Geology and Geomorphology of Jenolan Caves and the Surrounding Region David F. Branagan1 John Pickett2 and Ian G. Percival3 , NSW ‘Honorary Associate, School ofGeosciences, University of Sydney, 2006 ([email protected]); WB NSW 2Honorary Associate, Geological Survey ofNSW, Clarke Geoscience Centre, Londonderry 2753 ([email protected]); WB NSW 3Geological Survey ofNSW, Clarke Geoscience Centre, Londonderry 2753 ([email protected]) Published on 30 May 2014 at http://escholarship.library.usyd.edu.au/journals/index.php/LIN Branagan, D.F., Pickett, J.W. and Percival, I.G. (2014). Geology and Geomorphology ofJenolan Caves and the Surrounding Region. Proceedings ofthe Linnean Society ofNew South Wales 136, 99-130. Detailed mapping by university students and staff since the 1980s has significantly elucidated previously poorly known stratigraphic and structural relationships in the Jenolan Caves region. Apart from andesite of?Ordovician age, rocks west ofthe caves probably correlate with the lower Silurian Campbells Group. That succession is faulted against the Silurian (mid Wenlockian) Jenolan Caves Limestone, in which caves developed during several episodes from the late Palaeozoic. Immediately east of Jenolan Caves, siliciclastic sedimentary and volcaniclastic rocks with interbedded silicic lavas constitute the newly defined Inspiration Point Formation, correlated with the upper Silurian to Lower Devonian Mount Fairy Group. Several prominent marker units are recognised, including limestone previously correlated with the main Jenolan limestone belt. Extensive strike-slip and thrust faulting disrupts the sequence, but in general the entire Silurian succession youngs to the east, so that beds apparently steeply-dipping westerly are actually overturned. Further east. Upper Devonian Lambie Group siliciclastics unconformably overlie the Inspiration Point Formation and both are overlain unconformably by lower Permian conglomeratic facies. Carboniferous intrusions include the Hellgate Granite with associated felsite dykes. The regional geomorphology probably evolved from late Carboniferous-early Permian time, with ‘steps’ in the deep valleys indicating episodic periods ofvalley formation, possibly including Permian glaciation. Manuscript received 16 October 2013, accepted for publication 23 April 2014. KEYWORDS: Carboniferous, Devonian, geomorphology, Jenolan Caves, palaeontology, Permian, Silurian, speleogenesis, stratigraphy INTRODUCTION speleogenesis of the caves and their antiquity. In comparison, the regional geology surrounding the Jenolan Caves, located 182 km west of Sydney Jenolan karst area has been relatively neglected, by road (Fig. 1), are Australia’s best known and largely due to its rugged terrain and structural most spectacular limestone caves. Early geological complexity. Thus the geological context in which studies concentrated on the narrow belt of limestone Jenolan Caves formed, that is so crucial to an and mapping ofthe cave system it encloses, whereas understanding of how the cave system evolved, has more recent scientific research has emphasized the taken a long time to unravel, and indeed still requires GEOLOGY OF JENOLAN CAVES REGION 149°59'E 150°00'E 150°01'E 150°02'E 150°03'E 150°04'E — Allan 1986 Hallett 1988 Road - Track Stewart 1987 House 1988 Fire trail Drainage 0 IKm Fig. 1. The Jenolan Caves region, showing the main access road and natural drainage pattern; inset map shows location within the state ofNew South Wales. Also plotted are the outlines of student thesis maps reproduced as Fig. 5 (Allan 1986, in purple), Fig. 6 (Stewart 1987, in blue), Fig. 7 (Hallett 1988, in green) and Fig. 8 (House 1988, in red). 