glassy fragments ejected
into the air from a volcano. We don’t know where the volcano was, but such fine
volcanic ash can be carried hundreds of kilometres by wind. The volcanic ash must
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have fallen onto the sea or in a meltwater lake, sunk and accumulated on the bottom.
Rare isolated pebbles in the sandstone (seen on the way to Stop 5) suggest there were
floating icebergs that periodically dropped pebbles into this body of water as they
melted.
Fig. 32: Well-bedded conglomerate and pebbly sandstone, upper part of Cottons Breccia at Cottons
Beach (Stop 4).
Fig. 33: Dyke seen at Stop 4, with pale crystals of feldspar in a fine-grained groundmass.
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Fig. 34: Greenish sandstone, actually a volcanic ash deposit, forming part of the Cottons Breccia at
Stop 4.
Fig. 35: Honeycomb weathering in the sandstone at Stop 4.
Stop 5: Walk over the small headland to next beach to the south. Walk across the
beach (~100 m), bearing right (southwesterly) towards outcrops on the landward side
where a narrow creek bed disappears into the scrub. Outcrops around here,
stratigraphically well below the green sandstone, are massive (un-bedded), grey
diamictite, rich in carbonate rock fragments, that is the typical rocktype in the Cottons
Breccia (Fig. 36). Where the outcrop surfaces have been exposed to the weather for a
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long period, many of the carbonate clasts have dissolved away, leaving holes. This
rock is interpreted as a ‘lodgement till’, i.e. deposited directly beneath a glacier. A
few metres to the east, you will find that this diamictite is overlain by a laminated
(thinly layered) calcareous siltstone crowded with dropstones (Fig. 37). The
laminated sediment must have been deposited under water rather than by ice. The ice
sheet or glacier must have retreated, allowing a wedge of water to intercede between
the melting ice and the deposit. The ice dropped debris into the laminated sediments
accumulating on the bottom. This laminated rock containing dropstones is strong
evidence for the involvement of ice, and cannot have been deposited by a debris flow,
which has been proposed as an alternate mode of deposition for the Cottons Breccia.
Higher up (stratigraphically, going east), there is more diamictite, some with a
diffusely layered matrix (Fig. 38: a feature that also supports glacial, rather than
debris flow deposition), and one or two more laminated beds with dropstones. This
sequence of rocks, about 50 m thick in all, is then overlain by the green volcanic
sandstone first encountered at Stop 4. (You can also find within this sequence, the
southward continuation of the feldspar-bearing dyke seen at Stop 4). The rock types
that form the clasts (fragments) in this succession (and in the Cottons Breccia
generally) are a point of interest, as they tell us something about the terrain that the
glaciers were moving over and eroding before they dropped their load of sediment
here. The carbonate rocktypes, limestone and dolostone, are most abundant; these are
fine-grained, vary from dark grey to white, and are recessive on exposed outcrop
surfaces owing to the fact that they dissolve slowly in rainwater. Some of the clasts
are oolitic carbonate – a variety with small (2 mm) concentrically structured, spherical
grains (Fig. 39). Similar grains can be seen forming on the sea floor today, in some
tropical, shallow seas. There are common clasts of hard siliceous siltstone and shale,
and rare, red chert. The siltstone and shale are identical in appearance to the Fraser
Formation that underlies the Grassy Group. However the carbonate rocks and the
chert are unknown as bedrock on King Island, which indicates a more distant, though
unknown, source area, perhaps now in Antarctica which lay west of King Island at
that time.
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Fig. 36: Massive, carbonate-rich grey diamictite at Stop 5.
Fig. 37: Laminated siltstone with dropstones, abruptly overying massive grey diamictite, Stop 5.
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Fig. 38: Diamictite with diffusely layered matrix, Stop 5.
Fig. 39: A clast of oolitic carbonate in the Cottons Breccia, Stop 5.
Stop 6: Return north along coast to where the rough track reaches the coast (Stop 3)
but turn right to head eastward over the outcrop of pillow lava belonging to the City
of Melbourne Vocanics. After about 30 m you will encounter the second of the three
volcanic formations, called the Shower Droplet Volcanics, overlying the pillow lava
of the City of Melbourne Volcanics. The Shower Droplet Volcanics here consist of a
few metres of volcanic breccia (Fig. 17 (Part 1)), overlain by a great thickness of
pillow lava. These pillows are distinct from the ones in the City of Melbourne Bay
Volcanics, they are more irregular, and lack the marginal zone of vesicles. Some have
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large central voids caused by remnant lava flowing out of a largely solidified pillow
(Fig. 40). The Shower Droplet Volcanics are composed of an unusual magnesiumrich, dense type of lava known as a picrite. This lava would have erupted at a higher
temperature, and been less viscous (runnier) than the City of Melbourne Bay
Volcanics. Because of this, some of the flows (not seen here) are very thin (less than
100 mm). Another difference you may notice is that the outcrops of the Shower
Droplet Volcanics are greener. The pale greenish colour of these volcanic rocks is not
their original colour (which would have been black), but has come about by alteration
of most of their original minerals (olivine, pyroxene, plagioclase) to secondary
minerals such as chlorite and epidote, probably soon after they were erupted.
Fig. 40: Pillow lava in the Shower Droplet Volcanics, near Stop 6 (Excursion 1). Note the central void
in the pillow just below the hammer.
Excursion 2: City of Melbourne Bay to ‘the Gut’ and beyond
Shale with intrusive rocks, glacial tillite, and cap carbonate of the Grassy Group, and
faulting, of Cryogenian-Ediacaran age (~640 – 580 m.y.); semi-fossil logs and leaves
in Pleistocene sand (120,000 y.).
From the car park at the end of Skipworths Road, proceed to the beach and turn
northwards, crossing the mouth of Yarra Creek. The stop numbers follow on from
Excursion 1 (Fig. 26).
Stop 7: Immediately north of the mouth of Yarra Creek, a cliff several metres high has
been cut into semi-consolidated dark sand, topped by dunes. The black, peaty sand
forming the lowest metre or so of the cliff has layers that dip gently landward, and at
the top of this unit, the layers have been truncated by the overlying, horizontally
bedded brown sand above. Rounded cobbles are scattered along the contact (Fig. 41).
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This surface is an unconformity, and reflects a time-gap between the sediments below
and above. Coaly black semi-fossilised plant fragments including wood and leaves
are common in the lower peaty sand. (NB Please don’t excavate this site, as it is
scientifically important and limited in extent). Studies have shown the presence of
rainforest species including myrtle (Nothofagus cunninghamii) and celery-top pine
(Phyllocladus aspleniifolius), no longer present on King Island, as well as numerous
species of beetle. This sand has been dated using the thermoluminescence method,
to about 120,000 years old. This peaty sand is thought to have accumulated in a backdune swamp, fed by Yarra Creek, in the early stages of the last interglacial phase.
Sea-level was similar to today’s when this deposit was formed, and the plants and
beetles indicate that the climate was a bit colder and wetter. (At a later time in the last
interglacial, sea levels rose to 4-6 m higher than today). The unconformity shows that
the peaty sand was compacted and eroded before the brown sand above it was
deposited. The brown sand has been dated by thermoluminescence at about 75,000
years. This is windblown sand, deposited in the early part of the last glacial period,
when sea levels were lower than today.
Fig. 41: The sandy cliff-face just north of Yarra Creek (Stop 7), showing black peaty sand with plant
remains (120,000 y), unconformably overlain by brown windblown sand (75,000 y).
Stop 8: Walk along the beach (~150 m) to the second of two small rocky
promontories on the north side of City of Melbourne Bay. Here a sill belonging to the
Grimes Intrusive Suite intrudes the lower part of the Yarra Creek Shale. The sill,
about 5 m thick, is of a very fine-grained, very pale grey rock, rather featureless
except for small (~5 mm) spherical vesicles that appear as pockmarks on the
weathered outcrop surface. (On freshly broken surfaces, the vesicles are filled with
greenish black chlorite). The fine grain-size and vesicles show that this intrusion
cooled rapidly at shallow depths (probably less than 100 m, not deep in the crust).
