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Lower Palaeozoic Unconformities in South-Western Tasmania

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Lower Palaeozoic Unconformities in South-Western Tasmania PAPERS AND PROCEE'DINGS OF THB ROYAL SOCIETY OF TASMANIA, VOLtlME 94 LOWER PALAEOZOIC UNCONFORMITIES IN SOUTH-WESTERN TASMANIA By B. SCOTT Lyell-E.Z. Explorations* (With 2 text flgllres). IN'l'ItODlTCTION Jukesian Unconformiiy.-The Jukesian Uncon­ formity has been defined as the angular discordance Recent geological work in south-west Tasmania between the Dundas Group below and the Junee on a regional scale has allowed the compilation of Group above, as revealed at the northern end of existing information regarding Lower Palaeozoic Mt. Jukes, south of Queenstown. Stratigraphically unconformities, namely the description of new this unconformity may be considered as the localities and in some cases the amplification and erosional surface of the Dundas Group on which extension of previously described exposures. the <lunee Group was deposited. From the tectonic Carey and Banks (1954, p. 245) provide a review viewpoint .it may be considered ag the expression of information to 1954 and their classification of of an orogenic movement of post-lower Upper unconformities has been adopted in this paper. Cambrian but pre-Lower Ordovician age. This These are repeated here (op. cit. p. 264) : movement is referred to as the ,]ukesian Movement Tyennan lTnconformity.-The Tyennan Uncon­ of the Tyennan Orogeny. Hills (1914, pp. 45) was formity may be defined as an angular discordance the first to give a detailed description of an uncon­ between pre-Dundas rocks below and the Junee formity between the Junee and Dundas Groups. Group above as revealed on the south-eastern slope In the Mt. Sorell and Mt. Jukes area he describes of Tim Shea, at the head of Tyenna Valley. Strati­ an angular and a metamorphic discordance between graphically it may be considered as the erosional the two groups and the presence of a basal breccia surface in pre-Dundas rocks on which the Juneo dervide from the underlying Dundas Group. Un­ Group was deposited. The orogenic movements fortunately the section described by Hills at the occurring within the of time represented by north end of ML. Jukes (p. 45) and taken by Carey this is to as the Tyennan and Banks as their type locality for the Jukesian Orogeny. This is a minimum of that during Unconformity is not an unconformity but a meta­ which the Dundas Group was deposited (at least morphic contact (Bradley, 1954, p. 206 and Solomon, lower Upper Cambrian to upper Middle Cambrian, 1957, p. 35). There is no doubt that a tectonic Opi!~, 1951a, 1951bJ. The base of the Junee Group break exists at the top of the Dundas Group in (Owen type Conglomerate) indicates a Lower this area and the author would suggest the South Canadian age (Opik, 1951aJ so that the Tyennan Darwin peal{ as the type locality for this uncon·· Orogeny must precede this, formity. Here the regional trend in the Dundas Stiehtan Unconformity.-The Stichtan Uncon­ Group is north-south with a steep dip of 30 formity has been defined as the angular discordance degrees to the west; it contains the Darwin Granite between the Carbine Group below and the Dundas and associated mag'netite-hematite mineralisation. Group above as revealed on the western flank of the The overlying beds consist of a thickness of Jukes Sticht Range and its southern continuation to a Breccia containing angular particles up to 12 inches point east of Lake Dora in the West Coast Range. in length of the underlying Dundas Group, granite Stratigraphically the Stich tan Unconformity may and magnetite-hematite, as originaliy described by be considered as the erosional surface on wbich Hills 0914. p. 43). This Breccia gTaC!eS upwards into the Dundas Group was deposited. It is an expression a more normal siliceous chocolate coloured pebble or orogenic movement of pre-upper Middle Cam­ Owen Conglomerate. This well bedded Conglom­ brian age, followed by a period of erosion or non- erate is folded into a shallow syncline which deposition to the deposition of the Dundas to the south at approximately 20 Group. movement is referred to as the dips of up to 25 degrees on the east west SticMan Movement of the Tyennan Orogeny. limbs. Thus in this area there is a structural dis­ cordance between the Dundas Group and Owen I would one minor modification to this Conglomerate, with an intervening period of granite definition by and Banks in the substitution emplacement, hematite··magnetite mineralisation of "Precambrian for" Carbine Group" at the and erosion to form the basal Jukes Breccia. beginning of the paragraph above. At the present time the term Carbine Group as a division of the Throughout, I have found it difficult not to use Precambrian has never been sufficiently well defined the terms pre-Dundas Group and Precambrian to allow its use beyond the type area at Dundas. synonymously although it is realised that the oldest fossils so far found in the Dundas Group have only Published by permission of Mt. Lyell Mining: and Raihvay been dated as upper Middle Cambrian (Opik. Company Ltd., and the Eleetrolyt:ic Zinc Co. of Australasia Ltd. 1951a) . 