Australian Journal of Earth Sciences

(2004)

51

,

67–83

Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales

K. R. SHARP

8 Karawatha Circuit, Cooma, NSW 2630, Australia

Studies of Cenozoic lavas and associated sediments in the Kiandra–Cabramurra and Adaminaby–Cooma areas identify and date tectonic deformations responsible for differential uplift and drainagedevelopment of the region. Volcanic activity on the northern Monaro was mainly Eocene–Oligocenebut in the extreme north there are Early Miocene sediments and lavas. Volcanic activity and foldingbegan to rearrange the drainage in the Eocene–Oligocene. The headwaters of the MurrumbidgeeRiver originally flowed south into the Eucumbene River but Early Miocene folding and faulting upliftedthe Monaro Range and created a large lake near Adaminaby. Lake overtopping rerouted thedrainage east and then south along the basalt-filled valley of an old north-flowing tributary, the'Adaminaby River', forming the present-day Murrumbidgee River. The folding also produced a 300 mheight difference between the Berridale and Adaminaby Plateaus and formed a section of the GreatDivide. This fold displacement ranks with the largest Cenozoic fault displacements. In the Kiandra areatectonism associated with Early Miocene volcanism rearranged the drainage and tilted the Kiandraarea and Kosciuszko Block to the north.

KEY WORDS: Cenozoic, Eocene, folding, Miocene, Murrumbidgee River, Oligocene, palaeo-drainage, tectonics, Tumut River, uplift, warping.

INTRODUCTION

South of Canberra the Snowy Mountains and Monaro formpart of the highlands of southeastern Australia thatincludes Mt Kosciuszko (previously spelt Kosciusko),2229 m, the highest point in Australia. Unusual geo-morphic features include a 120 km offset of the GreatDivide along the Monaro Range between Kiandra andBrown Mountain. Also the drainage pattern has suddenchanges in direction that appear abnormal, especially theMurrumbidgee and Snowy Rivers (Figure 1).

Since the early 1900s there has been controversy as tohow and when this region was uplifted and whether theupper Murrumbidgee River flowed into the Snowy River.Advances in the second half of last century includedsystematic topographic and geological mapping. Morerecently K/Ar dating of the Cenozoic basalts and fissiontrack dating of bedrock have changed perceptions of theage of the uplifts.

The role of Cenozoic tectonics in the evolution of thearea remains controversial. Kohn

et al.

(1999), for example,in their regional geology of the Kosciuszko massif, pre-sented various ideas as to the nature and location ofCenozoic

faulting

and

warping

that

affected

its

upliftand geomorphology. In the author's opinion these ideasincorporate elements of truth and misconception andreflect the generally poorly understood nature of thetectonic deformations involved and how these deform-ations have affected the surface history.

The subject is too extensive to review in total. Thispaper addresses some of the author's concerns throughcase studies. The Background section briefly reviews the

main areas of controversy. The main text presents inform-ation on the nature and age of Cenozoic tectonic deform-ations and palaeodrainage based on a reappraisal of earlierstudies of Cenozoic lavas and sediments in the Kiandra–Cabramurra area and a new study of the Adaminaby–Cooma area. Where a finding conflicts with a specific the-ory or interpretation it is discussed in the text. In theDiscussion and Conclusions attention is drawn to differ-ences between the findings and the broader ideas coveredin Background.

BACKGROUND

Nowadays published and unpublished 1:100 000 geologicalmaps (or non-metric equivalents) cover the region. Thebasement geology is mainly folded Ordovician to Devoniansedimentary rocks and acid volcanics, intruded by Silu-rian and Devonian granites. Eocene–Oligocene basalticlavas covered much of Monaro, and Lower Miocene lavascovered the northern part of the Kosciuszko Plateau,although much has been eroded away.

The Kosciuszko Block or Plateau is mainly above1500 m, peaking at 2229 m, while to the east the Adaminabyand Berridale Plateaus on the Monaro are mainly between1200 and 900 m. The concept of differential uplift of theKosciuszko Block by step-faulting was first suggested byDavid (1908) and it was widely accepted that this simpletectonic model was correct. It was doctrine in most schoolswell into the 1950s and 1960s. The term 'Kosciuszko Uplift'or 'Epoch' became entrenched in the geological literature,but nowadays has no specific meaning. There are still few

68

K. R. Sharp

reliable data as to exactly how and when the KosciuszkoBlock was uplifted.

The investigation and construction of the SnowyMountains Hydroelectric scheme between 1950 and 1975produced much geological data. Early findings challengedthe validity of the simple gravity block-faulting model,including the discovery of major thrust faults on thewestern flanks of the mountains, the Tawonga Thrust inVictoria (Beavis 1960, 1962) and,

en échelon

, the Khan-coban–Yellow Bog Thrust (Moye

et al

. 1963). Both havethrust granite over alluvium. Also

in situ

testing showedhigh horizontal compressive stresses (Moye 1962). Horstsand grabens exist but they are bounded by high- or low-angle reverse faults rather than normal faults (Moye

et al

.1963). The fault geometry does not conform with thethrusts and transcurrent faults classically associated withthe stress ellipsoid, and is not discussed further here.

Despite

these

findings

Browne

(1967,

1969)

continuedto promote and defend the conventional gravity block-faulting model. Ollier and Wyborn (1989) also promotednormal faulting: their figure 2 was copied from Sharp(1980), a map of faults in the Snowy Mountains region thatwere probably active during the Cenozoic. However, theyincorrectly assumed that most were normal faults andeven suggested (Ollier & Wyborn 1989 p. 41) 'that the thrust

faults are a local complication after large scale normalfaulting'. The geological map of Wyborn

et al

. (1990) of theKosciuszko National Park omits the Khancoban–YellowBog thrust and other established major faults.

The Berridale 1:100 000 sheet (White

et al

. 1977) coversparts of the Kosciuszko Block and Monaro. The structuralthesis of White

et al

. invoked east–west compression toexplain the Palaeozoic and some Cenozoic structures.Their block diagram depicts the main geomorphic featuresthat have been attributed to differential uplift during theCenozoic.

Warping has often been suggested as an uplift mech-anism but rarely supported with substantive evidence.Sussmilch (1910) and Taylor (1910) postulated warping toexplain

the

unusual

course

of

the

Murrumbidgee

River,as shown on Figure 1. Moye

et al

. (1963) and White

et al

.(1977) invoked warping to explain the eastern side of theKosciuszko Block. The concept that the Great Divide is afold or warp axis is not new although it is currently pro-moted by Ollier and Pain (1994). Sharp (1994) reportedCenozoic folding between Adaminaby and Cooma andfurther details are given below.

The Bombala 1:100 000 sheet south of Cooma (unpub-lished, mapped by students and staff of the University ofCanberra) shows widespread warping (or folding) of the

Figure 1 Snowy Mountains andMonaro Region showing maindrainage and Great DividingRange. Outlined are map cover-ages of Figures 2, 3 and Tayloret al. (1985). Various arrows showthe course of the palaeo-Murrum-bidgee River according to: 1,Taylor (1910), who assumed thatthe Snowy River originallystarted from a palaeodivide nearTharwa and flowed south intothe present-day Snowy River viaWullwye Creek; 2, Sussmilch(1910), whose route is almostidentical to Taylor (1910) but thepalaeodivide is further southnear Bredbo; 3, Browne (1914),who connected the ancient drain-age in pre-basalt time; 4, the firststage of the pre-basalt drainageas detailed in the present paper.

Cenozoic tectonism, Snowy Mountains, NSW

69

Eocene–Oligocene Monaro Volcanics. Their structuraldata are shown on the Bega–Mallacoota 1:250 000 geologicalsheet (Lewis & Glen 1995).

Major differences of opinion exist as to the number andtiming of uplift events. Wellman (1979) highlighted thiswhen

he

presented

a

hypothesis

of

a

steady

rate

of

upliftfrom Late Cretaceous (~90 million years ago) to thepresent. He compared his model with five others (Andrews1911; Craft 1933b; King 1959; Browne 1967; Hills 1975), whichinvoked from one to four episodes of uplift, most in theCenozoic, with no commonality in timing. However, mostmodels pre-date K/Ar dating.

Jones and Veevers (1982 p. 1) put forward 'the hypothesisthat periods of more intense volcanism correspond withuplift

of

the

Highlands

and

concomitant

subsidenceof the flanking basins'. The present study finds a closerelationship between tectonism and volcanism.