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The Grimes Intrusive Suite is therefore only slightly younger (in geological terms)
than the Yarra Creek Shale. The Grimes Intrusive Suite has been dated (using the UPb technique on zircons) at 575 ± 3 m.y., which puts it in the middle Ediacaran
Period. The Yarra Creek Shale to the east (overlying) the Grimes Intrusive consists of
alternating pale yellow-brown and black shale beds (Fig. 42). These beds were laid
down as mud on the sea floor, in the early Ediacaran Period, not long after the melting
of the ‘Snowball Earth’ glaciation, thought to be 636 million years ago (see next
stop). Thin sections of the black shale beds, examined under the microscope, indicate
that the black, carbonaceous matter within them was formed by sea-floor microbial
mats, probably of filamentous bacteria. No other signs of fossil life are visible in
these rocks, which are somewhat older than the famous Ediacara fossils of South
Australia and elsewhere. The upper, red part of the Yarra Creek Shale can be seen
overlying the lower, non-red part, in the most seaward outcrops here.
Fig. 42: Alternating pale yellow-brown and black shale beds, forming part of the Yarra Creek Shale,
about 100 m N of Stop 8.
Take a short cut across the paddocks to ‘the Gut’, about 1 km to the north, bypassing
exposures of mainly picrite (Shower Droplet Volcanics) along the coast. You will be
walking along a terrace, about 10 m above sea level, between the coast and the main
escarpment. This feature, also seen at Naracoopa, Cottons Flats and many other
places, was produced by marine erosion (wave action) at times of higher sea level,
probably during previous interglacial phases of the Pleistocene Epoch.
Stop 9: ‘The Gut’ is a small inlet that came about by erosion along a fault. The fault
has moved the rocks on the northern side, eastwards, so that in crossing the fault you
go from the Shower Droplet Volcanics to the Cottons Breccia, lower in the
stratigraphic sequence. Then walk eastward through good exposure of the upper part
of the Cottons Breccia, somewhat different to the section seen at Cottons Beach
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(Excursion 1, Stop 4 & 5). The diamictite here contains one or two very large (up to 3
m) boulders of carbonate – the largest clasts seen in the Cottons Breccia, and evidence
of the power of moving ice. It may be necessary to deviate inland for 50 m or so to
avoid some particularly jagged diamictite outcrops. The matrix in the upper part of
the formation is a red mudstone. A metre or two below the top of the formation is a
conglomerate bed in which the rounded boulders are in contact with each other (Fig.
43). This suggests some reworking by glacial meltwater, which appears to have
carried away the muddy matrix of the uppermost part of the diamictite. A faintly
laminated, fine-grained mauve sandstone bed marks the very top of the Cottons
Breccia here. Zircons from this bed have been dated, using a very precise technique,
at 636 m.y. There are good reasons to suppose that these were ‘fresh’ zircons,derived
from volcanic activity that was happening at more or less the same time as the mauve
sandstone bed was deposited, rather than from older rocks. Therefore the end of the
‘Snowball Earth’ glaciation on King Island happened 636 m.y. ago. This date also
marks the end of the Cryogenian Period, and the beginning of the Ediacaran Period.
The date is in good agreement with dates from Namibia and China. This spot is only
the third place in the world where this important event has been dated.
Fig. 43: Conglomerate bed near the top of the Cottons Breccia, at Stop 8. The cap carbonate is the
yellowish outcrop in the top left. The round holes in some of the clasts are where samples have been
taken for paleomagnetic analysis using a portable drill.
The ‘cap carbonate’ overlies the Cottons Breccia here, and is about 5 m thick. (This
formation has been named the Cumberland Creek Dolostone on King Island.) ‘Cap
carbonate’ is a general term for the unusual carbonate rocks (limestone, dolostone)
that overlie Cryogenian glacial deposits all over the world. The ‘Snowball Earth’
theory, although not accepted by all scientists, offers an explanation for the existence
of the cap carbonates and some of their features, and their close association with
underlying glacial rocks (see Part 1). The thin layering and pale yellow colour are
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typical. There are several examples here of the so-called ‘teepoids’, upward
culminations in the layering that resemble sharp-crested folds (Fig. 44). The teepoids
here are in N-S alignment, as are those in the cap carbonate in South Australia (the
nearest place to King Island where this cap carbonate is exposed again). How these
teepoids formed is still a matter of debate, but water movement caused by wave action
impinging on the sea floor, and cementation (hardening) of the sediments on the sea
floor, seem to be important factors. The cap carbonates were deposited under rising
seas, immediately after the rapid melting of global (or near-global) ice cover.
According to Snowball Earth theory, the high levels of atmospheric CO2 that brought
about the end of the ice age were sequestered into the carbonate sediments on the sea
floor, by way of continental weathering, dissolution and precipitation in the oceans.
The juxtaposition of the glacial deposits and overlying cap carbonate beds reflects one
of the most extreme climate transitions in Earth history. If tide is low, at least 50 m of
Yarra Creek Shale can be seen to overlie the cap carbonate here.
Fig. 44: Sharp-crested fold (or ‘teepoid’) in the cap carbonate at Stop 8, accentuated by white dashed
line.
Stop 9: Walk around the little bay to north – near the northern end, you will cross over
another fault that (again) displaces rocks on the north side, to the east. There is a
small outcrop of the uppermost part of the Cottons Breccia on the beach. On the
eastern side of this, the cap carbonate is only ~20 cm thick. Walk eastwards across
the Yarra Creek Shale, which is here only about 15 m thick. The Yarra Creek Shale is
overlain by about 5 m of coarse volcanic sandstone belonging to the lower part of the
City of Melbourne Volcanics (this unit was 30 m thick at City of Melbourne Bay
(stop 2 of Excursion 1). This is then overlain by massive (non-pillow) lava of the City
of Melbourne Volcanics. Why are these units (- the cap carbonate, the Yarra Creek
Shale and the lower breccia of the City of Melbourne Volcanics) so much thinner here
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than elsewhere? We surmise that the fault just crossed was active (with a south-sidedown sense of movement) while these units were being deposited. More room was
available for their accumulation on the south side of the fault. The fault movements
would have been accompanied by earthquakes. Slumping and breccia beds can be
seen here in the Yarra Creek Shale, consistent with earthquake activity during
deposition. The active fault movement during deposition of these rocks shows the
crust was under tension, and continued rifting (pulling-apart) of the crust led directly
to the vast volcanic outpourings of the upper Grassy Group.
Excursion 3: Stokes Point
Metamorphic rocks, turbidites, folds and dolerite dykes in the Surprise Bay
Formation (Mesoproterozoic, ~1300 m.y. old); sand dunes, rhizoliths and tufa
(Holocene, less than 10,000 y).
Follow the South Road, and signs to Stokes Point. 400 m past the Seal Rocks Road
turnoff, a gate is reached with signs indicating private land belonging to the Surprise
Bay Pastoral Company. Go through the gate and bear right to get onto the Stokes
Point Road. From there the road winds through grass-covered sand dunes to reach the
coast again at Surprise Bay (Fig. 45).
Fig. 45: Geological map of Stokes Point area, showing Stops 1-4 of Excursion 3.
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Stop 1: Where the road reaches the coast, just before another gate, and 50 m before a
‘Stokes Point’ sign, pull off to the right down a short kelp track. The extensive
coastal outcrops immediately in front of you are of grey, banded schist (a
metamorphosed mudstone), with the metamorphic minerals, garnet and andalusite.
This is the middle part of the Surprise Bay Formation. The banding (alternating
shades of grey) in the schist is the original thin sedimentary layering, which is tilted
over steeply to the west. The schist consists mainly of small platy crystals of mica
(muscovite and biotite) that are in parallel alignment, so that the rock splits easily (it
has a cleavage) and has shiny cleavage surfaces. The garnet is hard and resistant to
weathering, and easily seen as scattered tiny (1 mm), prominent, roughly spherical,
pinkish grains. (Garnet’s typical habit is to form roughly spherical, multifaceted
crystals). The andalusite has all been altered (‘retrogressed’) to mica, but the
distinctive columnar crystal shapes, square or rhombic in cross-section, can still be
seen (Fig. 46). The mica, garnet and andalusite formed under conditions of high
temperature and pressure at many kilometres depth in the crust, about 1290 m.y. ago.
