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Sedimentology, weathering, age and geomorphologicalsignificance of Tertiary sediments on the far southcoast of New South WalesJ. F. Nott a b , R. W. Young a & M. Idnurm ca Department of Geography , University of Wollongong , PO Box 1144, Wollongong, NSW,2500, Australiab School of Chemistry and Earth Science , Northern Territory University , PO Box 40146,Casuarina, NT, 0811, Australiac Bureau of Mineral Resources , GPO Box 378, Canberra, ACT, 2601, AustraliaPublished online: 09 May 2007.

To cite this article: J. F. Nott , R. W. Young & M. Idnurm (1991) Sedimentology, weathering, age and geomorphologicalsignificance of Tertiary sediments on the far south coast of New South Wales, Australian Journal of Earth Sciences: AnInternational Geoscience Journal of the Geological Society of Australia, 38:3, 357-373, DOI: 10.1080/08120099108727978

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Australian Journal of Earth Sciences (1991) 38, 357-373

Sedimentology, weathering, age and geomorphologicalsignificance of Tertiary sediments on the far south coast of

New South WalesJ. F. NOTT1*, M. IDNURM2 AND R. W. YOUNG1

1Department of Geography, University of Wollongong, PO Box 1144, Wollongong,NSW 2500, Australia.

2Bureau of Mineral Resources, GPO Box 378, Canberra, ACT 2601, Australia.

The age of the NSW coastal lowland from Tuross to the Victorian border can now be shown to be at least mid-Tertiary.By this time the coastal plain had twice been partially blanketed by terrestrial sediments. Palaeomagneticdeterminations on the more recent of these sedimentary accumulations, the Long Beach Formation, reveal a minimumdepositional age of Early Miocene. Eustatic influences may be responsible for the aggradation of the Long BeachFormation and as a consequence the diversion of the lower Bega River from its former Tertiary valley. Markeddifferences in the pattern of weathering between the older sediments, the Quondolo Formation, and the inset LongBeach Formation are evident, with the former largely silicified and the latter ferruginized. Similarities to the Quondoloand Long Beach Formations in terms of their stratigraphic relationships, chronology and weathering styles are evidentwithin other Tertiary sedimentary formations north along the coastal plain to Ulladulla.

Key words: Long Beach Formation, Miocene, NSW south coast, Oligocene, palaeomagnetism, palynology, QuondoloFormation, Tertiary.

INTRODUCTION

The terrestrial sediments scattered along the coastof southern New South Wales have received littleattention, but provide important clues to the evol-ution of the coastal lowland and of the adjacenthighland. The sedimentology, weathering charac-teristics and magnetic age of weathering within Ter-tiary deposits at Long Beach, 8 km north of Eden,and at Tura Beach, 5 km north of Merimbula (Fig.1) form the basis of this paper. New evidence fromthese deposits throws light on four major aspects ofthe regional geomorphology. First, it provides evi-dence of the timing of the uplift of the adjacenthighlands. Second, it demonstrates the occurrenceof at least two major phases of regional stream ag-gradation, which together blanketed much of thecoastal lowland during the Tertiary. Third, it in-dicates the timing of several major weatheringevents, especially a change from dominantly sil-iceous to dominantly ferruginous induration. Andfourth, it yields an obvious explanation for ananomalous aspect of the regional drainage, theabrupt change of direction of the lower Bega Riverfrom a large abandoned valley into a narrow gorgethat leads to the sea at Tathra.

*Present address: School of Chemistry and Earth Science,Northern Territory University, PO Box 40146, CasuarinaNT 0811, Australia.

REGIONAL SETTING

South of Jervis Bay the coastal lowland of NewSouth Wales varies in width from 10 to 15 km. Tothe west it is bounded by the escarpment of theSouthern Highlands which rise steeply to a height of~ 600 m. Most of the coastal lowland is hilly, withonly relatively small areas of flat to undulating top-ography. These discontinuous patches of undu-lating land, which extend 2-4 km inland from thecoast, are partly mantled with Tertiary terrestrialsediments. The thickness of these sedimentary unitsvaries from 10 to at least 60 m, and possibly 150 m,and the majority occur at elevations near, or belowpresent day sea level. North of Batemans Bay theTertiary sediments unconformably overlie Permiansediments of the southern Sydney Basin, but farthersouth they rest on truncated strata of the LachlanFold Belt.

SEDIMENTOLOGY AND STRATIGRAPHY

At Long Beach the Tertiary sediments cover an areaof ~ 11 km2 and in places reach a thickness of up topossibly 150 m. They have been deposited within abroad depression cut into the extensively foldedLate Devonian Merrimbula Group. To the south,the Long Beach Tertiary sediments form a 10-30 mhigh sea scarp ~ 2.5 km in length, and to the north

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358 J. F.

(a)

Wollongong

Kiama ,

BENDALONG

MERINGO CREEKFORMA TION

BERGAL1AFORMATION

BERMAGUIBEDS

TURA BEACH \ LONG BEACHLONG BEACH] QUO«DOLQ

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Merimbula ...yS- Lake ./^

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PAMBULAtfeA.'27 BEACH4"

j =-< QuoraburagunPinnacles

to EDEN/

Urban Areas

Long Beach Formation

Quondolo Formation

Lennards- Is.

Fig. 1 (a) Location of Tertiary sedimentary units on the south coast of New South Wales, (b) Location of the LongBeach and Quondolo Formations.

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TERTIARY SEDIMENTS, SOUTH COAST NSW 359

and west the Long Beach outcrop is bounded byPambula River and the Princes Highway respec-tively (Fig. 1).

