Potassium‐argon ages on the Cainozoic Volcanic rocks of Eastern Victoria, Australia

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Terms http://www.tandfonline.com/loi/taje19 http://www.tandfonline.com/action/showCitFormats?doi=10.1080/00167617408728858 http://dx.doi.org/10.1080/00167617408728858 & Conditions of access and use can be found at http://www.tandfonline.com/page/ terms-and-conditions D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 http://www.tandfonline.com/page/terms-and-conditions http://www.tandfonline.com/page/terms-and-conditions POTASSIUM-ARGON AGES ON THE CAINOZOIC VOLCANIC ROCKS OF EASTERN VICTORIA, AUSTRALIA by PETER WELLMAN (With 5 Tables and 4 Figures) (MS received 18 February 1974) ABSTRACT The Cainozoic volcanic rocks of eastern Victoria comprise most of the Older Volcanic Series. K-Ar ages of volcanic rocks in South Gippsland range from middle Palaeocene to middle Eocene; the older ages are to the east. In the Eastern High- lands the majority of lavas are of late Eocene to middle Oligocene age, with some lavas of late Oligocene to Pliocene age. The Cainozoic volcanic activity in Victoria is thought to have been almost continuous since it started in the early Palaeocene, with peaks of activity from Palaeocene to Eocene and from Pliocene to Pleistocene. The uplift of the Eastern Highlands is thought to have taken place in three sub-equal stages, starting in the middle Mesozoic. The last uplift is inferred to have begun some time after the Oligocene, and to have resulted in less than 300 m of uplift in the central and southern parts of the highlands, and negligible uplift in the northern. In the Oligocene the highlands had a relief of over 1000 m, and a drainage system similar to that of today. INTRODUCTION Cainozoic volcanism, mainly basaltic, is widespread along the eastern margin of the Australian continent extending from north Queensland to Tasmania. In Victoria the Cainozoic volcanism occurs within about i50 km of the coast, as restricted lava fields and valley filling flows in the Eastern Highlands, as flows associated with sedimentation in the Gippsland Basin, and in western Victoria as a very extensive young lava field (Fig. 1). Traditionally the lavas have been included-in two mains groups, the Older Volcanic Series of Early Cainozoic age and the Newer Vol- canic Series of Late Cainozoic age comprising the large lava field of western Victoria. Except where lavas are associated with sediments yielding age-diagnostic fossils, control on the age has been poor. In this paper K-Ar ages are reported on lavas from the main areas of outcrop of the Cainozoic lavas in Victoria; these results indicate that volcanism has occurred throughout the Cainozoic, rather than being confined to the Early and Late Cainozoic as previously thought. As the Newer Volcanics of Western Victoria have been subjected to considerable geochrono- logical study, this study concentrates almost entirely on the measurement of age of the lavas east of the longitude of Melbourne. GENERAL GEOLOGY Predominantly, the Cainozoic volcanic rocks of Victoria are flows of olivine basalt, in units generally 1 to 10 m thick. The flows form lava fields a few metres to 300 m thick, covering areas commonly about 60 km in diameter, although the Newer Volcanic Series of western Victoria forms a lava field about 350 km across. Large central or shield type volcanoes are absent; eruption has mainly been from extensive dyke swarms or localized vents. Most of the lavas are undersaturated to slightly undersaturated with respect to silica (Edwards, 1939) although some of the Newer Volcanics are tholeiitic. The lava flows are almost all subaerial as they overlie and are interbedded with fluviatile sediments, and in some cases they are associated with lacu- strine sediments and diatomites. In southern Victoria there was extensive marine sedimentation during the late Oligo- cene to late Miocene. Commonly the Caino- zoic volcanic rocks have been divided on their relationship to this marine sequence; the Older Volcanic Series lie below and the Newer Volcanic Series above the mid-Caino- zoic marine sequence. The two series were considered to be respectively pre-Miocene and post-Miocene by Smyth (1858), Early Caino- zoic and Late Cainozoic to Recent by Skeats (1910), Oligocene to early Miocene and late Pliocene to Quaternary by Hills (1938) and Singleton (1941), and mainly Palaeocene and Eocene and Plio-Pleistocene by Singleton & Joyce (1969). The only widespread dating control on the Older Volcanic Series is the extensive occur- rence of leaf impressions of the Cinnamomum Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 360 PETER WELLMAN t--. 142'E 144* 146" 148° Murray "•• f. n •.•'•'..~:-' ; r \ A^> V' ,•:>-' . - ' > v v « 9 4 i 4 -~_ Highlands 38*S Western District Volcanic Plains 100 Fig. 1. Distribution of Cainozoic volcanic provinces and the physiographic subdivisions. Numbers refer to province numbers used in text. flora. This flora has been found in the sedi- ments associated with the volcanics in the Eastern Highlands, as well as those associated with the volcanics of the Gippsland Basin, so that it has generally been accepted that both are of the same general age. Within the Gippsland Basin the basal sedi- ments of the Latrobe Valley Group are re- ported to be accompanied by basaltic vol- canism throughout the whole period of the transgression—Late. Cretaceous to Early Mio- cene (Hocking, 1972). There is now excellent palaeontological control on the age of these basalts (James & Evans, 1971) but details of most of the work have not yet been published. Details of palaeontological dating have been given for Palaeocene-Eocene flows near Yarram in Gippsland (Cookson & Dettman, 1959), and for a Palaeocene flow north of the Otway Ranges (Cookson, 1954). Two groups of volcanic rocks in western Victoria have been dated isotopically. The flows at Aireys Inlet and Maude and a dyke near Portland have been shown to be late Oligocene to middle Miocene (Abele & Page, 1974; Harding, 1969), and the Newer Volcanic Series has been shown by K-Ar and radiocar- bon dating to range from 4.5 m.y. to 5000 years B.P. (McDougall et al., 1966; Aziz-ur-Rahman & McDougall, 1972; McDougall & Gill, 1972; Gill, 1967). I have therefore concentrated on the lavas of poorly known age in eastern Victoria. The results are given in terms of 14 geographic provinces. The large number of provinces is necessary mainly because the basaltic lavas in the Eastern Highlands of Victoria appear to form 6 provinces of late Eocene to middle Oligocene age, each containing