Before that, the rock was a mudstone, originally deposited as clay and silt on the floor
of the ocean. In spite of all it has been through, you can still see the original
sedimentary layering in this rock. There are some thin beds of fine-grained, lightercoloured quartzite, lacking the metamorphic minerals, that were originally finegrained sandstone and siltstone of predominantly quartz grains. Note that the
cleavage (splitting direction) in the schist is not parallel to bedding, but at an angle to
it. In general, cleavage has a particular geometric relationship to the folds that formed
in the same deformation event, so its orientation is important in working out the large
scale structure of an area. In these outcrops, small folds can be seen here and there,
which formed when the rocks were compressed at about the same time the
metamorphism happened.
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Fig. 46: Schist of the Surprise Bay Formation at Stop 1, with the metamorphic minerals garnet (small
dark reddish, rounded grains) and andalusite (larger, dark grey rhombic shapes). The andalusite
crystals here have actually changed back into mica and quartz while still preserving their shapes.
Stop 2: About 2.6 km further south along the track, turn right down the track
signposted ‘Sealers Wall’. The track ends next to the stone wall only 60 m further on.
Climb onto the prominent outcrop where the stone wall ends, and head seaward
(southward) a bit. You will see several thick (1 – 6 m) layers of massive brownishgrey rock, in places with spheroidal weathering, alternating with thinner layers (0.5 –
2 m) of dark grey schist (Fig. 47). Spheroidal weathering happens when the corners
of rectangular joint blocks become rounded, because the angular edges provide more
surface area for decomposition by weathering. Edges and especially corners of an
angular block weather faster than flatter surfaces. The end result of this process is the
rounded bouldery shapes seen here. The rock making up these thicker layers is
something of a mystery. It resembles a dolerite, and contains an abundant dark
mineral (amphibole) as crystals 2-3 mm long. Chemical analyses of the mystery
rocks are more like dolerite than rocks of sedimentary origin. These rocks have
previously been interpreted as dolerite sills. However we are now more inclined to
think they are in fact of sedimentary origin, with an added component in the form of
volcanic ash, which must have been incorporated into the sediments at the time they
were deposited. One indication in support of this idea is that on the western side of
the outcrop, a gradation (rather than a sharp contact) can be seen between the schist
and the amphibole-bearing mystery rock. The amphibole did not crystallise from a
magma, then, but is a metamorphic mineral that grew in the rock under conditions of
high temperature and pressure, like the garnet and andalusite of the last stop. Just to
make matters a little more complicated, real dolerite intrusions are also present here.
If you climb to the highest point of the outcrop, you will be standing on a fine-grained
dolerite dyke about 1.2 m wide that cuts obliquely across the layering in the enclosing
rocks. Narrow offshoots (a few cm wide) extend out from this dyke into the enclosing
rocks. This dyke is clearly an intrusive igneous rock because it cuts across the
sedimentary layering (bedding) of the ‘mystery rock’ and schist.
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Fig. 47: Stop 2, view looking south. Thick layers of a spheroidally-weathering rock of uncertain
origin (‘mystery rock’), with thinner conformable layers of schist.
Stop 3 (020): Back on the Stokes Point track, about 500 m further: in the coastal
outcrops here, thick, prominent beds of fine-grained, pale sandstone alternate with
grey siltstone and schist. The sandstone beds are probably turbidites. The bedding
dips steeply west, but some of the thinner sandstone beds here show cross-lamination
indicating that the beds are overturned, i.e. the original “up” was to the east (the rocks
have been rotated more than 90 degrees from the original horizontal). This is the case
all along this stretch of coast, for several kilometres to the north-west. The
overturning of such a huge mass of rock gives an idea of the immense tectonic forces
involved in the mountain-building event of 1290 m.y. ago. Note the large, rounded
voids distributed along some of thicker, more massive sandstone beds (Fig. 48).
These voids were previously filled with calcium carbonate (limestone) which has
mostly dissolved away. In some, there is still a bit of limestone remaining. This
limestone formed as concretions within the sandstone, soon after the sand was
deposited on the sea floor. Calcium carbonate was precipitated into the spaces
between the sand grains, while the spaces were still filled with seawater. Looking
closely at this limestone and in the harder rocks of the outer parts of the concretions
will reveal tiny (2 mm) flecks of a dark green metamorphic mineral, a variety of
amphibole (Fig. 49). Metamorphism causes different minerals to form in different
rock types. Particular minerals are usually characteristic of certain limited conditions
of temperature and pressure of formation. The combination of metamorphic minerals
in the Surprise Bay Formation has been interpreted to infer that the metamorphism
happened at temperatures of between 470° and 580°, and pressures of 1 to 3 kilobars.
Such conditions would be found at depths of 4 - 10 km in the crust.
20
Fig. 48: Large, rounded voids along the middle of a thick sandstone bed in the Surprise Bay Formation
at Stop 3. The voids were left by the weathering out of calcareous concretions.
Fig. 49: A small amount of amphibole-calcite rock remains in the core of this concretion, under the
pen.
Stop 4: This is the location of the hinge of an enormous, tight fold. All along the coast
since stop 1, and indeed almost as far as Surprise Point 5 km to the NW, bedding dips
to the west and is overturned. From about here onwards, at least as far as the end of
Stokes Point, bedding still dips to the west but is right-way-up. We can sometimes
tell what way up the beds are, from certain sedimentary structures such as crosslamination (see Part 1 and Excursion 4, stop 5). (On geological maps, different
21
symbols are used to show the way-up or facing of the beds, either right way up,
overturned, or unknown). The fold hinge here is a syncline (a downward-pointing
fold, as distinct from an anticline or upward-pointing fold). If you have a look at the
outcrops here you will find that there is not a simple, single large fold, but a more
complex zone of several smaller folds, spread over about 50 m, whose overall effect is
that of a large tight syncline (Fig. 50).
Fig. 50: Tight fold (syncline) in Surprise Bay Formation at Stop 4, accentuated by the white dashed
line.
Stop 5: On the way back up South Road, turn left on Seal Rocks Road. After 2.8 km
there is a small car park on the left; a walking track (600 m) leads to the ‘Calcified
Forest’. A dune blowout here has revealed some spectacular examples of rhizoliths,
that resemble the fossilised branches of a buried forest (Fig. 51). They are actually
encrustations of calcified sand, adjacent to thin roots. The rhizoliths have hollow
centres, or sometimes only a narrow (1 mm) tubular hole down their centres where the
actual root was located before rotting away. The outer surface of the rhizoliths is very
irregular, being controlled by nearly random fluctuations in the distance of chemical
transport and reaction surrounding an individual root. Parts of the sand dunes have
resisted erosion, having been hardened by small amounts of calcium carbonate
cementing the sand grains together. The dunes here are perched 50 m above sea level,
and may be of early Holocene age (about 5,000 – 10,000 years old).
22
Fig. 51: The ‘calcified forest’.
Stop 6: On the way back up South Road, north of Pearshape, turn left down Pearsons
Lane. Gates may have to be opened and closed. About 800 m south of where the
track reaches the coast, you will see a prominent outcrop that would be almost an
island at high tide, with a pyramidal cairn (the Cataraqui Memorial) on its landward
side. (Fig. 52). The rock making up this outcrop and the area around it, is the
Surprise Bay Formation again, with thick, prominent beds of fine-grained sandstone
like Stop 3, although here the beds dip west and are right-way-up (not overturned as at
Stop 3). Metamorphic garnet is present in the schist layers between the sandstone
beds. About 50 m seaward of the Cataraqui Memorial, there is a second prominent
outcrop, in which a dyke, cutting vertically across the dipping beds, is well displayed
(Fig. 53). The dyke, viewed close up, is a dark grey, fine-grained rock sprinkled with
elongate rectangular pale feldspar crystals (Fig. 54). A very similar dyke – probably a
continuation of this one – can be seen on the coast just south of Ettrick River, and
another at Netherby Bay where it intrudes the Loorana Granite. So we know they are
younger than the Loorana Granite (748 m.y.), but other than that we have little idea
how old these feldspar-bearing dykes are.
