Geological time
This field guide refers to units of geological time using
terms which might be unfamiliar to some people. It is
recommended that you download a copy of the time scale
(www.sa.gsa.org.au) and consult the relevant sections when
necessary.
MASLIN BAY AND
PORT WILLUNGA
Geological Trail
A walking trail through geological time
Introduction
Since late in the 19th Century, the scenically attractive cliff
sections at Maslin Bay and Port Willunga have inspired
painters, field naturalists and collectors, teachers and
students, and geologists and palaeontologists. From about
30 000 to 18 000 years ago Earth’s climate was intensely
cold; ice sheets covered much of North America and Europe,
the Antarctic ice sheet was at its maximum thickness
and extent, and glaciers formed in Tasmania and on the
southern highlands of eastern Australia. With so much
water frozen in place on the land, sea-levels were 120–
130 metres lower than they are today. With the onset of our
present globally warmer climate, substantial amounts of
glacial meltwater returned to the oceans, which reached
present sea-level about 7000 years ago. These coastal
cliffs are thus relatively new features that have formed as
a consequence of marine erosion. Today the west-facing
outcrops are kept fresh by the back-cutting action of the
sea and the washing effect of the winter storms. The rocks
are all sedimentary strata (layers) and their succession
through geological time is readily grasped as one traverses
north to south.
The strata offer a glimpse into the Willunga Embayment,
the rocks upon which the soils of the McLaren wine region
developed. Their broad scientific and environmental
significance is high, because no other locality in Australia
and very few on the planet display a critical episode in
geological time so well. This cycle spanning 42 to 32 million
years (myr) ago records the great transformation from a
greenhouse world, when polar ice sheets were small or
absent, to the icehouse world of today with its substantial
ice sheets.
The Willunga Embayment is a relatively small, triangular,
downwarped area which is bounded to the north by a high,
up-faulted block of very much older folded rocks, and to the
south by the high escarpment of the still active Willunga Fault.
Along the eastern margin of the parent St Vincent Basin, a
series of closely spaced, curved faults, roughly parallel to the
Willunga Fault, define and control several similar embayments
with matching sedimentary successions. However, the most
complete and most easily studied strata are those of the
Willunga Embayment which outcrop in the cliffs at Maslin Bay
and Port Willunga.
Figure 1 Top: Relative positions of Australia and Antarctica following
separation; heavy black lines show the position of the spreading ridge. The
landmass that includes New Zealand separated eastwards. Coloured areas
signify marine sediments which were deposited within the Australo-Antarctic
Gulf that formed between the Australian and Antarctic continents. The
position of the St Vincent Basin is shown.
Bottom: The form of the St Vincent Basin is controlled by north-south
trending faults, shown as heavy black lines. The basin is bound by the eastern
margin of Yorke Peninsula to the west and by the Mount Lofty Ranges to the
east. Also shown are the Noarlunga and Willunga Embayments, which lie
within the St Vincent Basin.
The geological succession is best walked from north to
south, for the strata dip and become younger southwards.
As shown on Figure 2, access from South Road is easy.
There is a good walking track above the beaches from (1)
to (6) and this sector covers most of the geology. The cliff
sections are reached easily, the walking is pleasant on the
sand with some scrambling over large rocks which can be
hazardous when wet. Numerous rock falls illustrate the
dangers of getting too close to cliffs. Blanche Point is seen
best at low tide, but is generally accessible at most other
times.
Maslin Bay
1
Sealed road
Rocla
sand
quarry
old
sand
quarry
Geological walking trail
Parking
Maslin
Beach
Watercourse
2
Looking south from Maslin Bay village or anywhere on
the cliff or beach, one can observe that the lower strata
dip gently towards Blanche Point and Gull Rock. However,
View from Site 2, Maslin Bay, looking south. Eocene South Maslin Sand and Tortachilla
Limestone in the foreground, Pleistocene units (see Table 1) in the background. (Photo 409036)
Blanche Point
3
ALDINGA
4
Road
Perkana Point
Chinaman
Gully
5
BAY
6
2
Port
Willunga
Quinliven Road
t
1
Cree
k
Thomas Road
Tuitt Road
caravan
park
Coach
Gull Rock
et
South
MASLIN
1000
BAY
Snapper Point
Figure 2 Location of sites along the Maslin
Bay and Port Willunga Geological Trail.
