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 (1997) 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.

i) Geological history:

    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 speleothems fromations eg 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 old.

ii) Cave development:

    Phreatic cave development in the ridge is thought to date from the mid-Miocene (Sprigg, 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.  Phreatic cave development in the ridge is thought to date from the mid-Miocene (Sprigg, 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 completely infilled 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.

Alexandra Cave, perhaps the most beautifully decorated of all 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 Cave:

    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.