100 Proc. Linn. Soc. N.S.W., 136, 2014 BRANAGAN, PICKETT AND PERCIVAT D.F. J.W. I.G. further study. Geological mapping by students and of the limestone belt at Jenolan were younger than staff (mainly at the University of Sydney) over the the limestone, for which he indicated a westerly dip, past thirtyyears, notably during the 1980s, has greatly and that their cherty nature was the result of contact improved knowledge ofrock types, their distribution metamorphism by intruding dykes. Consequently he and relationships in the vicinity of Jenolan, but this revised his opinion ofthe age ofthe “Cave Limestone” workhasremained largely inaccessible inunpublished to Early or Middle Devonian, younger than the theses and field compilations. The results presented limestones at Yass. However, David and Pittman here are primarily based on detailed field mapping (1899), in a further examination of the radiolarian- (at a scale of 1:10,000) and accompanying reports bearing cherty sediments, expressed uncertainty as to by Allan (1986), Stewart (1987), Hallett (1988) and whether the limestone was Silurian or Devonian. House (1988) (Figs 1, 5, 6, 7, 8), supplemented with Curran (1899) (Fig. 2) discussed some aspects of mapping over the same period by D.F. Branagan the Jenolan geology, placing the eastern succession and K.J. Mills (all of the University of Sydney), of sedimentary rocks in the Silurian, intruded by with additional thesis mapping by Doughty (1994). diorites, quartz- and felspar- porphyries, and included Unpublished studies by Stanley (1925), Chand a photo ofone ofthe cuttings on the road down to the (1963), Pratt (1965), McClean (1983), and E. Holland caves. (former Jenolan Manager) have also been taken into The matter rested there until Morrison (1912), consideration. Other geological investigations of the carrying out a reconnaissance trip to complete the area remain unpublished, although in preparation of proposed Geological Map of New South Wales this paper we have had the benefit ofdiscussion with (published 1914), placed the limestone inthe Silurian, various workers (particularly Ian Cooper) who have together with the adjacent rocks including ‘slates, mapped the Jenolan Caves Limestone and nearby radiolarian cherts, claystones ... and contemporary strata in considerable detail. lavas’, and assigned a post-Devonian age to the intrusive porphyries and felsite dykes observed by David. Morrison (Fig. 2) noted the unconformable WORK PREVIOUS nature of the junction between the Devonian sandstones and quartzites [Lambian rocks] and the The earliestgeological observations ofascientific ‘Upper Marine’ Permian beds, and the occasional nature on the Jenolan Caves area were made by staff occurrence of the younger strata abutting the New of the Geological Survey of South Wales (Fig. Devonian rocks, but several hundred feet below the 2), including Wilkinson (1884) and Young (1884) Devonian outcrops. (vide Havard 1933). Fossils in the limestone attracted Sussmilch and Stone (1915), following brief the attention of Government Palaeontologist Robert statements by Sussmilch (1911, 1913), presented the Etheridge Jr (1892) who described a pentameride results ofa study ofthe caves region undertaken over brachiopod and first assigned a late Silurian age to a number ofyears. Their paper remained the standard these rocks. Initial usage of the name ‘Jenolan Cave explanation of the geology until at least the 1960s. Limestone’ can be attributed to Etheridge (1894) but The study by Sussmilch and Stone (Fig. 2), based on the terminology was not formalized for another 77 the outcrops along the Mount Victoria road (almost to years. Inspiration Point), and the first few bends (Two Mile T.W.E. David (1894) (Fig. 2) concurred with the Hill) of the Tarana road, the Six-Foot track and the late Silurian age ofthe limestone, also assigning that Jenolan River, recognised the essential lithological agetothestratatotheeastthathelaterdescribed(David variations. They dismissed David’s contentionthatthe 1897a) as consisting of“several hundred feet of dark cherts occurring west of the limestone were formed indurated shales, greenish-grey argillites, reddish- by contact metamorphism and determined that they purple shale and coarse volcanic conglomerates with did not dip conformably with the limestone, but large lumps of Favosites Heliolites etc.”