Sherriff Road
Main
0 metres 500
Hill
When the Eocene sea swept in, it
formed estuaries in the inset valleys
into which the South Maslin Sand
was deposited (Site 1). Conditions
were marine but stressed, probably
by low oxygen and low salinity,
so marine life was not abundant.
However, when a normal, warm sea
was established the communities
flourished and the limey skeletons
are diverse and abundant —
bryozoans, molluscs and sea urchins
especially but many others too.
These conditions produced the richly
fossiliferous Tortachilla Limestone
sediments, which are best observed
at the base of the stepped pathway
from the car park at the top of the
Pioneer
sand
quarry
Road
Below the angular unconformity
Road
Maslin Beach Road
Bourke Road
Old
Access
Maslin Bay and
Port Willunga
nn
Our 19th Century geological pioneers admired these
spectacular strata but lacked the grand theories that inform
our modern observations. Such theories include plate
tectonics, global greenhouse-icehouse change, oceanic
circulation, the ages of rocks at the far ends of the Earth, even
organic evolution itself.
to Adelaide
De
When the St Vincent Basin was born, the incoming sea was
a large inlet from the Australo-Antarctic Gulf of the Indian
Ocean. At 42 myr ago that gulf had less than 10 myr to live,
for it was soon absorbed into the new Southern Ocean as new
ocean floor grew rapidly between Tasmania and Antarctica,
and the Indian and Pacific Oceans were connected for the
first time. There are strong reasons to identify this seaway
connection as the beginning of the modern ocean and
modern marine life.
about halfway up the cliff, these
gently dipping strata are truncated
by an overlying horizontal layer
of strongly cemented, in part
overhanging sandstone. This angular
unconformity represents a 20 myr
break in deposition between the
Paleogene sediments of the South
Maslin Sand, Tortachilla Limestone
and Blanche Point Formation below
and the Neogene sediments of the
Hallett Cove Sandstone above.
Bowering
The Willunga Embayment with its sediment infill is one of
several sub-basins on the eastern side of the St Vincent Basin
(Figs 1, 3 and 4). The basin and the sedimentary succession
which was subsequently deposited therein were initiated
when Australia and Antarctica began their rapid separation
(continental drift). During Eocene time, 45–42 myr before
the present, Australia accelerated its migration to the north
(from <2 to >5 mm/year). Changes in the Earth’s stress field
reactivated lines of weakness in the region of the ancient Mt
Lofty Ranges. The outcome was birth of the St Vincent Basin,
now about 42 myr old and currently two-thirds flooded by
the modern Gulf St Vincent.
Ald inga
Cr
e
ek
Aldinga
203874_002
cliff (Site 2). Many exposures of this formation are green
in colour due to the presence of grains of glauconite, a
distinctive mineral that is characteristic of low-oxygen
marine environments.
Point Formation is succeeded by the Chinaman Gully and
the Port Willunga Formations. Above the unconformity, the
Pliocene Hallett Cove Sandstone, the Burnham Limestone
and other younger strata remain accessible.
The Blanche Point Formation contrasts strongly with the
Tortachilla Limestone, even though it too is rich in fossils
(Site 3, adjacent Blanche Point and Perkana Point). Fossils
of turreted snails especially are extremely abundant in
some layers, intensive burrows in others, sponge gardens
in yet others. The unweathered sediments are dark grey
(not white as suggested by the word ‘Blanche’) and
rich in finely disseminated opal, derived from sponges
and diatoms. This points to abundant runoff forming a
brackish, low-density surface layer on the sea, inhibiting
circulation and gas exchange with the atmosphere. This
condition affected the marginal waters of the AustraloAntarctic Gulf whose northern and southern shores were
lined with subtropical rainforests in the Late Eocene.