At North Tura the Tertiary sediments cover anarea of ~ 2 km2. They abut the Middle Devonianrhyolite of the Boyd Volcanic Complex and overliethe Merrimbula Group. The sediments are well ex-posed along the beach where, like the Long Beachsediments, they form a scarp, 15-30 m high and1.5 km long. In outcrop they are at least 60 m thick,and Hall (1969) reported that a bore was sunk to adepth of 120 m before reaching bedrock to the westof the beach scarp.

The Tertiary sediments between North Tura andLong Beach are here subdivided into the LongBeach and Quondolo Formations. Both of these for-mations are best exposed at Long Beach and it isfrom this location that the following type sectiondescriptions are taken.

Quondolo Formation

The Quondolo Formation is composed of mediumto coarse well indurated pink (5YR 8/3) sand, withnumerous angular to subangular quartz clasts up to10 cm long. Layers of well rounded poorly sortedboulders of Devonian sandstone, up to 1.5 m acrosstheir b-axis, are also present (Fig. 2). Bedding withinthe unit is mainly horizontal, except for a suddendip southwards under the Long Beach Formation.At the northern end of the Long Beach sea scarp, theQuondolo Formation abuts against the incised

Devonian strata. Here, both the top of theQuondolo Formation and the planated surface ofsteeply dipping Devonian strata lie 22.5 m abovesea level.

Long Beach Formation

The Long Beach Formation is significantly less in-durated, more extensive, voluminous and textufallyheterogeneous than the Quondolo Formation. Atthe northern end of the Long Beach scarp it overliesboth the Quondolo Formation and the truncatedDevonian strata (Fig. 2). Two hundred metres to thesouth the Long Beach Formation rests against theincised southward-dipping Quondolo Formation,and 1.5 km farther south it abuts the Devonianstrata at Lennards Point. Like the Quondolo For-mation, the Long Beach Formation appears to ex-tend below present sea level.

Even within the exposures at Long Beach a greatdeal of textural variation is evident in the LongBeach Formation. Where the unit is inset betweenthe Quondolo Formation and a small outcrop ofDevonian siltstone and sandstone 300 m farthersouth, it is composed of fine sand and silt with oc-casional patches of coarse sand and small wellrounded pebbles. In places, these small pebblesform layers up to 10 mm thick. The eroded rem-nants of this unit form the Quoraburagun Pinnacles(Fig. 3) and is here named the Pinnacles Lens. ThePinnacles Lens gradually pinches out to the southabove the small southern outcrop of Devonian

IIP Long Beach FormationPinnacles Lens

Quondolo Formation

Devonian Merrimbula Group

Fig. 2 Schematic stratigraphy of the Long Beach Formation, Pinnacles Lens and the Quondolo Formation.

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Fig. 3 Quoraburagun Pinnacles. This constitutes the main part of the finely mottled well lateritized Pinnacles Lens.Note the sharp horizontal break between the upper mottled and lower pallid zones. This break dips suddenly towards theeast, as seen in the left of the photograph, marking the location of the sampled slumped sediments.

Fig. 4 Trough cross-beds within the Pinnacles Lens.

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TERTIARY SEDIMENTS, SOUTH COAST NSW 361

exposed in the Eden rubbish tip 150 m above sea level. These sediments show mashown in Fig. 3.

nsum

Fig. 6 Coarse mottle patterns in the southern cliffed sediments of the Long Beach Formation.

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strata. The bulk of the Long Beach Formation istexturally more diverse, with clay and sandy claylayers, 10-50 cm thick, interbedded with layers offine to coarse sand which contain scattered angularto well-rounded quartz clasts up to 5 cm across theiri-axis, and rare black quartzite clasts. Tabularcross-bedded sand units up to 30 cm thick are evi-dent. More prevalent, however, and especiallywithin the Pinnacles Lens, are trough cross-beddedunits up to 1 m wide and 20 cm thick (Fig. 4). Un-fortunately, too few flow structures are preservedfor reliable palaeoflow directions to be determined.

The Quondolo and Long Beach Formations atNorth Tura and other localities

Outcrops of the Quondolo Formation occur alongthe Princes Highway between Long Beach andMerimbula, especially near the Yowaka River.They also are found on the western shore ofMerimbula Lake and on the southern headland atNorth Tura Beach (Fig. lb), where the QuondoloFormation is disconformably overlain by the LongBeach Formation. Most of these outcrops displaysimilar induration, colouring and sedimentarycharacteristics. Only at Long Beach, however, arethe large boulders present, while at North Turamottling is better developed than in the otheroutcrops.

At Long Beach the surface topography of theLong Beach Formation is bench-like as it rises, atfirst gently then more steeply, towards the PrincesHighway 500 m to the west. The Long Beach For-mation is well exposed in the Eden refuse tip, theupper part of which lies 150 m above the beachsection (Fig. 5). The sediments here are similar tothose of the Pinnacles Lens exposed at theQuoraburagun Pinnacles. Farther north, near Pam-bula Lake, a 20 m thick sequence of weatheredgravel and sand disconformably overlies a wellsorted unit of medium to coarse sand. Similargravel and sand are exposed in small cuttings alongthe Princes Highway between this outcrop andMerimbula golf course. All of the gravel depositsalong the highway are at topographically higherpositions than the exposures of the QuondoloFormation, though no contacts with that formationare exposed.