. David were probably brought into contact by overthrusting. , , (1897b) further postulated that “the felsite dykes east Sussmilch and Stone (1915) suggested that the cherts of the limestone had assimilated much lime in their (‘Jenolan radiolarian cherts’) and associated dark passage through the limestone”, and suggested that claystones were of Ordovician age. They recognised the conglomerates exposed on the Jenolan road 6 the ‘Cave limestone’ and the geographically distinct miles (9.7 km) from the caves were Upper Devonian. (but then vaguely located) east-dipping ‘eastern Extrapolating from his work on similar rocks at limestone’, which appeared to be unfossiliferous, Tamworth of Devonian age, David surmised that the believing that the separated limestones belonged to cherty radiolarian-bearing rocks cropping out west a single ‘bed’ on opposite sides of a large anticline. Proc. Linn. Soc. N.S.W., 136, 2014 101 GEOLOGY OF JENOLAN CAVES REGION Fig. 2. A selection of geologists who have made significant contributions (discussed in the text) to the investigation or mapping ofthe Jenolan Caves region, spanning more than a century from 1884 to 1988. The final four photographs are those of students from the University of Sydney whose B.Sc. (Honours) thesis maps were used in the compilation of this paper. Some of the historic photographs are sourced NSW from Johns (1976) and Middleton (1991); others are from the image library ofthe Department of Resources and Energy. Top row (L to R): C.S. Wilkinson (NSW Geological Surveyor-in-charge 1875- 1891), T.W.E. David (University ofSydney), E.F. Pittman (NSW Government Geologist 1891-1916), Rev. NSW J.M. Curran. Middle row, left image: officers of the Department of Mines c.1893 (clockwise from top left L.F. Harper*, R. Etheridge Jr (Palaeontologist), O. Trickett (Inspector of Caves), M. Morrison (Assistant Geological Surveyor); right image: C.A. Siissmilch (seated) and W.G. Stone, both of the De- partment of Geology, Mineralogy and Mining, Sydney Technical College; Bottom row (L to R): J.E. Carne (NSW Government Geologist 1916-1920), G.A.V. Stanley (graduate of the University of Sydney 1925), T. Allan (B.Sc. Hons 1986, S.U.), W. Stewart (B.Sc. Hons 1987, S.U.), M. Hallett (B.Sc. Hons 1988, S.U.), M. House (B.Sc. Hons 1988, S.U.). *note that L.F. Harperwas engaged on geological investigations in areas other than Jenolan Caves. 102 Proc. Linn. Soc. N.S.W., 136, 2014 BRANAGAN, PICKETT AND PERCIVAT D.F. J.W. I.G. Sussmilch and Stone grouped all the variably- between the main and eastern limestones, Stanley coloured, thin-bedded, highly-jointed rocks, east of still regarded them as stratigraphically equivalent, the ‘Cave limestone’, as Silurian slates underlying the interpreting the eastern body as closer to the sediment limestone. The unit identified by them as a rhyolite- source, while the main body he surmised to be of porphyry, cropping out close to the Grand Arch, was reefal origin. He thus accepted the large anticlinal an important marker for their structural interpretation structure suggested by Sussmilch and Stone (1915), of an anticline, as it was located again west of the but believed it was complicated by cross faults and eastern limestone. Other igneous bodies were strike -slip faulting. Stanley regarded all the igneous identified as intrusive. These included the andesite bodies east of the main limestone as sills. Perhaps occurring west of the ‘Cave limestone’, and quartz Stanley’s major achievement was the preparation of porphyrites and felsites to the east. the first, surprisingly accurate, contour map of the The stratigraphic order set down by Sussmilch Jenolan region, using compass and tape, Abney level and Stone (1915), in addition to their interpretation and aneroid. of the geomorphic history, subsequently became Geological interpretation of the Jenolan region entrenched in the literature ofJenolan. Influenced by remained untouched for the next 40 years, until a Andrews’ (1911) concept of the Kosciusko Uplift, new round ofuniversity student studies took place in said to have occurred at the end ofthe Pliocene, they the mid-1960s, with work by Chand (1963), Gulson thought that the age of formation ofthe cave system (1963) and Pratt (1965). However, stratigraphic could only be less