PAPERS AND PHOCEEDINGS OF TlIfti ROYAL SOCIF~TY OF TASMANIA, VOLUME 91 THE PRECAMBRIAN ROCKS OF TASMANIA, PART I, DOLERITES OF THE NORTH-WEST COAST OF TASMANIA
By
ALAN SPRY Department of Geology, University of Tasmania
(With 4 Text Figures and 2 Plates)
ABSTRACT Previous Literature At least 30 small bodies of dolerite have been The presence of these basic dykes was first found intruding sediments of the Pre-Cambrian recorded by Twelvetrees 0903, 1905) who found Rocky Cape Group along the coast between Sulphur them at Burnie, Boat Harbour and Rocky Cape. Creek and Crayfish Creek. The igneous bodies are Stephens (909) also briefiy mentioned the same chiefly sills although dykes also occur. The bodies in his notes on a traverse along the north dolerites are characterized by strong deuteric alter west coast. ation which led to the formation of secondary tremolite-actinolite, chlorite, zoisite, albite, calcite, sericite, sphene, leucoxene, serpentine, and prehnite. COUNTRY ROCKS These rocks show petrological and chemical affini ties with dolerites of a similar age in Western The dolerites intrude a group of unfossiliferous and South Australia. They have certain features sediments which extend from Penguin to Smithton. in common with the lavas of the Cambrian Dundas These sediments consist chiefly of quartzites and Group, but the possibility that these bodies acted slates with siltstones, dolomites and a little con as feeders to the flows is discounted on structural glomerate. They were referred to broadly as the and chemical grounds. "Rocky Cape quartzites" by Twelvetrees (1903, 1905), but examination now shows that the sedi ments constitute a group. Thus the Rocky Cape INTRODUCTION Group is here defined as those sediments, chiefly quartzites, slates, dolomites and siltstones outcrop The dolerites occur as small bodies, ranging in ping intermittently from Penguin to Smithton and size from a few inches to 400 feet in thickness lying unconformably below the Dundas Group (at and, as they are less resistant to weathering than Penguin). Its thickness is in excess of 10,000 feet the sediments they intrude, most are only revealed and while the detailed stratigraphy is not yet in the coastal rock platform. They outcrop sporadi known, a number of formations have been recog cally along the coast from Blythe Heads to Crayfish nized. A small area of coarse mica-schist along the Creek, but the greatest concentration is between Inglis River, shown in fig. 1, may lie unconformably Burnie and Cooee Point so that it is intended to below this group but the base of the Rocky Cape use the term Cooee Dolerite to refer to this group Group has not yet been seen. The formations are of igneous rocks as a whole. It is believed that discussed in order (tentative only) from oldest to similar rocks occurring in the vicinity of French youngest. man's Cap are part of this group and that as mapping of the Pre-Cambrian continues, more will be found. The Cooee Dolerites thus represent a distinct period of igneous activity, probably late in .Burnie Quartzite and Slate the Pre-Cambrian. Evidence shown later in this This formation contains those quartzites and paper indicates that the Middle to Upper Cambrian slates outcropping along the foreshore at West volcanism was a later and distinct episode. A Burnie. It appears to outcrop from east of Howth widespread period of basic to ultrabasic activity to Doctor's Rocks, except where covered by a super earlier in the Pre-Cambrian is represented by ficial layer of later material. The formation is amphibolites, &c., and the Interview River dyke probably several thousand feet thick and consists swarm described by Spry and Ford (957) belongs mostly of a monotonous repetition of thin slates to this episode. and quartzites. The argillites are dark-grey to black siltstones or slates which show a cleavage which is strongly developed in some specimens but Acknowledgements lacking in others. Some contain abundant clastic The author is grateful to W. St. C. Manson of the mica and others are graphitic. Many of the coarser Tasmanian Department of Mines who carried out siltstones exhibit cross-bedding, scouring and other the four chemical analyses which are published by intraformational structures. The quartzites are courtesy of the Director of Mines. B. May and A. generally light-coloured, quartzose, and massive, Rundle assisted in the field work. flaggy or thinly bedded with occasional cross bed For "Pre-Cambrian" read .. Precambrian" throughout the paper.
