Geology

Ian Cooper

Spider Cave is contained within the Late Silurian (415-410 million years old) Jenolan Caves Limestone. This limestone is a composite unit with a thickness of 235m at Caves House (Allan 1986). Stratigraphy in vicinity of Spider Cave is as follows:

The limestone is interpreted to represent a shallow marine shelf with coarser reworked detritus, derived from small reefs. Spider Cave is developed in the three lowermost units of the Jenolan Caves Limestone.

The limestone in Spider Cave dips at 70° to the west-southwest and is overturned. On the surface, above the cave, bedding is right way up, dipping at 80° to the east-northeast. On the surface the west edge of the limestone is up to 30m east of the underground river. This variance in dip is a consequence of 1km wavelength, reclined, open folds that have a shallow southerly plunge (Allan 1986). An early cleavage, (S₁) is also folded by these folds.

The rose diagram is a graphical representation of the most common passage directions in Spider Cave based on the length of survey in particular compass directions. The diagram demonstrates the dominance of the NNW and WSW trending passage directions corresponding to bedding and joint control respectively.

Several geologic controls on the development of Spider Cave are apparent. In order of importance these are:

• strike control along steeply dipping bedding
• control along sub-vertical joints
• dyke control
• control along steeply dipping cleavage
• palaeokarst control
• fault control

An examination of the cave map shows that the Jenolan Underground River (JUR) largely is developed along bedding in the basal shaly section of the limestone. Shale bands are clearly visible along the streamway and constrain the stream course north of Wishing Well. Pike Lake Sump corresponds to "the point of breakthrough through a fairly substantial shale band" (Cox and Welch 1984). Based on the stratigraphy above, the stream is within 20m of the base of the limestone. The JUR has migrated down-dip and across-strike to the west as the cave has developed. The streamway only migrates back to the more massive and purer limestones once the bedding turns vertical or steeply east dipping in the vicinity of the tourist caves. East of the streamway Spider Cave is developed in massive limestone and the influence of bedding is less apparent.

The Jenolan Caves Limestone contains regular vertical east-west trending joints. The cliff face above the Spider Cave entrance and the northern side of Hennings Bluff are striking examples of landforms controlled by this structure. Similar east-west jointing is widespread throughout eastern NSW. In Spider Cave this jointing is the dominant control within the massive units of the limestone. Areas developed along jointing in Spider Cave include: the entire entrance series, the area between Caverna Alba and Terror Traverse, the JUR east of the downstream bifurcation (Innes 1979), parts of Glop Hole Gallery, passages leading to Inspired Point, the out flow sump north of Wishing Well, the initial parts of EndZone Sump and Frenchmans Cave. This jointing allows the JUR to quickly migrate across the strike of the limestone at both the upstream and downstream ends of Spider Cave. Jointing in a similar orientation is a major control on passage development in Serpentine Cave (Cooper 1993) and Wiburds Lake Cave (Cooper 1996).

Dark brown basalt dykes are common within Spider Cave, Henrys Hole and Frenchmans Cave and have only been seen developed parallel to the east-west jointing. These dykes have acted as aquacludes and further concentrated water flow along the jointing. It is probable that two ages of basaltic dykes are present at Jenolan. That some dykes intruded along jointing indicates that the jointing predates these dykes that are (?)Tertiary, in line with basaltic volcanism elsewhere in the Blue Mountains. Some of the basalt dykes may be Devonian in age and may be related to the overlying volcanics of the Jenolan Beds. The faulted mafic dyke in Wiburds Lake Cave is probably Devonian. Known Devonian mafic dykes and sills are present in the limestone of the Jaunter area, 20km to the southwest of Jenolan. Dykes have controlled passage development of Spider Cave at the entrance streamway, Upper Helictite Chamber, the out flow sump north of Wishing Well, and Frenchmans Cave. Basaltic dykes are also seen at Pike Lake and in Upstairs Rockpile. No evidence has been observed for granitic dykes or white calcite hydrothermal veining as seen in Mammoth Cave (Cooper 1990) and Serpentine Cave (Cooper 1993).

Parts of Tinkling Tunnel, The Jail, and Khan Passage are developed slightly oblique to bedding within massive limestone. These passages appear to be developed parallel to an early cleavage (S₁) created during initial deformation of the area.

Clearly visible in the Colosseum are two areas of brown dolomitic / ankeritic lithified cave fill. This material indicates that in the past this area had cave development that was then infilled, compacted and lithified. Similar material is present throughout the tourist caves and represents (?)Permian cave fill associated with initial deposition of the Sydney Basin (R.A.L. Osborne pers. comm.). Once exposed to the atmosphere this palaeokarst "rusts" and breaks down more quickly than the primary limestone. This process has largely controlled the shape of the Colosseum and is responsible for the boulder pile filling the bottom half of this chamber.

Cox and Welch (1984) provide a thorough description of the geomorphology of Spider Cave as known at the time. They recognised that cave development is dominantly phreatic (below the water table) with only minor recent vadose modification. They also considered that the cave was largely fault controlled. Evidence for faulting is scant and the "fault" planes are mostly joints which may have minor (<1m) movement. The supposed displacements on the limestone margins are largely explained by the interaction of changing dips and steep topography. The variation in dip through the limestone previously explained by faulting is more fully explained by the reclined folding recognised by Allan (1986). The "fault breccia" in Helictite Chamber is in fact a section of primary rudaceous limestone with coarse fragments of reef detritus.

Both the current and fossil stream gravels in the cave contain rocktypes from both east and west of McKeowns Valley (chert, shale, sandstone and volcanics) indicating that the JUR and Jenolan River supply sediment throughout the cave's development. This implies that Spider Cave has been part of the main JUR system for its entire history.