After deposition of the Blanche Point Formation, there
are places under Adelaide and on Yorke Peninsula where
there were valleys, up to 50 metres deep, cut into the
Eocene sediments. These valleys were then backfilled with
non-marine sands and muds, before the sea returned. This
happened exactly at the time when (on global evidence)
the ice sheet on Antarctica first grew to seriously large
dimensions and global sea level was lowered accordingly.
At Port Willunga this deposition of non-marine sediments
is represented by the Chinaman Gully Formation (Site 4).
Blanche and Perkana Points separate two beautiful
sandy beaches. Blanche Point is floored by the very well
cemented Tortachilla Limestone, an Eocene sea floor
now being exhumed by the modern sea. To the north the
sea erodes the much less resistant South Maslin Sand.
Perkana Point is floored by the very hard layers of the
Gull Rock Member. Further south the sea erodes the very
Perkana Point, showing Blanche Point Formation (banded Gull Rock and
soft Perkana Members). (Photo 409039)
soft Perkana Member. So, the dip of the strata and their
varying resistance to erosion determines the shape of the
coastline.
Above the angular unconformity
The cemented Pliocene Hallett Cove Sandstone juts
out from the cliff and is found in abundant fossiliferous
slabs that have fallen onto the beach. Around the coasts
of South Australia and locally, it is variable in hardness
and fossil content, but its fossils indicate very warm
conditions — not as warm as in Eocene time, but warmer
than in the following Pleistocene time.
The crumbly, poorly fossiliferous Burnham Limestone,
the red-white Ochre Cove Formation and olive-green
Ngaltinga Clay, and surface calcrete are also easily
viewed from the stepped concrete pathway leading
from the carpark to the beach (Site 2). These sediments
represent contrasting environments during the
Pleistocene Epoch, somewhat warmer and wetter in the
earlier part and cooler and drier in the later part. They
also reflect the rise of the young (Pleistocene) modern Mt
Lofty Ranges, increasing erosion and delivering sediment
here and further out into the St Vincent Basin at times of
lower sea level.
Port Willunga
Blanche Point and Gull Rock soon after low tide. The hardened Eocene sea
floor on the Tortachilla Limestone is just below the modern sea surface. Lower
half of cliff, Gull Rock Member, then the soft, easily eroded Perkana Member.
Overhang with patches of deep shadow is the Hallett Cove Sandstone. White
at top, Burnham Limestone. (Photo 409038)
3
4
5
6
The base of the cliffs along the coastline from Blanche
Point, south to Port Willunga township, reveal
progressively younger strata that were deposited during
Late Eocene, then Oligocene time. The top of the Blanche
Following deposition of the Chinaman Gully Formation,
the sea returned to record a succession of marine
environments — shore sands, deeper water sands and
muds, increasingly limey (Aldinga Member of the Port
Willunga Formation; Site 5). The bryozoan- and echinoidrich fossil assemblages that are preserved in these
sediments of the new Southern Ocean are much more like
modern shallow-water marine faunas than those of the
Blanche Point Formation that lived in the old AustraloAntarctic Gulf. This is strong corroborating evidence that
the modern ocean, with stronger water circulation and
better ventilated with more oxygen, took a major step
forward during this time at the expense of the poorly
oxygenated marine environments of the late Eocene. The
replacement of sponge gardens by bryozoan gardens is
a telling sign of a shift from a greenhouse world in the
Eocene to an icehouse world in the Oligocene.
Para
Fault
Eden
Fault
Clarendon
Fault
Adelaide Plains
Meadows
Fault
10 km
Willunga
Fault
Noarlunga
Embayment
Willunga
Embayment
203874_003
Figure 3 Simplified block diagram of the Adelaide region showing the
uplifted fault blocks and sedimentary basins.