At North Tura, the Long Beach Formation is gen-erally finer grained than at the type section. Cyclicsedimentation patterns are evident. Beds of coarsesand grade upwards into fine sand, silt and clay.These cycles vary in thickness from 0.5 to 2.5 m.They are not always laterally continuous becausepost-depositional incision and infilling has left

trough shaped lenses of coarse sands within thesequence. These coarse sand units vary in width andthickness from 1 to 15 m and 1 to 20 m respectively.Trough cross-beds up to 3 m wide and 0.5 m thickare also evident within these lenses. The present daytopography is similar to that at Long Beach, with abench-like surface rising westward. Exposures alongthe Merimbula-Tathra road to the west of theNorth Tura sea scarp reveal gravel deposits within acoarse sand matrix.

Discussion

Both the Long Beach and Quondolo Formationsappear to have been deposited within fluvial en-vironments. Evidence of such an origin, especiallyin the well sorted and bedded gravel and sand de-posits immediately west of the sea cliffs, is clearer inthe Long Beach Formation. In general, the textureof these sedimentary units fines eastward and atfirst sight this may be taken as superficial evidenceof deposition by these streams within an estuarinereach in the vicinity of the present day coastline.However, as mentioned previously, the dominantsedimentary structures revealed in the cliffs at Longand North Tura Beaches are trough cross-beds indi-cating a reasonably high unidirectional flow regime.This together with the presence of pollen and sporesand the absence of marine micro-fossils in the sedi-ments of the Long Beach Formation suggests thatdeposition occurred upstream of an estuary.

The origin of the Quondolo Formation is less cer-tain but the cobble lenses and dominantly coarsesand deposits infilling distinct bedrock channelspoint to fluviatile deposition. Immediately to thenorth of the Pinnacles Lens, large poorly sortedboulders of Devonian sandstone appear indicativeof high-energy channel base deposits, althoughsome may have slumped from the adjacent valleywall cut in the Devonian strata. Unfortunately, nofossils are evident in the Quondolo Formation.

WEATHERING

Differences in patterns of weathering are clearlyevident between the formations. Most of the sedi-ments comprising the Quondolo Formation havebeen well silicified, with silica levels of 96% (Table1), making them comparable with silcrete fromother parts of Australia (Young 1985). However,layers of less indurated sediment occur within theupper levels of the Quondolo Formation at LongBeach. Ferruginous mottling patterns occur in theQuondolo Formation only at the small outcrop onNorth Tura Beach.

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Table 1 Mineral and elemental analyses, and grain sizes of sediments comprising the Long Beach Formation (LBF) andQuondolo Formation

XRD Mineral(order of %Coarse %Fine

Location %Fe2O3 %A12O %SiO2 dominance) sand sand %Silt %Clay

LBF mottled horizon

LBF pallid horizon

Pinnacles lens mottled horizon

Pinnacles lens pallid horizon

Pisolite layer

Quondolo Formation

12.73 19.13 59.0 KaoliniteIlliteVariscite

3.53 12.82 90.0 KaoliniteIlliteVariscite

4.95 17.95 66.0 KaoliniteHydrobiotiteVermiculiteGoethite

3.89 10.39 86.0 KaoliniteHydrobiotiteVermiculiteGoethite"

6.38 6.71 40.0 QuartzGoethiteHaematiteKaolinite

0.31 2.13 96.0

73.0 8.6 3.2 13.6

44.4 16.2 9.2 28.6

0.0 84.4 0.7 19.3

3.2 73.2 0.0 17.96

52.4 21.74 4.0 20.36

Coarse sand >0.25 mm; fine sand 0.06-0.25 mm; silt 0.004-0.06 mm; clay <0.004 mm.

The Long Beach Formation has experiencedextensive diagenetic alteration to a depth of at least60 m. Indeed, the Pinnacles Lens at theQuoraburagun Pinnacles forms a striking exampleof a lateritic soil (Fig. 3). Much of the A2 horizon ofloamy red ferruginized soil has been stripped.However, the remainder of the lateritic profile iswell preserved. It includes a B horizon consisting ofiron and aluminium oxide concretions or pisolites,weakly cemented together and showing varyingdegrees of development. The mineral assemblage ofthis pisolitic layer, which is dominated by quartzand also contains goethite, hematite and somekaolinite (Table 1), is typical of lateritic B horizons(Gilkes & Suddhiprakaran 1981). Much of thepisolitic layer capping the Pinnacles Lens hasnodular concretions surrounded by an earthymatrix. Stripping of this matrix is indicated by theconsiderable areas of loose, though very welldeveloped, pisolitic gravels scattered over the sur-face of the site.

The pallid and mottled zones within the Pin-nacles Lens are separated by a distinct horizontalboundary. For the main part, this boundary seemsto be a weathering imprint, apparently indicating aformer water table. Caution, however, is needed ininterpreting the very striking section opposite themain lookout at this site. From a distance, the

weathering boundary seems to dip seaward, where-as a marker for a former water table such as thiswould be expected to continue horizontally acrossthe outcrop (Fig. 3). Here the originally horizontalboundary has been eroded, and then buried underred sediments which have slumped from higherparts of the profile. The slumped sediments can bedistinguished from those within the mottled hor-izon because of their stronger colour, their lack ofmottles and the inclusion of pisolites.

Unlike the Pinnacles Lens, no clear separationbetween mottled and pallid horizons is presentwithin the remaining sediments of the Long BeachFormation at either Long or North Tura Beaches.But like the Pinnacles Lens, a weakly developedpisolitic crust is evident and mottling is moreprominent within the higher levels where the mot-tling pattern varies considerably. Mottles range insize up to 20 cm; these larger mottles being clearlyseparated from each other (Fig. 6). Within the lowerlevels of the profile, horizons of more strongly indu-rated sediment are separated by unconsolidatedlayers that also become better indurated with depth.Induration has resulted from the translocation ofsilica downwards in the profile where it has repre-cipitated in discrete layers (Table 1). The lowerlevels of these sediments, like the pallid zone of thePinnacles Lens, exhibited smaller concentrations of

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iron and aluminium hydroxides. However, X-raydiffraction analyses of the mineral constituentsfrom the lower part of the formation differed fromthe Pinnacles Lens by the presence of illite andvariscite.

