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Structural evidence of the age offolded rocks on the south coast of NewSouth WalesM. A. Etheridge a , D. M. Ransom b , P. F. Williams c & C. J. L.Wilson da Department of Geology and Mineralogy , University ofAdelaide , Adelaide, South Australia, 5001b Cominco Exploration Pty Ltd , 8 Greenhill Road, Wayville, SouthAustralia, 5034c Department of Earth and Atmospheric Sciences , StateUniversity of New York , Albany, New York, U.S.A.d Geologisch en Mineralogisch , nstituut de Rijksuniversteit ,Garenmarkt, Leiden, HollandPublished online: 03 Aug 2007.

To cite this article: M. A. Etheridge , D. M. Ransom , P. F. Williams & C. J. L. Wilson (1973)Structural evidence of the age of folded rocks on the south coast of New South Wales, Journal ofthe Geological Society of Australia, 19:4, 465-470, DOI: 10.1080/00167617308728814

To link to this article: http://dx.doi.org/10.1080/00167617308728814

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STRUCTURAL EVIDENCE OF THE AGE OF FOLDED ROCKSON THE SOUTH COAST OF NEW SOUTH WALES

By M. A. ETHERIDGE, D. M. RANSOM, P. F. WILLIAMS & C. J. L. WILSON

(With 1 Plate and 2 Text-Figures)

(Received 27 July, 1972; read by title in Perth, 4 September 1972)

ABSTRACTThe pre-Devonian sedimentary and volcanic sequence exposed along the south

coast of New South Wales has previously been divided into three stratigraphicgroups: (1) Upper Ordovician graptolite-bearing slate which is conformable with(2) interlayered thinly-bedded greywacke and pelite of undifferentiated Ordovician ageand (3) Cambrian successions of interlayered chert, pelite and volcanic rock at Bate-mans Bay and Narooma. The main bases for this subdivision are a change in rocktypes between (1) and (2), and the unconformity between (2) and (3) formerlyproposed on the basis of changes in rock type and fold style across the boundary.New structural data are presented which refute the presence of the unconformity, andconformity of (2) and (3) with the fossiliferous slates is established.

INTRODUCTIONStrongly deformed rocks exposed along the

south coast of New South Wales from northof Batemans Bay to Eden (Fig. 1) have beendescribed by Brown (1928, 1930, 1933),David (1950), Hobbs (1962), and Wilson(1968). The monotonous rock type of thethinly-bedded greywacke and pelite whichdominate the sequence, the lack of strati-graphic markers through most of the sequence,and the paucity of outcrop away from thecoastal exposures make regional mapping andcorrelation almost impossible. In addition,apart from a (?)retiolid graptolite of indefin-able age from near Batemans Bay (Sherrard,1962), the rocks are unfossiliferous, althoughthey are overlain to the west in some areas byUpper Ordovician graptolite-bearing slate.

Major outcrops of chert, pelite and basicvolcanic rocks at Batemans Bay, Narooma andBermagui are the only other rock typesexposed, apart from scattered, very minorbands of chert. The boundary between theseand the dominant greywacke sequence isobscured in all but one place by the paucityof outcrop, but previous workers, Brown(1928, 1930, 1933), David (1950) and theGeological Survey of New South Wales (1965,1:250,000 Bega Sheet) have claimed that thechert-volcanic sequence underlies the grey-wackes unconformably. They have referred tothe change in rock type and the apparentgreater structural complexity in the cherts.

With this somewhat sketchy information, thegreywacke sequence has been assigned anOrdovician age, and the chert-volcanic

sequence a Cambrian age. It is the purpose ofthis paper to review these ages in the light ofrecent structural data, and especially to investi-gate the proposed unconformity, whosepresence is the only justification for the Cam-brian age assigned to the cherts and volcanics.Since these exposures make up over half theareal extent of the Cambrian in N.S.W.recorded in Packham (1969), and are over320 kilometres south-southeast of the nearestoccurrence at Stuart Town (Packham, 1969),it is important to establish their credentials.

THE NATURE OF THECHERT-VOLCANIC SEQUENCE/GREYWACKE SEQUENCE BOUNDARY(1) Batemans Bay

There is an exposure of the boundarybetween the two sequences at Sunpatch, 12kilometres south of Batemans Bay. A cliffoutcrop (Plate 26) reveals thick (1 to 2metres), massive greywacke layers interbeddedwith tightly folded chert and siliceous pelite.No facing criteria are available in this par-ticular exposure, so the macroscopic structureof the exposed rocks cannot be determined,but the following points are clear:

(i) There is only one group of mesoscopicfolds in the chert sequence, and thesefolds have very similar geometry to thesingle group of folds in the greywackesequence immediately to the south andwest of this exposure.