81 R.S.-6. 00 l"
RECENT
~ ALLUVIUM d BEACH SANDS
SEE FIG TERTIARY ~BASALT, SCALE 0,, $E.DlfiJENrS o 2 4 6 8 MILES ! I ! j =:d PERMtAN
SEDIMENTS
." ORDOVICIAN :>;l t
FIG. I.-Regional geological map showing the, distribution of the Precambrian reeks and in particular of the dolerites. Some boundaries after Loveday (1956). ALAN SPRY 83 ding. Thin sections of many quartzites show that Bluff Quartzite they contain angular quartz fragments and this fact, together with their association with some beds This is a 1500 foot thick quartzite, light in colour, of sub-greywacke, indicate that they are not normal thinly bedded at the top, but massive towards the ortho-quartzites (Pettijohn, 1949). Some arenite base, with coarse cross-bedding and ripple marks layers contain abundant angular slate fragments (plate D. It overlies the Cowrie Siltstone and lies up to eight inches long and the location of these below the Port Quartzite and Slate. It is litho intraformational breccias is shown in fig. 3. logically indistinguishable from the Cave Quartzite (defined below) and this causes great difficulty in The lack of recognizable marker horizons, to mapping this area. gether with the close folding and faulting (see fig. 3) prevents the accurate measurement of the' thickness of this formation. The beds generally dip to the west (see fig. 1), but small folds are abundant and these are asymmetrical with the steeper limb Port Slate and Quartzite either to the east or west, but with a general fiat plunge in a direction 230'. Faults are common and This formation is named after the small harbour cause difficulty in interpreting the structure, and on the eastern side of Rocky Cape, this being shown are frequently associated with zones of contortion. as "The Port" on old maps of the area. It lies between the older Bluff Quartzite and the younger The phyllites at Somerset are more altered and Cave Quartzite. It consists of 1500 feet of alter are strongly cleaved with a distinct lineation due nating slates and quartzites. The argillites are grey to crenulation, but there appears to be a complete to black laminated slates and slatey siltstones con transition into the less altered beds at Burnie. taining abundant intraformational structures such These phyllites are overfolded towards the south as sandy lenses, mud pellet conglomerate and slump in the quarry on the Somerset-Waratah road, half structures. The quartzites are generally thinly a mile south of Somerset and several other places bedded. on the wave-cut platform near Somerset.
Black River Dolomite The Burnie Slate and Quartzite is separated Cave Quartzite from the sediments in the Rocky Cape area by a cover of Permian sediments and Tertiary lacustrine The quartzite which forms the extremity of Rocky beds and volcanic rocks, consequently the relation Cape is called the Cave Quartzite from the Rocky ships between the sediments of the two areas is not Cape Caves which occur within it. It overlies the known. The oldest bed named in this' paper from Port Slate and Quartzite but the upper limit is not the Rocky Cape area is the thin (50 feet) grey to visible. It consists of 1500 feet of light coloured, buff dolomite which outcrops at the bridge where thickly bedded quartzite with some cross-bedding the Bass Highway crosses the Black River. It is and ripple marks. The quartzite on the cliffs east brecciated and silicified in parts. A small area of of Sisters Beach and at Jacob's Boat Harbour is dolomite at the eastern end of Sisters Beach may be probably this formation. Exposed in a raised sea the same formation. The Black River Dolomite cave half a mile east of Sisters Beach is a conglom underlies the Cowrie Siltstone and overlies the un erate which is named the Sisters Conglomerate, named siltstones which occur to the south up the being a member within the Cave Quartzite. The Black River where they are folded into a series conglomerate occurs as two closely spaced horizons of anticlines and synclines about half a mile across, one of which is about 18 feet thick and the other with axes running approximately east-west. about three feet thick. There are coarse and fine phases but the well-rounded boulders (which are always quartzite identical with that above and be Cowrie Siltstone low) range up to 18 feet long. The type of locality for this and the following The quartzite at the eastern end of Sisters Beach three formations is on the southern part of the is of interest as it contains the only trace of organic western side of Rocky Cape as shown in fig. 3. life in the Rocky Cape Group. Plate II shows a The name is taken from Cowrie Pt. and it is con specimen of "worm tracks". sic:ered that the formation occurs all the way along the coast from Rocky Cape to Black River although The Cave Quartzite may be equivalent to the it cannot be proved at present that there is not a Bryant Hill Quartzite named by Carey and Scott fault separating the Cowrie Pt. rocks from those (1952) at Smithton where it underlies the Smithton at Rocky Cape where the sequence was established. Dolomite. These two formations were referred to Thus the Cowrie Siltstone probably lies above the as "Carbine Group" by correlation with Elliston's Black River Dolomite and below the Bluff Quart (1954) section at Dundas. zite. It consists of at least 1500 feet of laminated to fiaggy siltstones which are buff, grey and black in Field mapping has shown that the stratigraphY colour with a distinct cleavage at an angle to the of the Pre-Cambrian rocks is very complicated and bedding. They are characterised by possessing in consequently, any correlation of the extensive sedi traformational structures such as cross-bedding ments around Smithton and Rocky Cape with the and slumping, with pyrite nodules in some bands. imperfectly known Carbine Group of Dundas, is Thin layers of quartzite occur throughout. unWise at present.