Sections of the Long Beach Formation show sev-eral distinct weathering features which post-datethe lateritic weathering phase and which can be at-tributed to an overprinting during the Pleistocene.Pleistocene sands up to 4 m thick, which overlie thepisolites in places, have been well podsolized. Thepisolitic horizon may have been influential in thedevelopment of this ground water or hydromorphicpodsol by creating a perched water table which, asnoted by Farmers et al. (1983), is essential for thevertical growth of hydromorphic podsols such asthis. Where piping has removed parts of the pisolitichorizon, tongues of the organic Bh horizon of thepodsols have penetrated into the underlying later-itic mottled zone. Organic matter has also moveddown from the podsolized sands in dissolved andcolloidal forms, and has then precipitated, forminghardened organic horizons in the Long BeachFormation.

In a few locations, where organic matter has pen-etrated the Tertiary sediments to depths of at least10 m, dark spherical concretions ranging in diam-eter from a few millimetres to 10 cm have dev-eloped. X-ray diffraction analyses revealed theseconcretions to be composed of quartz, pyrite andmarcasite. Sand surrounding the concretions withinthe pallid/mottled zone has weathered to a distinctpale yellow colour and smells strongly of sulphur. Itis presumed that these concretions are diageneticand have formed under anoxic conditions withinsaturated zones of the sediment.

AGE

Where Tertiary sediments occur in association withradiometrically dated basalt their ages can be deter-mined with reasonably close precision. Approxi-mately 200 km south of Sydney, sediments in theBendalong/Ulladulla district (Fig. 1) are inter-bedded with Oligocene basalt (~ 30 Ma; Wellman& McDougall 1974; Young & McDougall 1982).Farther south, near Moruya, the Oligocene (30 Ma)Coila Basalt (Wellman & McDougall 1974) overliesthe Tertiary Bergalia Formation and underlies theMeringo Creek Formation (Chalker & Bembrick1977). Beyond the limit of the Tertiary basalt,which only extends as far as Bodalla, the age of thesediments is more conjectural. From <1 km southof the last certain outcrop of the Bergalia Formationa series of Tertiary sediments, known as the

Bermagui Beds, crop out intermittently betweenLake Birroul and Tanja, a distance of 60 km (Fig. 1).Chalker and Bembrick (1977) suggested thatsimilarities in weathering features between theBermagui Beds and the Bergalia Formation mayindicate that the two are similar in age.

Age of the Long Beach Formation

The Long Beach Formation between Eden andMerimbula has been mapped on the Mallacoota1:250 000 geology sheet (SJ 55-8) as Pliocene inage. As with the eastern Victorian Tertiary out-crops, the Pliocene age has been derived throughextrapolation from the Haunted Hill Gravels inGippsland (Gemuts etal. 1976). Palynological andpalaeomagnetic evidence shows that this extra-polation into southern New South Wales was incor-rect, for the Long Beach Formation was depositedwell before the Pliocene.

PALYNOLOGY

Palynological investigations of the Long Beach For-mation at North Tura (Morgan 1974) revealed anassemblage which was assigned to the Proteadditestuberculatus zone of Stover and Partridge's (1973)Gippsland Basin sequence, thus implying adepositional age of Oligocene to Early Miocene.This assignation was based on the occurrence ofFoveotriletes lucunosos and rare Chenopodipollissp., and the lack of any diagnostic indicators of theoverlying Triporopollenites bellus zone. The assem-blage from North Tura differed from the GippslandBasin assemblages in lacking the diverse suite oflarge Proteaddites pollen and a number of import-ant spores such as Cyatheaddites annulatus.Morgan (1974) further suggested a tentative corre-lation with the upper P. tuberculatus zone indicat-ing an Early Miocene age for the sediments;however, this correlation was based upon the ab-sence of species (extinction points) in the NorthTura assemblage compared with the Gippsland as-semblages. Because of this, Morgan (1974) statedthat an Oligocene age could not be ruled out untilmore regional differences within the P. tuberculatuszone are described.

Palynological studies at Long Beach gave equi-vocal indications of the age of the deposits. Samplesfrom a i m thick grey clay layer within the LongBeach Formation were examined by H. A. Martin(University of NSW). She identified an unusualassemblage containing a variety of long rangingTertiary genera, namely Cyathea, Casuarina, Podo-

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TERTIARY SEDIMENTS, SOUTH COAST NSW 365

carpus, Dacrydium and Myrtaceidites sp. C, andmodern day Pinus, Eucalyptus and Casuarina spp.These latter contemporary species are indicative ofcontamination (H. A. Martin pers. comm.). Myrta-ceidites sp. C. has formerly been recognized to havea range of Early Pliocene in coastal sites in WesternAustralia (Bint 1981) and Victoria (H. A. Martinpers. comm.). However, away from the coast in theMurray Basin it is found in an assemblage which istypically Early Miocene or possibly Late Oligocene(Truswell etal. 1985; Martin 1986). It also occurs inOligocene sediments at the Deep Sea Drilling Pro-ject Site 254, Ninety East Ridge, in the IndianOcean (Kemp & Harris 1977). In light of the pal-aeomagnetic age of the Long Beach Formation it issuggested that the range of Myrtaceidites sp. C. incoastal sites in New South Wales is equivalent tothat found in the Murray Basin. Even more unusual,considering the palaeomagnetic age, was the com-plete absence of any Nothofagus sp.