Fission track dating is telling much about the earlieruplift story. Dumitru

et al

. (1991) recognised that the SnowyMountains has younger fission track ages than the sur-rounding region. Further study by Kohn

et al

. (1999 p. 181)provides 'strong evidence for two episodes of accelerateddenudation commencing in Late Permian–Early Triassic(

ca

270–250 Ma) and mid-Cretaceous (

ca

110–100 Ma) times,and a possible third episode in the Cenozoic'.

The controversy over rearrangement of the Murrum-bidgee River is discussed in the introduction to theAdaminaby–Cooma area.

KIANDRA–CABRAMURRA AREA

Kiandra and Cabramurra are at the northern end of theKosciuszko Block or Plateau. The Cenozoic geology of thisarea described by Gill and Sharp (1957) requires updatingand expanding. Figure 2 covers the southern two-thirds oftheir map, but in more detail. Their study dealt mainlywith sedimentary deposits exposed in old gold workings atKiandra and to a lesser extent subsurface exploration ofthe Eight Mile deposits east of Cabramurra. Additionaldata are in Moye

et al

. (1969) while Snowy MountainsAuthority geology reports include maps, sections and drilllogs of the Eight Mile (Moye 1953; Stapledon & Moye 1957).Basalt talus normally obscures the sediments, and strati-graphic data comes from gold workings and subsurfaceexploration. The 8 km-long Eight Mile basalt plateau wasextensively explored for sand deposits by mapping [1:15 840(4 in to 1 mile) and larger scale], drilling and bulldozertrenching. Nineteen drillholes around the southern andwestern flanks and the northern end of the plateau providefive complete and several partial stratigraphic sequences.

Approximately 75 m of sediments lie beneath the mainbasalt caps at Cabramurra and Kiandra. These palaeo-valleys are 10 km apart on opposite sides of the present-dayGreat Divide, but the stratigraphic sequences are similar,exhibiting three main cycles of sedimentation each withsand (or gravel) grading up into lignite. Because similarepisodes of tectonic ponding and quiescence affected bothvalleys, the cause was regional.

Several features at the Eight Mile deposits indicate thatthe area was tilted to the north during and after theirformation. A change in sedimentary facies from sandy in

the south to silty and clayey in the north and currentbedding indicate sediment transport from south to north.At the southern end the three cycles of sediments areuninterrupted whereas at the northern end at least threebasalt flows are randomly interposed in the cycles. Theseflowed north from two large dykes east of the deposit. The30 m-thick plateau cap apparently originated from RoundMountain (Figure 2).

Aside from the northward tilting, the Eight Mileplateau has a synclinal deformation with the north–southaxis along the buried valley, so that the Cenozoic strata andbedrock surfaces have similar dip directions. In the south-west corner stratigraphic correlation (mainly betweendrillholes) indicate an average dip of 7

�

NE, while theunderlying

weathered

granite

bedrock

surface

dips

20

�

Eor less. At the northern end the dip is 3

�

WSW (Figure 2).Near Eight Mile Diggings correlations between drillholesindicate

a

gradient

to

the

north

on

the

main

lignites

of5 m/km or more. The lignites were presumably laid downsubhorizontally in swamps, so this is further evidence ofpost-basalt tilting. (The present-day Wingecarribee Swampnear Moss Vale has a gradient of 1 m/km or less.)

These data come from the northern end of theKosciuszko Plateau. The remnants of the basalt-cappedplateau and the Kosciuszko Plateau to the south present afairly even skyline over a distance of more than 50 km, witha declivity to the north of ~10m/km (<1

�

). However, thedeclivity

decreases

north

of

Kiandra.

The

inference

isthat the Cabramurra tectonism embraced the entireKosciuszko Plateau. Hence there was differential uplift ofthe Kosciuszko Plateau during this epoch of volcanism andtectonism.

Age of uplift and deformation

The sediments were initially assigned an Early Tertiaryage, possibly Late Eocene or Early Oligocene, on the basisof

the

fossil

flora

(Cookson

&

Pike

in

Gill

&

Sharp

1957)but later K/Ar dating by Wellman and McDougall (1974)showed the basalts to be Early Miocene. The SnowyVolcanic

Province

basalts

range

from

23

to

17 Ma(Wellman

&

McDougall

1974;

Owen

&

Wyborn

1979;Young & McDougall 1993). (For the ages listed here, thosefollowed by * have been recalculated by McDougall usingnewer constants; published ages are in parentheses.)The ages in the Kiandra–Cabramurra area are 22.1*(21.5) ± 0.6 Ma for the basalt cap 2 km north of Kiandra;22.3* (21.7) ± 0.4 Ma for flow no. 3 located within the thirdmain sedimentary cycle 5 km north of Cabramurra; and20.9* (20.4) ± 0.6 Ma on Section Ridge between the first two(Figure 2).

The statement in Moye et al. (1969) that the plateau wastilted in Early Cenozoic time requires amending to EarlyMiocene time.

Other models based on old erosion surfaces cannotbe so easily adjusted. Browne (1967, 1969) thought that,in common with the rest of the eastern highlands, theKosciuszko block experienced uplifts in the Eocene, LateMiocene and Pliocene, which he called the Kiandra,Macleay and Kosciuszko Epochs. He dated his peneplain onthe premise that the Kiandra basalt was Eocene to EarlyOligocene (presumably based on Gill & Sharp 1957) and his

70 K. R. Sharp

Cenozoic tectonism, Snowy Mountains, NSW 71

train of logic collapses if the revised age is substituted. Itshould be noted that Early Miocene uplift does not featurein any of the models listed by Wellman (1979).

It is not implied that Early Miocene tectonic eventswere solely responsible for the uplift of the Kosciuszkoblock.

Evolution of palaeodrainage

Young and McDougall (1993) included the Kiandra–Cabramurra area in their study of long-term landscapeevolution since the Early Miocene. Their assumption(Young & McDougall 1993 p. 37) that 'post-basaltic deform-ation has been insignificant' is at odds with the foregoing,and there is other evidence of post-basaltic deformationand stream derangement.

Young and McDougall's study adopted the base of theMiocene basalt as the Early Miocene topography whereasthe author considers that it is the interface betweenbedrock and the overlying Miocene deposits whethersediment or basalt. Spot heights on this interface areshown on Figure 2. But this is not the prime issue. Youngand McDougall show approximate contours of the sub-basaltic surface and make several statements about them,including (p. 39) 'In the Tumut valley sub-basaltic contoursdecline northward from ~1640 m in the headwaters belowMt Jagungal to 700 m east of Batlow' and (p. 40) 'Moreover,basalts in the headwaters of the Tumut River near Cabra-murra descend without any apparent dislocation from over1600 m to the 1200 m level of the basalt on the Bago Plateau.Indeed, nowhere in the region is post-basaltic dislocationevident'. These statements are not consistent with the fielddata.

The contours in the Upper Tumut valley as shown byYoung and McDougall (1993) are in places incorrect by asmuch as 300 m, as is the Tumut River pre-basalt profilederived from them. Gill and Sharp (1957) (a paper they cite)gave some palaeo-stream-bed levels in the Upper Tumutvalley that demonstrate that their profile is incorrect,while Moye et al. (1969 p. 549) reported that 'The RoundMountain Lead has been seriously affected by faulting, andis difficult to reconstruct'. At key points along their pre-basalt profile the lowest bedrock and basalt are The Gulf(1360 m and 1370 m), Fifteen Mile (1300 m and 1370 m: Moye1953) and Eight Mile South (1450 m and 1530 m). The palaeo-valley floor level and basalt base actually fall ~300 m thenrise ~150 m. The section on Figure 2 shows the actual pro-file along the Round Mountain Lead of Andrews (1901) orpre-basalt Upper Tumut River of Young and McDougall(1993).

Furthermore, the map of Gill and Sharp (1957) andFigure 2 show that the Tumut Pond Fault forms the easternboundary of the Emu Plain basalt and its level is ~100 mlower than the adjacent basalt plateau.

The development of the palaeodrainage in the UpperTumut area is complex. Figure 2 shows a 300+ m-deeppalaeovalley between The Gulf and the Fifteen Mile thatdeveloped along a major fault zone. The section showsthat the valley fill has two parts. The upper 100 mappears to be a continuation of the Eight Mile plateausequence. Beneath this there is 200 m of older strata thatare found only south of Tumut Pond, terminating at orclose to the Fifteen Mile. There is no topographicevidence of an exit to this palaeovalley at this level(1300 m) in any direction. It probably flowed west fromFifteen Mile into the Tooma River, rather than continuingnorth through the Eight Mile valley, but was truncated byfaulting prior to the formation of the Eight Mile deposits.However, the rivers are entrenched for many hundreds ofmetres, forming deep gorges and destroying most of theevidence.