PLIOCENE
RUWARUNG
MEMBER
EARLY
OLIGOCENE
6
PORT WILLUNGA
FORMATION
NGALTINGA CLAY
OCHRE COVE FORMATION
BURNHAM LIMESTONE
HALLETT COVE SANDSTONE
PLEISTOCENE
5
ALDINGA
MEMBER
4
CHINAMAN GULLY FORMATION
TUIT MEMBER
50 m
PERKANA
MEMBER
LATE
EOCENE
3
PALEOGENE
GULL ROCK
MEMBER
2
BLANCHE POINT
FORMATION
NEOGENE
HOLOCENE
non-marine clays, clayey
sands, calcrete and other
soil horizons: marine
limestone at base
calcareous sandstone,
sandy limestone
bryozoal limestones,
silts, clays
chert nodules
bryozoal limestones,
clays, sands
varicoloured clays, silts,
sands
hard (silica-rich) and soft
(carbonate-rich) layers
spicule-rich
opal/carbonate/clay
hard (silica-rich) and soft
(carbonate-rich)
TUKETJA MEMBER
TORTACHILLA LIMESTONE
calcareous glauconitic clay
rubbly limestone and sandy
ferruginous limestone
SOUTH MASLIN SAND
ferruginous quartz sands,
cross-bedded, glauconic
1
Chinaman Gully Formation at Chinaman Gully. This is the brown and green
unit separating the Blanche Point Formation (covered under the pink-flowering
bushes) from the Port Willunga Formation (flanking the footbridge). (Photo
409040)
MIDDLE
EOCENE
0
NORTH MASLIN SAND
cross-bedded quartz
sands
clay lenses with
terrestrial flora
PERMIAN
CAPE JERVIS FORMATION
sands and clays with
pebbles and boulders
OLDER FOLDED ‘BASEMENT’ ROCKS
203874-001
Figure 4 Geologic section showing the succession of strata which
comprise the rocks of the Maslin Beach and Port Willunga areas. The older
folded ‘basement’ rocks and the glacial sediments of the Permian Cape
Jervis Formation do not outcrop in the cliff exposures. These units were
once exposed in the excavated base of an old sand quarry immediately
north of Maslin Beach. This quarry has now been rehabilitated and today
none of the stratigraphic units can be observed in outcrop. This diagram
should be interpreted in conjunction with the Geological Time Scale (www.
sa.gsa.org.au). A wavey line between an underlying and an overlying unit,
e.g. Tortachilla Limestone (underlying) and the Tuketja Member of the
Blanche Point Formation (overlying), signifies an erosional surface.
Cliffs at Port Willunga, north of remains of jetty (off photo to right). The
Port Willunga Formation is faulted—the brown layer at head of tallest
figure is displaced upward by about 2 m to the left. Large slabs on beach are
fossiliferous Hallett Cove Sandstone, fallen from strata at lowest green bush
on skyline. White at top, Burnham Limestone. (Photo 409041)
Additional sites to visit
An understanding of the geology can be extended at
two additional localities, one north and the other south
of Maslin Bay:
• A low cutting on the southern side of Chapel Hill
Road, 2 km east of Chapel Hill winery, McLaren
Vale, where ferruginised sands and gravels of North
Maslin Sand overlie deeply weathered white clay,
The hard sandstone of the Port Willunga Formation juts out above the
Chinaman Gully Formation. (Photo T015640)
which once resembled the purple siltstones that
outcrop at Hallett Cove. The sands and gravels are the
oldest sediments in the St Vincent Basin and are not
exposed on the coast. The latitude was about 60°S
(25° further south than now), but the climate was
both warmer and wetter than that of today. Fossil
plant remains preserved in fine sediments, exposed in
quarries at Maslin Beach and at Golden Grove, signify
plant communities which resembled the subtropical
rainforests of today. This outcrop is perched above
most of the Willunga Embayment due to Pleistocene
uplift of the Clarendon-Moana fault block. It was also
upstream of the meandering waterways and billabongs
of Eocene times, the sediments of which were exposed
during sand quarrying.