86 PRECAMBRIAN ROCKS OF TASMANIA, PART I
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/ "- / \ A / \
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~ ANTICLINAL AXIS SHOWING PLUNGE RAILWAY
~ SnfCLlNAL AXIS SHOWlNG PLUNGE LITHOLOGICAL BOUNDARY
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FIG. 3.-Precambrian sediments (Burnie Slate and Q uartzite) with dolerite' intrusions on the foreshore between Cooee Point and Park Point, West Burnie. STRUCTURAL RELATIONS of Rocky Cape, &c. The Detention River Sill shows It is possible to divide the dolerites into two characteristics common to each group. groups on structural, geographic and petrological The evidence suggests that the easterly group grounds. The intrusions west of Wynyard (Le., were intruded either prior to, or contemporaneously Boat Harbour, Sisters Beach, Rocky Cape, Deten with, the folding, whereas the westerly group were tion River and Crayfish Creek) are chiefly dykes intruded during the faulting which followed the and show very marked secondary alteration. It folding. The two phases are considered to be parts is noticeable that practically all of these intrusions of one period of igneous activity. strike north-easterly and only a few diverge even as much as 30° from this direction. Not only is MINERALOGY this the direction of a prominent set of faults in this The dolerites probably originally consisted chiefly area, but the dykes themselves actually lie along of pyroxene and labradorite with minor amounts faults on the northern end of Rocky Cape. of brown hornblende, biotite and olivine in some specimens. Accessories are apatite and ilmenite. The intrusions east of Wynyard (Le., Somerset, Secondary minerals are tremolite-actinolite, Burnie and Blythe Heads) are all sills and show chlorite, zoisite, sericite, prehnite, albite, leucoxene, secondary alteration to a lesser degree than those sphene, calcite and serpentine.
88 PRECAMBRIAN ROCKS OF TASMANIA, PART I low, as is the lime. The alumina is high (9(/<;) Hornblende as most augites range from 2-8 per cent. The Several rocks contain large laths of hornblende total of 1· 05 per cent alkalis is appreciable although which are so well formed as to be probably prim Hess 0949, p. 654) recorded an augite with ary. This is abundant in the coarse phase of the Na,O = 1·61 per cent and KeO := ·06 per cent. The dolerite (4856) near Cooee Pt. where crystals up 2'85 per cent of water is unexpected as the sample to 1·5 cms. long are visible in the hand specimen. appeared to be completely fresh but the slight The mineral is pleochroic with X pale-green, Y = amount of P,O-, is due to a small but persistent reddish-green and Z == deep green to rusty-brown apatite contamination. and has Z/\C = 13°. A little olive-green horn The pyroxene is compared with one from a Cam blende is developed in other specimens but its brian spilite of the Dundas Group, analysed by somewhat ragged shape suggests that it may be Scott (1954). The similarities (e.g., high alumina secondary. and low iron) are pronounced although there are slight differences in 2V, titania content and colour. Olivine Some dolerites (4855, 7276) from just west of Burnie contain a few crystals which have the TABLE 1 typical shape of euhedral olivine but which now PYROXENE ANALYSES consist of serpentine.
