Naracoorte Caves, South Australia – Background briefing on landscape and natural history

A megastore of mammal fossils in south eastern South Australia at Naracoorte

Naracoorte is in south eastern South Australia in a state region called The Limestone Coast. This region has many coastal attractions including the recent evidence of volcanic activity and limestone formations at Mount Gambier, a beautiful coastline, wine producing areas with the most famous vineyards based on a narrow (15 km x 2 km) strip of limestone derived Terra Rossa soils and the Naracoorte Caves. In the Naracoorte Cave area there are some 25 caves, several of which are open to the public.

Caves like those near Naracoorte can only develop on limestone. Locally this limestone is derived from two adjacent sources of marine sediments broadly termed the Gambier Limestone and the Calcarenite dunes. The first of these is the oldest, being formed in the Tertiary Period, from about 37 to 12 million year ago. Cave formation has been most active in this limestone. The limestone dominated dunes that made the Calcarenite are from the Pleistocene Epoch (2.6 million to 10,000 ya (years ago) within the more recent Quaternary Period and these overly the cave area with their limestone (Calcarenite) helping preserve the integrity of the caves beneath. However, in some places, solution tubes have penetrated the Calcarenite and these have become the pitfall traps as described below. All this area along with the limestones has been subject to uplift about 800,000 ya and Naracoorte is about 50m above sea level.
Limestone is subject to weathering via weak acid such as that formed when carbon dioxide naturally dissolves in water or when weak organic acids from decaying vegetation moves through the soil and into the joints of the underlying limestone rock.

Terra Rossa soil profile with the limestone subsoil in the famous Coonawara wine growing area. South Australia
Attractive coastal scenery at Beachport dominated by limestone calcarenite rocks.
The Umpherston sinkhole in Mt Gambier. The Mt. Gambier Limestone a formation also occurs at Naracoorte.
Over long periods chemical weathering develops cave systems, with passages and caverns as the solid Calcium Carbonate limestone undergoes dissolution (i.e. the limestone going into solution) and is then removed via underground streams. While the emphasis in this article is on fossils, we must not forget the attractiveness of dripstone formations that develop in limestone caves when carbon dioxide is diffused into the atmosphere from the solution resulting is the precipitation of Calcium Carbonate crystals which en masse form wonderful formations like stalagmites, stalactites, ribbon, shawls and flowstone.

The roof of caves may be penetrated by what are referred to as a sink-hole or doline and when large enough these not only allow runoff water to carry surface sediments underground but become pit-fall traps for animals. While some animals suffering such a fall may be injured or die, others trapped deep underground have died from starvation then adding their decaying bodies to the growing hills of sediments falling from the sink-hole above to become fossils. Sink-holes in the Naracoorte cave area have been recipients of such fossils for over 500,000 years.
A rich find was made in the late 1960s when two underground cave explorers squeezed through a passage leading from Victoria Cave. Victoria Cave has a depth of 20 m and is made up of several collapsed caverns and some 3 km of surveyed interlinking crawl ways or passages. The explorers discovered a previously un-known cavern and found it to be filled with mountains of red sediment and countless jaws and skulls of previous lost megafauna. The particular sink-hole that had produced the build-up had been blocked off about 15,000 ya, but Fossil Cavern/ Chamber, as it was then named by palaeontologists, had been collecting skeletons for over 200,000 years before the closure. In places fossils are found in 3 to 4 m deep sediments.

Megafauna are in many cases oversized species related to many of today’s fauna. Australian megafauna includes the giant short-faced kangaroos, the giant monitor lizard and the Diprotodon, Zygomaturus that looked like a giant wombat. As well, a marsupial lion (Thylacoleo carnifex) was found and added to a previously discovered family of extinct carnivorous marsupials. Palaeontologists were able to reconstruct much of the marsupial lion’s skeleton but were missing the hind limbs. As luck would have it, workers at a nearby Naracoorte limestone quarry came across a cavern littered with many fossils during their operations. Work stopped in this section and a recovery team of palaeontologists documented and collected all the fossilised material found in the cavern. Among the specimens recovered were some vertebrae and the hind leg of Thylacoleo carnifex!

Mounds of sediments and fossils. Fossil Cavern Victoria Cave, Naracoorte. Wikipedia.

The fossil finds at Naracoorte were scientifically significant enough for the area to be accepted and listed as a World Heritage site. While these fossils account for the animals that existed during and prior to the arrival of Aboriginal people, particularly the Australian megafauna, fossils found Riversleigh in NW Queensland had mostly formed well before Aboriginal arrival. Up to 1986 early studies found fossils at 30 different sites and these ranged in age from existing 50,000ya to 15 Mya (million years ago): in geological terms from the Pleistocene back to the mid Miocene epochs. However, by 2011 on-going research reported fossil material from 25 Mya, in the Oligocene epoch, and in 2012 fossils had been found in 200 Riversleigh locations. The richest of these sites yielded 35 fossil bat species and another important discovery were Monotreme fossils representing the primordial platypus.

The Australian megafauna become extinct between 60,000 and 40,000 ya and there is considerable debate about what caused their demise. Two major arguments put forward are firstly that the environment was getting drier through climate change so changing the kind of food available to the megafauna. The other argument is that Aboriginal people, the first humans to occupy the country, were changing the environment mainly through burning practices to manage their country and hunting. Of course, megafauna extinction does not have to be one or the other of these factors and it is likely that both climate change and the Aboriginal practices contributed but not necessarily in equal proportions.

Marsupial Lion skeleton on display in Victoria Cave, Narracoorte

A giant short-faced kangaroo in a diorama, Naracoorte Caves visitor centre

Many of the other animal fossils species were from groups similar to those being found at Naracoorte, but the majority had been living in much wetter environments that supported rainforest and wet sclerophyll forests and the majority had been preserved in limey mud in the bottom of pools. Unlike Narracoorte, the majority of Riversleigh fossils are from the mid Miocene and with exception of protruding bones are mostly ‘entombed’ in limestone rock originating from the limey mud. Before the fossils may be fully exposed the lumps of rock containing them are treated in a weak acetic acid bath to remove the limestone and so release to delicate fossil bones.
In the early days Riversleigh fossil site stood alone but it has now been enclosed within the nearby Lawn Hill National Park and the whole area is now called Boodjamulla National Park. Together Riversleigh and Narracoorte fossil sites have provided palaeontologists and zoologists with a lot of information particularly about the evolution of Mammals and because of their significance were co-listed as a World Heritage Sites in 1994 as Australian Fossil Mammal Sites (Riversleigh / Naracoorte).

Fossil bones and the gizzard stones of a bird embedded in limestone from Riversleigh
Limestone rocks at Riversleigh, the other part of the World Heritage site.