Table 1 Succession of strata
North Maslin Sand, iron-stained to ochre colours, overlies deeply weathered
siltstones having an age of about 600 myr. The contact of the two units is an
unconformity representing an hiatus of more than 550 million years .Chapel
Hill Road, east of Chapel Hill winery. (Photo 409037)
• South of Sellicks Beach, Esplanade south of Gulf View
Road, beach walk of~2 km. Here the Port Willunga
Formation is exposed at low tide in the shore platform
and in the adjacent cliffs. The platform with rock
pools is cut into the limestone by the modern sea. The
limestones lying unconformably on much older strata
have been dragged up to high angles by movements
on the Willunga Fault — the boundary between the
Willunga Embayment and the Willunga Fault block
and associated with the uplift of the modern Mt Lofty
Ranges. This uplift caused invigorated erosion and
deposition of an alluvial fan comprising gravel, sand
and mud, sediments that are exposed in the cliff face
above the beach, and which are again being eroded to
form the modern beach sediments.
References
Talbot, J.L. and Nesbitt, R.W. 1968. Geological excursions in the Mount
Lofty Ranges and the Fleurieu Peninsula. Angus and Robertson,
Sydney, 60p.
Acknowledgement
Original text prepared by B. McGowran and Geological Society of
Australia (SA Division) Field Guide Subcommittee.
Steeply dipping Port Willunga Formation as occurring at the additional
locality at the southern end of Sellicks Beach. (Photo T015642)
Be prepared when bushwalking:
• Allow at least half a day to walk the trail.
• Wear sturdy shoes, hat and apply sunscreen even during winter
months.
Strata (youngest at top, oldest at bottom)
Age (Ma=million years
before present)
younger group (flat lying strata )
Calcrete crust (soil limestone)
Ngaltinga Clay (olive-coloured) probably
aeolian
Ochre Cove Formation (red-white mottled)
warm and wet conditions
Burnham Limestone (thin crumbly
limestone)
Hallett Cove Sandstone (very fossiliferous,
large slabs on the beach, sometimes soft
sand). Shallow seas, warmer than today,
marking the last Neogene warm period
before the late Neogene or Quaternary ice
age set in after 3 Ma.
Unconformity (low angle, representing a
break in the succession—an hiatus)
Neogene (Miocene to Today)
Pleistocene (<0.5 Ma)
Pleistocene (>0.5 Ma)
older group (strata dipping at about 2° to
the south)
Port Willunga Formation (sandstones,
limestones) Ruwarung Member (above cave
ceiling at former jetty) Aldinga Member
(below ceiling)
Chinaman Gully Formation (clays with
bright weathering reds browns below,
greens above)
Blanche Point Formation
Tuit Member (thin, snail-rich, often under sand)
Perkana Member (soft, sponge spicule-rich)
Gull Rock Member (hard, banded, snail-rich
layers)
Tuketja Member (thin, green glauconitic)
Tortachilla Limestone (very rich in fossils)
Upper member (green, snail-rich)
Lower member (bryozoan-rich)
South Maslin Sand (sparse marine fossils)
North Maslin Sand (sands, gravels, clays,
rich terrestrial plant fossils in oxbow lakes)
Palaeogene (Paleocene to
Oligocene)
Late Oligocene (24–26 Ma)
Sellicks Beach
Early Oligocene (30–33 Ma)
Port Willunga
Earliest Oligocene
(33.5 Ma)
Pleistocene (>0.8 Ma)
Pleistocene (~1.7 Ma)
Pliocene (about 3 Ma)
Hiatus of ~32 myrs
(Maslin Bay) or ~28 myrs
(Pt Willunga) or 18 myrs
(Sellicks Beach)
Late Eocene (34–36 Ma)
Middle to Late Eocene
(37–38 Ma)
Middle Eocene (38–39 Ma)
Middle Eocene (40–42 Ma)
• Carry sufficient food and drinking water.
• Safety First! Keep to the defined walking trail except when on the
beaches; avoid overhanging rock strata which could potentially fall;
stay well back from the tops of cliffs.
• Inform a responsible person of your proposed route and expected
time of return.
• Weather conditions can change quickly, ensure you have
appropriate wet weather clothing.
• Be advised that the southern end of Maslin Bay is reserved for both
clad and unclad bathing.
South Australian Division
www.sa.gsa.org.au
2011
203874