The Geology of Hallett Cove Conservation Park

Introduction

At Hallett Cove, evidence of episodes of the Earth’s history from the last 600 million years can be seen, from a total Earth age of about 4,600 million years.

Much of the evidence for the past 600 million years has been eroded from the Hallett Cove landscape and lost from the geological record there. However, some results of the Earth’s continuous geological change remain for us to see and piece together.

At Hallett Cove, one can see five different rock types, formed at five different periods of Earth’s history, under five different climates, and when Australia was on different parts of the Earth’s surface.


Mountain Building

About 600 million years ago the area now occupied by the Mount Lofty and Flinders Ranges was part of a long "trough” in the seafloor (the Adelaide Geosyncline) where sediments such as silt, sand and limestone were deposited and buried.

At about 500 million years ago this region was squeezed by movements in the Earth’s mantle and crust and these sediments were deeply buried and hardened to form rock (siltstone, quartzite and limestone). The compression continued, folding the rocks and uplifting them to form an enormous mountain range called the Delamerian Highlands, which stretched from the present Fleurieu Peninsula across to western Victoria.

These Highlands eroded down to form an undulating landscape. The eroded silt and sand was washed eastwards out into the sea, and we now see them as some of the rocks in New South Wales, Victoria and Tasmania.

At Hallett Cove we can see the folded siltstone, sandstones and quartzites of the Delamerian Highlands at Black Cliff, and in the cliffs and shore platform to the north.

 

Permian Ice Age

At about 280 million years ago Australia was part of a supercontinent called Gondwana which included all the (present) southern continents and India. Gondwana was then moving over the South Pole and was covered by an icecap.

That icecap covered the southern two-thirds of Australia from about 280-270 million years ago (Permian times) and was at least one kilometre thick. The icecap moved in a northerly direction over Australia.

Evidence left by the ice includes chatter marks, crescentic gouges, scratched and polished bedrock. The remains of a U-shaped valley can be seen along the coast south of Hallett Cove Beach. The glacial striations on Black Cliff indicate that the ice sheet at Hallett Cove moved in a north westerly direction.


Permian Deposition

About 270 million years ago the climate warmed and the ice sheet melted. Meltwater laden with fine rock flour and coarser material accumulated in layers on the floor of lakes, and, boulders in icebergs were released and dropped into very fine sediments. This evidence can be seen throughout Hallett Cove Conservation Park.


Late Pliocene Times

The Gondwana supercontinent later broke up and the fragments dispersed, with Australia finally separating from Antarctica about 43 million years ago. The next geological evidence to be seen at Hallett Cove relates to a warm, shallow sea about 4 million years ago.

In that much warmer climate, shellfish and forminifera proliferated, and their shells accumulated on the sandy shallow sea bed. Over time a fossiliferous sandstone was formed. A shelf of this sandstone can be seen at a number of spots around the Park.

 

Pliocene to Pleistocene

At about 3 million years ago, the region was again squeezed and the preset Mount Lofty Ranges and Flinders Ranges were uplifted. The uplift still continues at the present time, as evidenced by the earthquake at Darlington in 1954.

The Hallett Cove region was also uplifted, and so that fossiliferous Pliocene sandstone is now about 50 metres higher than when it formed.

This uplift initiated another cycle of erosion, which resulted in the red-green mottled sediment in the Amphitheatre.

 

Pleistocene to Recent Times

Up to about 30 thousand years ago, now-extinct mega fauna roamed the area; Diprotodon and giant kangaroo remains have been found in the banks of the Field River inland from the southern end of Hallett Cove beach.

About 20 thousand years ago, during the last Ice Age, the sea level was about 130 metres lower than at present. This exposed the continental shelf and the strong winds blew calcareous dust (from broken shells) over the land.

Subsequent wet and dry seasonal climatic changes caused a calcrete layer to develop in surface soils and this rubbly limestone can be seen around the rim of the Amphitheatre.

The Amphitheatre was eroded into its present shape about 10 thousand years ago, revealing glimpses of 270 million years of geological time. The Hallett Cove region and the Mount Lofty Ranges are still being uplifted and the resulting processes of erosion and sedimentation continue.


Evidence of geological phenomena to be observed at Hallett Cove

  • Alluvial Sediments
  • Anticlinal Fold
  • Calcrete (Kunkar)
  • Chatter Marks
  • Climate Change
  • Crescentic Cracks
  • Crescentic Gouges
  • Dropstones
  • Erratics
  • Folding
  • Fossiliferous Sandstone
  • Glacial Clay
  • Glacial Silt/Sand/Clay
  • Glacial Striations
  • Graded Bedding
  • Granite Erratics
  • Ice Age Evidence
  • Lunate Fracture
  • Permian Sediments
  • Pleistocene Clays
  • Pliocene Sandstone
  • Proterozoic Rocks
  • Quartzite
  • Ripple Marks
  • Sandstone
  • Shore Platform (wave-cut) 
  • Siltstones
  • Slickensides
  • Slump Folds
  • Striated Pavement
  • Syncline Folding
  • Tillite (Sturtian) Erratics
  • Uncomformities
  • U-Shaped Valley

While preserving the unique geological evidence at Hallett Cove is the Park’s primary purpose, there are other important environmental aspects to be conserved and explored.

The Park has the largest area of publicly accessible remnant coastal vegetation along the metropolitan coastline and it is a sanctuary for birds, reptiles and insects.

Disturbance or removal of rocks, fossils or biological specimens is not allowed


Further Reading

Cooper, H M et al (1970): Hallett Cove, a Field Guide , South Australian Museum, Adelaide.

Giesecke, R (Ed., 1999): A Field Guide to the Geology of Hallett Cove and other Localities with Glacial Geology on the Fleurieu Peninsula (1999), Field Geology Club of SA - available through their website: www.fieldgeologyclubsa.org.au.

Hasenohr, P. & Corbett, D. (Eds., 1986) A Field Guide to the coastal Geology of Fleurieu Peninsula, Field Geology Club of SA, Adelaide - available through their website: www.fieldgeologyclubsa.org.au.

Hallett Cove and Marino Conservation Parks Management Plan (2010), Department for Environment and Heritage, Adelaide.

 

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