103 104 LOWER PALAEOZOIC UNCONFORMITIES IN S.W. TASMANIA UNCONFORMITY .. BETWEEN .. JUNEE . GROUP TABLE I. AND PRE-DUNDAS GROUP ROCKS Lower Palaeozoic Stratigraphy a. Andrew River Area (Mather, 1953, p. 15) In this area the Dundas Group is absent and rocks correlated with the Owen Conglomerate, which is overlain by the Gordon Limestone, rest Gordon Limestone directly on the Precambrian (Fincham Group, see Florentine Valley Mudstone Table D. The actual contact between the Owen Caroline Creek Sandstone JUNEE GROUP Conglomerate and the Fincham Group has only Owen Conglomerate been observed in one stream section. In this section Jukes Breccia coarse grained Owen Conglomerate overlies a pink­ ish coloured quartz schist of the Fincham Group. The rocks on either side of the boundary have com­ DUNDAS GROUP parable attitudes. Along the regional strike to the north-west at about 1200 feet distant part of the Precambrian Stratigraphy-Frenchman's Cap Area base of the Owen Conglomerate is exposed. It con­ sists of a coarse grained breccia conglomerate (after McLeod, 1955 and Spry, 1957) which contains angular and rounded fragments of quartz and white micaceous quartzIte up to nine Groups Grade of Metamorphism inches in size. The particles of micaceous quartzite Fincham Group Chlorite zone are considered by Mather to have been derived from Franklin Group Garnet zone the underlying Fincham Group and to indicate Mary Group Chlorite zone erosional unconformity. Mather also considered Joyce Group Garnet zone the marked decrease in the stratigraphic thickness of the Fincham Group as it is traced along strike to the south-east to be due at least in part to c. Elliott Range erosion which took place between the late Pre­ cambrian and the deposition of the Owen Conglom­ The Tyennan unconformity at the north end of erate (Lower Ordovician) . The apparent conformity Elliott Range was originally described by Carey as expressed by the similar attitudes of the two and Banks (1954, p. 255). The unconformity is well is considered to be due ,to the shallow dip of the exposed in a near vertical cliff section and consists Precambrian in this area at the time of the de­ of well bedded white quartzites and pebble con­ position of the Owen Conglomerate. This would glomerates (Owen Conglomerate) which overlie suggest broad folding of the Fincham Group prior Precambrian siliceous schists. to the deposition of the Owen Conglomerate; later folding has produced the present steep attitude of these sediments. d. Sprent River The exposures north and south of Sprent River indicate a broad anticlinal structure with a central b. Mt. Fincham Area (McLeod, 1955, p. 13) portion of metaquartzites and siliceous schists and Similar conditions apply here as in the Andrew margins of quartz pebble conglomerate and sand­ River area. The actual contact betwen the Owen stones which have been correlated with the Owen Conglomerate and the Fincham Group was only Conglomerate. The latter sediments form the seen in one place. Here breccias and conglomerates prominent cuesta of Sprent Ridge which is striking 100 0 and dipping 50 0 to the south occur immediately to the north of the Sprent River. On within twelve inches of a purple quartzite of the the western margin the strike of the Precambrian Fincham Group whose strike varies from 090 to and Owen Conglomerate is comparable but with 1200 with a dip of 70 to 80 0 to the south. At a the suggestion of a steeper dip in the former sedi­ number of other places rocks on either side of the ments. On the eastern margin the Owen Con­ contact were farther apart and the evidence for glomerate trends nearly north-south with the Pre­ a structural unconformity between the Owen Con­ cambrian horizons trending east-west. The overall glomerate and Precambrian is less conclusive owing structure can be best explained by an anticline in to the varialble dips which occur in the latter. existence prior to the deposition of the Conglom­ erate with refolding after their depOSition about an A correlate of the Jukes Breccia was seen, axis some one to two miles to the west of the occurring as half a dozen loose blocks which con­ previous one. tained ·a predominance of rock particles up to three inchs in size but with some to six inches. The The above descriptions allow the establishment fragments include massive and schistose quartzites, of an unconformity between the Junee Group and micaceous schistose quartzites, and some reddish pre-Dundas Group rocks for a distance of 35 miles, massive quartzite. from the north end of the Engineer Range (Mt. McLeod concludes that usually the strike of the Madge) southwards to Mt.
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