23
Fig. 52: Location of Stop 6, seaward of the Cataraqui Memorial.
Fig. 53: Stop 6: dark dolerite dyke (right) intruding west-dipping sandstone beds of the Surprise Bay
Formatiion (left).
24
Fig. 54: Close-up of the dolerite, showing pale yellow feldspar crystals in a dark fine-grained
groundmass.
Stop 7: Boggy Creek tufa terraces. The track ends about 200 m further south. The
tufa terraces are a walk of another 800 m to the south. Groundwater that has migrated
through the calcareous dune sands is laden with dissolved calcium carbonate. The
groundwater emerges here as springs, and the calcium carbonate is deposited as tufa,
and here has built several small rimmed terraces (Fig. 22 (Part 1)).
Excursion 4: Netherby Point - Currie Harbour
Sedimentary rocks of the Surprise Bay Formation (Mesoproterozoic, 1300 m.y.);
Loorana Granite (Cryogenian, 748 m.y.); fault zone; granitic dykes and sills.
From central Currie, turn south on Netherby Road and after 1 km, turn right on the
kelp track (just after the kelp factory) (Fig. 55). After 500 m, the track reaches the
coast at a plaque commemorating the wreck of the Netherby in 1866.
25
Fig. 55: Map of stops, Excursion 4.
Stop 1: At the Netherby plaque, turn left and walk down to the beach, then along the
cobbly spit that leads to an outcrop on the south side of the small bay (Fig. 56). The
outcrop around here belongs to the Loorana Granite, which has been dated at 748
million years old. It is a typical granite, a rather uniform, massive, pale grey rock
consisting of crystals of whitish feldspar, grey quartz, and a little black biotite. The
crystals, 1-3 mm wide, are much larger than those seen in volcanic rocks such as
basalt, a result of slow cooling and crystallisation at a depth of some kilometres down
in the crust. In this outcrop, you will find a dyke (narrow vertical igneous intrusion)
of a rock type known as feldspar porphyry, within the Loorana Granite. The dyke is
about 1 m wide and can be followed for about 40 m in a southwesterly direction (Fig.
57). It is a very fine-grained, pale grey rock with scattered white feldspar crystals up
to about 8 mm long. It is fine-grained because it cooled rapidly against the
surrounding Loorana Granite which must itself have been relatively cool at the time
the intrusion happenned. The feldspar crystals were suspended in the magma when it
was intruded. A ‘porphyry’ is a fine-grained igneous rock with larger crystals (in this
case, feldspar) scattered through it. This porphyry dyke has been radiometrically
dated at 350 million years, the same age as the Sandblow Granite of the Grassy area
(see Excursion 7). It is the only known representative in western King Island, of the
Carboniferous granitic intrusions that are so important in the east of the island.
26
Fig. 56: Location of Stop 1, seaward of the Netherby memorial plaque.
Fig. 57: Feldspar porphyry dyke, 350 m.y. old, intruding Loorana Granite (748 m.y. old), at Stop 1.
The vertical dyke contacts are indicated by the white arrows.
Stop 2: Go back to the road and proceed along the coast to the N for 400 m. Head
across the narrow isthmus towards Netherby Point. The outcrops around here belong
to the Surprise Bay Formation. You have crossed a major fault that separates these
rocks from the Loorana Granite of the last stop. (We will see this fault at Stop 4). In
the middle of the isthmus, you will see another dyke, here intruding the Surprise Bay
Formation. It looks identical to the Carboniferous dyke seen at Stop 1, and has a
27
similar trend (orientation). It is quite likely to be a continuation of the same dyke,
offset by movement along the fault we have just crossed. If it is the same dyke, there
must have been about 800 m of horizontal, west-side-north movement (and an
unknown amount of vertical movement) on the fault, in the time since the dyke was
intruded 350 m.y. ago. A little further west you will see that the outcrops of Surprise
Bay Formation making up Netherby Point are of laminated, grey-green siliceous
siltstone, in which the bedding dips steeply west (Fig. 58). An identical rock-type
makes up part of the Fraser Formation in eastern King Island (see Excursion 7), and
this is part of the evidence behind the suggestion that the two formations are in fact
one and the same.
Fig. 58: Laminated siliceous siltstone at Netherby Point.
Stop 3 (003): Proceed north along the coast track for another 600 m. You will see
some striped, pale and dark grey outcrops about 50 m seaward (Fig. 59). The paler
layers are sandstone, the darker ones mudstone. The Surprise Bay Formation seen in
this excursion is only weakly metamorphosed, unlike the schists of the Cape
Wickham and Stokes Point areas. The reason for this is unclear. The banding in these
outcrops is the original sedimentary layering, and dips steeply west. The beds are
offset by small faults here and there (Fig. 60). The sandstone beds are turbidites –
that is, they were deposited rapidly by turbidity currents in deep water. There are
some subtle clues here that tell us which way up the beds are. Within some of the
sandstone beds, there is cross-bedding (inclined lamination, truncated by overlying
lamination). This, and the sharp bases and gradational tops of some of the turbidite
sandstone beds, show that they are right-way-up, i.e. get younger to the west.
28
Fig. 59: Sandstone turbidite beds in the Surprise Bay Formation at Stop 3, dipping and younging to the
west.
Fig. 60: Small fault, offsetting bedding by about 15 cm, at Stop 3.
Stop 4 (415): Go along the coast track another 800 m, to the south side of the next
small bay north of Stingray Bay. A 3 m wide sill (intrusion parallel to bedding) in the
Surprise Bay Formation is well exposed here. It is a brownish rock contrasting with
the dark grey mudstone either side (Fig. 61). It is porphyry, with small (1 mm)
crystals of quartz and feldspar in a fine-grained groundmass. The rough surface of the
outcrop is due to the small, hard, prominent quartz grains. Marginal zones 150 mm
29
wide (adjacent to the mudstone) lack the small quartz and feldspar crystals,
presumably because the magma cooled too quickly, against the surrounding
mudstone, for the quartz and feldspar crystals to form. Similar sills nearby have been
dated at about 780 m.y., from the uranium-lead ratios in the zircons within them.
These are the oldest dated igneous rocks on King Island, although some of the dolerite
and amphibolite intrusions, which cannot be dated, are probably much older. About
10 m to the west, there is another smaller (1 m wide) sill that looks superficially
similar, but which lacks the quartz grains of the larger sill. There are some nice
examples of small-scale honeycomb weathering on this sill.
Fig. 61: Porphyry sill, 780 m.y. old (brown), with dark grey mudstone on either side, at Stop 4.
Stop 5: (308): The bedding in the Surprise Bay Formation here dips steeply west, and
here you can see a few thin (50 mm) beds of white siltstone in predominantly grey,
slaty mudstone. The siltstone beds have well-developed cross-lamination produced
by migrating ripples on the sea floor. The cross-lamination shows that the beds here,
as at Stop 3, right-way-up (they get younger to the west). The form of the ripples
shows the currents moved northwards (Fig. 5 (Part 1)). Similar right-way-up
indicators are seen at several other spots in this coastal tract of Surprise Bay
Formation, between Netherby Point and Three Rivers Bay. From such small clues,
the large scale geological structure of an area can be deciphered.
Stop 6: (293): just east of the breakwater, the major fault is here exposed, between the
Surprise Bay Formation to the west, and the Loorana Granite to the east. The fault
zone is about 40 m wide, and is of a pale grey, fine-grained, very fissile (flaky) rock,
in places with tiny (<1 mm) remnant crystals of quartz and feldspar that suggest that
it consists at least in part, of highly sheared granite. There has probably been a
significant amount of movement on this fault – kilometres perhaps – because in the
30
Surprise Bay Formation to the west, there is little or no sign of contact metamorphism
(heating) that you would otherwise expect from its proximity to the granite. Further
to the east, the rock becomes less fissile, and there is a gradual transition to more-orless undeformed Loorana Granite (similar to Stop 1). As you walk towards Currie
Harbour you will notice several dark, fine-grained intrusions or dykes, 0.4 – 2 m
wide, within the granite. The excursion may be concluded with a cup of tea at the
Boathouse Restaurant.