PALAEOMAGNETISM

Palaeomagnetic measurements were made on sedi-ment samples to help determine which of thealternative ages for the Long Beach Formation iscorrect — Pliocene or Oligocene/Early Miocene.

Sampling and measurements

The sediments were too friable to sample by coringand the samples were collected instead in plasticcontainers using the following procedure to mini-mize disturbance to the fabric. A flat surface wasprepared on the sediment; then, using a scribedhorizontal line as a guide, a slightly oversize cubewith one horizontal plane was carved out. The plas-tic holder was pressed lightly onto the cube, and thecube was oriented and detached. The orientationwas with a magnetic compass and an inclinometer.At each locality the possibility of a magneticanomaly was checked by comparing magnetic andsun compass orientations; none was found.

A total of 82 samples were collected from four keylocalities: (1) the southern cliffs of Long Beachwhere the mottling is coarse; (2) the finely mottledparts of the Pinnacles Lens; (3) slumped red sedi-ments at the seaward slopes of the Pinnacles Lens;(4) coarsely mottled sediments at the cliffs of NorthTura Beach. In the initial analysis, the results fromthese four localities were treated independently.The remanent magnetization was measured at theBlack Mountain Laboratory in Canberra using anScT cryogenic magnetometer, a Schonstedt two-

axis GSD-5 alternating field demagnetizer and aSchonstedt TSD thermal demagnetizer.

Results

Preliminary studies showed that, when demagnet-ized in alternating fields, the samples generally givescattered orthogonal projection plots, whereaswhen demagnetized thermally the scatter is usuallysmall (Fig. 7). Therefore, thermal demagnetizationwas adopted, even though the maximum tempera-ture for the heatings was limited to 220°C by defor-mation of the plastic specimen containers.

Three components of remanent magnetizationwere isolated by principal components analysis(Kirschvink 1980): a low blocking temperaturecomponent which was eliminated by 140°C; an in-termediate blocking temperature component whichforms the bulk of the remanence and persists be-yond 220°C; and a hard component whose existencewas deduced in many specimens from small inter-cepts on the axes when the orthogonal plots wereprojected towards the origin. The soft component isinterpreted as a recent viscous overprint, and theintermediate component as the remanence as-sociated with the chemical weathering. The originof the hard component is not clear; one possibility isthat it is due to random magnetic moments of thedetrital grains. An unusual feature of the results isthat all remanences have normal polarities, unlikeweathered profiles generally in which both normaland reversed polarities are observed (Schmidt &Embleton 1976; Idnurm & Senior 1978; Schmidt &Oilier 1988). This suggests that the magnetizationsat the south coast were acquired during a perioddominated by normal polarity of the geomagneticfield, implying that weathering occurred rapidly.

Seventeen samples were rejected because themean angular deviations of their orthogonal plotsexceeded 5°. This selection criterion is more strin-gent than usual (e.g. Kirschvink & Rozanov 1984used 10°C for Cambrian strata): only specimenswith well defined directions were considered likelyto yield age estimates of acceptably small un-certainty when compared with the Australian Ceno-zoic path that spans an arc of only ~30°. Theremanence directions of the remaining samples atthe four key localities are listed in Table 2 andshown in Fig. 8. The directions in the coarse mottlesat Long and Tura Beaches are closely similar; there-fore, these two sets of results were combined for thefinal analysis. However, the directions in the finelyand coarsely mottled sediments differ appreciably:Watson's F test for comparing mean directions(Watson 1956) gives the statistic 12.1 with 2 and

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1 1 . c0.5 1.0 1.5

E/UP

mA/m

10.6

0.4

0.2

1 "11

-A*

NSR

/

i

25 1

NRM

G> "

I I

oc.NRM

I l i0 0.2 0.4

(d) BBRC 4.1

0.6 0.8 1.0E/UP

N

0.5

.NRM

}0.5

(f)TBTC8.1

NRM-2 .0

-1 .0

1.0 1.5-E/UP

NRM

2.0 4.0-E/UP

mA/m

Fig. 7 Representative orthogonal projections of remanence vectors, (a) and (b) show demagnetizations in which thefirst 18 steps are in alternating fields at peak values ranging from 5 to 90 mT, and the remaining steps are heatings at 60°,90°, 120°, 150°, 180°, 210°, 220° and, for (b) only, 23O°C. Note reduced scatter for thermal steps. At the 230°C heating in(b) the plastic sample container deformed, resulting in scatter. The treatments in diagram (c) were 60°, 80°, 100°, 120°,150°, 180°, 210°, 220°C and in (d), (e) and (f) were 60°, 80°, 100°, 120°, 140°, 150°, 180°, 210° and 220°C. NSR21.1 andBBRCl 8.1 are from fine mottles in the Pinnacles Lens; NSR25.1 and BBRC4.1 are from slumped sediments; BBRC3.1is from coarse mottles at Long Beach; and TBTC8.1 is from coarse mottles at North Tura Beach. Open (filled) symbolsindicate projection in horizontal (vertical) plane.

102 degrees of freedom, indicating that the prob-ability of the directions belonging to the same popu-lation is much less than 1%. The directions from theslumped material are intermediate between those ofthe finely and coarsely mottled sediments, and forma slightly elongated group. It is not possible todistinguish the slumped material directions statisti-cally from either of the other groups. The intermedi-ate angles are consistent with the evidence that thesediments have been redeposited by slumping.