This interpretation is partly based on the fact that thepresent-day Tooma and Upper Tumut Rivers initially headnorth on opposite sides of the Toolong Range, then con-verge to within 4.5 km and then suddenly diverge. TheTooma turns west to join the Murray while the Tumutturns abruptly north to join the Murrumbidgee (Figure 1).This configuration suggests that they were joined, asshown on Figure 2. The divide where the two rivers nearlymeet is a broad (several kilometres wide) low section of theToolong Range that in two places drops below 1400 m, thatis, only 100 m above the Fifteen Mile bed level, yet 50 mlower than the Eight Mile bed level. Flattened spurs arecommon in the Tumut valley between the Fifteen Mile andthe Tumut–Tooma divide mostly around the 1400 m level,and these probably represent valley-in-valley structuresrelated to this palaeovalley (Gill & Sharp 1957 photograph2). Furthermore, sub-basaltic levels in the Tooma valley(1430 m at Musical Hill and 1420 m at Out Station Hill)suggest that the pre-basalt Tooma valley had a similarlevel.

The tectonic setting suggests that faulting was themain mechanism for reorganising the drainage, probablycoupled with northward tilting. West of Fifteen Mile is acomplex network of intersecting faults, some with evidenceof Cenozoic movement. Only the larger faults are shown onFigure 2, but tunnels have intersected ~10 significant faultsbetween the Fifteen Mile and T1 power station, and thereare more to the west. There are also hidden thrust faultsaround Emu Plain that were intersected in the Tooma–Tumut Tunnel.

On the western edge of the major fault zone that under-lies the Fifteen Mile basalt cap and sediments is a 1 m-widefaulted granite–diorite contact. It is in the right position toraise the egress level to the west and at the same timetectonically ponding the Fifteen Mile Creek area. There isno apparent displacement of the Eight Mile plateau by thisfault, so the Eight Mile deposits post-date the fault move-ment.

The Tumut Pond Fault zone is a major structure thatdisplaces the Early Miocene Emu Plain basalt by ~100 mbut not, apparently, the Eight Mile deposits. The fault zoneincludes clay gouge as well as mylonite, which is consistentwith several generations of movement.

The fault east of T1 power station is loose and blocky,suggesting that it developed under shallow cover, which is

Figure 2 Geological map of Miocene sediments and basaltssouth of Cabramurra and Kiandra. The section is along Andrews’(1901) Round Mountain Lead and Young and McDougall’s (1993)Early Miocene sub-basaltic stream profile in the upper Tumutcatchment. The map is a portion of Snowy Mountains Authoritymap T1/101 (Svenson & Moye 1957). See Figure 1 for location.

72 K. R. Sharp

consistent with a Cenozoic age. It could have been involvedin rearranging the drainage.

In this reconstruction the valley beneath the Eight Miledeposits was not part of the palaeo-Upper Tumut River, asassumed by Andrews (1901) and Young and McDougall(1993). It was an upper tributary, possibly the head, of theancestral Tumut River system.

Section Ridge is an 11 km-long string of basalt outcropsrepresenting a palaeovalley that lies between the EightMile and Kiandra deposits. It is a separate entity and,unlike them, has no associated sediments. The basalt baseis almost the same as the floor of the Eight Mile palaeo-valley (1300 m) and more than 100 m below the top of theEight Mile deposits. It then descends to 1030 m, well belowthat on Young and McDougall's Tumut River pre-basaltprofile. The age of this basalt (20.9* ± 0.6 Ma) is ~1 millionyears younger than the Eight Mile and Kiandra basalts(22.1* ± 0.6 and 22.3* ± 0.4, respectively). On its own this agedifference might not be considered significant but coupledwith the different geological setting it clearly indicates anew drainage system that post-dates the Eight Mile depo-sits. It appears to be a fairly minor drainage feature and nota river.

To sum up, the Early Miocene was not a period oftectonic stability as envisaged by Young and McDougall(1993). The geological history is complex and significanttectonic deformation and stream rearrangement can belinked to different ages of Early Miocene basalt lavas.Tectonism preceded, accompanied and followed variousperiods of eruption.

ADAMINABY–COOMA AREA

Introduction and background

The Murrumbidgee River is one of the abnormal-lookingdrainage features of the Monaro, with diverse explan-ations (Figure 1). Sussmilch (1910) and Taylor (1910)developed similar hypotheses that the Murrumbidgeewas a tributary of the Snowy River. Sussmilch envisagedan ancient divide near Bredbo, while Taylor placed it40 km north (at Tharwa). The U-turn, the most southerlysection of the Murrumbidgee, was where south-flowingsections of the Murrumbidgee supposedly united at pointA (Figure 3) then flowed through a low point (940 m) in theGreat Divide (Cooma Airport), Wullwye Creek and intothe Snowy River. They attributed this river capture toupwarping of the present-day Great Divide.

Browne (1914) postulated that the pre-basalt drainageflowed south, also recognising that the Murrumbidgeevalley east of point A was post-basalt. The two arms of theU were separate entities, only the western arm beinglinked to the Snowy via Wullwye Creek. The basalt-filledvalleys of Pilot and Cooma Creeks were a main tributary ofthe eastern arm.

Craft (1933a) was not in favour of any of these theories.He perceived Late Palaeozoic or Early Mesozoic uplift,followed by a series of epeirogenic uplifts, and stated(p. 243) that '. . . the probability of post-basaltic warping andfaulting during the most recent uplifts is not excluded, it isbelieved to have had little influence on the development of

the modern surface'. He also stated (p. 235, my italics) that'...post-basaltic erosion has tended to reproduce pre-basaltfeatures in detail', a generality that is often true but cannotbe applied to the entire Murrumbidgee River. Taylor et al.(1985) prepared a basalt basement contour map and con-cluded that there was no pre-basalt connection between theMurrumbidgee and Snowy Rivers. They stated (p. 65) that'Comparison of the sub-basaltic and present-day topo-graphy shows there has only been minimal change intopography and drainage patterns during the Cainozoic'.They also concluded (p. 68) that '. . . the MurrumbidgeeRiver occupies the same valley as it did before the extru-sion of the basalt' and gave undue credence to Craft’sfindings to support these statements.

Sharp (1994) rejected the statement by Taylor et al.about the course of the pre-basalt river. The present-daylink between the eastern and western arms of the U is the20 km section of the Murrumbidgee River downstream of A(Figure 3). This valley is 150–200 m deep and steep sidedwith no basalt remnants except where the river cuts acrossthe basalt-filled precursor of Pilot Creek (Upstream of Q onFigure 3). Basalt is at river level where Pilot Creek inter-sects the present-day Murrumbidgee River, so Taylor et al.in effect claimed that basalt filled the Pilot Creek Valleywithout flowing upstream or downstream along the pre-basalt Murrumbidgee valley. This is not feasible hence itwas concluded that the valley was post-basalt: this meantthat their sub-basaltic contour map showed the westernside of the U as a basalt-filled depression with no apparentegress.

Sharp (1994) examined this problem and concluded thatthe pre-basalt river flowed north, not south, and thatreversal of river flow resulted from Cenozoic tectonicdisplacements.

Further study of this complex area has been under-taken including K/Ar dating of basalts.

Pre-basalt Murrumbidgee or Adaminaby River

GEOLOGICAL SETTING

The basalt-filled Murrumbidgee valley is near the north-ern end of a large lava field––the Monaro Volcanics—thatflooded most of the Monaro. The area is covered by1:100 000 geological sheets [Berridale (White et al. 1977);Tantangara (Owen & Wyborn 1979) and Cooma (GeologicalSurvey of New South Wales unpubl.)], but the basaltboundaries are often unreliable. The 1:63 360 reconnais-sance map of Adamson (1955, 1958) is more accurate andalso shows occurrences of associated sediments andbauxite.

The basalt was remapped using 1:25 000 and 1:50 000scale base maps with 10 and 20 m contours (Figure 3). Mostlocations are identified by letters (A–Q) rather than names.

The explanatory notes on the Bega–Mallacoota 1:250 000geological sheet (Lewis et al. 1994) provide an overview ofstudies of the Monaro Volcanics (see also Brown et al. 1993).

The Monaro Volcanics mainly consists of nondescriptbasaltic flows. The Bondo Dolerite is a distinctive markerthat extends more than 20 km north of the Great Divide to5 km south of Cooma. Similar dolerites are found in thestudy area.