than 500,000 years. relationships and actual geological ages remained The regional geological interpretation of uncertain and there was certainly some confusion Sussmilch and Stone (1915) was accepted by Carne introduced by the incorrect assignment of bedding to and Jones (1919), who outlined more accurately the structures in the limestone and other units. Chand’s outcrop of the limestone, showing it extending for 1963 work, extending a considerable distance east some distance bothnorthand southofthetouristcaves. from the limestone belt beyond Black Range, was the Of particular interest on their map is the marking of most painstaking, including the collection of nearly m two pods oflimestone approximately 400 westerly 900 rock specimens from widespread documented ofthe almost continuous main belt at its northernmost localities. extent, close to McKeowns Creek, suggesting a Branagan and Packham (1967) were the first to possible offsetting by faulting. However, mapping of recognise the overturning of the sequence east of these bodies was clearly affected by the inadequate the limestone belt, and Packham (1969) revised the base maps available to these earlier workers, as more stratigraphic relations, with the western units being recent mapping shows that despite poor outcrop, the oldest, followed by the limestone and the younger limestone does in fact swing westerly away from eastern beds. the creek, by flexure, and encloses these two pods. Pickett (1969, 1970, 1981, 1982) provided much Came and Jones also located the eastern limestone ofthe modern palaeontological data available on the more accurately than was shown on earlier maps (e.g. limestone. His early reports identified macrofossils Sussmilch and Stone 1915). submitted by C. Mitchell and T. Chalker who Sussmilch (1923) expanded a little on his remapped the limestone (Chalker 1971). Additional previous work, with a revised cross-section, and fossils, predominantly corals, stromatoporoids and provided a geological history beginning with a deep- algae, were identified for that paper by J. Byrnes. sea environment in which the radiolarian cherts were The age of the Jenolan Caves Limestone was given deposited, shallowing to a warm sea in which the as Ludlovian (late Silurian). Conodonts diagnostic eastern sediments were deposited, followed by clear, of Silurian biostratigraphic zones, however, proved shallow seas in which lime-secreting organisms built elusive, despite 20 samples being processed from up a mass of limestone (but not a reefal body). The throughout the extent of the western limestone belt successionwasthoughttohavebeenfolded attheclose (Pickett 1981). Talent et al. (1975, 2003:198), citing ofthe Devonian or during the early Carboniferous. unpublished work by P.D. Molloy, mentioned the G.A.V. Stanley (Fig. 2) carried out considerable presence of conodonts referable to the Ozarkodina mapping for an Honours thesis at the University of crispa Zone (oflatest Ludlow age: Strusz 2007) from Sydney in 1925, but this work was never published, the upper well-bedded part ofthe main limestone belt. so the results were ignored formanyyears. He thought Unfortunately these specimens were not illustrated. the western succession was probably Devonian Strata underlying the Jenolan Caves Limestone were with a gradational boundary against the limestone. assigned to ‘equivalents ofthe Campbells Formation’ Though pointing out the considerable differences (now Group) of late Silurian (Ludlow) age by Talent et al. (1975: fig. 1, column 23). Proc. Linn. Soc. N.S.W., 136, 2014 103 GEOLOGY OF JENOLAN CAVES REGION Lishmund et al. (1986) presented a generalized our revised interpretation of the rock succession map of the limestone occurrences in the vicinity of east of the Jenolan Caves Limestone. In Figures Jenolan and the immediately surrounding geology, 5-8 depicting the detailed geology as mapped by modified from mapping by Chand (1963), Gulson Allan (1986), Stewart (1987), Hallett (1988) and (1963), Pratt (1965) and Chalker (1971). Lishmund House (1988), we retain the informal stratigraphic et al. (1986) also thought