All three poles plot slightly east of the Late

Mesozoic-Cenozoic path (Fig. 9): the poles fromthe finely mottled and slumped sediments deviatedby less than their respective 95% confidence limitsfrom the path; the pole for the coarsely mottled sedi-ments deviates by more than those limits. Theeastward bias could be explained by an eastward tiltof the region by 3-4°; indeed, an easterly tilt of sedi-ments of the continental shelf is reported byChalker and Bembrick (1977) for the Naroomaregion to the north of the study area.

Assuming that the bias is purely eastward, the

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TERTIARY SEDIMENTS, SOUTH COAST NSW 367

Table 2 Summary of palaeomagnetic results

Unit

Remanence directionNo. Decl. Incl. a.95

Samples (deg.) (deg.) k (deg.)

South PoleLat. Long. A95

(deg.) (deg.) K (deg.)

Coarse mottles: Long BeachCoarse mottles: Tura BeachFine mottles: Pinnacles LensSlumped sediments: Pinnacles LensCoarse mottles: combined Long and Tura Beach

1917171236

1.03.77.26.52.3

—63.9—63.9—69.2—66.9—63.9

230.6230.0173.0144.3234.0

2.2•2.4

2.73.61.6

81.281.073.176.281.2

146.3134.0135.1132.7140.4

99.9107.067.366.5

104.6

3.43.54.45.42.4

k, K are precision parameters (Fisher 1953).«95, A95 are semi-angles of 95% cone of confidence.

330

300 60

270

240

210 150

Fig. 8 Equal-area projection of directions obtained from principal components analysis (Kirschvink 1980). Alldirections are in the upper hemisphere, (a) Fine mottles in the Pinnacles Lens; (b) The Pinnacles Lens slumpedsediments; (c) Coarse mottles at Long Beach; and (d) Coarse mottles at Tura Beach.

magnetic age of fine mottles at the Pinnacles is esti-mated as 19 Ma (Early Miocene — see Harland etal. 1989) using a polynomial regression relationshipbetween the latitudes of the calibration poles on thepath and their ages. The 95% confidence limits forthis age are estimated by the method of Renfrewand Clark (1974) as ± 9 Ma. Similarly the magneticage of the Long and Tura Beach coarsely mottledsediments is estimated as 7 ± 4 Ma (Late Miocene— see Harland et al. 1989).

Fig. 9 Palaeomagnetic poles and their 95% confidencecircles for fine mottles (BB), coarse; mottles (T-BB) andslumped sediments (BBS), plotted on the Late Mesozoic-Cenozoic pole path for Australia (Idnurm 1985).Age-calibrated poles are indicated by large symbols,weathered profile poles by small symbols. OP denotes aCretaceous overprint pole.

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368 J. F. NOTT ETAL.

It should be noted that an alternative LateMesozoic-Cenozoic pole path has been derivedrecently for Australia (Musgrave 1989). That pathgives appreciably older ages than those estimatedhere: the Early Miocene minimum age becomes midOligocene, and the Late Miocene estimate becomesmid-Miocene. However, as discussed by Idnurm(1991) the approach used in deriving the alternati vepath is questionable, and the estimates based on theearlier path are preferred.

The magnetic age of the fine mottling and, hence,the minimum age for the Long Beach Formation isestimated as Early Miocene, contrary to the Plio-cene age suggested by Gemuts et al. (1976), but inaccord with the Oligocene-Early Miocene palyno-logical age assignation at North Tura Beach (Mor-gan 1974). The magnetic age of the coarse mottlingappears to be Late Miocene or possibly Pliocene.

DISCUSSION: GEOMORPHICIMPLICATIONS

Although the Oligocene K-Ar determinations onbasalt (Wellman & McDougall 1974; Young &McDougall 1982) demonstrated the great age of thecoastal lowland between Ulladulla and Tuross, theage of the lowland farther south remained uncer-tain. The uncertainty was heightened by attempts tocorrelate sediments south of Bega with those ofPliocene age in Gippsland. The evidence presentedhere indicates a general continuity in age of thecoastal lowland from the Ulladulla district to atleast the Victorian border, and indeed raises thepossibility that some sediments in east Gippsland,which have weathering profiles similar to the LongBeach Formation, may be substantially older thanpreviously thought.

Age of the Quondolo Formation

Apart from the fact that the Quondolo Formationmust be older than the overlying Long Beach For-mation, its age remains uncertain. Indeed, the greatcontrast in the degree of induration between the twoformations, together with the extensive dissectionof the Quondolo Formation prior to the depositionof the Long Beach Formation, especially at Turawhere only a small fragment of the former remains,indicates that there may have been a substantialhiatus separating these two depositional phases. At-tempts to define the age of the Quondolo Formationmore precisely by comparing it with sediments far-ther north along the coast have been inconclusive.At first consideration it is tempting to regard theQuondolo as equivalent to the Bergalia Formation,the lower of the two Tertiary sedimentary units atTuross, especially as that formation pre-dated theextensive basaltic extrusion dated at 30 Ma.However, this conjecture seems doubtful when thecontrast in texture and especially diagenesisbetween these formations are taken into account;whereas the Bergalia Formation contains numerouspebbly lenses and is relatively weakly induratedwith dominantly ferruginous cement, the QuondoloFormation is sandy and is highly indurated with sil-iceous cement. If anything can be inferred from thiscomparison, and from the discussion of the ap-parent ages of weathering given below, it is that theQuondolo Formation may be older than the Ber-galia Formation.

