The geology of the area has been described by a number of authors including
Sprigg (1952), Ludbrook (1961), Wopfner and Douglas (1971), Cook et al.
(1977) and Idnurm and Cook (1980). These are summarised by Wells
et al. (1984). An updated interpretation in preparation by Moriarty
et al. (in press) has thrown new light on the geological history and cave
development and some of the results are summarised here.
The fossil bearing caves are situated in a linear ridge of Oligo-Miocene
Gambier Limestone known as the Naracoorte East Range. This ridge
forms the easternmost edge of a limestone plateau uplifted along the nearby
Kanawinka Fault originating within the underlying basement rock.
It is distinguished from the Naracoorte West Range by its Gambier Limestone
core. A number of lithological units are recognised within the Gambier
Limestone. The caves have been formed in a shelly bioclastic limestone
known as the Naracoorte Member of the Early Miocene Gambier Limestone.
This is a series of beds of coarse shelly matter alternating with finer
bryozoal limestone and is dated to the early Miocene. In this member,
bedding is subhorizontal and jointing is well developed in two dominant
directions: parallel to the ridge direction (north-north-west) and at right
angles to the ridge (east-north-east). The joint pattern has controlled
horizontal cave development while roof collapse has been along bedding
planes. The East Range is capped by Pliocene and Pleistocene beach
and dune deposits comprised of bioclastic calcarenite and calcareous sands
with prominent dune bedding.
The top of limestone cave ridge is up to 20 metres above the current intervening
valley floors. This ridge is a relic of an Early Pleistocene age
land surface, uplifted along the Kanawinka Fault and preserved from erosion
by an overlying beach dune (Wells et al., 1984). Two limestone ridges
to the east, the Joanna and Hynam/Koppermurra ranges, are also relics of
Plio-Pleistocene high sea stands.
Highly vertically exaggerated cross section through sedimentary sequence
near Naracoorte town quarries. Caves are exaggerated horizontally
for effect. "CFL" refers to clay filled honeycomb dissolution in mixing
zone at top of phreatic palaeo-watertable levels.
The Naracoorte Limestone was deposited in shallow warm ocean waters with
abundant marine life up to the Early Miocene. Corals, sponges, echinoderms,
brachiopods and molluscs are common as fossils in the limestone although
larger marine species such as sharks and whales have been reported by Glaessner
(1955). Following the Early Miocene, a marine regression (Sprigg,
1952) resulted in sub-aerial weathering of the limestone and phreatic cave
formation . In the Early Pliocene a marine transgression deposited
a series of dune ridges across south-eastern Australia (Sprigg, 1952; Blackburn
et al., 1965) and the regressive phase which followed saw the deposition
of fluvial sands (Parilla Sands) in the intervening valleys and watercourses.
This fluvial phase was accompanied by high phreatic water levels in the
limestone which dissolved to form the main caverns visible at Naracoorte
(for example, Blanche and Alexandra Caves). Minor uplift and low
sea levels (possibly associated with the onset of glaciation in the northern
hemisphere) allowed water tables to drop for a period in the Late Pliocene
(about 2 million years ago). This was also a time of intense weathering
in the Naracoorte area and further east, causing the leaching of the upper
10 metres of the sediments and limestone exposed at that time. Solutions
which penetrated to the caves deposited extensive speleothem fromations,
e.g. stalactites and stalagmites. Sea level rose again in the Early
Pleistocene (about 1.2 million years ago) inundating the caves and as it
receded high water tables caused much of the earlier speleothems to dissolve.
The remnants of these can be best seen in Blanche and Alexandra Caves.
These caverns were enlarged by about half to one metre at the same time.
About 800,000 years ago the Kanawinka fault caused sudden uplift of the
limestone in the Naracoorte area, stranding the caves well above water
table level and also out of reach of stream flow. Additionally they
were protected by a great dune of carbonate sand thrown up by the retreating
sea. Dense calcrete formed on the top of the dune to inhibit erosion
and limit water percolation into the underlying limestone caverns.
The interdune areas which were not protected by the calcrete-capped dunes
commenced to erode and the limestone surface has now been lowered by 20-30
metres, leaving the ridge and its caves well above the general land surface
of the plateau. However, on the ranges, solution tubes gradually
penetrated down through the dune sands and into the limestone, in places
forming pitfall traps for animals which accumulated in the bone deposits.
Thus, the deposits could contain fossil material in excess of 500,000 years
ii) Cave development:
by a green manganiferous clay. This dissolution records a fossil
water table about 15 metres below the limestone surface and is probably
the youngest of the cave forming events.
Phreatic cave development in the ridge is thought to date from the mid-Miocene
1952) following marine regression and occurred as a result of limestone
solution in the upper zone of an ancient water table (Wells et al., 1984).
However there is no extant evidence that this occurred, probably because
major disolution events in the Pliocene and Pleistocene have obliterated
the earlier phases.
Owing to uplift in the Pliocene and Pleistocene, the caves are now above
the present water table. Originally, most of the larger caverns formed
well beneath the water table level, which must have been at, or above,
the current limestone surface. There is no evidence of stream flow direction;
in fact the dissolution features are typical of very slowly moving phreatic
water. The larger caverns formed along a predominant north-north-west
joint direction and are connected at floor level by networks of tunnels.
There are extensive speleothem deposits which have undergone another period
of phreatic dissolution, also extending to the limestone surface. This
surface is overlain by extensively weathered sands which infill karst features
and are truncated by Late Pliocene marine transgressive sands.
Some caverns (e. g. parts of Victoria Cave) have formed at the upper phreatic
and capillary zones (Esteban and Klappa, 1983) by roof collapse and dissolution.
These caves contain extensive areas of honeycomb porosity partly or
Cave, perhaps the most decorated of the Naracoorte caves.
One of the implications of this sequencing of events is that the some cave
fill sediments could date back to the Early Pliocene. However, dissolution
event 3 removed more than half a metre of limestone and reduced large speleothems
to jagged remnants. It extended throughout the present cave system
and is likely to have dissolved carbonate shells and bone in any cave deposits.
Therefore, cave fills prior to the Middle Pleistocene uplift will be siliceous
residues devoid of fossils.
Victoria Fossil Cave is one of 26 known phreatic caves in Naracoorte Caves
Conservation Park, representing approximately one-third of caves documented
for the Naracoorte dunes (Lewis, 1976; Pilkington et al., 1982).
The Cave has a total depth of 20 m and consists of numerous large, domed
collapse chambers connected by a series of low crawlways. Passages
are generally wide with either silt or clay floors. Some 3 km of
passages have been surveyed, 350 m of which comprise the tourist section
of the cave. Many of the chambers are extensively decorated with
active and inactive speleothems including examples of straws, stalactites,
stalagmites, columns, fluted columns, sprinkled columns, pendulite, flowstone
pavement, shawls, helic-shawl and calcite flakes. Also in the cave,
are many encrusted Oligo-Miocene invertebrate fossils in the ceilings,
as well as the Pleistocene fossil vertebrate deposits of the Fossil Chamber
and the Ossuaries.
in Victoria Fossil Cave. Courtesy: Alpha.