Blythe Heads Lynch Creek Biotite (Scott, 1954) Analyst: Analyst: Some dolerites, particularly those at Burnie W. St. C. Manson B. Scott (4855, 4860, 4864) contain a little well-formed biotite which appears to be primary. It is fresh Si02 46'10 48·53 AI,O" 9'00 7-10 and pale-green in 4858 but is generally strongly FeeO, 1·90 0·70 pleochroic from pale rusty-yellow to deep brownish FeO 7·00 5·71 black (4864) with peripheral alteration to MnO 0·18 not det. chlorite. The crystals are not greater than ·2 mm. TiO, 1·60 0'85 in length and the chilled dolerites are rich in very PeO, 0·15 not det. tiny crystallites of biotite. CaO 17·70 21·24 MgO 12'60 15·90 Quartz NacO 0·84 not det. Apart from very minor amounts in the dolerites K,O 0·21 not det. from Detention River (4861> and Rocky Cape (4853, H20 0·14 not det. 4854), the only rocks in which quartz is import H,O+ 2·71 not det. ant are the leucocratic varieties from Burnie. Quartz veins cut the dolerite at Burnie. 100·13 100'03 Ilmenite This is an accessory in all of the dolerites. It ATOMIC PERCENTAGE forms irregular crystals which measure as much Blythe Heads Lynch Creek as 2 mm. across and which are partially or entirely (Scott, 1954) replaced by either leucoxene or sphene. Ca 42'0 46'7 Mg 41·9 42·3 Apatite Fe 16·1 1l·0 Percentage Al in Z 14·3 This is an abundant accessory in all of the 0 0 dolerites. It appears as colourless hexagonal 2V +66 +51 prisms and needles which reach 3·3 mm. long in 4864 and 7265. Plagioclase Secondary Minerals No primary plagioclase was found in any of these The primary minerals show various degress of rocks and the albite, zoisite, calcite and prehnite alteration and this is made a basis of subdivision are considered to have been derived from original of the dolerites in a later section. basic plagioclase. Amphibole Orthoclase This is well developed in some specimens where it has either replaced pyroxene or forms peripheral It is difficult to recognise orthoclase among the fringes around it. The amphibole is fibrous and strongly altered feldspar, all of which is crowded varies from colourless through various shades of with tiny inclusions but the analyses in table 2 pale-green to olive-green. A tremolitic variety is indicate its presence. The mineral is usually present in the leucocratic dolerite (4851>; it is sericitized, e.g., 7265, but occurs in a fresh condition pleochroic from colourless to palest-yellow, has an as an intergranular graphic intergrowth with quartz optic axial angle of approximately 80° (negative) in the Detention River sill (4861>. and an extinction angle ZAC of 16°.
92 PRECAMBRIAN ROCKS OF TASMANIA, PART I
CONCLUSIONS TABLE 3 The petrological and structural evidence suggests ANALYSES OF SOME AUSTRALIAN CAMBRIAN AND very strongly that the Cooee Dolerites and the PRE-CAMBRIAN BASIC HOCKS Dundas spilites represent separate periods of igneous activity. This could be proved if the 1 2 3 4 5 Dundas was found to overlie unconformably one SiO, 52·47 43·42 49·33 49'33 52'27 of the dolerites but the chances of finding this seem ALO, 16·07 14·93 13·23 16·39 14'63 remote, due to limitations of outcrop. Radioactive Fe,O" 1·29 4·21 2·54 2·33 3·33 age determinations do not seem likely to help be FeO 6·24 6·21 10'71 3·64 7·70 cause of the lack of the appropriate minerals. MnO 0·15 0'16 0·33 0·15 0'25 CaO 361 7·47 9·43 7·95 7'61 The dating of this period of igneous activity MgO 4·33 6·09 6·37 4·36 5·09 is difficult but it is probably pre-Dundas, I.e., earlier Na,O 2·60 2'62 2·21 4·62 2·35 than Middle Cambrian, and as the Rocky Cape K,O 2·23 1'05 1·01 0'70 1'97 Group is probably Upper Proterozoic, then the TiO, 1·36 1'69 1·73 0·64 1·30 dolerites could belong to the Australia-wide group P,O" 0·31 0·15 0·24 0·10 of basic rocks ranging from Late Pre-Cambrian to H,O 0·14 2·23 0·14 0·23 0·62 very Early Proterozoic. H,O 332 1'56 1·83 2'56 1·03
1. Average of three dolerites from Burnie and Detention River (see Table 2). 2. Average of 10 dolerites and basalts from the Flinders Ranges, South Australia. 3. Average of 10 dolerites from the Pre-Cam brian of Western Australia (Prider, 1945). 4. Average of five spilites from the Dundas Group, Tasmania (Scott, 1954). 5. Average of seven basalts from the Kimberley area, Western Australia (Edwards, 1942).