that the sedimentary rocks nomenclature oftheir studies, but on the compilation west of(underlying) the main limestone belt might be map (Fig. 4) the formal stratigraphic terminology Silurian, based on lithological similarity to potential as described below is employed. It should be noted regional equivalents, notably the Kildrummie that there are some differences apparent between the Formation. compilation map (Fig. 4) and those of the student Subsequent detailed mapping by Allan (1986), theses (Figs 5, 6, 7 and 8). These differences are Stewart (1987), Hallett (1988) and House (1988), due to additional field observations by Branagan whichhas remained largely unpublished till now(Figs and K.J. Mills and consequent reinterpretation. The 5-8), forms the basis of our current understanding main stratigraphic sections presented (Figs 5-8) are of the geology of the Jenolan region, and is fully Five Mile Hill to Jenolan Caves, Navies Creek, Bulls discussed below. Osborne and Branagan (1985) Creek (with Pheasants Nest Creek), and the Jenolan- indicated a likely karstification age at least as old Kanangra Road (Two Mile Hill section). These as Permian for the development of the caves, and sections reveal unequivocally that this stratigraphic subsequently (Osborne and Branagan 1988) included —sequence is overturned (with but few exceptions) a brief description of the Jenolan karst in an overall the succession younging to the east. Numerous New review ofkarst in South Wales. Detailed studies strike (and thrust) faults in the area separate the ofthe Jenolan Caves Limestone, concentrating on its rocks into distinct lithostratigraphic and structural karstificationhistory, have beenpublished by Osborne domains but because of the paucity of fossils so far (1991, 1993, 1994, 1995, 1999), Osborne etal. (2006) found, the relative age of these domains cannot be and Cooper (1990, 1993). For a project directed to stated with certainty. However, reinterpretation ofthe developing tourism at Jenolan, Branagan (in Hunt scant palaeontological evidence provides the basis for 1994) compiled a geological map based largely on the revised correlations ofrock units west and east ofthe detailed Honours thesis mapping undertaken between main limestone belt, as well as reassessment of the 1986 and 1988 mentioned above. Branagan et al. age ofthe Jenolan Caves Limestone. (1996) summarized the results from this mapping together with that ofDoughty (1994). A. Lower Palaeozoic rocks west ofthe main Apart from the maps by Sussmilch and Stone Jenolan Caves Limestone belt (1915), Carne and Jones (1919), Chalker (1971) and As indicated above, these rocks, with a few Lishmund et al. (1986), other generalised maps ofthe exceptions, have generally been regarded as older boundary of the limestone to have been published than the limestone and probably of Ordovician (or include those of Trickett (1925), Shannon (1976), alternatively Silurian) age. Pratt (1965) informally and Kelly and Knight (1993), the latter which also referred to these beds as the ‘Oberon Hill Chert’, shows adjacent geology, based on unpublished thesis including the andesitic volcanic unit exposed behind mapping by Allan and Stewart. Osborne (1999) Caves House (and in the Lower Car Park), which he in illustrating the limestone belt used mapping by thought belonged within the ‘Jaunter Tuff’ of Shiels Shannon (in Welch 1976), but attributed it to Welch. (1959). Pratt noted closely-spaced concentric folds within the chert sequence. Doughty (1994) followed Pratt (1965) to some extent, informally naming the STRATIGRAPHY succession ofshales, siltstones, sandstones, cherts and andesite, west of and underlying the limestone in the Despite the rather formidable topography, some vicinity of Jenolan, as the ‘Oberon Hill Formation’. excellent road and creek exposures can be measured Doughtycommentedonthegenerallackofchertwhere in the Jenolan area, providing the key to much of he examined the unit as the basis for the modification the understanding of the stratigraphy presented in of the name, and gave its minimum thickness as this paper (Figs 