Excursion 5: Cape Wickham
Metamorphic rocks of the Surprise Bay Formation (Mesoproterozoic, 1300 m.y.);
dykes of dolerite, granite and pegmatite; deformation and contact metamorphism
associated with granite intrusion; Cape Wickham Granite (760 m.y.).
Drive to the car park at the end of the Cape Wickham Road, in the far north of the
island. The lighthouse here was built in 1861, using stone quarried nearby, belonging
to the 760 m.y. old, Cape Wickham Granite.
Walk downhill to the coastal outcrops (Fig. 62). These outcrops are of Surprise Bay
Formation that has been strongly affected (heated and deformed) by the intrusion of
the Cape Wickham Granite, which is found a few hundred metres inland (mainly
covered by sand, but seen at Stop 6).
Fig. 62: Map of stops for Excursion 5, Cape Wickham area.
Stop 1 (1): The rock here is a schist, with very thin, parallel continuous bands or
layers of darker and lighter grey. This is a metamorphic rock, originally thinly
bedded mudstone and siltstone, now composed of quartz, feldspar and mica. The
31
bedding dips steeply west here, and many short, discontinuous veins cut obliquely
across the layering, best seen on vertical, north-facing outcrop surfaces (Fig. 63). The
veins are filled with coarse-grained quartz, feldspar and tourmaline. These minerals
must have been deposited by hot fluids that pervaded the rock at the same time that
the granite was being intruded nearby, as tourmaline is a mineral that typically forms
in the late stages of granite intrusion. The layers in the schist are deflected around
these veins, in a way that indicates that they formed as tensional tears, or ‘tension
gashes’, that opened up by stretching of the rock, which was evidently softened by
heat but still able to be fractured (think of pulling a bit of putty apart). The oblique
angle between the tension gashes and the overall direction of layering indicates that
the rock was also being subjected to shear forces while these structures were being
formed. The Cape Wickham Granite intruded the much older Surprise Bay
Formation here, 760 m.y. ago. The molten granite mass rose and expanded into the
surrounding rocks, softening them with heat almost to their melting point and
shouldering them aside. Expansion or ‘ballooning’ of the intrusion, as it was fed by
more magma from below, probably caused the stretching and shearing of the Surprise
Bay Formation schists seen here.
Fig. 63: Tension gashes (arrowed), oblique to bedding, in banded schist, similar to those at Stop 1.
Stop 2(2): Walking north along the coast, this outcrop shows more of the thinly
layered schist, with tension gashes (like Stop 1) but here with folds, and at least two
granitic dykes (Fig. 64). Examining complex outcrops such as this can reveal the
time-order, in which the tension gashes, folding and dykes formed. There is usually
plenty of scope for argument even (or especially) amongst professionals. It appears
here that the folding happened after the tension gashes, because the latter change their
orientation around the main fold (they have been rotated by the folding). And the two
32
granite dykes intruded the rocks after the folding, one cutting across, and therefore
after, the other. We can infer that all these features formed during the emplacement of
the Cape Wickham Granite. The stretching that caused the tension gashes was
followed by compression (which produced the folding), presumably also a result of
the expansion of the intrusion. The dykes probably represent the ‘last gasp’ of
magmatic activity associated with the Cape Wickham intrusion. A few metres to the
left (east) of Fig. 64, a dolerite sill about 1.2 m wide is cut by a pegmatite vein. The
pegmatite, essentially a very coarse variety of granite, consists of large (several cm)
crystals of feldspar, quartz, tourmaline and muscovite (Fig. 65). A variety of granitic
dykes, from a few cm to a few m wide, can be found along this stretch of coast.
Pegmatites are common, while others are fine-grained quartz and feldspar
(microgranite).
Fig. 64: Outcrop at Stop 2, showing fold (F) and granitic dykes (D1 and D2).
33
Fig. 65: The minerals tourmaline (T), muscovite (M), quartz (Q) and feldspar (F), in a pegmatite dyke
at Stop 2.
Stop 3(27): Following the coast, about 400 m to the north, cross a tumbledown fence
made of thick aluminium cable. About 25 m north of that, in bedding dipping steeply
east, here is a rare example of cross-lamination, indicating that the beds are right-wayup (get younger to the east) (Fig. 66). This outcrop may be difficult to find. It is rare
to find way-up evidence in rocks as metamorphosed and deformed as these, because
sedimentary structures such as cross-lamination are often obscured or destroyed by
metamorphism.
Fig. 66: Cross-lamination in steeply dipping bedding at Stop 3. Up in the photo is to the east.
34
Walking north, you will notice abundant beach cobbles at a level several metres above
the level of present day high tide. These high level coastal gravels are seen in many
places around the King Island coast, and reflect a period of slightly higher sea level,
perhaps in the last interglacial period. Amongst these cobbles, or in the present-day
cobbly beaches lower down, you will see the occasional one of a highly porous,
lightweight rock - pumice (Fig. 67). Pumice is frozen froth from particularly gassy
volcanic eruptions. It is so full of gas bubbles that it floats, and pieces of it regularly
wash up on coasts all around Australia. The pumice can drift on ocean currents for
thousands of kilometres; most of it probably comes from undersea eruptions in the
Tonga-Fiji area. A lot of pumice also came ashore from a 1962 eruption in the South
Sandwich Islands in the south Atlantic.
Fig. 67: A rock that floats: A pumice cobble north of Stop 3.
Stop 4 (80): About 600 m further north, the schist here contains scattered blocky
shapes a few cm long, which formed as the metamorphic mineral andalusite, but
which have changed back (retrogressed) into fine-grained mica while still maintaining
the original crystal shapes of the andalusite (Fig. 46). About 10 m to the west, a 1 m
wide granite dyke intrudes the schist, and a further 10 m to the west, fine-grained
quartzite (originally a fine-grained quartz sandstone) crops out, also part of the
Surprise Bay Formation.
Stop 5 (86): About 200 m further up the coast: Here the schist is intruded by a northtrending dolerite dyke, about 10 m wide, forming a prominent dark grey outcrop on
the eastern side of a small rocky inlet. The dyke cuts across thin granitic veins in the
surrounding rock, that are likely to be associated with the final phases of granite
intrusion, so it is probably younger than the Cape Wickham Granite. It contains pale
yellowish crystals of feldspar up to 7 mm long, set in a darker greenish fine-grained
groundmass. It looks identical to the north-trending dyke seen at Cataraqui Memorial
35
(Stop 6, Excursion 3) and Ettrick Bay – could it be the same dyke that has cut right
across the island? To the east of the dyke (88), the schist contains curious ovoid
bodies, about 50 mm across, of a darker mineral, rimmed by white (Fig. 68). These
are very large examples of ‘metamorphic spots’, caused by the growth of certain hightemperature minerals in contact-metamorphic zones adjacent to intrusions. They
indicate that we are approaching closer to the granite contact - the source of the heat
- as we head east around Cape Wickham.
Fig. 68: Large ‘metamorphic spots’, caused by strong contact metamorphism, at Stop 5.
Stop 6 (101): The contact with the Cape Wickham Granite is encountered in the first
small bay east of Cape Wickham. The contact is north-trending, and there is a zone of
chaotically deformed schist immediately adjacent to the contact, resulting from
movements of the granite mass against the heat-softened metamorphic rocks. The
granite to the east of the contact is a massive, pale grey coarse-grained rock composed
of quartz (grey, translucent), feldspar (whitish) and a little biotite (flaky, black, shiny).
The largest crystals are blocky feldspar up to 40 mm long. They are weakly aligned,
probably resulting from magma flow during crystallisation. With the angle of the light
just right on these feldspar crystals, many can be seen to have an exact division into
two halves (Fig. 69). This is known as ‘simple twinning’ of the crystals and is a
characteristic of this particular type of potassium-rich feldspar, known more precisely
as orthoclase. The Cape Wickham Granite has been dated at 760 m.y., from a sample
collected from the old quarry 800 m SE of the lighthouse.