(ii) A fold in the chert/greywacke boundaryhas the same geometry as those in bothsequences.

Journal of the Geological Society of Australia, Vol. 19, Pt. 4, pp. 465-470, PI. 26, January, 1973.

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466 M. A. ETHERIDGE, D. M. RANSOM, P. F. WILLIAMS & C. J. L. WILSON

35*30'

PERMIAN

DEVONIAN(Mid-Upper)

ORDOVIClAN

CAMBRIAN

DEVONIANGranites

10 20i i i

30

Kilometres

Fig. 1. Regional geological map of the coastline between Batemans Bay andEden. The data are taken from the N.S.W. Geological Survey1:1,000,000 sheet of New South Wales.

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AGE OF FOLDED ROCKS ON SOUTH COAST OF N.S.W. 467

(iii) The chert bands immediately adjacent tothe boundary are parallel to it.

Along the coastline north of Sunpatch, amappable sequence of chert, pelite andsiliceous pelite, with minor basic volcanic rocksand breccia can be traced to within 6 kilo-metres south of Batemans Bay, where the grey-wacke reappears. Once again the boundariesbetween the rock-types are obscured, but thegeometry of the first group of folds in each isidentical. Beyond these greywackes the chertsequence reappears and continues to Durras,9 kilometres north of Batemans Bay where itdisappears under the flat-lying Permian sedi-mentary rocks.

In the whole sequence from Sunpatch toDurras, the sedimentary structures, the cleav-age/bedding relationships and the vergence(Wood, 1963) of the mesoscopic folds, pointto an upward-facing, uniformly steeply wes-terly dipping structure on the macroscopicscale. Thus the coastal section over this wholeextent progresses down through a conformablesequence of greywacke, chert-volcanics, grey-wacke and finally chert, all containing peliticstrata.

(2) NaroomaWilson (1968) mapped a major anticlinal

structure in a sequence of greywacke, pelite,chert and volcanic strata at Narooma. Theoldest rocks in this structure are a series ofslates, phyllites and arenites with minor chert(Bogolo Formation), overlain by the chertsand volcanic rocks of the Wagonga Formation,and finally by the ubiquitous thinly-beddedgreywacke and pelite which continue some 60kilometres northwards to Sunpatch.

In outcrops in the hinge of the anticlinemost of the chert bedding strikes NNW (paral-lel to the axial plane of the macroscopic struc-ture), and that in the greywacke strikesparallel to the mappable folded surface (cf.Wilson, 1968, Fig. 2). It was this and similaroccurrences of the obvious discordance inbedding trends that first prompted Brown(1930) to propose that the two units wereunconformable. However, structural analysisshows that the first (defined by facing criteria)folds in all three mapped units have the samefold-axis and axial-plane geometry. Further,comparison of the chert and greywacke inother localities shows that the apparent bedd-ing discordance in the fold hinge is a functionof the fold style in the different rock types(Williams, 1969). The chert is deformed into

very tight to isoclinal folds, and in small out-crops where mesoscopic fold hinges are rare,all the observable bedding is parallel to themajor structure's axial plane (Fig. 2). Thissituation is analogous to that of transposedbedding in other highly deformed regions(Baird, 1962; Hobbs, 1965), where the meso-scopic layering is also axial plane to macro-scopic folds.

Fig. 2. Sketch map of the types of relationshipfound between the greywacke and chertsequences at Sunpatch (A), Narooma(B). The greywacke sequence (stippled)is not folded mesoscopically, and appearsto be unconformable with the underlyingcherts in the hinge of the fold (B).

(3) BermaguiThe other major chert occurrence is at

Murunna Head, three kilometres north ofBermagui, where thinly interbedded chert andsiliceous pelite outcrop over about one kilo-metre of cliff exposures. The northeasternboundary with the greywacke sequence is afault and the southwestern boundary isobscured, so that direct evidence on its natureis again lacking. Also evidence of multipledeformation in both sequences and lack ofinformation on the fault movement geometrymake normal structural methods somewhatambiguous. This situation is especially pre-valent in the chert, where overprinting poly-clinal folds (cf. Hobbs, 1962) make geometricanalysis tenuous. However, the geometry ofthe first recognisable folds in the greywackeindicates that the chert underlies it in the coreof an anticlinal structure, with much of thetightly folded chert layering parallel to its axialplane (Williams, 1971).