Uplift

The Oligocene to Early Miocene ages determinedfor the sediments between Bega and Eden addfurther weight to claims for at least an EarlyTertiary uplift of the highlands to the west (Young1977; Young & McDougall 1982; Lambeck &Stephenson 1986). The sediments fill the lowerreaches of valleys draining eastwards from the high-land; the gravels and cobbles in some of the valleyfills seem indicative of considerable local relief andthere is neither stratigraphic nor topographic evi-dence of tectonic dislocation between the highlandsand the lowlands (Dixon & Young 1980). More-over, recent work by Taylor et al. (1990) showedthat the highlands immediately to the west had arelief of at least 400 m, and possibly 800 m, by theLate Paleocene. Fission track dating of apatites inbatholiths immediately to the north in the hinter-land of the Bega Valley indicates that substantialuplift and stripping had occurred by the EarlyMesozoic (Moore et al. 1986)

Aggradation

The evidence from Tura and Long Beach also indi-cates that the Oligocene to Early Miocene was atime of major fluvial aggradation in this region.This aggradation may well have been a response tobase level change due to sea level rise. The sea levelcurves for southern Australia summarized by Loutitand Kennett (1981) and the world sea level curve byVail et al. (1977) show significant rises in sea levelfrom Early to Middle Oligocene and from Early to

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TERTIARY SEDIMENTS, SOUTH COAST NSW 369

Middle Miocene. The Murray Basin (Brown 1983)and Gippsland Basin (Stover & Partridge 1973) alsoexperienced significant deposition of sediments as aresponse to eustatic sea level rise at these times, and,as noted by Loutit and Kennett (1981), the sedi-mentary records in these basins show a hiatus insedimentation during the mid Oligocene (30 Ma)that is indicative of a major fall in sea level. Thedisconformable relationship of the Quondolo andLong Beach Formations may well be a response tothat fall. Moreover, Partridge et al. (19 81) have rec-ognized estuarine sediments 25 m above presentsea level at Little Bay, Sydney, which have a micro-fossil assemblage indicating an Early Miocene age.Thus, like other areas of southeastern Australia,high sea levels along the southern coast of NewSouth Wales may have caused significant fluvial/estuarine aggradation during the mid-Tertiary.

Weathering

Several periods of intense weathering have beenpreviously reported from farther north on thecoastal lowland. Near Ulladulla silcrete appears topredate, and ferricrete to post-date Oligocene (30Ma) basaltic extrusion (Young & McDougall 1982).Other weathering profiles on the lowland, though ofuncertain age include ferricrete around Nowra(Young 1977) and deeply kaolinized rhyolite nearTathra (Dixon & Young 1980). The silicification ofthe Quondolo Formation clearly predates the onsetof deposition of the Long Beach Formation, forwhich palynological evidence indicates anOligocene to Early Miocene age. It might, therefore,be tentatively correlated with the siliceous weather-ing near Ulladulla. The palaeomagnetic evidence ofEarly Miocene age for the ferruginous profiles dev-eloped in the Long Beach Formation likewise seemcompatible with the maximum age determined forferricrete at Ulladulla. Nonetheless, caution isneeded because, as noted previously, the QuondoloFormation was substantially eroded prior to thedeposition of, and may be much older than, theLong Beach Formation. Be that as it may, theQuondolo Formation itself filled valleys cut belowthe deeply kaolinized and bleached rhyolite nearTathra, thereby indicating the kaolinization prob-ably represents the oldest weathering profile on thecoastal lowland. Additional evidence of a great agefor the weathering of the rhyolite, which at WhiteRocks extends to a depth of ~ 50 m, can be seenfarther north near Tuross, where the freshness of theOligocene basalt shows that it must have post-datedthe period of intense kaolinization.

Diversion of the Bega River

The most anomalous feature of the drainagesystems crossing the coastal lowland occurs down-stream from Bega township where the Bega Riverturns suddenly from its broad valley to cut througha narrow gorge to reach the sea at Tathra. The broadvalley continues straight on beyond this abruptbend, providing a ready path to the sea via the pre-sent day Jellat Jellat Swamp and Wallagoot Lake(Fig. 10). Topographic evidence leaves little doubtthat the swamp and lake occupy the former courseof the lower reaches of the Bega River, a conclusionsupported by the fact that bore logs indicate thedepth to bedrock increases from 29 to 58 m (pointsA & B on Fig. 10) below present sea levelsouthwards from the Bega River through JellatJellat Swamp towards Wallagoot Lake (NSW WaterResources Commission pers. comm.).

The diversion of the lower reaches of the BegaRiver was probably caused by the extensive aggrad-ation of valleys described previously. The old valleymouth lies only a few kilometres north of the exten-sive outcrops of the Long Beach Formation atNorth Tura. An extensive deposit of similar sedi-ments occurs along the prior course of the lowerBega River, separating Jellat Jellat Swamp fromWallagoot Lake, and sand and gravel are scatteredacross the divide separating the old valley from themodern gorge. Like the nearby valleys, the lowerBega Valley aggraded as a response to rising sealevels. Freed from the bedrock constraints of theprior valley, the lower reaches of the river appear tohave migrated northwards from their former pos-ition and then incised in response to a subsequentfall in sea level, cutting the modern gorge. The priorcourse of the Bega is incised below and presumablypost-dates the intensely kaolinized profile weath-ered in the rhyolite near Tathra. Moreover, giventhe age determined for the Long Beach Formation,this valley was apparently filled in the Oligocene/Early Miocene and abandoned for the new courseduring the mid- to Late Miocene. Notwithstandingthe age of these events, their topographic expressionis still strikingly preserved.