HEFERENCES CAREY, S. W., 1946.--Ann. Hept. Govt. Geologist for 1945. Dept. Mines. Tas. '2 '1 CAREY, S. W. AND SCOTT, B., 1952.-« Revised Interpretation of • 3 the G'"'ologv 0 the Smithton District." Pap-. & Proc. '5 Roy. Soc. Ta8., S6, 63-70. EDWARDS, A. B., 1942.-" Some Basalts from the West Kim berley, Western Australia." Journ. Roy. Soc. West. Aust., Fe ° Mg ° 27, 79-93. EDWARDS. A. B. AND CLARKE. E. DE C., 1941.-<4 Some: Cambrian Basalts f'rom the East Kimberley, Western Australia," Journ. Roy. Soc. Wcst. Aust., 26, 77-94. LOVEDAY, J .. 1956.-" Reconnaissance Soil Map-Burnie" Table Cape." C.S.I.R.O., Vivo Soil.•. ELLISTON, J., 1954.-" Geology of the Dundas District." Pap. &- Proc. Roy. Soc. Tas., 88, 161-163. HESS, H. H., 1949.-" Chemical Composition and Optical Pro- peTties of Common Clinopyroxenes," Am. Min., 84, 621-666. PETTIJOHN, F. J., 1949.-" Sedimentary Rock::;." New York. PIUDER, R. T., 1945.·~" Igneous Activity, Meta.morphism, and Ore-Formation in Western Australia." Journ. Roy. Soc. W cst. A ust., 33, 43-R4. SCOTT, B., 1954.-" Metamorphisrn of the Cambrian Volcanic '4 llocks of Tasmania, &c." Pal). &- Proc. Roy. Soc. Tas., SR, 129-149. SPH.Y, A., 1952.--" Basic Igneous of the Worumba Area." Tmns. Roy. Soc. South Aust., 75, 97-114. '2 " '3 SPRY, A. AND FORD, R. J., 1957.-" Reconnaissance of the • 5 Corinna-Pieman Heads Are-a, Geology", this volume. STEPHENS, J., 1909.--" Notes on the Geology of the North-West Coast of Tasmania." Proc. Linn. Soc. New South Wales, 3:3, 4, 752-767. MgO SUNDIUS, N., 1930.-" On the Spilitie Hocks." Geol. Mag. Land., "2° 57, 1-17. FIG. 4.~Diagrams illustrating the similarities and differences 1903.~" between the compositions of basic rocks from (1) Burnie: TvVELVBTREES, W. R., Report on the Dial Range,,', Tas. (2) South Australia; (3) Western Australia; (4) Kim Dept. Mines Pub. berley, Western Australia; (5) Dundas Group, Tasmania. ------, 1905.-" Report on the North-West Coast See details in table ;~. 'Mineral Deposits." Sec. Min. Pap.
PAPEKS AND PROCEJ:mINGS OF THE ROYAL SOCIETY OF TASMANIA. VOLUME 91 PLATE 11
2
3 4
Nos. 1, 2, 3.-Minor folds in the Burnie Quartzite and Slate. These folds have been mapped and are shown in fig. 2. No. 4.,~" Worm tracks" on a specimen of quartzite from Sisters Beach. Specimen is six inches across.