36
Fig. 69: The Cape Wickham Granite. The white arrows indicate a couple of crystals of orthoclase
feldspar with simple twinning.
Excursion 6: Naracoopa-Fraser Bluff
Sedimentary rocks of the Fraser Formation (Mesoproterozoic?: ~1300 m.y.); volcanic
rocks of upper Grassy Group and dolerite sill intruding lower Grassy Group
(Cryogenian-Ediacaran, 650 -570 m.y.).
In Naracoopa, park on the grassy verge opposite the Naracoopa Holiday Units, and
walk down to the small sandy beach (Fig. 70).
37
Fig. 70: Geological map of Naracoopa area, showing stops for Excursion 6.
Stop 1: The rocks exposed here, and along Fraser Beach as far east as just beyond the
Naracoopa jetty, belong to the Fraser Formation. This is the very thick succession of
mudstone and siltstone that makes up most of eastern King Island. It has not been
directly dated. It must be older than the Cryogenian (~650 m.y.) rocks of the Grassy
Group that overlies it. We suspect it is the same age as the Surprise Bay Formation
(~1300 m.y.) and that it only differs in being less deformed and metamorphosed (see
Part 1). At this stop, the small beach probably conceals a fault between two slightly
dissimilar parts of the Fraser Formation. To the east, there is monotonous pale grey
laminated siliceous (quartz-rich) siltstone, with bedding that dips to the east. The rock
is dotted with small (1 – 2 mm) rectangular dark green spots. These are composed of
chlorite, a metamorphic mineral that indicates relatively mild conditions of
temperature and pressure – milder, at least, than the garnet-bearing metamorphic
rocks of the Surprise Bay Formation. 50 m west of the beach, siliceous siltstone
occurs as graded beds, 0.3 to 0.7 m thick, with sharp bases, and tops that grade up into
dark mudstone (Fig. 71). These beds are very likely turbidites, just like those in the
Surprise Bay Formation. Near the beach, there are thin beds of black shale (which
probably developed as sea-floor microbial mats, see also excursion 2, stop 8) and
clastic dykes.
Fig. 71: Graded beds in siltstone of the Fraser Formation, near Stop 1.
Stop 2: 100 m E of Naracoopa Jetty: Outcrop at the Jetty is monotonous, laminated
siltstone like that seen east of the small beach at Stop 1. Just here, the rocks are crisscrossed by many small fractures. Going east, the outcrop stops at a small sandy beach
about 40 m wide. East of the beach the outcrops are of a different rock type: a very
fine-grained, greenish basaltic rock, also quite strongly fractured. This rock belongs
to the Shower Droplet Volcanics, the second volcanic formation of the upper Grassy
38
Group (also seen at Stop 6, excursion 1). All of the other formations of the Grassy
Group that are normally found between the Fraser Formation and the Shower Droplet
Volcanics (Fig. 11 (Part 1)) are missing. A major fault must underlie the beach here.
The fractured nature of the rocks on either side is a clue to the presence of a concealed
fault. Movement on the fault has brought the two originally separate formations
together. Generally, faults are only rarely exposed at the Earth’s surface, because the
highly fractured fault rocks tend to be easily eroded and then get covered by surficial
deposits, such as the beach sands here.
Stop 3: (10 m E of 56): The overall layering in these volcanic rocks is due to a
succession of thin (<1m) lava flows, typical of the low-viscosity picrite lavas of the
Shower Droplet Volcanics. The layers dip southeast, so in walking eastward along the
coast we are walking up through the sequence into progressively younger rocks. Here
is a slightly prominent outcrop consisting of irregular lumps of lava, that may have
been deposited as ‘volcanic bombs’ in a semi-molten state: such deposits are known
as ‘agglomerate’ (Fig. 72). 10 m to the west, there is a 4 m wide, NE-trending dyke
that cuts across the lavas. This is a slightly prominent, brownish rock contrasting with
the greenish picrites around it. The dyke contains abundant small (5 mm) rounded
feldspar crystals. This dyke is the same chemical composition as the third volcanic
formation (Grahams Road Volcanics) and very likely it is a feeder dyke for those
basalts, which we will encounter at Stop 5.
Fig. 72: Volcanic agglomerate at Stop 3.
Stop 4: (72) Walking up through the volcanic sequence we encounter mainly thin
flows, with occasional layers of volcanic breccia and ash. The latter tend to have a
vertical cleavage, resulting from a phase of mild deformation in the Cambrian period
(see also Stop 1, Excursion 1). 300 m E of Stop 3, near a fence post set into the shore
39
platform, the picrites take on a reddish hue, for reasons not clear (Fig. 73). Perhaps
the lavas were exposed to the air (and oxidised groundwater) for a time, leading to
oxidation of the iron content and hence reddening. In support of this idea, the upper
surface of one of the flows has a pattern resembling, with some imagination, coiled
rope. This so-called ‘ropy lava’ phenomenon is today seen on the surfaces of flows
that erupt on land – that is, not under water. (More convincing examples of ropy lava
are seen elsewhere in the Shower Droplet Volcanics: Fig. 74). Most of the evidence
suggests the volcanic rocks of the upper Grassy Group were erupted under the sea
(e.g. the thick pillow lavas, see Excursion 1) but here, at least, the volcanic pile must
have breached the surface.
Fig. 73: Unusual red picrite at Stop 4. The dark flecks are probably chlorite fillings of gas bubbles.
40
Fig. 74: ‘Ropy lava’ top of a flow in the ShowerDroplet Volcanics, thought to indicate subaerial
extrusion, as distinct from subaqueous extrusion shown by the pillow lavas.
Stop 5: (80): Approaching this point (Fraser Bluff), the lava flows have become
progressively thicker until parts of the sequence are massive and uniform for many
metres at a time. At this stop the rock has taken on an entirely massive appearance,
although there are many fractures and veins, some with pale green (epidote) alteration
along them. In places, quartz-filled vesicles (originally gas bubbles in the lava) are
abundant. These are light-coloured and prominent on outcrop surfaces. A chemical
analysis of this rock has confirmed that it is a basalt belonging to the youngest
formation of the Grassy Group, the Grahams Road Volcanics.
Stop 6: About 160 yards south of Fraser Bluff, another fault is crossed (but again it is
not exposed). The fine-grained, dark greenish basalt is replaced by outcrops of a
coarser-grained, grey dolerite (Fig. 75) which forms extensive coastal outcrops further
to the south. Another 300 yards south, at Stop 6, the eastern contact of this dolerite is
exposed at low tide. It is overlain to the east by laminated, green, black and brown
mudstone, probably belonging to the Robbins Creek Formation, the oldest of the
formations in the Grassy Group. This mudstone is very hard and flinty, a result of
being baked by the heat of the intrusion immediately below it. The dolerite is a thick
sill that intrudes this formation. The base of the sill is on the coastal escarpment some
250 m inland. Chemical analyses show that the dolerite belongs to the Grimes
Intrusive Suite, also seen at City of Melbourne Bay (Excursion 2, stop 8). Mapping
shows that the Grimes Intrusive Suite is thickest here at Fraser Bluff, and gets
progressively thinner going south and climbs upward through the stratigraphy of the
Grassy Group until it reaches its southernmost, thinnest and stratigraphically highest
point in the Yarra Creek Shale at City of Melbourne Bay. The date on the Grimes
Intrusive Suite (575 m.y.) is an important age constraint for the Grassy Group (see
Part 1). The chemical composition of the Grimes Intrusive Suite is distinctive and
rather unusual. It is high in certain trace elements, such as nickel and chromium, that
are also high in basaltic lavas and picrites (such as the Shower Droplet Volcanics) that
have erupted rapidly from a mantle source. However it is much too high in silica
(60% SiO2) and other crustal elements to have formed as a simple mantle melt. It is
postulated that the rapidly ascending, very hot magma, melted and incorporated silicarich crustal wall rocks on its ascent through the crust.