(4) BodallaCoastal outcrops east of Bodalla consist

almost wholly of thinly interbedded greywacke

Journal of the Geological Society of Australia, Vol. 19, Pt. 4, pp. 465-470, PI. 26, January, 1973.

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468 M. A. ETHERIDGE, D. M. RANSOM, P. F. WILLIAMS & C. J. L. WILSON

and pelite deformed by only one episode ofmesoscopic and macroscopic folding. Four thinbands of chert (maximum 60 cm thickness)have been found within this sequence, andthey are everywhere conformable with it. Suchoccurrences are also found within the grey-wacke sequence in most other localities. How-ever, one particular outcrop lucidly contraststhe style of the folding in the chert with adja-cent greywacke. There is very complex foldingin one chert band less than 30 cms wide be-tween two planar greywacke beds. Once again,however, notwithstanding their apparent com-plexity, these small folds are coaxial with thesomewhat larger ones in the surrounding grey-wacke-pelite sequence, and all lithologies areobviously conformable.

THE RELATIONSHIP OF THEGREYWACKE SEQUENCE TO THEUPPER ORDOVICIAN SLATES

Along most of its exposure, the greywackesequence is unconformably overlain by gentlyfolded Upper Devonian quartzites, or intrudedby Middle Devonian granite masses. It is onlyin a few localities that the immediately over-lying Upper Ordovician slates crop out, anddue to the almost complete lack of outcropaway from the coast, the contact between thetwo formations is always poorly exposed. Onceagain, however, a comparison of their struc-tural geometries reveals that the dominantfolding in the slate is coaxial with and has thesame axial planar orientation as the first fold-ing in the greywacke. It is thus concluded thatno obvious angular unconformity existsbetween the Upper Ordovician slates and thegreywacke sequence.

DISCUSSIONCriteria for establishing conformity between

two adjacent rock formations are that:(i) The original sedimentary layering in the

older unit is parallel to the surfacebetween them.

(ii) The structural elements of the firstperiod of deformation in each are paral-lel. They do not have to possess thesame symmetry,

(iii) There is no drastic change in:(a) metamorphic grade;(b) lithological association, across the

boundary.Of these, (i) and (ii) are both necessary

and sufficient, while (iii) (a) and (b) are

necessary but not sufficient to establish con-formity.

The existing 'unconformity' and agesrecorded in the literature are based merely onthe criteria of:

(i) difference in deformational stylesbetween the cherts and the greywackes;and

(ii) the changes in rock type across theboundary.

The use of fold style to separate folds ofdifferent deformational phases is deep-rootedin the literature, to the extent that Turner &Weiss (1963, p. 79) inferred that it is the mostimportant criterion in determining the agerelationships between various folds. Morerecently, however, Means (1963, 1966) foundthat two main fold styles in the Otago Schistsof New Zealand could be recognised in twoof the three phases of deformation defined byoverprinting criteria. More significantly in thiscontext, Williams (1969), in a detailed studyof the rocks exposed at Bermagui, concludedthat the many fold styles present there haveno chronological significance in a series ofdeformational events separated by unambig-uous overprinting criteria. On this basis, thefirst criterion of Brown (1930) is disregardedas being neither necessary nor sufficient.

Brown's second criterion is related to (iii)(b) above, which is not sufficient on its own.In addition, the association of chert, basicvolcanic rock and pelite and greywacke ofturbidite origin is extremely widespread, andis indeed characteristic of flysch successions.Thus, the second premise is also dismissed asneither necessary nor sufficient.

The present study has produced evidence atBatemans Bay and Narooma which complieswith all the criteria mentioned above. In bothareas the facing evidence shows that the firstrecognisable folds represent the first episodeof deformation in both the chert sequence andthe greywacke. Also the fold axes and in mostcases axial planes of these earliest folds arecoincident in both sequences. The parallelismof the chert sedimentary layering to the boun-dary and the parallelism of folds in this boun-dary to those in the rocks on either side of itfulfil criterion (ii). Finally, the metamorphicgrade of the whole sequence (away fromintrusive bodies) is uniformly low, defined bya chlorite-muscovite-albite-quartz assemblagein the greywacke, and chlorite-albite-epidotein the volcanic rocks.

The chert exposures at Bermagui are con-

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AGE OF FOLDED ROCKS ON SOUTH COAST OF N.S.W. 469

sistent with conformity, but criteria (i) and(ii) cannot be fulfilled beyond doubt. TheBodalla example further emphasises theinapplicability of the fold style argument as achronological guide.