rocks that are relatively uniform but differing from rocks in neighbouring provinces in petrography and average potassium content. METHODS Samples were collected from many of the major areas of outcrop of the Cainozoic vol- canics. Generally several samples were obtained from each area, so as to provide some test of consistency of the results with the local stratigraphy. Each sample was examined in thin section to determine its suita- bility for age determination. The ideal sample is one in which the rock is holocrystalline and free of alteration. Unfortunately very few D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 361 were of this kind; many contain glass that shows some degree of devitrification or altera- tion, which could result in loss of radiogenic argon by diffusion, leading to measured K-Ar ages that are too low. Thus most of the ages must be regarded as minimum estimates of the time of eruption, and the reliability is gauged by the degree of agreement between the results on specimens from the same lava sequence. In the tables of K-Ar ages (Tables II to IV) the suitability of the rocks for dating is indi- cated by letters—A, where potassium-bearing phases are fresh and the K-Ar age is likely to be correct; B, where they are slightly altered and the K-Ar age could be low; C, where they are considerably altered and the age is likely to be low; and D, where they are extensively altered and the measured K-Ar age is expected to be much too low. The techniques of measurement of the K-Ar ages have been described by McDougall (1966). Potassium is determined by flame photometry and argon by isotope dilution using Ar 3 s as a tracer, the isotopic measure- ments being made in a Reynolds-type mass spectrometer. The precision is given at the level of two standard deviations as deter- mined from the statistics of the individual measurements in the manner given earlier (McDougall el al., 1969). Duplicate measure- ments were made in some cases and generally the agreement is to within that expected from the statistics. Constants used in the calcula- tions are: Ae = 0.585 X 10-1° y r - i ; A/g = 4.72 X 10-10 yi-l; K40/K = 1.19 X 10-2 atom percent. The measured K-Ar ages can be related to the geological time scale by referring to the physical time scale given in Table I (after Berggren, 1972). TABLE I The physical time scale Age of boundary (m.y.) 0 Epoch Quaternary Pliocene Miocene Oligocene Eocene Palaeocene Cretaceous 1.8 5 22.5 38 54 65 THE VOLCANIC PROVINCES Flinders Province (1) Near Flinders, in the southern portion of the Mornington Peninsula, occurs a sequence of basaltic lavas over 300 m thick (Keble, 1950). Individual lava flows are between 5 and 30 m thick, dip slightly to the east, and are displaced by north-south faults. Away from Flinders the basaltic sequence is thinner. Related thin sequences probably extend for about 40 km, cropping out on the Bellarine Peninsula to the west, in the northern Morn- ington Peninsula, and to the east on French Island, and on Phillip Island (Edwards, 1945). Edwards (1939, 1945) has described the lava types on Bellarine Peninsula, Morn- ington Peninsula, and Phillip Island as under saturated olivine basalts and undersaturated titanaugite basalts. The lava sequence overlies thin fluviatile sediments and in places is inter- bedded with fluviatile sediments and lignites. The macroflora in these underlying sediments were thought by Douglas (in Jenkin, 1962) to be probably Oligocene in age. The basaltic sequence and sediments rest on a Palaeozoic basement, and are overlain by mid-Miocene marine sediments and younger non-marine sediments (Gostin, 1966). The lavas in the southern portion of the Mornington Peninsula were nominated by Edwards (1939) as the type locality of the Older Volcanic Series. The three lavas that were dated give ages between 47 and 42 m.y. or middle Eocene (Table II). Two flows near the top of the 300 m section near Flinders gave ages agree- ing within experimental error at 42 m.y. A flow on Phillip Island, 15 km to the east, gave a slightly older age of 47 m.y. As this flow is only about 170 m above the basement, it probably stratigraphically underlies the others. Hence the flows were probably erupted from at least 47 to 42 m.y. ago. A plug 2 km south of Leongatha, 80 km to the east, gave a simi- lar age of 39 m.y.; so it may be related to these lavas. Thorpdale Province (2) In South Gippsland a thick sequence of basaltic lavas called the Thorpdale Volcanics is interbedded with the basal sediments of the Latrobe Valley Group. The lavas crop out around the margins of two uplift areas (Strzelecki and Bullock Uplifts) over an area of about 70 km by 45 km. Edwards (1939) recognized extensive titanaugite and olivine basalts, with less widespread iddingsite basalts and crinanites. The volcanic rocks seem to be Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 TABLE II K-Ar ages for basaltic whole rock samples from the Flinders,1 Thorpdale and Bachus Marsh Provinces Sample No. K Rad Ar 10 cc STP/g Age 2xs.d.Latitude m.y. south Longitude Alter- Locality east ation 1. Flinders Province 69-1458 0.641,0.638 69-1459 0.561,0.540 69-1463 1.417,1.410 69-1466 1.086,1.086 2. Thorpdale Province 69-1468 1.280,1.273 70-140 1.357,1.359 69-1465 1.045,1.050 1.083 77.5 42.0 0.946 62.5 42.6 2.688 88.3 47.1 1.702 91.1 38.9 2.970 93.3 2.951 94.2 3.005 97.8 2.O89 95.2 57.4 57.1 54.7 49-3 0.7 0.7 l.i 0.6 38° 38° 38° 38° 2 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 363 associated with a northeast striking swarm of dykes and plugs of similar composition (Edwards 1934). Sediments near Yarram, in the same strati- graphic position as the volcanic rocks in this area, give Palaeocene to Eocene microfloral ages (Cookson & Dettman, 1959). To the northwest in the Latrobe Valley, Hocking (1972) reports that the volcanics near the base of the sequence are of middle Eocene age. Only three samples have been dated by the K-Ar method in this important area (Table II), as there is a lack of unweathered outcrops and consequently a lack of fresh samples. Two basaltic flows near Yarram gave ages of 57 and 55 m.y. (late Palaeocene), and a flow near Leongatha 55 km to the west gave an age of 49 m.y., or early Eocene (Table II). Hence both palaeontology and K-Ar dat- ing give ages between middle Palaeocene and middle Eocene for these volcanic rocks . Bacchus Marsh Province (3) Fifty-five kilometres northwest of Mel- bourne a basaltic flow sequence crops out withm the Ballan Graben, a small fault- bounded block 30 km long and 20 km wide. The basaltic sequence and associated dyke swarm in the