PAPERS AND PROCEEDINGS 01<' 'rHE ROY AI, SOCIETY OF TASMANIA, Vq'LUME 94 GEOLOGY AND STRUCTURE OF THE MIDDLE DERWENT VALLEY By M. A. ANAND ALWAR Geological Survey of India' and University oj Tasmania (Comrnunieated by Maxwell Banks. University of Tasmania) (With 8 figures and 4 coloured maps) ABSTRACT PHYSIOGRAPHY An area of 130 square miles of the Middle The physiographic development of the area has Derwent Valley lying between New Norfolk and been dominantly controlled by the structure and Hamilton has been geologically mapped. The the complex intrusion of Jurassic (?) dolerite. The Permian sub-greywacke type sequence consists of Derwent Valley is a graben. Permian and Triassic 1300 feet of sandstone, siltstone, mudstone and rocks with minor dolerite and Tertiary basalts limestone deposited on a stable shelf. Disconfor­ and lake sediments form the valley, while dolerite mably overlying the Permian are rocks belonging hills like Mt. Spode, Mt. Belmont and Moogara to the Triassic System which have been grouped Plateau border the valley on either side and rise tentatively into three formations. The Permian to heights of 1,500 to 2,000 feet. Small hills of and Triassic are extensively and complexly injected resistant Triassic sandstone and Tertiary basalt by dolerite; mineralogical variations from the rise to 500-600 feet elevation in the valley. The normal tholeiitic dolerite to granophyre have been area is drained by the Derwent and its tributaries traced. Evidence for liquid immiscibility is present i.e., Russell 1'"a11s, Styx and Plenty Rivers and towards the base of the sill. The Jurassic structure smaller rivulets such as Allen Vale Rivulet. Belmont. is interpreted as a cauldron subsidence. Faulting Rivulet and Dry Creek. The drainage is controlled Is accompanied by dolerite intrusion of transgres­ for the most part by either the structure or resis­ sive to concordant sills, which are like cone-sheets tant rocks. Though the elevation in the area does near the root zone. Various Tertiary faults com­ not exceed 2,000 feet, the gradient of the hills is bine to form a stepped graben, the axis of which quite steep and deep gullies have been cut in the trends north-west, resulting in the formation of resistant rocks. The physiography could be said a chain of lakes and lowlands. In late Tertiary to be in a late youthful stage. times, large outpourings of olivine basalt and associated tuffs, at least 350 feet thick, overran the lakes and partly filled the valley. Several flows PERMIAN SYSTEM of basalt have been mapped and five periods of volcanism have been recognised. The i.nter­ General basaltic periods are marked by either lake sediments Rocks belonging to this system have received or fossil forests. The graben faulting controlled great attention in the Hobart district and several the drainage pattern both during pre-basaltic geologists have studied them. Voisey (1938), Lewis times and subsequently. (1946), Banks (1952, 1955) and Banks and Hale (1957) are responsible for the present l~nowledge of the Permian rocks in Tasmania. Permian rocks INTRODUCTION from the Bundella Mudstone upwards are exposed The area mapped lies in the middle Derwent in the area on the upthrown side of the Glen Fern Valley comprising four ten-kiloyard grid squares, Fault and west of Glenora up to westerway. namely, Mt. Spode, Macquarie Plains, Glenora and Plenty and is covered by the aerial photographs BundeHa Mudstone Ellendale runs 1 to 10 [J,nd Styx runs 1, 2 and 4. Tertiary basalts were mapped on a scale of 1 inch The top 35 to 40 feet of this formation are equals 400 feet, using a base map, provided by exposed on the up thrown side of the Glen Fern courtesy of the Hydro-Electric Commission, Fault at grid 489.000, 733.050. The rock is an olive Hobart. Details from this have been incorporated grey, non-fissile siltstone, with angular fragments in the 1 mile = 1 inch map. Specimen and thin of quartz, feldspar, muscovite and rock fragments section numbers quoted refer to material lodged with glacial erratics of quartzite and phyllite in the Geology Department, University of Tas­ ranging in size from a fraction of an inch to a mania. couple of inches. --------------------------- *- Published by the permission of the Director. Geological Survey of India. 