Cenozoic tectonism, Snowy Mountains, NSW 73

Figure 3 Geological map of the basalt-filled Murrumbidgee Valley downstream of Adaminaby showing Cenozoic geology, structurecontours on the fold deformation and four main stages in the evolution of the palaeodrainage. Bedrock geology of Ordoviciansedimentary rocks and Silurian granites are not shown. Only Cenozoic faults are shown. See Figure 1 for location.

74 K. R. Sharp

The Bega no. 7 bore on the Great Divide 19 km south ofCooma intersected 198 m of volcanics, including the BondoDolerite and eight bauxitic horizons, overlying 8 m of sedi-ments (Brown et al. 1992). Volcanics nearby rise 70 m abovethe bore, making the total thickness at least 270 m. How-ever, there is only one bauxite in the study area.

Cenozoic sediments mostly occur as small pocketsbeneath flows or thin interbeds, but are locally extensive.Sediments up to 150 m thick occur in the southern Monaro(Taylor et al. 1990). In the northern Monaro they areuncommon except between Adaminaby and Cooma, asshown on Figure 3. These thicker deposits are oftenassumed to result from damming by lava flows (Brownet al. 1993) although the author believes that tectonicponding is the more common cause.

ADAMINABY RIVER

The findings of Sharp (1994) are summarised. Many basaltflows are dipping, some as steep as 4�. A broad syncline wasobserved in a prominent dolerite lava and the deformationquantified by structure contouring the top of the dolerite.The contours delineate a large basin centred on A withmaximum displacement of ~300 m on the northwest flank.A gentle anticlinal flexure was also inferred and approxi-mately positioned (Figure 3).

Occurrences of boulder-sized gravel beneath basaltdefine the old river course but the bed levels are irregular:1000 m at G, rising to 1080 m at E then falling to 800 m at B.South of B the river bed is obscured by basalt but is at orbelow 790 m at A. Further south the bed level rises to 890 mat P, but at the same time the strata beneath the doleritewedge out and dolerite sits directly on bedrock (Figure 4a).

The irregular bed levels result from the palaeoriver bedbeing folded along with the dolerite, and the highest gravelat E coincides with the inferred anticline. (Figure 4a).

Undo this folding and the river bed becomes relatively flat,so the river could have flowed north rather than south.

This alternative concept was examined traversing upthe Murrumbidgee valley with interesting outcomes.Beyond H, the last basalt outcrop, the valley graduallyopens out and becomes alluviated. At I is a striking geo-morphic feature where the broad alluviated valley goesstraight ahead as Goorudee Morass while the present-dayMurrumbidgee enters from the north through a narrowV-shaped valley. Goorudee Morass extends 5 km beyond theMurrumbidgee confluence then stops abruptly against theAdaminaby Fault escarpment. The Morass was interpretedas the westward continuation of the north-flowing riverbefore it was severed by this fault. It is also evident onFigure 3 that the Murrumbidgee River further upstreamhas been deranged by the Cotter–Adaminaby Fault. This isnot directly related to the derangement caused by thefilling of the valley with basalt and the folding, and isdealt with below under Miocene Geology.

The 'pre-basalt' river was a very different drainagesystem to the modern Murrumbidgee River and requireda different name. Ollier and Taylor (1988) coined the name'Adaminaby River' for a hypothetical northwest-flowingancestor of the Upper Murrumbidgee River. Although theirtheory is different, their name has been retained.

Chronological development of the topography, stratigraphy and structure

Some features of the Cenozoic stratigraphy seem related tothe fold geometry, indicating that tectonism was somehowinvolved in their development and was not just a post-volcanic event. Clues to how and when tectonism affectedthe topography, stratigraphy, drainage, may be locatedanywhere in the system so, as far as practicable, systemevolution is examined in chronological order.

Figure 4 (a) Profile along the bed of the ancient Adaminaby River comparing its folding with the folding of the Bridle Creek Doleriteand the profile of the present-day Murrumbidgee River. (b) Section across the Murrumbidgee Valley showing the first three stages ofpalaeodrainage.

Cenozoic tectonism, Snowy Mountains, NSW 75

The evolution of various features is described by refer-ence to Figure 3 and the main events, including four stagesof palaeodrainage, are summarised in Figure 5.

The stratigraphic succession was difficult to establishbecause there are few marker beds. The main markers, thedolerite (which was later shown by K/Ar dating to consistof two separate flows) and an intermittent bauxite, arerestricted to the Adaminaby River valley southeast of theanticline. Sediments occur on both sides of the anticlinebut not in random fashion. Northwest of the anticline onthe west side of the Murrumbidgee River the lavas havethin interbeds of sediment, while sediments southeast ofthe anticline are less common and are found mostly alongpalaeovalleys. The basin defined by the dolerite structurecontours did not become a major sediment trap, an indi-cator that the final stages of folding post-date the MonaroVolcanics, probably taking place in the Miocene.

TOPOGRAPHY

The Adaminaby Plateau is ~300 m higher than the Berri-dale Plateau but the nature of the interface was disputed.Craft (1933a p. 231) referred to 'Bolairo plain' near Adam-inaby and the deeply incised Murrumbidgee valleydownstream as 'basin and narrow' topography, which heconsidered to be a normal feature of the region, a balancebetween erosive forces and resistance. Prior to this Suss-milch (1910 p. 339) stated that '...at Rhine Falls where theCooma road drops from the Adaminaby tableland to theBerridale tableland two faults occur, with combined throwof ~900 feet (275 m)'. White et al. (1977) found no evidencefor these faults and suggested warping. An inferred warpis shown on their block diagram but not their geologicalmap.

The folding shown by the structure contours differssignificantly from White et al. (1977) and it explains theheight difference better. The plateau interface is essen-tially a 5–10 km-wide ramp and dipping basalt flows showthat this continues to the southwest, but when the basaltspeter out due to erosion the fold ramp merges into thescenery. It probably extended 35 km southwest to theBerridale Fault to mutate into the Barneys Range Fault,which has a similar displacement. To the northeast theanticline heads toward Roberts Mountain and Mt Flinders,

and this high ridge could well be the continuation of thefold structure.

That such a significant structural element went unrec-ognised for so long demonstrates the difficulty of identi-fying folding, and the poor understanding of the nature ofCenozoic tectonics.

This fold deformation has significantly modified theearlier topography. The present-day relief on the west sideof the Murrumbidgee valley is ~500 m, the river beingbelow 800 m near A while the Great Divide is just over1300 m at N (Figures 3, 4b). A and N also represent thelowest and highest basalts and this could be misinter-preted as 500 m of 'pre-basalt' relief. But ~300 m is tectonicdisplacement, leaving 200 m or less as true pre-basalt relief.There was ~100 m of uplift on the east bank. The topo-graphy of the Adaminaby River valley at the onset ofvolcanism was not that depicted by the sub-basalticcontours of Taylor et al. (1985).

The present-day relief to the north of the Murrum-bidgee River exceeds 700 m and much of this terrain,including Yaouk Peak 1725 m, is much higher than theGreat Divide. Cenozoic tectonic movement was probablyinvolved in the development of this terrain but the detailsare not known.

STRATIGRAPHY

The oldest succession is in the Adaminaby River valley,and was initially subdivided into pre-bauxite, post-bauxite–pre-dolerite and post-dolerite successions, butlater it was recognised that there were two separatedolerites. The strata north of the anticline that did notinclude bauxite or dolerite were of unknown age prior toK/Ar dating, which showed that much of it was not evenMonaro Volcanics but Miocene.

GEOCHRONOLOGY

When the present study commenced there were 11 radio-metric ages of the Monaro Volcanics as reported byWellman and McDougall (1974) and Taylor et al. (1990),ranging from 56* (54.5) to 34 Ma (mainly Eocene and EarlyOligocene), although only one is close to the study area.Furthermore, north of the Murrumbidgee River near

Figure 5 Sketches showing the evolution of the palaeodrainage and geomorphology of the Adaminaby–Cooma area during theCenozoic. Each figure has the same coverage as Figure 3. In (d) <19 Ma–18 Ma> means that the time span for the stage 4 drainage andMiocene deposits is not known precisely but pre-dates 19 Ma, the oldest Miocene basalt, and post-dates 18 Ma, the youngest dated basalt.

76 K. R. Sharp

Shannons Flat, Owen and Wyborn (1979) dated some smallbasalt outcrops as 18–15 Ma (Early Miocene), slightlyyounger than the Snowy Province basalts near Kiandra.Field data could not differentiate these basalts from theMonaro Volcanics.