eastern part of the graben were mapped and described by Jacobson & Scott (1937). The basaltic rocks, 160 m thick, over- lie thin non-marine sediments and Palaeozoic basement and are overlain by non-marine sediments containing coal seams, and by the Newer Volcanic Series (Thomas & Barag- wanath, 1950). Away from the area of the basalts the coal seams are overlain by, and interbedded with, early to middle Miocene marine sediments (Parr, 1942). Only three bodies were dated (Table II): a flow near the base of the Korkuperrimul Creek section gave ages of 60±2 and 63±1 m.y., a boulder from the roadside slightly higher up the section gave an age of 79 m.y., and a ?flow cropping out 8 km to the north- west gave an age of 53 m.y. The three measured dates are highly discrepant, but flows in the eastern part of the graben are at least early Palaeocene in age, while flows near Ballan are at least late Palaeocene. Bonang Province (4) A small area of basaltic flows covers the top of a hill 3 km west of Bonang, which lies between the tributaries of the Deddick and Brodribb Rivers, about midway between Orbost and Bombala. The elevation of the hill is about 800 m whereas that of the surround- ing peaks is near 1000 m. The basaltic pile is about 10 km long, 2 km wide, and apparently in the order of 50 m thick. The flows con- tain olivine-rich nodules, and appear to be alkali olivine basalts. The two dated flows gave ages of 38 and 41 m.y., indicating a late Eocene age (Table III). Gelantipy Province (5) Basaltic lavas crop out extensively between the Snowy and Tambo Rivers. Most of the flows form lava fields near Gelantipy and on the Nunnion-Nunnet Plains; the remainder are remnants of flows down narrow valleys (Hills, 1938; Fletcher, 1963). All these lava sequences are probably not much more than 30 m thick, and their petrography (Hills, 1938) and low potassium content suggests that they are tholeiitic or transitional between tholeiitic and alkaline. In many places they are associated with stream sediments, and near Mount Weeler the sediments contain a Cinna- momum flora (Talent, 1969). Samples were collected for dating from most of the major areas of basaltic flows. The measured ages range from 42 to 33 m.y., adjacent flows giving ages that differ by much more than experimental error (Table III). Petrographically the rocks can be divided into three groups—a group of four holocrys- talline fresh rocks with ages of 42 to 38 m.y., a group of two holocrystalline, but weathered, rocks with ages of 36 and 34 m.y. (they were dated because of their physiographic impor- tance), and three rocks whose groundmass is partly isotropic or anisotropic glass with ages of 34 to 33 m.y. It is most probable that the weathered and partly glassy rocks have lost a proportion of their argon, and that the age of crystallization of all these lavas is in the range 42 to 38 m.y., or late Eocene. Bogong Province (6) On the Bogong and Dargo High Plains, and the high plains between them, is an extensive province of volcanic rocks, extending 40 km north-south and 20 km east-west. This pro- vince contains the greatest volume of volcanic rocks in the Highlands. The volcanic sequences form the highest portions of a series of flat-topped mountains that were originally continuous at about 1500 m altitude, but which are now separated by deep valleys. The exposed sequences consist of flows and tuffs overlying and intercalated with fluviatile and lacustrine sediments. From drilling it is known that the volcanic and sedimentary section is about 150 m thick in the Bogong High Plains at Mount Jim (Beavis, 1962), and over 200 m thick in the Dargo High Plains (Hunter, Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 TABLE III K-Ar ages for basaltic whole rock samples from Bonang, Gelantipy, Bogong, Howitt, Aberfeldy and Toombullup Provinces Locality 4. Bonang 70-1028 70-1029 K * Province 1.223,1.236 1.001,1.013 5. Gelantipy Province 70-l49gl 69-1469 70-1553w 70-1554w 69-1470 69-147lgl 69-1473 69-l474gl 69-1475 6. BoRonff 69-1476 69-1478 69-1481 69-148O 70-1328 7. Howitt 69-1491 70-1037 . 69-1492 69-l493gl 0.380,0.396 0.384,0.385 0.395,0.399 0.364,0.362 0.280,0.278 0.476,0.460 0.303,0.301 0.588,0.590 0.240,0.240 Province 1.282,1.280 1.370,1.363 1.451,1.440 1.90O,li900 1.182,1.183 Province 0.673,0.675 0.570,0.576 0.542,0.541 1.627,1.635 Had ti 10 °cc STP/g 1.783 1.681 0.511 0.646 0.581 0.494 0.434 O.616 0.479 0.805 0.363 1.709 1.999 1.458 2.315 1.470 0.866 0.802 0.725 O.895 Ar"° 94.1 79.9 41.1 34.0 54.7 56.2 63.8 63.8 56.6 62.3 78.7 93.2 95.7 93.0 96.3 93.7 87.6 62.1 80.1 70.0 Age m.y. 37.7 41.4 33.6+ 41.7 36.3+ 33.8+ 38.6 32.7+ 39.3 33.9+ 37.6 33.1 36.3 25.I 30.3 30.9 31.9 34.8 33.3 13.7+ 2xs.d. m.y. 0.9 1.3 0.6 0.8 0.9 0.8 0.6 0.5 0.7 0.6 0.6 0.8 0.6 0.4 0.5 0.7 0.8 0.8 0.8 0.3 Latitude s outh 37°12.5' 37°12.O' 37°O4.5' 37°O4.8" 37°O7.8' 37°O7.8' 37°l4.8' 37°l4.8- 37°19.4' 37O12.5' 37°09.2' 37°O1.4' 36°59.O' 37°O6.4' 37°10.9- 37°O9.1' 37°13.8' 37°10.5- 37°10.5' 37°10.5' Longitude east 148°42.5' 148°42.3" 148°34.5' 148°21.O' 148°19.5' 148°19.5' 148°15.2' 148°15.2' 148°O5.3' 148°01.3' 147°57.4' 147°14.O' 147°O8.8' 147°O9.3' 147°11.8> 147°11.3' 146°41.8- 146°4O.5> 146°4O.5' 146°4O.5' Alter- ation B B 0 B C C B C B 0 B B B B C C B B B D Location 4 km SW of Bonang 2 km SW of Bonang near Tubbut Post Office 18 km NNE of Gelantipy 12 km NNE of Gelantipy same flow as 70-1553 3 km south of Gelantipy overlies 69-1470 20 km SW of Gelantipy 21 km west of Gelantipy 22 km west of Gelantipy 10 km SE of Hotham Hotel near Hotham Hotel N side of Dargo High Plains S side of Dargo High Plains S side of Dargo High Plains 6 km SE of Mt Howitt 2 km E of Mt Howitt (top) 2 km E of Mt H(Twitt( middle) 2 km E of Mt Howitt (base) 1 D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 365 a. "3 XJ E c 0 d- i c W 1 CO - CO m so c\ o \D r- H H O o o r° i O C\ o n 0 i 6 ll i E H « M w c o _ o so r> O H m CO r j so m i n C " l H CO 1 SO E c o W I c\ X c_ en i n 0 "O en cc o CJ •sO cA CO C\ o 0 \ m o H O X r j i > o ! 