Some Formations Close to the Permo-Triassic Boundary in Tasmania
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PAPERS ANn PROCEEDINGS OF' 'l'HI<J ROYAL SOCIJilTY 01" TASMANfA. VOLUME 101. SOME FORMATIONS CLOSE TO THE PERMO-TRIASSIC BOUNDARY IN TASMANIA MAXWEL.L. R. BANKS, University oj Tasmania and IKRAM H. NAQVI, Department oj Mines, Tasmania, formerly Universi.ty oj Tasmania. (With three text. figu.l'cS and one plate.) ABSTRACT boundary elsewhere. Lewis (1940) recognised that The Coal Measures is a unit of carbol1- the occurred coal and feldspathic sandstone in at the not top Cygnet Measure Upper Permian age. It is overlain but was not consistent (1946, pp. 22-3). gradationally or disconi'ormably by the Springs The difficulties are increased by lack of definition Sandstone. the lower member of which, Barnetts and reasonably precise knowledge of the so-called Sandstone, is a thinly-bedded, fine-grained feJd basal Triassic units. The Ross Sandstone of spathic to arkosic sandstone, and the upper, the Ronwick (870) and later authors is not properly Mountain Lodge Sandstone. a more thickly-bedded, defined in its type area and may be undefinable medium-grained protoquartzite. The upper part there. The outcrop is certainly not good in the of the Springs Sandstone is probably Otoceratan. Ross area. The Rhyndaston Sandstone (David The Permo-Triassic boundary lies within these 1932) is much better exposed at Rhyndaston than gradational non-marine units and cannot yet be the Ross Sandstone at Ross, but its relationships fixed accurately. The base of the Mountain Lodge to the Permian on the one hand and to younger Member is probably the most convenient boundary units on the other are not yet clear. The Springs to use as the Permo-Triassic boundary in local field Sandstone (Lewis 1940) is moderately well-exposed mapping. in its basal portions and its relationship to under lying rocks in the type area determinable but not INTRODUCTION that to younger sedimentary rocks. No accurate section of this formation has been published. Milligan 0849 pp. 17-18) llrst described the coal Recently the basal 'rriassic unit in the Lake St. measures at Southport subsequently correlated with Clair area has been called the Gould Formation the Cygnet Coal Measures. Thureau in 1881 men (McLeod et al, 1961, p. 26), the relationships of tioned the coal at Cygnet and his description of which to underlying and overlying units are clear, the section at the coal mines may still be the best. if gradational, but of which no detailed sectIon has Subsequently Johnston (887) descrIbed coal been published. There are, in addition, innate measures of similar age at Adventure Bay on Bruny diffi.culties in defining and correlating units such Island and in 1888 described the section at Cygnet as the Springs, Ross, Rhyndaston and Gould Forma and correlated both successions with the New tions, because of their depositional environment. castle Coal Measures of New South Wales. Subse probably fiood plains with braided channels. quently freshwater beds at this level have been recognised widely in Tasmania and sections In t.his paper, definitions and deta.iled sections measured in several places (eg. Mount La Perouse, of formations near the Permo-Triassic boundary in Hobart, Zeehan, Lake St. Clair, Mole Creek). the Cygnet and Hobart Districts will be offered During 1965, a good succession near the nominal and the problem of the Permo-Triassic boundary type area was established in detail by I. Naqvi and discussed in some detaiL this led to a review of formations adjacent to the Permo-Triassic boundary. CYGNET COAL l\iEASURES Difficulty is experienced in many parts of Tas Stra.tigraphy mania in delineating the Permo-Triassic boundary. The nomenclature and synonomy of this formation There is a macrofioral change near the boundary have been dealt with by Smith (1959, pp, 47-8). from the Glossopteris to the - DicTOidium flora but The first Significant report is that of Thureau (1881) pinpointing this change in the field is very difficult who used the term "Mt. Cygnet measures" and because of the poorly fossHiferous character of the recorded a sandstone fioor, 3 ft. of coal, approxi formations overlying the Cygnet Coal Measures mately 64 ft. of sandstone, a 5 ft. seam of coal and their correlates. The standard field convention within borders of carbonaceous clay, and a mica has been to regard the base of the first thickly ceous, laminated sandstone roof. His map and bedded quartz sandstone above carbonaceous shales section clearly show two seams separated by sand (with or without a Glossopteris fiora) of the Cygnet stone but it is not quite certain that the thickness Coal Measures as a convenient place for the Permo quoted for the inter-seam sandstone is not Triassic boundary with full recognition that this excessive due to faulting. Twelvetrees (1902) may not correspond exactly to the Permo-Triassic reported a slightly different section at the Mt. R.S.