3, 4). The road exposures were the between 1200 and 1500 m. The succession continues basis ofthe mapping by Sussmilch and Stone (1915), north from the tourist area, beyond Dillons Creek (the although some sections of the roads have since been first main stream northerly from the Jenolan-Oberon relocated. Allan (1986) mapped the Inspiration Point Hill road, draining from Oberon Hill and joining road section in great detail, providing the basis for McKeowns Creek opposite South Mammoth Bluff), 104 Proc. Linn. Soc. N.S.W., 136, 2014 BRANAGAN, PICKETT AND PERCIVAL D.F. J.W. I.G. Ma. Trough Fig. 3. Palaeozoic stratigraphy and intrusion history of the Jenolan Caves region; cross-hatched areas represent intervals ofnon-deposition and/or erosion (see text for discussion). Timescale from Gradstein et al. (2012). Proc. Linn. Soc. N.S.W., 136, 2014 105 GEOLOGY OF JENOLAN CAVES REGION 149°59'E 150°00'E 15G°01'E 150°02'E 150°03'E 150°04'E REFERENCE -33°44'S Recent i Debris flow/ I alluvial fan Permian Conglomerate Carboniferous Microsyenite, felsite dykes Hellgate Granite Upper Devonian Lambie Group Quartzite Undated intrusives xxx Hornblende dolerite Lower-Upper Silurian Inspiration Point Formation Tuffaceous sandstone Spilite Limestone Rhyolitic volcanics Siltstone Conglomerate Interbedded siltst./sandstone Silicic & dacitic flows 33 49S Quartz porphyry Tuff& tuffaceous sandstone Lower Silurian Jenolan Caves Limestone Lower- Upper Silurian Campbells Group Cherty siltstone Quartz sandstone Km 1 —33°51'S ? Ordovician Andesite Fault Road s -* Thrustfault Fire trail — Geological boundary ^ ' Track C(haegretu/nMkundoswtno)ne — Anticline, Syncline Drainage Fig. 4. Geological map of the Jenolan Caves region, compiled by D.F. Branagan and K.J. Mills, based on B.Sc. Honours thesis mapping especially as shown in Figs 5-8, and personal observations. Note that there are minor inconsistencies between this map (which shows the formal stratigraphic nomenclature adopted in this paper) and those ofthe students. 106 Proc. Linn. Soc. N.S.W., 136, 2014 BRANAGAN, PICKETT AND PERCIVAT D.F. J.W. I.G. m where Stewart (1987) mapped a sequence more than sequence. This sequence, about 500 thick, is made m 500 thick that he informally named the ‘Western up of wide bands of thinly-bedded radiolarian-rich Jenolan Beds’. black siltstones, interbedded with slates and minor beds ofquartz sandstone. The siltstone bands contain 1 ?Ordovician andesite tight slump folds, show graded bedding, small- . The andesite (informally referred to as ‘Caves scale erosional features, and flame structures, which House andesite’ on some maps), which has puzzled all indicate an easterly facing. Inthin section the siltstone observers since the area was first examined, abuts the consists mainly of a dark chlorite and quartz matrix, Jenolan Caves Limestone over a short distance in the withlargerspheroids ofmicrocrystalline quartz. These vicinity of Caves House (Fig. 5). Chemical analysis are casts ofradiolaria, often visible to the naked eye, by Stone (in Sussmilch and Stone 1915) showed it but they are generally poorly preserved and cannot was originally ofbasaltic-andesitic composition. Two be readily identified. Occasional specimens display a rock types are present: a fine-grained augite-andesite, relict internal structure, and some bear short robust and a porphyritic augite-andesite which occurs as spines. inclusions within the fine-grained rock. Chalker Although evidence is slight in the immediate (1971) suggested that the andesite represented an vicinity of Jenolan, exposures to the north (in intrusive body, although it has more generally been McKeowns Valley) show that these ‘Western Jenolan interpreted as aflow, apparently conformable with the Beds’ have a faulted, and probably unconformable, limestone. However, it is probable that the andesite contact with the overlying limestone succession. unit has been brought into position by faulting along Sussmilch and Stone (1915) recognized an overthrust McKeowns Fault and that its stratigraphic position is, fault, subsequently mapped by Stewart (1987) as