1 -B Z I c Tn r j 0 -d- -3" O SC O r - l CM A en X Ci m m H O X X o C \ C\ X o f - CM O C to o c a 0 to ra p lo , E T3 0 0 n 0 s E -3 ro u n W to ii to Journal of the Geological Society of Australia, Vol. 21, Pt 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 366 PETER WELLMAN 1909). In the Bogong High Plains, and near Hotham, the volcanic rocks are mainly alkali basalts with minor limburgites, phonolites, and tuffs, with associated doleritic and alkaline dykes-—the Bogong Volcanics of Beavis (1962). To the south of the Dargo High Plains the rocks range from alkali olivine basalt to phonolite and camptonite (Edwards, 1939). In the sediments associated with these volcanic rocks are numerous plant beds and some coal. The included flora has not been studied in detail, but Beavis (1962) suggested that it indicated an Oligocene to Miocene age. The five flows that have been dated give ages ranging from 36 to 25 m.y. (Table III). Flows from near Hotham Hotel and 10 km to the southeast give ages in rough agreement at 36 and 33 m.y., and slightly younger ages of 31 and 30 m.y. are given by two flows at the southern margin of the Dargo High Plains 20 km to the south. A flow between these two areas gave a distinctly younger age of 25 m.y., but in the absence of confirming dates it seems probable that this young age is due to argon loss. Hence the activity in the central and southern portions of the Bogong Province is thought to have been in the period 36 to 30 m.y. ago in the early and middle Oligocene. Howitt Province (7) Near Mount Howitt in the eastern High- lands lie two small areas of basaltic flows. The lavas rest on flat topped ridges that form the widespread high erosion surface in this part of the Eastern Highlands at about 1500 m altitude. Most of the lavas are olivine basalts with a low potassium content, so that they are possibly transitional between alkaline and tholeiitic. In the largest area, just south of Mount Howitt, there are several flows with a total thickness of at least 100 m. Three flows were dated from this section; the two upper flows gave ages of 35 and 33 m.y., whereas the lowest flow, consisting mainly of glass, gave an age of 14 m.y. and must have lost a considerable proportion of its radiogenic argon (Table III). A flow collected 6 km to the south gave an age of 32 m.y., and is in approximate agreement with the two older ages. From these data it seems likely that the volcanism in the Howitt Province occurred about 35-32 m.y. ago, in the early Oligocene. Aberfeldy Province (8) Basaltic lavas near Aberfeldy and Conners Plain are olivine basalts; two chemically analysed rocks (Baragwanath, 1925) have minor nepheline in the norm and are there- fore alkali olivine basalts. The lavas occupy small areas less than 2 km in diameter on the hill tops; some are near the present main divide at about 900 to 1200 m altitude, and others are on a ridge south of Aberfeldy at 820-650 m altitude. The lower flows have been described as former valley flows of the Aberfeldy-Thomson River (Baragwanath, 1925); so because of their importance in defining the physiographic history of the Eastern Highlands, two samples have been dated even though they are slightly altered. The samples gave ages of 28 and 26 m.y. (Table III): thus the flows are at least as old as middle Oligocene. Toombullup Province (9) Thin basaltic flow sequences occur on the northern margin of the Eastern Highlands between Mansfield and Wangaratta. These sequences rest on flat-topped hills near the main divide in the vicinity of Toombullup (Kitson, 1900), and also on the two major ridges extending 40 km to the north. The petrography of the flows suggests that they have alkaline affinities, and Edwards (1939) lists a titanaugite and an olivine basalt from this area. The four flows that have been dated give ages ranging from 43 to 36 m.y. (Table III). Three flows within 8 km of Toombullup agree in age to within experimental error at 37 m.y., whereas a single flow from one of the ridges 15 km to the north gave an age of 43 m.y. The basaltic volcanism in the Toombullup Province is therefore thought to be of late Eocene to early Oligocene age. Neerim Province (10) Basaltic lavas occupy isolated areas at the southern margin of the Eastern Highlands between Traralgon and Berwick, over a dis- tance of 110 km. The lavas form the summits of flat-topped spurs 300-100 m above the present rivers. Within the highlands the lavas rest directly on Palaeozoic sediments and granites, whereas on the plains they rest on thin gold-bearing gravels (Whitelaw, 1899; Murray, 1916) and are in some places over- lain by equivalents of the Haunted Hill Gravels of probable Pliocene age. Edwards (1939) has described the lavas as titanaugite and olivine basalts, and he considers that they have alkaline affinities. The three dated lavas are up to 40 km apart; they were collected from Nerrim South, from near Icy Creek, and from near Yallourn North. The samples gave ages of 24, 22, and 19+ m.y.: they are late Oligocene to early Miocene in age (Table IV). A similar age of 22 m.y. was given by an olivine nephelinite ?plug near Drouin West D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 367 (Mahony, 1931), just south of the highlands. The age found by K-Ar dating is the same as the palaeontological age given by Hocking (1972) for flows (in the Maxwell Formation) that are probably the southern extension of this province. Melbourne Province (11) The basaltic lavas near Melbourne of the Older Volcanic Series are up to 35 m thick, and extend over an area 30 km across. The lavas between Melbourne and Keilor, to the south and west, are within the non-marine Werribee Formation (Bell et al., 1967), and are overlain by marine sediments of early Miocene to middle Miocene age. The flows at Keilor, Broadmeadows, and Essendon are glassy olivine basalts (Edwards, 1939) that are not suitable for K-Ar dating. In the north at Plenty occurs a holocrystalline basaltic flow that is probably the same age (Geol. Surv. Viet., 1964) although it is not definitely associated with the Werribee Formation. This flow gave an early Miocene age of 17 m.y. (Table IV) which is consistent with extrusion before the early to middle Miocene sediments overlying the flows between Melbourne and Keilor wece deposited; so it is.possible that all the lavas of this province are early Miocene. Woodend Province (12) Underlying the olivine basalts of the Newer Volcanic Series in the Western Highlands is a group of small flows and small intrusions of differentiated rocks. The more felsic rocks are flows of anorthoclase trachyte and small laya domes of soda trachyte near Mount Macedon (Skeats & Summers, 1912; Edwards, 1938), and small domes of trachyte and trachyphono- lite near Trentham, 30 km to the west (Mahony, 1931; Coulson, 1954). Associated with these trachytic rocks is a wide range of other differentiated rock types such as oligo- clase trachybasalt, oligoclase basalt, and alkali limburgite. There appears to be a general time progression from felsic to mafic among the differentiates. From the Macedon-Trentham area seven bodies have been dated, three trachyte domes and three limburgite flows from near Mount