Ian McDougall (Australian National University) made amajor contribution to the present study by carrying out 13K/Ar age determinations on the author's samples (prefixedKS in Appendix 1).

McDougall analysed material only if thin-sectionexamination showed that the high-temperature phases(plagioclase, clinopyroxene etc.) were fresh and the inter-sertal glass or mesostasis, where much of the K occurs,was generally fresh and usually <10%. Having passed theacceptance criteria he expected the measured ages to begood minimum values. The onus was then on the author tocheck these ages for consistency with the stratigraphy andinterpret the data as discussed below.

Prior to the K/Ar dating the local dolerite was thoughtto be the equivalent of the Bondo Dolerite on the northernflanks of the Great Divide south of Cooma. Pratt (in Lewiset al. 1994 p. 112) referred to Bondo Dolerite at Rhine Falls(Point O on Figure 3, or Wambrook Hill). Later on BondoDolerite from the Bega no. 7 bore was dated at 48.9 ± 0.4 Ma(Roach 1996).

Dolerite from Wambrook Hill (KS9) gave an age of30.4 ± 0.4 Ma, but the author queried this and additionalage determinations were carried out. The dolerite actuallyconsists of two flows, both younger than the type areaBondo Dolerite.

The section across the Murrumbidgee Valley(Figure 4b) shows the two flows aligned like a singleflow. KS18 from the dolerite above the bauxite near theMurrumbidgee River gave an age of 44.5 ± 0.6 Ma. This flowhas been named the Bridle Creek Dolerite (Appendix II),the type area being the ridges west of A between Bridle andPeak Creeks that are capped with this flow.

Basalt sample KS23 from just above the Wambrook Hilldolerite (KS9) gave an age of 40.6 ± 0.4 Ma. The 30 Ma agefor KS9 could be real if it was a plug and localised flow butthis was ruled out by additional mapping. This doleritethus appears to be ≥40.6 Ma while 30 Ma is, for no apparentreason, a defective minimum age. This age does notcorrelate with the type area Bondo Dolerite, 48.9 Ma,hence this flow has been named the Wambrook Dolerite(Appendix 2). The type area is west and north of point O,Wambrook Hill.

Field checks have confirmed that the Bridle Creek andWambrook Dolerites are not one flow despite their prox-imity. The Wambrook Dolerite characteristically containslarge (up to 10 mm) phenocrysts of titanaugite. Alsobauxite underlies the Bridle Creek Dolerite but not theWambrook Dolerite.

The precision of the dolerite structure contours hasbeen reviewed in the light of this mapping error. Each flowprobably experienced slightly different fold deformationbut the contouring is not precise enough to detect this. Alarge part of the fold deformation apparently post-datesboth dolerites.

Most ages are shown on Figure 3, but KS17 (Tilla-budgery Trig: 41.2 ± 0.4 Ma) and KS27 (Gourock Plug: 44.9 ±0.6 Ma) lie off this map. Seven ages are shown in profile on

Figure 4b, some being projected. Much of the age data canbe evaluated in relation to the stratigraphy and structureby reference to this cross-section.

The 30 Ma dolerite age is in parentheses because it hasbeen judged as too low. The basalt resting on the bauxitegave two ages: 47.7 ± 0.5 Ma for KS10 and 42.5 ± 0.5 Ma forKS12. The latter is in parentheses because it is inconsistentwith the surrounding data and considered too low. (Itwas difficult to locate unweathered rock in this area.)On the right (eastern) side of the section the strataappear to be superimposed in a regular manner. Thebasalt above the bauxite (47.7 Ma) is overlain by the BridleCreek Dolerite (44.5 ± 0.6 Ma), and 10 km to the northeastKS15 (44.2 ± 0.5 Ma) is higher than the Bridle Creekdolerite and 50 m below the ridge top. This age progressionis realistic.

On the left (western) part of the section the ages do notdecrease regularly with height because this is not a simplestratigraphic succession but represents tectonic deform-ation and three stages in the reorganisation of the palaeo-drainage. These stages are identified on plan and section(Figures 3, 4) by one, two and three barbs, and discussedin detail below; they are also shown separately onFigure 5a–c.

The stage 2 and 3 palaeovalleys are uphill of stage 1, aconsequence of fold deformation. To interpret the section itshould be tilted so that the dolerite and the west-banktopography are essentially horizontal, thus more or lessrestoring the palaeovalleys to their relative positionsbefore the folding.

Stage 1 palaeodrainage

The Adaminaby River was the original north-flowingdrainage and its basalt-filled valley and several smalltributaries extend 30 km. The strata are divided into pre-bauxite, post-bauxite–pre-dolerite and post-dolerite. Thereare no post-dolerite lavas on the section line (Figure 4b)but they occur north of this line on the east bank of theMurrumbidgee River, and south of point A on SpringPlain.

The south flank of Peak Creek (west of A) provides aprofile through most of these strata. The river bed is notexhumed here but there are pre-bauxite basalts (50 m),intermittent bauxite and sediments (0–5 m), basalts (35 m)and Bridle Creek Dolerite (5 m).

Along the valley there are two variants in thissequence: 1 km south of A the bauxite and Bridle CreekDolerite disappear beneath basalt, while further south at Pthe dolerite rests on bedrock. The wedging out of the pre-dolerite strata indicates a rising valley floor consistentwith a north-flowing river. To the north near the anticlinethe bauxite and dolerite cut out between D and E.

The eastern valley wall is well defined and forms theeastern limit of most flows. Only post-dolerite flows over-top it at one place east of C. The western valley wall is notwell defined.

PRE-BAUXITE SUCCESSION

The pre-bauxite succession consists of nondescript flowsexcept for a fresh-looking columnar basalt that overlies the

Cenozoic tectonism, Snowy Mountains, NSW 77

river gravel. It also extends up a tributary and appears inBulga Creek east of C.

Samples from this sequence were not suitable for K/Ardating. However, the sequence is older than the basaltoverlying the bauxite, 47.7 Ma, while the oldest K/Ar agefor the Monaro Volcanics is 56 Ma. The estimated age isthus around 50 Ma.

BAUXITE AND PALAEOWEATHERING

There is one discontinuous horizon of bauxite, unlike theMonaro Volcanics south of Cooma with eight interflowweathering profiles in the Bega (BMR) no. 7 bore (Brownet al. 1992). Most probably this bauxite correlates with oneof three above the Bondo Dolerite.

The development of bauxite indicates a long pause involcanic activity and stable conditions over many thou-sands of years. Many of the bauxites to the south outcropcontinuously over many kilometres whereas this has inter-mittent outcrops. This results from partial erosion, indi-cating a change in conditions before volcanism resumed.

Apart from the underlying river gravel there are nosediments in the pre-bauxite succession. The absence ofsediments in this relatively narrow valley suggests thatthe Adaminaby River became defunct or was diverted else-where. Basalt ages indicate that the stage 2 drainagedeveloped to the west of this valley around 46.0 ± 0.5 Ma,while the main valley was still being filled with lavas.

The first sediments in the valley fill are scatteredpockets of clay, silt and sand a few metres thick, mostlybetween bauxite outcrops. An exposure 1 km north of pointA explains this association, with sediments deposited in ashallow erosion channel in the bauxite. The bauxite wascontinuous, but widespread subtle erosion removed muchof the bauxite and not much else. Sediments filled theshallow erosion depressions between the bauxite rem-nants.

There is an apparent relationship between the abun-dance of bauxite and the tectonic deformation. Foldingproduced gradients and an erosion potential varyingaccording to position in the fold basin. Bauxite is mostabundant (that is erosion was least) in the lowest part of thebasin. There are scattered outcrops (greater erosion) on theeastern limb of the basin, but only as far north as theanticline, and none has been found on the steeperwestern limb. This dates the probable onset of folding at≥47.7 Ma.

In places the sedimentary basement rocks beneath thebasalt are intensely weathered and red or orange to depthsof ~10 m. This is found around the level of the bauxite andup to the Bridle Creek Dolerite, but not in the lowest partsof the valley. This palaeoweathering was associated withthe bauxite formation and its distribution more or lessmimics that of the bauxite, becoming sparser instead ofmore common at higher levels.

Perhaps even these explanations are too simplistic. Thepre-bauxite infill of the Adaminaby River valley is an inlierapparently isolated from lavas of similar age, and this isnot easily explained without extensive erosion of itssurroundings. There is no bauxite in the Pilot Creekvalley-fill east of the Adaminaby River valley althoughtheir levels are similar. This valley presumably post-dates

the bauxite, possibly making it the same age as the stage 2palaeodrainage.