-3 17 .... A Distl'ibution and Thickness of Cygnet Coal B-Sections in Cygnet and C·-Map of Springs Area. 00 Measures and Barnetts Member in Hobart Area Mt. Wellington Cygnet District o 2.0 Chains Q 4 Miles L== J Q Metres f·5 " I)" l 10 Ulo ·15 Is: /Pl nnacle Rd)? t<1 (PiUinger .. i o"l Mountain j Pi i:ti Lodge , is: Member >,., F (5 Z to o Medium ot" grained Ul ., Springs t<l ,Ss .• o>-l ..., ~ t<1 'tJ t<l o~ :., :;::0 Ul orJl 2 oto q Cool Mine Mt.Cygnet x x Bay Coal Mine Z ><~ z tn~ ~ Z 30 Thickness of Bametts Member > 5 Thickness of Cygnet Coal Mine Thickness in Cradac Area after Leaman (pers. comm.) 3 4 5 Section on Mt. Wellington Top FIG. L Barnetts A~ Map of the Cygnet al'ea showing distribution and thickness of Cygnet Coal Measures and Barnetts rvlember. i H. Columnar sections of Cygnet Coal Measures and Springs Sandstone, Cygnet and lvit. Wellington. 500Ft·1 1. Section at Coal Mine Bay. 2. Section at Mt. Cygnet Coal Mine. :)~ 4~ 5, Section on Mt. "'''ellington. t·!\op of D ='===;:::;j C. Sketch map of part of Mt. """"ellington showing positions of sections. CcoL Mine Boy Areo, Cygnetj D. Map of Coal Mine Bay area; Cygnet~ showing position of type sections. MAXWEl.l. R. BANKS AND IKRAM H. NAQVI 19 Cygnet Mines, a small 2 inch seam, another seam aceous section is six metres thick and is overlaIn 1 ft. thick 12 feet above the first and the main disconformably by conglomerate with a quartz rich seam 3 ft. thick, 25 ft. above the second. The matrix. main and two minor seams of which only one was No macrofossils occur in this section and un Reeous siltstone, the lower siltstone resting on dark fortunately specimens submiUed to Dr. G. Play sandstone and the upper with a roof of grey hard. ford. University of Queensland, for palynological glistening, quartzose sandstone with a conglomerate study yielded no recognizable spores. in places. Reid (in Hills et al. 1922) reported two main and two minor seams of which only one \vas A thin section (33626) of carbonaceous siltstone being worked. At Berry's, north-west of the main from the middle of the formation revealed a modal working (fig. 1 A) a sandstone roof caps a seam grainsize of the clastic particles between 0.03 12" -14" thick resting on 12 inches of black shale. and 0.045 mm. with variation from very fine silt This shale rests on 8 feet of shale and 20 feet to very fine sand. Aggregates up to about 0.4 mm. of sandstone which in turn overlies mUdstone. Thus long of clay grade carbonaceous matter comprise it would appear that in the nominal type area the about 30% of the rock, the only other clay grade coal measures are at least 6.8 m. thick and may be material being a very minor amount of interstitial over 12 m. thick on Thureau's figures. Recent clay mineral. Clear unstrained quartz is the investigations by the authors show that detailed commonest constituent of the rock, about 55%, information in the type area is not available because but a little quartz with globular inclusions in lines of thick vegetation and deep soil cover. The also occurs. Plagioclase (oligoclase) is about 1 % Cygnet Coal Measures appear to overlie Ferntree of the rock and there is a surprising amount of Mudstone, to be between 12 and 18 m. thick and biotite (about 12%) and some muscovite (about to be overlain by quartz sandstone and thinly 3 % ) . The quartz and plagioclase grains are pre bedded (beds about 1 cm. thick) micaceous sand dominantly angular and very few are as rounded stones and siltstones. as subrounded. Their sphericity varies from equi dimensional to very elongate and many have The best section now available is that in shore lengths exceeding three times their breadth. The platforms at Coal Mine Bay and immediately south quartz and feldspar grains lie in a felted matrix of <summarised as fig. 1 B, 1). The Ferntree Mudstone biotite and muscovite laths and elongate stringers with rare erratics and some marine fossils is over of carbonaceous matter all held by a minor clay lain gradationally by fine-grained carbonaceous cement. siltstone with some sets up to 1.5 cm. thick of interbedded very-fine grained sandstone and car This section is thinner than that recorded from bonacous siltstone in beds up to 2 mm. thick but the Mount Cygnet Coal Mine area (fig. IB, 2) and mostly less than 1 mm. in thickness. Such apparently differs from it in containing less sand sets form less than 10% of the basal metre. stone. The section on Balfs Hill near Cradoc is From 0.9 m.