Macedon and a trachyandesite from Spring Hill 25 km to the west (Table IV). The samples were collected with a view to Rb-Sr dating; they are not ideal for K-Ar dating .as the soda trachyte from Hanging Rock and the trachyandesite appear altered in thin section, and the limburgite from Hesket contains con- siderable glass. Samples from the three soda trachyte lava domes give whole rock ages ranging from 5.7 to 5.3 m.y., and significantly older potassium feldspar ages ranging from 6.3 to 6.0 m.y. The potassium feldspars are fresh and the ages agree to within experi- mental error, so that ages given by them are more likely to be correct than the discrepant ages given by the altered whole rock samples. This potassium feldspar K-Ar age is the same to within experimental error as that given by Rb-Sr dating of whole rock soda trachyte samples (Dasch et al., 1972). The two flows of limburgite from Woodend gave ages that agreed to within experimental error (6.8 and 6.6 m.y.) so that they may approximate closely to the age of extrusion. The two other samples that were dated give younger mini- mum ages of 3.2 and 4.5 m.y. Two differentiated rocks from north of this area have also been dated. One sample came from what is probably an isolated flow 5 km west of Dookie within the Murray Basin Plains, 180 km northeast of Macedon. This basaltic rock contains olivine phenocrysts and appears to have considerable nepheline in the groundmass. The measured age of 5.6 m.y. is similar to the age of the differentiates in the Macedon area. A sample from the Seven Creeks limburgite flow was dated because of its physigraphic significance. This flow is about 7 km long and 10 m thick and has an appearance similar to that of recently extruded flows (Dunn, in White, 1954; Hills, 1938). It occurs 7 km south of Euroa on the floor of the Seven Creeks Valley, where it is deeply incised in the granites of the Strath- bogie Ranges. A sample from the plug at the head of the flow gave an age of 6.8 m.y. (late Miocene), but as the rock contains about 5 percent devitrified glass this age should be considered a minimum. All these flows are considered to be related, so the range in age of the lavas of the Wood- end Province is at least from 6.8 to 5.8 m.y., or late Miocene. Newer Volcanic Province (13) The extensive lava field of Newer Volcanic Series rocks, west of Melbourne, is con- sidered here as a single widespread province, with the alkaline rocks near Macedon and the basalts near Uplands in the Eastern Highlands forming two distinct provinces of similar age. The volcanic rocks of the Newer Volcanic Province crop out over much of the Southern Lowlands and valleys of the Western High- lands between Melbourne in Victoria and Mount Gambier in eastern South Australia. The rocks are lavas and tuffs, and they cover Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 TABLE IV K-Ar ages for samples from Neerim, Melbourne, Woodend and Uplands Provinces Sample No. Rad Ar STP/g Age m.y. 2xs.do Latitude south Longitude east Alter- ation Location 10. Nerrim Province 70-145 7O-l43gl 70-141 70-1025 11. Melbourne 69-1457 12. Woodend : 7O-469Sa " WR 7O-47OSa " WR 7O-474Sa " wWR 69-334 69-335 69-331gl 69-336W 69-1498 69-1497gl 1.290,1.285 1.261,1.255 1.402,1.400 1.115,1.116 Province 1.247,1.241 Province 5.311,5.353 4.227,4.186 4.389,4.396 3.616,3.647 4.661,4.660 4.125,4.101 2.636,2.648 1.307,1.300 0.886,0.893 3.175,3.184 2.373,2.361 1.868,1.863 14. Uplands Province 70-1030 69-1495 1.575,1.568 1.479,1.482 I.269 0.971 1.214 0.972 0.873 1.343 0.967 1.078 0.808 1.120 0.870 0.719 0.346 0.162 0.413 0.546 0.506 0.142 0.134 80.3 85.1 73.3 86.1 88.6 71.1 67.3 87.8 73.0 35.1 83.6 83.3 78.9 42.6 59.8 75.8 78.7 59.2 59.3 24.5 ' 19.3+ 21.6 21.7 17.5 6.31 .5.75 6.14 5.57 6.01 5.29+ 6.81 6.63 4.55+ 3.25+ 5.77 6.79+ 2.26 2.27 0.6 0.3 0.4 0.5 0.3 0.15 0.10 0.10 0.09 C.ll 0.09 0.12 0.11 0.08 0.07 0.14 0.16 0.06 0.06 38 02.5' 37°53.1' 38°O7.8' 38°O9.8' 37°41.2> 37°20.6" •• 37°22.4" •' 37°2O.O' '• 37°21> 37°21- 37°21' - 36°20.8' 36°52.3' 36°50.5' 36°50.5' i45 54.8' 146°O5.9' 146°23.4' 145°51.9' 145°O7.O' 144°40.8" " 144°35.7' 144°35.5' " 144°32' 144°32' 144°39' - 145°35.8> 145°36.7' 147°42.3' 147°42.3' B C B B B B B B C C B C B D A A Ik km N of Warragul 35 km NE of Warragul 13 km NE of Moe plug, 6 km W of Warrugul 2 km south of Plenty- Brock1 s Monument in Macedon area Camels Hump in Macedon area Hanging Rock in Macedon area Woodend in Macedon area Woodend in Macedon area Hesket in Macedon area Spring Hill near Trentham 8 km west of Dookie Ik km south of Euroa 2 km north of Uplands P.O. 3 km north of Uplands P.O. ! p gl contains glass, w = weathered, Sa. = sanidine, WR = whole rock, + = age is probably too low Except where indicated all samples were run as whole rocks. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 369 about one half of an oval area 350 km east- west and 150 km north-south, with a thick- ness that is normally less than 60 m but is locally up to 150 m (Singleton & Joyce, 1969). The lavas are predominantly olivine basalts with alkaline affinities (Edwards, 1938), with small volumes of hawaiites, mugearites, and tholeiitic rocks. K-Ar dating by McDougall et al. (1966), Aziz-ur-Rahman & McDougall (1972), and McDougall & Gill (1974), and radiocarbon dating by Gill (1967) have shown that the volcanism was mainly restric- ted to between 4.5 m.y. and 5000 years ago. Uplands Province (14) In the Eastern Highlands, north of Benam- bra. several lava flows extend for 20 km along the floors of the valleys of Deep Creek, Morass Creek and Mitta Mitta River. The lavas are similar to those of the Newer Vol- canic Province of western Victoria in their physiographic settings (Hills, 1938), in the common occurrence of iddingsite, and in the vesicular nature of the flows. The similarity is strengthened by the late Pliocene age found for two flows (both 2.3 m.y.) (Table IV). DURATION OF VICTORIAN VOLCANIC ACTIVITY In Table V the range in the measured ages is tabulated for each of the 14 volcanic pro- vinces. In most provinces the volcanism lasted less than 5 m.y., and argon loss seems to be a minor problem except in rocks that have a groundmass largely of glass; the mean age for the volcanism can be fairly reliably esti- mated to within 5 m.y. However, in the Bacchus Marsh and Thorpdale Provinces some of the samples could have lost argon, and the mean age of the province is probably only correct to within 10 m.y. The histogram of Figure 3 a shows that the volcanic activity in Victoria was nearly con- 10 141 22\ 143 19 \ 145 24 OA •>. 