POST-BAUXITE–PRE-DOLERITE SUCCESSION

The post-bauxite–pre-dolerite succession begins with thescattered sediments between bauxite outcrops. There areno other sediments or palaeodrainage features close to thepresent-day river, but 3–4 km west sediments were beingdeposited in the stage 2 palaeodrainage system in essen-tially the same time period.

BRIDLE CREEK DOLERITE

The Bridle Creek Dolerite is a relatively coarse-grainedbasaltic rock in a sequence of fine-grained basalts. It isconfined to the valley of the Adaminaby River south of theanticline. It is not clear whether it did not flow across theanticline or whether it was there and was subsequentlyeroded. It did not flow across the interfluve into Pilot Creekeast of C but some overlying lavas did.

On Figure 4b the dolerite structure contours and thebed of the Adaminaby River appear to converge toward theanticline. It is not an accurate indicator but it suggestsuplift of the anticline of ~100 m by the time the BridleCreek Dolerite erupted, that is, at 44.5 Ma. Hence the anti-cline may have been a height barrier that the dolerite couldnot cross.

POST-DOLERITE SUCCESSION

Considerable thicknesses of basalt overly the Bridle CreekDolerite, 50 m above KS15 (44.2 ± 0.5 Ma) east of C and~80 m across Spring Plain between A and P. The top of thelatter is probably about the same age as the basalt atTillabudgery Trig (KS17: 41.2 ± 0.4 Ma), the highest (andyoungest) basalt north of Cooma.

Stage 2 palaeodrainage

Valleys filled with ~30 m of sediments, and minor basaltflows, form a palaeodrainage system ~11 km long on thesoutheast flank and crest of the anticline (Figures 3, 5b).These relatively thick sediment deposits are localised andcontrast with thin sediments between flows that occurnorthwest of the anticline (not all are shown on Figure 3).It appears that drainage to the north was diverted to thesouthwest by progressive filling of the Adaminaby Rivervalley with lavas and the rising anticline.

Near the western end, a sediment-filled palaeovalleycapped with 44.6 ± 0.5 Ma basalt crosses the present-dayGreat Divide at N near 'Muniong'. The dolerite structurecontours show that uplift was uneven, being greatest nearthe Great Divide. This decreased the stream gradient to thewest and created a sediment trap. Continuing uplift createda divide (the proto-Great Divide) and diverted the drainagesouth and formed the stage 3 palaeodrainage.

The original gradients have been severely distorted bythe folding so bed levels cannot be used to verify this, butfacies changes support this model. The most southerlysediments (on the road to 'Peak Valley') are at 940 m whereclay and silt overlie the 46.0 ± 0.5 Ma basalt. A nearby farm

78 K. R. Sharp

bore penetrated 30 m of sand. North of this (in Peak and PatAnne Creeks) similar thicknesses of sandy sediments withinterbedded basalt overlie bedrock. At the eastern endthere are gravels at 1020 m but it is not absolutely certainthat these underlie the basalt. Near the western end a farmbore was drilled in 1959 at 'Muniong' to 31.4 m, stoppingshort of bedrock at approximatel 1230 m. Disturbed drive-tube samples were mainly silt and clay that often containedplant fragments and lignite, the only lignite known in thisarea. There thus appears to be a facies change along thechannel, gravel in the east, sandy facies in the centre andswampy conditions in the west. E. M. Truswell (pers.comm. 1996) found the pollens in the lignite to be too poorlypreserved to warrant study.

The development of this part of the drainage systemis essentially constrained between 46.0 ± 0.5 Ma and 44.6 ±0.5 Ma, that is, the flows beneath and above the mainsediments.

Stage 3 palaeodrainage

The Wambrook Dolerite has an unusual relationship withthe sediments of the stage 2 drainage that is best describedas an unconformity. The eastern part of the doleritedirectly overlies sediments while on the western side thedolerite and associated basalts lie in a separate valleysouth of and ~100 m lower than the sediments at Muniong(Figure 4b). This stage 3 drainage flowed south and has noassociated sediments. Uplift progressively converted theMuniong area from a valley to a sediment-filled valley andthen a divide. Next, the stage 3 valley was excavatedmainly in deeply weathered granite and the WambrookDolerite and associated lavas filled it. The uplift of theMuniong area of ~100 m post-dates the basalt capping at'Muniong' (44.6 ± 0.5 Ma) while the valley pre-dates theWambrook Dolerite and overlying basalt (40.6 ± 0.4 Ma).Four million years is a realistic time for all these events.Subsequent deformation tilted this area and redirected themodern day Wambrook Creek to the southeast.

This 40.6 Ma basalt is close to the top of the strati-graphic sequence near and south of Wambrook Hill, but afew younger ages exist elsewhere.

Younger Monaro Volcanics

Near and north of the anticline there are thin beds ofsediment between most basalt flows. Those on Dry Plainproved to be Early Miocene, but in the middle of a group offlows further south (5 km northeast of 'Muniong') KS19gave an age of 36.0 ± 0.4 Ma. These flows appear to bestratigraphically above the Wambrook Dolerite. A knoll ofcolumnar basalt 1.5 km south of the sample location maybe a plug and the source of these flows.

Furthermore this younger age does not stand alone.Basalt 12 km west of point N was dated by Wellman andMcDougall (1974) as 37.0* (36.0) ± 0.9 Ma. This is not shownon Figure 3 but is west of the Great Divide, filling a shallownorthwest-trending valley in the old land surface of theAdaminaby Plateau.

Near F at the entrance to the Murrumbidgee Gorge aflow at the top of the sequence (KS5) was dated at30.0 ± 0.3 Ma. Importantly this is not part of the Miocene

volcanics. Whether this represents even younger MonaroVolcanics or a spurious minimum age has little effect onthe overall history.

From Monaro Volcanics to Miocene volcanics

The youngest Monaro volcanism was 36 Ma or possibly30 Ma, while the Miocene volcanism began at 19 Ma. Thistime interval represents more than a lull in volcanicactivity, it involved erosion, tectonism and widespreadsediment deposition as detailed below.

Miocene geology

EXTENT OF MIOCENE DEPOSITS AND ORIGIN OF LAKE ADAMINABY

Early Miocene deposits became known in the northernMonaro when Owen and Wyborn (1979) dated small basaltoutcrops near Shannons Flat as being 18.0 ± 0.4, 18.2 ± 0.3and 15.2 ± 0.3 Ma. The lowest of approximately four flows atDry Plain (KS1) was dated as 19.2 ± 0.2 Ma, indicating thatmuch of the basalt northwest of the anticline is also EarlyMiocene. Sediments underlying or interbedded with thesebasalt flows are clearly Miocene.

West of these established Miocene deposits are wide-spread high-level alluvial and lacustrine deposits inclu-ding the Bolairo Plain around Adaminaby and north upthe Murrumbidgee valley to Yaouk (10 km north of thecoverage of Figure 3). All these deposits are in one sectionof the drainage basin between the Great Divide and theBoboyan Divide (the ACT–NSW border). Their westernlimit more or less coincides with the Adaminaby–CotterFault [the Adaminaby and Cotter Faults mapped by Owenand Wyborn (1979) are parts of a single fault].

Sharp (1994) postulated that the ancient AdaminabyRiver flowed north and then west to J where the GoorudeeMorass ends abruptly against the Adaminaby Fault. As thewest side is up-throw, faulting would dam the west-flowingAdaminaby River, hence the sediments were interpreted aslake deposits. Such faulting would not dam an east-flowingriver, that is, the Murrumbidgee. The concept of LakeAdaminaby is portrayed on Figure 3 by the present-day1100 m contour as the approximate lake perimeter. This100 m-deep lake extended ~30 km, comparable in dimen-sions to the man-made Lake Eucumbene to the west.

Subsequently the simple fault-damming model has beenvaried slightly. Drainage south into the Eucumbene Riverwas disrupted by uplift of the Monaro Range, includingCooloowye Saddle, by faulting and folding.

The resulting Miocene deposits in the eastern sector(with basalt) and western sector (without basalt) arediscussed separately.

EASTERN SECTOR

Near the change from Monaro Volcanics to Miocenedeposits the topography was modified by erosion beforeMiocene sediments were deposited. At F, the entrance tothe present-day Murrumbidgee Gorge, 100 m or more ofMonaro Volcanics plugged the valley, but a few kilometresnorth between G and H, remnants of Monaro Volcanics(gravel and basalt) and Miocene sediments occur at an

Cenozoic tectonism, Snowy Mountains, NSW 79

elevation of 1000 m in close proximity. Exhuming thisportion of the Adaminaby River valley floor required atleast 100 m of erosion and required an egress at 1000 m forthe Early Miocene drainage.