The abandoned course of the lower Bega River ismore than just a local oddity. It demonstrates thateven the lowest reaches of the coastal valleys can beof great antiquity, and that incision triggered by lowstands of the sea during the Quaternary may havedone little to modify older topography. The impli-cations for our understanding of the evolution ofthe coastal lowland and of the erosion of the high-land escarpment are considerable. The degree ofpreservation of the Oligocene-Early Miocene land-

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370 J. F. NOTT ETAL.

to COOMAV. to NOWRA i Metasediment

Bega -Tathra

GorgeJBega Ft.Delta

Palaeo-Bega River

Coastal Hills

Tertiary Sediments(Long Beach Formation)

rhyoliteA190

• truncated Al-Jsjjy gravels^ (kaohmzed cliffs^ ^ l - 4 v Jjdown to 20 m.a.s.l.)

km

Fig. 10 Northward diversion of the lower Bega River near Tathra. The Early Miocene Bega River flowed to the seaimmediately north of Bournda Island in the valley now occupied by Jellat Jellat Swamp and Wallagoot Lake.Re-excavation of this valley has since occurred leaving only traces of its former, more extensive, alluvial fill. Note thealluvial sediments lying at high levels between the palaeovalley and the present day outlet of the Bega River.

scape along the coastal fringe, is remarkable. More-over, it is at odds with the still widely acceptedcyclical models of landscape evolution which pre-dict the advance of successive erosional cycles toproduce a stepped array of planation surfaces withthe youngest nearest the sea. This was the manner inwhich King (1967) depicted the evolution of thesouth coast of New South Wales when integrating itwith his intercontinental model of pediplanation;

all these mid-Tertiary fragments were shown asQuaternary on his map of erosion surfaces in south-eastern Australia. It can now be demonstrated thaterosional surges have been concentrated along themajor streams and have had their main effect in theheadwater areas along the highland escarpment,bypassing older features nearer the coast. This styleof erosion has been discussed in detail by Young(1977) for the Shoalhaven and Clyde River valleys.

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TERTIARY SEDIMENTS, SOUTH COAST NSW 371

The implications of the preservation of these oldfeatures along the coastal fringe are also of consider-able importance for the evolution of the continentalshelf. Disconformities within the sediments on theshelf, the now partly buried offshore extensions ofriver valleys, and the benched topography of theshelf itself (Davies 1979) may be of considerableantiquity. Clearly, there is great scope for correla-tive studies of the onshore and offshore records.

CONCLUSIONS

This study has shown that, like the coastal lowlandbetween Ulladulla and Tuross, the lowland ex-tending south to the Victorian border is also mid-Tertiary in age and, therefore, uplift of the high-lands to the west must have occurred by that time.Although Tertiary basalt is absent from this south-ern portion of the coastal plain, palaeomagneticdeterminations reveal that phases of deep weather-ing occurred within the Long Beach Formation dur-ing the Early Miocene and Late Miocene. Theseages agree well with palynological evidence whichindicates that deposition of the Long Beach For-mation occurred during the Oligocene to EarlyMiocene.

The Quondolo Formation probably pre-datesmost other Tertiary units along the south coast ofNew South Wales, for its disconformable relation-ship with the Long Beach Formation reveals a per-iod of significant post-depositional erosion which isnot recorded in the Bermagui Beds or the Bergaliaand Meringo Creek Formations. The QuondoloFormation may, however, be similar in age to thewell silicified sediments which dip below presentsea level at Bendalong (Young & McDougall1982).

Three common characteristics can be shown toexist between Tertiary sedimentary units along thiscoast. First, the majority have been deposited dur-ing the Oligocene to Early Miocene. Second, with noevidence for any regional tectonic movements dur-ing this time, aggradation of this belt of sedimentscan probably be attributed to eustatic influences,for trends in available sea level curves indicate asignificant increase in world ocean volumes duringthe Oligocene to Early Miocene. Finally, at leastfour phases of weathering during the Tertiary cannow be recognized in these deposits; deep kaolin-ization, well preserved in the deeply weatheredrhyolite near Tathra, was followed by silicification,exemplified in the Quondolo Formation and in thesilcrete near Moruya and Ulladulla which, in turn,was followed by two phases of ferruginization, as

recorded in the Long Beach Formation and in theferricrete near Ulladulla and Nowra.

As a consequence of the findings reported here aclearer view of the sequence of geomorphic eventsalong the far south coast of New South Wales duringthe Tertiary is now beginning to emerge. Thissequence can be summarized as follows. Deepweathering of country rock, as evidenced by thekaolinized rhyolite at White Rocks near Tathra, wasfollowed by a period of erosion which occurredeither before or during the Early Tertiary. Depo-sition, silicification and erosion of the QuondoloFormation took place prior to or during the mid-Tertiary. Further sediment accumulation occurred,from the mid-Oligocene to the Early Miocene. ThisOligocene to Early Miocene aggradation was prob-ably triggered by a eustatic rise in base level. As aresult, the lower Bega River was diverted and de-veloped a new exit to the sea further north nearTathra. Intense ferruginization then occurred in theEarly Miocene and was followed by a second periodof ferruginization in the Late Miocene to Pliocene.Sea levels were at this stage beginning to fall,allowing incision of streams into the landscape, andin the case of the lower Bega River this was throughOrdovician strata forming a narrow gorge unlike itsformer, deeper and broader valley. This latter stageof landscape incision probably also produced thelower reaches of other valleys crossing the coastallowland.

ACKNOWLEDGEMENTS

We thank Dr E. A. Bryant for reading the originalmanuscript and our reviewers for their highly con-structive comments. We also thank Jeana Nott forfield assistance, Julie Kamprad for X-ray diffrac-tion analysis of the marcasite nodules, RichardMiller for drawing Figs 1,2, 10, and Gail Hill fordrawing Figs 7-9. M. Idnurm publishes with thepermission of the Director, Bureau of MineralResources, Canberra.

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