1025 22/A Fig. 2. Distribution of Cainozoic volcanic rocks (black and shaded) in eastern Victoria, and locality of samples. Large numbers give number of volcanic province used in text; small numbers give latter part of sample number, followed by measured age in million years. Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 370 PETER WELLMAN 10- (a) TABLE V Summary of K-Ar Ages from Victorian Cainozoic Volcanic Rocks Province No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Volcanic Province Flinders Thorpdale Bacchus Marsh Bonang Gelantipy Bogong Howitt Aberfeldy Toombullup Neerim Melbourne Woodend Newer Volcanic Uplands Number of bodies dated 3 3 3 2 8 5 4 2 4 3 1 8 28. 2 Range in age of lavas (m.y.) 47-42 57-49 79-53 41-38 42-33 36-25 35-14 29-26 43-36 24-19 17 6.8-3.2 4.5-0.5 2.3 Range of preferred ages (m.y.) 47-42 57-55(749) ??63 41-38 42-38 36-30 35-32 (29-26) + 38-36(742) 24-22 717 6.8-5.8 4.5-0.5 2.3 Estimated original volume (km1!) 700 700 10 2 20 140 7 T 10 10 8 8 1300 2 10 (b) if] 20 40 60 „ 80 20 40 Age (m.y.) 60 80 Fig. 3. Distribution of Victorian volcanic activity with time, according (a) to mean age of volcanic provinces, and (b) to measured ages. Duration of activity in each province shown as 2 m.y. tinuous throughout the Cainozoic, but with little or no activity in the early Cainozoic (55- 45 m.y.) and the middle Miocene (18-7 m.y.). A histogram of the measured ages (Fig. 3b) also suggests that the volcanic activity was virtually continuous, but also reflects the greater sampling of some provinces and the anomalously low ages obtained from some samples because of argon loss. The Cainozoic lavas of Victoria fall naturally into four groups: (a) Alkaline basaltic lavas of the Southern Lowlands and Western Highlands with early Palaeocene to middle Eocene ages (?63-42 m.y.); (b) Alkaline and tholeiitic lavas of the Eastern Highlands with late Eocene to middle Oligocene ages (42-30 m.y.); (c) Alkaline and strongly alkaline lavas in Central Victoria with late Oligocene to early Miocene ages (24-17 m.y.); (d) Slightly alkaline olivine basalts in eastern and western Victoria with late Miocene to Pleistocene and Recent ages (7-0 m.y.). The most voluminous groups of lavas are the oldest and youngest. These are the two usually called the Older and the Newer Vol- canic Series. The two groups of lava of inter- mediate age have a much smaller volume, and their age was not well established before this study. The original volumes of lavas in the vol- canic provinces have been estimated from their present-day thickness and extents (Table V). Large volumes are estimated for Flinders D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 371 Province (700 km^), Thorpdale Province (700 km3), and Newer Volcanic Province (1300 km3); all the other provinces are likely to have volumes less than 200 km^. These estimates indicate that the early and late Cainozoic volcanic activities were approxi- mately equal in volume, although the period of activity was several times longer. Volcanic activity in Victoria after the mid- Cretaceous appears on the dating evidence to have started about early Palaeocene, and lasted almost continuously until the present day, with maxima during the late Palaeocene to late Eocene, and during the Pliocene to Pleisto- cene. Wellman & McDougall (1974a) have shown that in New South Wales similar vol- canic activity started about the Late Creta- ceous and finished in the middle Miocene, and was at a high level from about mid- Palaeocene to mid-Miocene. Volcanic activity in Queensland started in the Late Cretaceous and has lasted until the present day (Harding, 1969; Webb & McDougall, 1967; Webb et al, 1967; Wyatt & Webb, 1970; McDougall & Slessar, 1972), with maxima in the Oligocene and Pliocene to Pleistocene. The volcanic activity in Tasmania is thought on strati- graphic evidence to have extended at least from early Oligocene to middle Miocene, but the only K-Ar ages published are late Oligo- cene/early Miocene (Sutherland et al., 1973). The volcanic activity extended over a very similar period in Queensland, New South Wales, and Victoria, although the maxima were at different times. This changing pattern of volcanic activity in eastern Australia is discussed by Wellman & McDougall (19746). The pattern of activity can be explained, at least partly, by relative Iithospheric plate movements, the formation of the Tasman Sea 80 to 60 m,y. ago (Hales & Ringis, 1973), and the development of new sea floor between Australia and Antarctica which started about 50 m.y. ago (Weissel & Hayes, 1972). On a smaller scale the Victorian vol- canic activity can be related to the changing tectonic patterns in Victoria. In particular the basalts of the Gippsland Basin are associated with the beginning of a sedimentation phase (Hocking, 1972) and hence probably the be- ginning of downwarping in the Gippsland Basin. PHYSIOGRAPHIC IMPLICATIONS It has long been recognized that the high- lands adjacent to the margin of eastern Aus- tralia result from several epeirogenic uplifts in the Cainozoic and the Mesozoic. The axes of the warping lie approximately along the present drainage divide, subparallel to the coast. In Victoria the Eastern Highlands, which are continuous with the highland region of southern New South Wales, rise to an altitude of about 2000 m at Mount Bogong. The high- lands decrease in altitude to the northwest and south where basins of Cainozoic sedimentation are found. The highlands consist mainly of the Palaeozoic sediments and volcanics in- truded by numerous granitic batholiths that form the southern part of the Lachlan Geo- syncline of the Tasman Orogenic Zone. The pre-Middle Devonian rocks are folded and faulted with north-northwest trends dominant. In central Victoria during the Late Devonian there was widespread silicic volcanism with related intrusives, but these rocks are virtually undeformed; so this is regarded as the final stage of consolidation of the southern part of the Tasman Orogenic Zone. The physiographic history of Victoria has been discussed extensively, notably in papers by Gregory (1903), Skeats (1910), Jutson (1911), Fenner (1918) and Baragwanath (1925). Hills (1934) summarized the earlier work and contributed much additional infor- mation (Hills, 1938; 1940; 1955), and more recently King (1959) and Singleton (1968) have discussed the geomorphological evolu- tion of Victoria. These workers have recog- nized a number of more or less well-developed erosion surfaces in the Eastern Highlands of Victoria. Hills (1934; 1955) implies that after the eruption of the Late Devonian lavas and the deposition of the relatively thin