therefore, uncertain. Doughty (1994) noted that close a high-angle reverse fault (the McKeowns Fault, to CavesHouse,theJenolanCavesLimestone contains interpreted as a near-vertical thrust defined by a thin clasts of andesite, indicating an unconfonnable or layer offractured rock) that separates this succession disconformable relationship with the andesite body. from the Jenolan Caves Limestone. This fault is Presence of an unconformity is supported by the noted also on the western end ofthe detailed section observation that in the eastern Lachlan Fold Belt, measured by Stewart (1987) alongNavies Creek (Fig. andesitic rocks are characteristic of the Ordovician, 6 ). rather than the Silurian. Accordingly, the andesite is On the western side of McKeowns Valley there m most likely of Ordovician age, making it the oldest is a 90 wide zone ofbrecciation, consisting mainly rock unit exposed in the Jenolan region. ofcherty clasts (Fig. 6). Neither the displacement nor the amount of strata missing can be determined, but 2, Campbells Group equivalents (Lower Silurian! there appears to be no angular discordance between The ‘Western Jenolan Beds’ of Stewart (1987) the two units. However, in view ofthe apparent lack consist of two broad units, an older quartz-rich of chert in the succession as it is mapped south to sandstone unit, and a younger ‘cherty’ sequence (Fig. Jenolan and beyond, it may be that this fault runs 6). The sandstone unit includes fine and medium- slightly obliquely to the general strike of the beds grained sandstones, with very minor slates, and a thin and cuts out the cherts. The width of the fault zone tuffaceous layer (possibly more than one). The unit is certainly suggests that the effect ofthe fault could be dark to light grey with a distinctive blocky outcrop, quite significant. and occupies the ridge tops. In thin-section it is seen The ‘Western Jenolan Beds’ have previously to be composed mainly ofrounded, strained grains of been assigned an Ordovician age by some authors quartz, with 5 to 10% of lithic fragments, and about (e.g. Stewart 1987), although Pratt (1965) thought 1% of mica fragments, and about the same volume they might range into the early Silurian. Packham of matrix, composed of white mica, calcite, sphene, (1969) suggested that they could be correlated with chlorite and epidote. Iron oxide cement is present, the RockleyVolcanics, cropping outto the west. Other usually only about 1%, but in exceptional cases it authors (Chalker 1971; Talent et al. 1975; Lishmund may make up about 20% of the rock, imparting a et al. 1986) have regarded the rocks underlying the dark colour to some hand specimens. The tuffaceous main limestone belt to be of Silurian age. layer is mainly composed of weathered felspar. This Recent mapping by the Geological Survey of NSW sandstone unit continues west ofthe mapped area and suggests that much of the Rockley Volcanic its thickness exceeds 450 m. Belt should nowbe regarded as Silurian, withreported There is a distinct, but not sharply delineated, evidence of Ordovician ages (e.g. Fowler and Iwata lithological change to the overlying finer-grained 1995) from this tract to the west of Jenolan being Proc. Linn. Soc. N.S.W., 136, 2014 107 GEOLOGY OF JENOLAN CAVES REGION 150°01'E 150°02'E REFERENCE ? Upper Silurian - ? Lower Devonian Eastern Beds Spilite and keratophyre Quartz porphyry Phyllites and cleaved mudstone Massive to impure shaly limestone Mudstone Cleaved mudstone Quartz sandstone Upper Silurian - Lower Devonian Jenolan Beds Massive quartzo- J :• feldspathic sandstone Interbedded siltstone/sandstone Conglomerate Dacitic crystal tuff Quartz porphyry Cleaved mudstone Upper Silurian Jenolan Caves Limestone ? Ordovician to Silurian Black radiolarian-rich mudstone L. > V Andesitic lava L. Fault Geological boundary Dip and strike Dip and strike inverted Road West Ridge East Fire trail Track Drainage Huts Fig. 5. Geological Map and cross section, modified from Allan (1986). Note that stratigraphic names utilized in the Legend (except for Jenolan Caves Limestone) are informal. 108 Proc. Linn. Soc. N.S.W., 136, 2014