41
Fig. 75: Dolerite south of Fraser Bluff, near Stop 6. This medium-grained igneous rock, belonging to
the Grimes Intrusive Suite, is composed mainly of feldspar (pale) and pyroxene (dark), and is in the
upper part of a thick sill.
Excursion 7: Grassy Open-Cut Mine
Metamorphosed skarns and hornfels of the Grassy Group (Cryogenian-Ediacaran,
650 - 570 m.y.), intruded by Sandblow Granite (Carboniferous, 351 m.y.); cobbles,
sand and peat on raised wave-cut platform (Pleistocene, 2.6 m.y. – 10,000 y).
Scheelite was discovered on the coast below Grassy in 1904, and was mined almost
continuously from 1937 to 1990. The open cut mine was developed on the ‘No. 1
orebody’, and was mined out in 1975. The subsurface Dolphin Mine continued until
1990 from an entrance at the eastern end of the open cut, 17 m below sea level. The
Dolphin Mine is mainly under what was originally Grassy Bay (but is now land
reclaimed using mine tailings). The abandoned open cut mine is now filled to sea
level with water, and most of the benches are overgrown and difficult of access.
Good exposures of the Sandblow Granite and metamorphosed Grassy Group can still
be seen in places.
The scheelite ore formed within a 150 – 200 m thick sequence of strongly
metamorphosed sedimentary rocks and volcanics belonging to the Grassy Group. The
metamorphism and the formation of the ore is a direct result of the intrusion of the
Sandblow Granite (see Part 1).
Drive north along the coast road from Grassy Harbour. It may be best to park at the
gate about 300 m N of the harbour.
Stop 1: Just past the gate, on the left there are outcrops of the Sandblow Granite
beside the road. These massive brownish outcrops can be seen, up close, to be made
42
up of crystals of yellow-orange feldspar, grey quartz and black biotite 2-4 mm across,
with some larger blocky feldspars up to 30 mm wide. 351 million years ago, this
granite was a slowly crystallising magma, releasing superheated, mineral-laden fluids
that formed the scheelite ore in the rocks to the north.
Stop 2 (007): About 400 m north of the gate, take the second turn on the left, which
leads to the southern edge of the old open cut (permission may be needed from the
mining lease holders to access this area). About 200 m along, looking across to the
opposite (northern) side of the open-cut, the dark-coloured rocks in which the benches
are cut are metamorphosed sedimentary rocks of the Grassy Group, dipping
moderately (45 degrees) southwards, towards you (Fig. 76). These rocks underlie the
main ore horizon, which has been more or less completely removed along the E-W
length of the open cut. The underground Dolphin Mine continued along the main ore
horizon to the east, below sea level. The ore is hosted in the calcareous rocks
belonging to the Cottons Breccia and the cap carbonate (see Part 1). The rocks on
which you stand are metamorphosed volcanics at a higher stratigraphic level
(overlying the ore horizon). They are massive, fine-grained dark green rocks known
as pyroxene hornfels. Their mineralogical make-up has been altered by the great heat
of the granite intrusion, although this cannot be perceived by the unaided eye.
Beneath you, in the subsurface, drilling and mining shows that the south-dipping
Grassy Group sequence is cut off by the steeply north-dipping contact of the
Sandblow Granite. Looking across at the southern side again, you can just see the
trace of a north-west trending fault between the darker Grassy Group and the paler
brown-weathering rocks, which are quartzite belonging to the Fraser Formation (Fig.
76). This is one of the main faults along which mineralising fluids migrated outward
from the granite into the surrounding rocks.
43
Fig. 76: View looking north across the abandoned open cut mine at Grassy. The dashed white line is a
fault separating the Grassy Group (below left) and the Fraser Formation (upper right).
At the top of the cutting behind you, about 5 m above the level of the road, the
greenish metamorphosed volcanic rocks are overlain by well-rounded granite cobbles
(Fig. 77). This cobble bed marks a bench cut by marine erosion, about 15 m above
present day sea level. The cobbles were transported and rounded by wave action at a
time when sea level was at least 15 m higher than today. Some uplift of the land may
also have been involved. We do not know when this higher sea level occurred, but it
was presumably during one of the Pleistocene (2.6 m.y. – 10,000 y) interglacial
phases. Similar, though somewhat lower, wave-cut platforms are widespread around
the King Island coast.
About 10 m further west, in the cutting, a vertical dyke about 5 m wide of pale
granite, can be seen intruding the darker greenish volcanics (Fig. 77). Looking closely
at this dyke, the granite is quite heterogeneous, with many fragments, angular and
rounded, of mainly darker, finer granite, set in a matrix of coarser, paler granite. This
dyke is an offshoot of the Sandblow Granite, and many of the darker fragments may
be bits of the surrounding rock, or earlier-solidified granite, that broke off and became
incorporated into the magma as it moved along the dyke.
Stop 3 (013): By retracing your steps about 50 m, you can get onto the partly
overgrown bench above the road. About 300 m west along this bench, the volcanics
and the cobble layer are overlain by well-bedded sand about 15 m thick, with some
peat beds. This is a Pleistocene windblown sand (and possibly beach) deposit, with
lagoonal peat deposits. In turn, this is overlain by a large mine waste dump.
44
Fig. 77: A dyke of granite (d) intruding greenish metamorphosed volcanic rocks at Stop 3. At top left,
the high level beach cobbles can just be seen.
Stop 4: Retrace your steps almost to the gate (Stop 1) and walk across to the boulder
beach. The rounded cobbles and boulders on this beach, and around the point to the
north, are mainly mine waste excavated from the open-cut mine. (This demonstrates
that it only takes a few years for originally angular, hard rocks to become rounded by
wave action). A variety of interesting rock types, that are now inaccessible in the
open cut mine, can be found amongst these boulders. These are all rocks belonging to
the Grassy Group that have been to a large degree, changed (metamorphosed) by the
heat and mineralising fluids from the intrusion of the nearby Sandblow Granite.
There are dark greenish grey fine-grained metamorphosed basalt, dark grey massive
fine-grained hornfels and also spotted and laminated varieties, banded, white, green
and pink hornfels, and dark brown, garnet-rich rocks. The colourful banded rocks,
which could also be called skarns (metamorphosed calcareous rocks), have alternating
layers of greenish colour, rich in epidote (yellow-green) or pyroxene (dark greenblack), white layers (sugary-textured calcite), and layers rich in pale pink or dark
reddish, coarse garnet (Fig. 78). There are some boulders of metamorphosed Cottons
Breccia in which the originally limestone or dolostone clasts have been transformed to
coarse white calcite and dark red garnet (Fig. 79). Mineral collectors will be well
rewarded with a fossick here. There are also common boulders of a variety of granite
types, all belonging to the Sandblow Granite, including a rare red variety with
hornblende (narrow columnar black crystals).
Fig. 78: Banded skarn: the dark green diopside rich layers are separated from the white marble layers
by a band of pinkish garnet.
45
Fig. 79: Metamorphosed Cottons Breccia. The white fragments are marble with a bit of garnet.
Appendix 1: Glossary
Amphibolite: a dark green-black metamorphic rock made mostly of the mineral
amphibole (a complex iron-magnesium silicate), and generally derived from basalt or
dolerite.
Andalusite: A metamorphic mineral, an aluminium silicate, common in
metamorphosed sedimentary rocks, such as schist, and with a distinctive elongate
crystal shape with a nearly square cross-section.
Basalt: A fine-grained dark volcanic rock that originated as lava that erupted on land
or at the sea floor.
Biotite: a soft, flaky, shiny dark brown or black mineral, a variety of mica, common
in granite and metamorphic rocks such as schist.
Cap carbonate: The enigmatic carbonate (dolostone and limestone) formation that
nearly everywhere overlies ‘Snowball Earth’ glacial deposits wherever they are found.
Carbonate: Geologically speaking, rocks made up of carbonate compounds, most
commonly limestone (made of calcite or calcium carbonate, CaCO3), and dolostone
(made up of dolomite, or calcium-magnesium carbonate, CaMg(CO3)2).