The Miocene Adaminaby River between H and JonesPlain is indicated by intermittent outcrops of grey billy-type silcrete (silicified sand and gravel) at or above 1000 m(most are too small to show on Figure 3). The same baselevel of 1000 m applies to scattered outcrops and sheets ofsilcrete in the western sector as far as J where GoorudeeMorass is truncated by the Adaminaby Fault.

In the eastern sector the main palaeotributary streamswere similar to the present-day Alum and Caddigat Creeks.The Miocene, or stage 4, palaeodrainage is shown with fourbarbs on Figure 3 and summarised on Figure 5d.

Miocene deposits beneath basalt are thickest southwestof H on the edge of Dry Plain in palaeo-Caddigat Creek with80 m of unconsolidated sand and gravel (with sporadicsilcrete and ferricrete), and 50 m in palaeo-Alum Creeknear Shannons Flat. No major facies variations, such aslignite, are known. The sediment fill was around 1100 mwhen the first basalt erupted, and this representsminimum lake level. Being highly erodible, the sedimentshave mostly disappeared, taking the overlying basalts withthem.

Across Dry Plain some sediments are higher than1100 m, which might be attributable to higher lake levels,but probably due to tilting associated with uplift of theMonaro Range.

Silcrete is most abundant near the base of the Miocenesediments in the eastern and western sectors, but smalloutcrops are also fairly common in close association withMiocene basalts across Dry Plain. In contrast silcrete israre in association with the Monaro Volcanics within thearea of Figure 3.

WESTERN SECTOR

The Miocene sedimentary deposits shown on Figure 3 aremostly silcrete and gravel after White et al. (1977) andOwen and Wyborn (1979). Adamson (1955) showed moreextensive deposits because he included fine-grainedunconsolidated materials. Their precise extent is not animportant issue here, just that they are associated with theformation and demise of Lake Adaminaby.

Silcretes on the Monaro are usually associated withbasalt, but the silcrete on Bolairo Plain appears to haveformed in a shallow lake or semi-inundated environment,that is, the earliest stage of Lake Adaminaby. This wouldhave been a backwater of the Murrumbidgee River.

It is obvious on Figure 3 that the upper MurrumbidgeeRiver has been affected by the Adaminaby–Cotter Faultand to a large extent follows it.

Just north of the map the Murrumbidgee debouchesinto the Yaouk Valley. East of the fault the river simplydumped its bed load on entering Lake Adaminaby andgravel deposits built up to 1110 m, which is virtuallyidentical with the minimum lake level (1100 m) indicatedby the Miocene sediments in the eastern sector.

Scarps are associated with the Cotter Fault north of Kand with the Adaminaby Fault for some distance south ofJ. The gap between these scarps relates to the gradual

divergence of the courses of the ancient and modern rivers.The Murrumbidgee hugs the fault on its east or downthrowside from L to K and the western palaeovalley wall is partlypreserved. Near K the fault bisects the valley floor and thegravel base is 1140 m on the west and ~1060 m on the eastside, that is, 80 m displacement. Further south the scarpdisappears because the fault ran along the eastern valleywall, and the scarp and valley topography cancel eachother.

At K the palaeovalley makes a big loop to the west viawhat are now Atkinsons Creek and Goorudee Rivulet,then back to J to be joined by the Adaminaby River. TheMurrumbidgee eventually took a short cut and cut a newgorge between K and I, as discussed later.

Coarse gravel between J and M indicate the route of thepalaeoriver to the south. At J the gravel base is 1060 m andthe deposit is capped with fine-grained alluvium. At M thegravel base is 1080 m and the top is 1110 m. The gravel trailpeters out but the evidence is that the river veered west andpassed through a saddle now at 1165 m where CooloowyeEmbankment has been constructed.

No river gravel has been found near Cooloowye Saddlebut other data indicate that the saddle is part of the palaeo-river. It is the lowest point in the divides surrounding LakeAdaminaby, apart from the Murrumbidgee Gorge whereLake Adaminaby eventually drained to the southeast.

Cooloowye Saddle has a 400 m-wide almost flat floorwith distinct walls, the form of an abandoned valley, awindgap, unlike the usual U-shaped saddles at creek heads.Snowy Mountains Authority exploration records show thatthe floor has 1–1.5 m of colluvial soil over sedimentarybedrock. Also the palaeovalley west of the gravel trail hasa fairly well-defined western flank that curves around intothe saddle.

Beyond Cooloowye Saddle the terrain is drowned byLake Eucumbene but maps show an alignment of thesaddle and part of Frying Pan Creek that leads to a sharpbend in the Eucumbene River, then straight down theEucumbene. It was a tributary of the Eucumbene River.

COOLOOWYE SADDLE AND LAKE ADAMINABY

Cooloowye Saddle was the original outlet or spillway ofLake Adaminaby and as such its changing level controlledthe lake level.

The Early Miocene Adaminaby River east of theAdaminaby–Cotter Fault had an elevation of 1000 m andthe saddle was part of that palaeodrainage at approxi-mately the same level. The saddle was raised to minimumlake level, ~1100 m, or higher. But the lake could not haverisen above 1165 m, the present-day level of the saddle, butpossibly got close. To test this idea the 1160 m present-daycontour was added to Figure 3 around the southern lakeshore and interestingly this contour takes in the higherMiocene sediments around Dry Plain. Nevertheless thesehigher sediments are more likely due to subsequent upliftthan a higher lake level.

The uplift of the saddle area was not a direct result offaulting but of the associated deformation. The AdaminabyFault truncates Goorudee Morass but peters out to thesouth as it nears the Monaro Range. The fault step isscissor-like and the eastern side was tilted up to the

80 K. R. Sharp

south while the western side remained relatively horiz-ontal. The fault dies out to the south but not the associatedtilting.

The area east of the fault is what Craft (1933a) referredto as the Bolairo Plain. This remarkably planar feature atthe foot of the Monaro Range dips 1.5–2�N, visible evidenceof the fold deformation that uplifted the range eventuallytransforming it into the Great Divide. The hills west of thefault have also been uplifted by warping or folding but theevidence is less tangible. The gradient from the highestriver gravel at M to the saddle is ~2�, essentially the sameas the tilted Bolairo Plain.

Uplift of another section of the Great Divide has alreadybeen discussed. The Muniong area was initially uplifted100 m or more to form a new divide and the stage 3 drainageprior to the 40.6+ Ma Wambrook Dolerite. But much of the300 m or more of fold-deformation deduced from thedolerite structure contours came later. The basalt flowson Dry Plain have a northerly component of dip and someMiocene sediments are at ~1150 m. The most probableexplanation is that this later folding, and the faulting andfolding near Adaminaby, are closely related. In otherwords, the 25 km section of the Great Divide betweenMuniong and Bolairo Plain, and probably much more,was uplifted by Early Miocene tectonism.

The Murrumbidgee River was rerouted when tectonicdeformation raised Cooloowye Saddle above the basaltfilling the Murrumbidgee Gorge near the anticline,forming a new lake outlet. This may have been as early asthe tectonic deformation that produced the main lake orlater. In other words, Cooloowye Saddle may have ceased tobe a spillway before reaching its present level. Regardlessof exactly when it happened, it repositioned the GreatDivide and created a new drainage system.

GOORUDEE MORASS

Goorudee Morass is an odd feature identified by Sharp(1994) as part of the west-flowing Adaminaby River, but thefeature also has a younger history. The Morass and thealluviated section of the Murrumbidgee River downstreamis a relatively broad channel lying 10–30 m below the levelof the Miocene silcretes that represent the old lake floor.This deeper channel was excavated after the draining ofLake Adaminaby. The Murrumbidgee River in its presentposition could not have excavated the Morass, whileGoorudee Rivulet is a minor stream with limited erosioncapability.

The deepening of the Morass can be explained if thepalaeo-Murrumbidgee River continued to flow between Kand J after Lake Adaminaby was formed and began todrain. This may mean that upstream of K a separate lake,Lake Yaouk, existed for a period of time at a higher levelthan Lake Adaminaby. Atkinson's Creek has someabnormal features supporting this interpretation. Itscatchment is much smaller than Goorudee Rivulet yetupstream of their confluence its channel is lower and notas steep. Unlike the rivulet its bed is alluviated andswampy. All these features make sense if Atkinson's Creekis the abandoned valley of the palaeo-Murrumbidgee.