It is concluded, therefore, that no uncon-formity exists between the Upper Ordoviciangraptolite-bearing slates and the lowermostmembers of the chert-volcanics sequence.Thus, without independent criteria, there is nojustification for assigning a Cambrian age toany of the rocks exposed along the south coastof New South Wales. It is suggested that allthe rocks below the fossiliferous slates be

grouped into an undifferentiated Lower Palaeo-zoic sequence.

ACKNOWLEDGMENTS

The authors wish to acknowledge the helpof Professor B. E. Hobbs throughout thework, which arose from honours and Ph.D.projects carried out at the University ofSydney. The financial assistance of the Aus-tralian National University is also acknow-ledged during completion of the field work.Mr R. G. Wiltshire is thanked for helpful dis-cussion.

REFERENCES

BAIRD, A. K., 1962: Superposed deformations inthe Central Sierra Nevada Foothills, east ofthe Mother Lode. Univ. Calif. Publs geol.Sci., 42, pp. 1-70.

BROWN, I. A., 1928: The geology of the SouthCoast of New South Wales: Pt. 1, ThePalaeozoic geology of the Moruya District.Proc. Linn. Soc. N.S.W., 53, pp. 151-192.

, 1930: The geology of the South Coastof New South Wales: Pt. II, Devonian andolder Palaeozoic rocks. Proc. Linn. Soc.N.S.W., 55, pp. 145-158.

, 1933: The geology of the South Coast ofNew South Wales with special reference tothe origin and relationships of the igneousrocks. Proc. Linn. Soc. N.S.W., 58, pp.334-362.

DAVID, T. W. E., 1950: The Geology of the Com-monwealth of Australia. London, EdwardArnold.

HOBBS, B. E., 1962: Structural analysis of a smallarea in the Wagonga Beds, New South Wales.Aust. J. Sci., 9, pp. 71-86.

, 1965: Structural analysis of the rocksbetween the Wyangala Batholith and theCopperhannia Thrust, New South Wales. / .geol. Soc. Aust., 12, pp. 1-24.

MEANS, W. D., 1963: Mesoscopic structures and

multiple deformation in the Otago Schist.NZ. Jl Geol. Geophys., 6, pp. 801-816.

, 1966: A macroscopic recumbent fold inschist near Alexandra, Central Otago. N.Z.Jl Geol. Geophys., 9, pp. 173-194.

PACKHAM, G. H., 1969: ? Cambrian System; inPackham, G. H. (Ed.), The Geology of NewSouth Wales. / . geol. Soc. Aust., 16, pp.73-76.

SHERRARD, K., 1962: Further notes on assemblagesof graptolites in New South Wales. / . Proc.R. Soc. N.S.W., 95, pp. 167-178.

TURNER, F. L, & WEISS, L. E., 1963: StructuralAnalysis of Metamorphic Tectonites.McGraw-Hill, New York.

WILLIAMS, P. F., 1969: Tectonic studies of rocksexposed along the South Coast of New SouthWales. Ph.D. Thesis, Univ. Sydney [unpub-lished].

, 1971: Structural analysis of the Berma-gui Area, N.S.W. / . geol. Soc. Aust., 18, pp.215-228.

WILSON, C. J. L., 1968: The geology of theNarooma area, N.S.W. J. Proc. R. Soc.N.S.W., 101, pp. 147-157.

WOOD, B. L., 1963: Structure of the OtagoSchists. N.Z. Jl Geol. Geophys., 6, pp.641-680.

M. A. Etheridge,Department of Geology and Mineralogy,University of Adelaide,Adelaide,South Australia 5001.

D. M. Ransom,Cominco Exploration Pty Ltd,8 Greenhill Road,Wayville,South Australia 5034.

P. F. Williams,Department of Earth and Atmospheric Sciences,

State University of New York,Albany,

New York,US. A.

C. J. L. Wilson,Geologisch en Mineralogisch,

Instituut de Rijksuniversteit,Garenmarkt,

Leiden,Holland.

Journal of the Geological Society of Australia, Vol. 19, Pt. 4, pp. 465-470, PL 26, January, 1973.

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470 M. A. ETHERIDGE, D. M. RANSOM, P. F. WILLIAMS & C. J. L. WILSON

EXPLANATION OF PLATE

PLATE 26

Photograph of the contact between the cherts and a massive greywacke bed in a cliff exposureat Sunpatch. The section shown in the photograph is about 5 metres high.

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M. A. ETHERIDGE, D. M. RANSOM, P. F. WILLIAMS & C. J. L. WILSON PLATE 26

'•SAmPfflt

Journal of the Geological Society of Australia, Vol. 19, Pt. 4, PI. 26, January, 1973.

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