Permian and Triassic sediments, physiographic relief of Victoria was low. Uplift of the highlands is considered to have begun in the Late Triassic, when basins of sedimentation developed to the south and north of the present highlands, resulting in deposition of thick sequences of sediments of Jurassic and possibly Early Cretaceous age. Thus the oldest erosion sur- face in the present context was pre-Jurassic, possibly Triassic. The highest preserved ero- sion surface in the Eastern Highlands is regarded as the somewhat reduced remnant of the pre-Jurassic surface. The inferred Jurassic uplift initiated an erosion cycle dur- ing which broad mature valleys were formed in the highland regions, and these contain remnants of basaltic flows regarded by Hills as belonging to the Lower Cainozoic Older. Volcanic Series. Throughout the Early Caino- zoic warping continued and sediments were Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 372 PETER WELLMAN deposited in the Gippsland, Otway, and Mur- ray Basins. A Late Cainozoic uplift, corre- lated with the Kosciusko Uplift of New South Wales, resulted in further rejuvenation of the streams. Hills (1955) emphasizes that tectonic movements in the highlands and adja- cent regions were probably more or less con- tinuous, viewed on a regional basis, so that the physiographic history for one region may not necessarily apply in detail to another region. There is now much less certainty that all the erosion surfaces in the highlands of Vic- toria were developed as peneplains graded to sealevel; processes such as pediplanation may also be of importance (c/. King, 1959). Nevertheless, the age data presented in this paper on the basalts found in the highlands can be used to provide more precise limits on the timing of the major events in the physio- graphic history of the region. Basaltic flows of particular significance are those of Gelan- tipy, Aberfeldy, and Glenmaggie districts on the southern margin of the highlands, and Uplands and Seven Creeks flows on the northern margin of the highlands. In these areas basaltic flows are preserved at the base of a river valley or part way up the side of a valley. A subaerial flow that filled the bed of a river is of considerable geomorphic impor- tance, because the base of the flow where it is in contact with river gravels is the surface of the lowest land at that time. By dating the flow a limit is set on the net amount of uplift of the area, because the net uplift cannot be greater than the height of a subaerially erupted basalt above sea level. The actual amount of uplift is generally impossible to infer. However, on the margins of the high- land, rivers are often close to grade because of their considerable erosive power. In this section of a river the net amount of uplift of the highland relative to the lowland, since extrusion of the basalts, should be given approximately by the height of the 'valley filling' basalt above the present river bed. In the Gelantipy area, near the eastern end of the Eastern Highlands, there are extensive outcrops of basaltic lava flows of late Eocene age (42-38 m.y.). These basalts occur over a wide range of present altitudes (Fig. 4) from about 1200 m to about 600 m (Hills, 1938; Ringwood, 1955). The highest basalts lie on the undulating Nunniong Tableland (1200 m altitude), an area 30 km to the northwest of Gelantipy, that is regarded as a reduced rem- nant of the Mesozoic surface. Most basalts lie within broad to narrow highland valleys cut into this Mesozoic surface and ranging in alti- tude from 900 m in the highland near Gelan- tipy to 600 m near the southern margin of the highland in the Buchan area. The broad mature highland valleys are the kind described by Hills (1955). Basalts at a lower physio- graphic level ocur within the Snowy River gorge, 10 km northeast of Gelantipy (Ring- wood, 1955). They rest on thin gravels and occur as residuals on the side of the gorge at about 600 m altitude, well above the present river bed at 200 m (Fig. 4). In the Gelantipy area east-west warping or faulting is not thought to have been significant since the Palaeozoic, so that the amount of uplift will be approximately that inferred from altitude differences. From the evidence given above for the Gelantipy area it is believed that the reduced Mesozoic surface had a net uplift of 600 m before the late Eocene, and that later net uplift has been about 400 m. The broad highland valleys had been uplifted 300 m before the late Eocene. In the Aberfeldy area in the southwest of the Eastern Highlands there are valley-filling basalt flows giving minimum K-Ar ages of 27 m.y. (mid Oligocene). The highlands sur- rounding the area have a minor erosion sur- face at 1350 m, and a more extensive surface at about 900 m (Baragwanath, 1925). The Aberfeldy basalts define a river valley that had a base with a present day altitude of 820 m at Aberfeldy, and 650 m at Beardmore. The present Aberfeldy and Thomson rivers are twin lateral streams at the position of the sides of the former valley. They have cut down so that they are now at altitudes of 500 and 350 m respectively near Aberfeldy (Fig. 4). It is inferred from these data .that the Mesozoic surface was uplifted about 700 m before the Oligocene, and that the net uplift since then is about 350 m. If the extensive erosion sur- face at 900 m represents a broad highland valley of Hills (1955), then in this area these valleys had been uplifted about 250 m before the Oligocene. An important area of basalt occurs at Glen- maggie, just southeast of the Aberfeldy area, where the Macalister River leaves the high- lands (Murray. 1878). In this area no fresh samples were obtained, so that no isotopic dates were secured. However the basalts are dissected and deeply weathered, and thus are likely to be no younger than the other basalts in the southwestern parts of the Eastern High- D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 373 neor Mt. Cooper -500m W Om Charlestown Creek Mitta Mitta River Gibbo River 2-3m.y.basal ts u p L A N D S P R O V I N C E I i 43m.y.basalts W h i t f leI3~K i n g River TOOMBULLUP PROVINCE J i . I WheelersCreightons W Creek Hi J0m Mt. Wombat even Creeks 7 m.y. basal ts Strathbogie SEVEN CREEKS BA'SALT Seven Creeks Buchan River basal ts Wulgu lmerang / 42 to 38m.y. basa l ts G E L A N T I P Y P R O V I N C E Abe r fe l dy River 28 + m.y. b a s a l t s ABERFELDY P R O V I N C E 10 15 Dl STANCE ( K ILOMETERS ) Fig. 4. East-west sections across valleys in Eastern Highlands showing relation between basalt occur- rences and present and past erosion surfaces. Section for Gelantipy Province approximate only. lands, and are probably early Oligocene to early Miocene in age. In.the Glenmaggie area the surrounding highland rises to 600 m, the basalts and associated sediments have a base near 250 m, and the present river level is 100 m. The amount of uplift since extrusion of the basalts is therefore about 150 m. Basaltic flows 2.3 m.y. old occur in Morass Creek and Mitta Mitta River valleys, in the eastern part of the Eastern Highlands just north of the main divide. In this area the highest land is near 1200 m altitude, and the basalts have a base between 700 m and 600 m altitude (Fig. 4 ) . Just upstream of the Journal of the Geological Society of Australia, Vol. 21, Pt. 4, pp. 359-376, December, 1974. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 374 PETER WELLMAN basalts, near Benambra, is an area of faulting and warping, with swamps, a lake, and present day aggradation. The basalts in Morass Creek valley, and downstream in the Mitta Mitta River valley, form an extensive terrace that covers most of the valley floor. The base of the basalt sequence forming the terrace is below the level of the creek bed near Uplands, but above the level of the creek bed farther downstream at Deep Creek, and about 200 m above the bed of the Mitta Mitta River. It seems that in this part of the highlands the changes in topography in the last 2.3 m.y. consist of incision of the rivers by about 200 m, and local warping and faulting. The total uplift of the Mesozoic surface has been about 800 m. Basaltic flows also occur near river level in the Seven Creeks valley at the northwestern margin of the Eastern Highlands. In this area there are extensive surfaces near 700 m alti- tude, and the present valley floor and a 7 m.y. basaltic flow are at 250 m altitude (Fig. 4). Therefore in the last 7 m.y. there appears to have been little or no uplift of the area rela- tive to the Murray Basin. There are major differences between the northern and southern margins of the Eastern Highlands of Victoria. To the south the high- lands have a well defined margin where alti- tudes decrease rapidly to the level of the Gippsland Basin, whereas the position of the northern margin of the highland is not well defined as altitudes decrease slowly to the level of the Murray Basin, after an initial step down about 40 km north of the main divide. The south-flowing rivers occupy deep V-shaped valleys with narrow floors and no aggradation gravels, whereas north-flowing rivers often occupy U-shaped valleys with a wide floor covered by aggradational gravels. The uplift history of the southern and cen- tral portions of the highland can be inferred from the physiography and distribution of basalts at Gelantipy, Aberfeldy, Glenmaggie, and Uplands. It is suggested that the highland was uplifted in three stages, each of about 300 m, the first resulting in the cutting of the high- land valleys, the second in narrow valleys cut in the wide highland valleys by the mid-Caino- zoic, and a third in further downcutting in the Late Cainozoic. On the northern margin of the highland there does not seem to be any evidence for a local major Late Cainozoic uplift; the Late Cainozoic basalt at Seven Creeks occupies a wide valley near present stream level, and many other valleys are wide and aggraded. For example in the Toombullup area the physiography is consistent with two periods of uplift, corresponding to the two earlier periods of uplift on the southern mar- gin of the highland, followed by a long period of valley widening. Thus the present uplift of the Victorian highlands may only extend from the southern margin of the highlands to about 40 km north of the main divide. As a result of K-Ar dating of the valley- filling flows it has been shown that by the Oligocene the highlands were already in exis- tence and possessed a relief in the order of 1000 m, with a drainage system similar to that of the present day. This reinforces the view, expressed by Hills, that much of the uplift of the Eastern Highlands of Victoria occurred in the Early Cainozoic or earlier, and is consistent with the conclusions reached by Wellman & McDougall (1974a) for the history of the highlands in southern New South Wales. In the Eastern Highlands of Vic- toria uplift after the Oligocene was nowhere more than about 300 m, the amount of down- cutting of the Snowy, Macalister, Thomson, and Mitta Mitta Rivers after eruption of the basalts. When this latest uplift started cannot be established, but it seems to be continuing at the present day (Beavis, 1960; Cleary et al., 1964). Hills (1934; 1955) regards the latest period of uplift and downcutting to be the result of the Kosciusko Uplift in Plio-Pleisto- cene times, but it seems likely that the uplift began in about the mid-Miocene, at the time of regression of the sea in the Gippsland Basin, just to the south (Hocking, 1972). ACKNOWLEDGMENTS This work would not have been completed without valuable discussions and encourage- ment from Dr Ian McDougall and Dr O. P. Singleton. Information about outcrop areas was received from Mr M. A. H. Marsden and the staff of the Geological Survey of Victoria. Technical assistance with the dating and sample collection was given by Mr D. J. Ed- wards, Mr Z. Roksandic, Mr D. Burmann. and Mrs R. Maier. D ow nl oa de d by [ U ni ve rs ity o f G la sg ow ] at 2 3: 18 2 0 D ec em be r 20 14 K-AR AGES ON VICTORIAN CAINOZOIC VOLCANICS 375 REFERENCES ABELE, C., & PAGE, R. W., 1974: Stratigraphic and isotopic ages of Tertiary basalts at Maude and Aireys Inlet, Victoria, Australia. 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WELLMAN, P., & MCDOUGALL, I., 1974a: Potas- sium-argon ages on the Cainozoic volcanic rocks of New South Wales. J. geol. Soc. Aust., 21, pp. 247-272. WELLMAN, P., & MCDOUGALL, I., 1974b: Caino- zoic igneous activity in eastern Australia. Teclonophysics, 23, pp. 49-65. WHITE, D. A., 1954: The geology of the Strath- bogie Igneous Complex, Victoria. Proc. R. Soc Vic., 66, pp. 25-52. WHITELAW, O. A. L., 1899: Report on a geologi- cal survey of portions of Nerrim, Jindick, Noogee. Nayook and North Neerim dis- tricts. Geol Surv. Vic. Mon. Prog. Rep. 3, pp. 18-21. WYATT, D. H., & WEBB, A. W., 1970: Potassium argon ages of some northern Queensland basalts and an interpretation of Late Caino- zoic history. J. geol. Soc. Aust., 17, pp. 39- 51. Peter Wellman, Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2600. Present Address: Bureau of Mineral Resources, Geology and Geophysics, P.O. Box 378, Canberra City, A.C.T. 2601. 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