Concretion: An ovoid body typically rich in calcite, that forms usually in sandstone,
soon after the sand is deposited.
Cross-lamination: A type of small-scale sedimentary layering produced by migrating
ripples, in which thin, inclined layers are overlain and truncated by flat-lying layers.
46
Diamictite: An uncommon type of sedimentary rock made up of fragments of a wide
range of sizes, set in a fine-grained (mudstone) matrix.
Diopside: A greenish magnesium-calcium silicate mineral, common in many igneous
rocks but also in skarns formed from the heating of carbonates.
Dolerite: Usually dark greyish or greenish, igneous rock that has solidified from a
magma in the subsurface. Chemically and mineralogically similar to basalt, but tends
to be coarser-grained (1-2 mm).
Dolostone a sedimentary rock made of the mineral dolomite: calcium-magnesium
carbonate (with the chemical formula CaMg(CO3)2).
Dyke: A narrow intrusion of igneous rock, that cuts across bedding where it intrudes
sedimentary rock. Most dykes have intruded along vertical fissures in the crust.
Epidote: a complex calcium-aluminium silicate mineral, typically grass-green or
yellow-green, common in metamorphosed carbonates.
Feldspar: A very common variety of minerals, silicates of aluminium, sodium,
calcium, and potassium, usually pale in colour and with a pearly lustre; especially
common in granites.
Formation: A recognisable body of rock, usually of layer form, that is extensive
enough to be shown on a map. Usually named after a local geographic feature, e.g.
‘Yarra Creek Shale’.
Garnet: A very hard, usually reddish, metamorphic mineral, forms roundish crystals,
found in schist and hornfels on King Island.
Graded bedding: A sedimentary bed with a sharp base, and an upward decrease in
grain size, is said to be graded.
Granite: A coarse-grained igneous rock, mainly of feldspar, quartz and mica, that
typically forms from the slow solidification of a large (several km) mass of magma at
considerable depth.
Igneous: Rock formed from the molten state.
Limestone: a sedimentary rock made of calcium carbonate (with the chemical
formula CaCO3). Precambrian limestone on King Island is very fine-grained and may
be a chemical precipitate, but the Miocene limestone is formed mainly from the shells
of marine organisms such as molluscs and bryozoans. The shells accumulate as a
sediment on the sea floor; over time, the sediment hardens to form the limestone.
Magma: Molten, or partly molten rock, formed deep within the Earth, that can be
intruded at depth in the crust (to form, for example, granite) or extruded at the surface
as volcanic lava.
47
Marble: Coarse-grained, usually white, carbonate rock formed by the metamorphic
recrystallisation of limestone or dolostone.
Metamorphic: Rock modified in the solid state by heat, pressure and/or deformation.
Mica: A common variety of minerals that are soft, flaky, grey, brown or black and
can be peeled into thin sheets. Muscovite and biotite are the commonest micas.
Mudstone: A fine-grained sedimentary rock formed from mud (clay and/or silt).
Muscovite: A pale brown variety of mica, common in metamorphic rocks
(particularly schist) and granite.
Orogeny: A mountain-building event in geological history, usually caused by
collision of tectonic plates, and usually recognisable in the geologic record by
widespread deformation, metamorphism, granite intrusion and other phenomena.
Paleomagnetism: Most rocks contain small amounts of magnetic minerals that retain
vestiges of the Earth’s magnetic field at the time they formed. Paleomagnetists study
these ancient magnetic directions, mainly to work out the past positions of continents.
Pegmatite: Very coarse –grained (25 mm or more) granitic rock, usually with easily
identifiable crystals of quartz, felspar, mica, and sometimes tourmaline.
Pillow lava: A distinctive type of basalt flow consisting of stacked pillow-shaped
masses, formed by extrusion under water.
Porphyry: A variety of igneous rock consisting of larger crystals, such as feldspar,
dispersed in a fine-grained groundmass.
Pyrite: Iron sulphide (FeS2), a brass-coloured mineral also known as ‘fool’s gold’,
but mostly formed as tiny grains dispersed through marine sedimentary rocks soon
after their deposition.
Quartzite: A sandstone made of mainly quartz grains, in which the grains have been
cemented together by more quartz to form a hard siliceous rock, often as a result of
heat and pressure.
Radiometric dating: The process of working out the age of a particular rock or
mineral, by measuring the relative amounts of a radioactive element of known decay
rate and its daughter product.
Scheelite: Calcium tungstate, a translucent, yellowish, dense mineral, one of the two
main ores of tungsten (the other being wolframite).
Schist: A metamorphic rock type, mainly of small platy mica grains that are aligned
such that the rock can be easily split, and most commonly formed by metamorphism
of mudstone.
48
Sedimentary: A rock originally deposited as a sediment, derived from erosion of preexisting rocks at the Earth’s surface.
Shale: A common variety of mudstone in which the rock splits easily, parallel to
bedding.
Sill: An igneous intrusion into sedimentary rock, that is parallel to bedding.
Siltstone: Sedimentary rock mainly composed of silt grains.
Skarn: a type of metamorphic rock most often formed at the contact zone between
granite intrusions and carbonate sediments, and composed of a variety of
metamorphic minerals, including garnet, epidote, diopside and many others.
Tectonic plates: Earth has an outer shell made of a number of discrete, slowly
moving tectonic plates floating on a mobile, convective mantle. Most tectonic activity
(i.e. earthquakes, volcanoes, mountain building) happens at the boundaries between
plates.
Thermoluminescence dating relies on measuring the (very tiny) amount of energy in
a sample of quartz sand grains, accumulated as a result of background radiation
(cosmic rays and local radioactivity). This energy only accumulates in the absence of
light, so the method gives you the time when the sand was last exposed to sunlight
(i.e. when it was deposited and buried by overlying layers).
Tillite: Diamictite deposited by ice.
Tourmaline: A complex boron-bearing silicate mineral, forming elongate, usually
black crystals and usually found in granite and pegmatite.
Tufa: A limestone, usually soft and porous, produced by precipitation at the surface,
at normal surface temperatures.
Turbidite: Sedimentary rocks deposited by turbidity currents, produced by
underwater avalanches, usually in the deep ocean. The beds thus deposited are usually
graded, and have a variety of other telltale characteristics.
Unconformity: An ancient erosion surface separating two sedimentary rock
sequences of different ages. Typically, the older sequence was tilted before being
eroded, so that its layers are truncated at the unconformity.
Volcanic breccia: A rock made up of broken, angular fragments of solidified lava
(such as basalt), generally produced during the eruption process, by, for example, the
explosive effects of expanding volcanic gases.
Zircon: A very hard translucent mineral, zirconium silicate, very common but mainly
as very small grains; important as the most common mineral that can be dated using
the uranium-lead method.
49
Appendix 2: Table of GDA co-ordinates of excursion stops
Metric co-ordinates, accurate to within about 10 m, are given here for the excursion
stops, in GDA 94 datum, Zone 55. Note that western King Island, including
Excursions 3, 4 and 5, is in Zone 54, while eastern King Island is in Zone 55. The
two zones have different grids. The 1:25,000 Tasmaps and geological mapsheets
extrapolate the Zone 55 grid over the whole island. However in the west a handheld
GPS gives you Zone 54 co-ordinates.
Excursion
Stop
Zone 55
mE
Zone 55
mN
1
1
1
1
1
1
2
2
2
2
3
3
3
3
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
7
7
7
7
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
253765
253882
253780
253197
253050
253824
253650
253791
254364
254457
236157
237241
237454
237654
234132
234047
234337
230725
230354
229970
229902
230011
230067
237094
237130
237247
237618
237776
238224
253401
254471
254588
254825
255187
255364
249457
249285
249058
249587
5567134
5566988
5566218
5565769
5565700
5566214
5567366
5567386
5568483
5568564
5553000
5551586
5551128
5550784
5555684
5564219
5563107
5573777
5573961
5574738
5574929
5575305
5575401
5613350
5613394
5613797
5614300
5614424
5614241
5577663
5577108
5577040
5576869
5576689
5576253
5561467
5561955
5562050
5561591
50