Late (post-lake) movement of the Adaminaby–CotterFault presumably caused the river to take its new route

between K and I, this relatively straight course being influ-enced by an old fault. This 100 m-deep gorge could not havebeen excavated while this area was submerged by the lakeand hence post-dates the draining of Lake Adaminaby. Itwould have taken a significant period of time to cut thisgorge.

Modern Murrumbidgee River

The anticline probably became a divide when the stage 2 orstage 3 drainage developed. This divide would be thehead of the south- and east-flowing proto-Murrumbidgee,a small stream with little erosion capability until itsuddenly became a river when Lake Adaminaby over-topped it. The course downstream is structurally con-trolled where it deviates east at point A, the lowest point inthe rim of the basin defined by the dolerite structurecontours.

Near the anticline the river has incised ~270 m in18 million years or less, averaging 15 m/106 years. At thisrate it would have taken 7 or 8 million years to lower thevalley enough to completely drain Lake Adaminaby. Thissuggests that the development of Goorudee Morass and theriver deviation between K and I should be assigned to theMiddle Miocene or younger.

DISCUSSION AND CONCLUSIONS

Cenozoic tectonism, faulting and folding, resulted insignificant differential uplift and affected the evolution ofthe drainage and geomorphology throughout this region.In particular Young and McDougall (1993) did not identifythe role of tectonism in the Kiandra–Cabramurra areanor did Taylor et al. (1985) in the Adaminaby–Cooma area.The findings also affect many of the theories covered inBackground.

A number of faults in the Kiandra–Cabramurra areaand one in the Adaminaby area were active during theEarly Miocene and they contributed to major streamderangement.

The uplift of the Kosciuszko Block is at least partly dueto northward tilting during the Early Miocene. Cenozoicfolding or warping played a significant role in the develop-ment of the Adaminaby–Cooma area. It produced 300 mor more differential uplift between the Adaminaby andBerridale Plateaus, created Lake Adaminaby and radicallymodified the drainage system. It also created a section ofthe Great Divide.

This large fold displacement is matched by few majorCenozoic faults. Folding is not readily identifiable, hence itmay be that folding is a major, but largely unrecognised,deformation mechanism responsible for differential upliftin other areas.

The American Geological Institute Glossary of Geology(Bates & Jackson 1987) defines warping as 'slight flexing orbending of the Earth's crust on a broad or regional scale'.This deformation is better described as folding rather thanwarping.

Ideas on the number and timing of uplift events varywidely. In this region the evidence is of tectonic activity atintervals during the Cenozoic, with an important tectonic

Cenozoic tectonism, Snowy Mountains, NSW 81

epoch associated with Early Miocene volcanism. The EarlyMiocene tectonism affected both study areas so it waswidespread, but very variable as to its nature andeffects. However, this age does not coincide with any ofthe uplifts postulated in previous models listed underBackground.

There were smaller tectonic movements within theperiod during which the Monaro Volcanics erupted (mid-Eocene to Early Oligocene) that progressively altered thedrainage system. It is not known whether these were fairlylocal movements or widespread.

Cenozoic tectonism and volcanism in the Adaminaby–Cooma area progressively rearranged the upper Murrum-bidgee River system. In the Early Miocene a large lake wasformed and when this was overtopped the river followed apre-basalt river, but in the reverse direction. What tookplace is quite different to earlier hypotheses of streamcapture and river reversal.

The Tumut River was also rearranged in the EarlyMiocene. The palaeo-upper Tumut River probably flowedinto the Tooma River rather than the Tumut River.

All this shows that the story of when and how thisregion was uplifted is more complex than generallyappreciated and is far from being understood.

ACKNOWLEDGEMENTS

Ian McDougall, Australian National University, carriedout the K/Ar age determinations. The Snowy MountainsHydroelectric Authority gave permission to use data,including maps, from unpublished reports, accompaniedby a disclaimer that it is not exhaustive or necessarilyconclusive. Figure 2 is a portion of Map T1/101 (Svenson &Moye 1957), annotated with spot heights and additionallocality names. Journal reviewers are thanked for theircomments.

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Received 26 September 2001; accepted 6 November 2003

APPENDIX 1: POTASSIUM–ARGON AGE DETERMINATIONS

Field no. Locality: AMG coordinates: elevation Age ± 1� (Ma)

KS1 Dry Plain (Caddigat Creek): 671800E 6006000N: 1120 m 19.2 ± 0.2KS5 SE of Jones Plain, above Murrumbidgee (‘Kuroona’): 67700E 6010900N: 1130 m 30.0 ± 0.3KS9 Wambrook Dolerite near Wambrook Hill: 671700E 5991700N: 1120 m 29.4 ± 1.7

30.4 ± 0.4KS10 Flow overlying bauxite (Bridle Creek): 680150E 5994600N: 850 m 47.7 ± 0.5

47.4 ± 0.5KS12 Flow overlying bauxite (Kissops Flat): 679400E 5997500N: 880 m 42.5 ± 0.5KS14 Beneath sediments and Wambrook Dolerite (‘Peak Valley’):

676100E 5992400N: 920 m46.0 ± 0.5

KS15 Above Bridle Creek Dolerite, 0.5 km W of ‘Murrumbucca’: 685300E 6003550N: 985 m

44.2 ± 0.5

KS17 Tillabudgery Trig, 4 km NE of Cooma: 692850E 5991050N: 875 m 41.2 ± 0.4KS18 Bridle Creek Dolerite, Dry Plain Road near Peak Creek:

679000E 5996100N: 940 m44.5 ± 0.6

KS19 ‘Glenbernie’, Dry Plain Road: 673300E 5999000N: 1200 m

36.0 ± 0.4

KS20 Main Divide near ‘Muniong’: 669000E 5996400N: 1290 m 44.6 ± 0.5KS23 Above Wambrook Dolerite, near sample KS9: 671400E 5991600N: 1160 m 40.6 ± 0.4KS27 Plug, ‘Gourock’ quarry, 29 km SW of Cooma: 702800E 5961700N: 1160 m 44.9 ± 0.6

Basalt samples collected by K. R. Sharp.Age determinations carried out by I. McDougall, Australian National University.

Cenozoic tectonism, Snowy Mountains, NSW 83

APPENDIX 2: STRATIGRAPHIC NOMENCLATURE

Monarco Volcanics and Bondo Dolerite Member

The Monaro Volcanics and a distinctive flow member, theBondo Dolerite, were formally named by Pratt et al. (1993)and later by Pratt (in Lewis et al. 1994). Dolerite nearWambrook Hill (or Rhine Falls) was considered to beBondo Dolerite but K/Ar dating has shown that thiscorrelation was incorrect. The two dolerite flows mappedin the northern parts of the Monaro Volcanics are youngerthan the type Bondo Dolerite and are here named theWambrook Dolerite and Bridle Creek Dolerite.

Wambrook Dolerite (previously considered to be Bondo Dolerite)

Type section No clearly definable section known.Type area Typical outcrops extend 1 km west and 3 kmnorth of Wambrook Hill (AMG 673000E, 5991000N), with atotal mapped extent of ~10 km.Designation A readily identifiable flow within thenorthern Monaro Volcanics.Thickness Estimated as ~20 m, rather than ~50 m givenby Lewis et al. (1994: p. 112).

Description A dolerite flow distinguished from sur-rounding basalt flows by grainsize and the presence oflarge phenocrysts of titanaugite. Petrographic descriptionand chemical analysis of this dolerite are given in Whiteet al. (1977: p. 89–91).Age K/Ar dated indirectly as ≥40.6 ± 0.4 Ma. Late Eocene.

Bridle Creek Dolerite

Type section This dolerite outcrops extensively as cap-pings of the ridges between Bridle Creek (AMG 679000E,5993500N), Wambrook Creek and Peak Creek. The typesection is the South flank of Peak Creek (AMG 679000E,5995000N) where ~5 m of Bridle Creek Dolerite overlies90 m of volcanics representing much of the lower strati-graphy of the northern Monaro Volcanics. These are basaltapart from an intermittent horizon of bauxite and sedi-ment (0–5 m), 35 m below the dolerite.Designation A relatively coarse-grained lava mega-scopically distinguishable from the associated fine-grained basalt lavas. It has been mapped along the valleyof the Murrumbidgee River for 20 km.Age K/Ar dated as 44.5 ± 0.6 Ma. Middle Eocene.