13 14 GEOLOGY AND STRUCTURE OF HIE MIDDLE DERWENT VALLEY :Faulkner Group underlying or overlying formations. Towards the This group is exposed overlyIng the Bundella base and 50 feet from the top fossiltferous siltstones Mudstone, The first membel' is a quartz-rich, occur. The top horizon forms a marker bed and is fissile, bedded sandstone, noticeably mic- exposed near grid 489.900, 731.000, aceous and parts eross-bedded. Overlying this Risdon Sandstone is a six feet thick, well-sorted, olive grey carbon-, Overlying the" Woodbridge Glacial Pormation aceous sEtstone which is unfossiliferous except is an 8 to 10 feet thick bed of feldspathic sandstone for vague plant The section above this with eoarse, sub-rounded quartz and feldspar; it is covered for a ot 60 to 70 feet. The next is unfossiliferous. member seen is a yellow to orange coloured feldspathic sandstone with angular to sub-angular I"erntree .Mudstone fragments of quartz and feldspar with of bryozoa. This formation is with the The Ferntree :Mudstone lies itbove the H.isdon Rayner Sandstone (Banks and Hale, 1957). Sandstone and below the Coal 1\10asure or where the latter is absent the Triassic Hoss The correlation of these formations from tho and Formations. :B'erntree Mudstone Bundella through the Faulkner to the side of Creek and attains Sandstone is open to are a thickness of 400 feet and is against the fossil dolerite. In the and \Vesterway area evidence This attains a thickness of feet, Good exposures correlation because these formations are seen in the railway and P.W.D. quarry neal' underlie the ~J''''''''C'C;'' Group with diagnostic fossil Westerway. West of Karanja a certain amount of evidence and within the area there appears to be baking is noticed due to the of the no structural discontinllity. dolerite rooL North of formation is disconformably overlain Cascades GrlluP asslc sandstone. Besides the Hisdon A good sec1;ion of this exposed on the three more sandstone horizons are noticed in the going south-south-east the Glen Fern- Ferntree Formation, along the numerous timber road from 450 feet to no feet tracks nort.h of and t.he track and overlies the Group. The Dry Creek west of 488.500, Fault throws down the section on the west side members the Ferntree F'ormation are at least 150 feet The Cascades in by (1957, pp. 8-9) from the Great Lake locality Is 320 feet thicl: consists area. of Nassau Limestone and TRIASSIC SYSTEM Grange Mudstone. General The Nassau Siltstone is 10 to 15 feet thick and Disconfonnably the Permian System is contains angular of quartz, and little feld­ of Triassic age. This spar. Numerous are present, especially relation by of and Fenestella. The other several workers spiriferids and fragments 1949; and Banks, of Towards the top the formation seen at grid 475.000. 746.400, and becomes more calcareous. and west of Hollowtree Road. The calcareous siltstone and limestone about 150 localities an angular feet thick form the B"!rriedale Formation, In­ the discordant attitudes of the dividual beds are a few feet thick separted by thin assic beds. fissile siltstones. The limestones range from calci­ The subdivision of Triassic rocks into dHIerent rudite to calcilutite. They are generalIylmpure and formations is difficult owing to rapid fa.cies va.ria- consist of clastic grains of quartz and erratics of tions and lensing of beds. Jennings and other rocks, Important 175) expressed a opinion with are species of StenopOTn, Wayatinah area.. General lithology fossil evi­ ferids, AviculoJ)ecten and corals. Towards the dence, at the present. state of knowledge, do not the limestone passes into Grange Mudstone. allow definite correlation, However, two distinct Grange lV[udstone both on the east formations can be recognised, the Sand­ and west side of and attains a thickness stone and Shale and the" F'eldspathic" '-"O'W""',,"v,,'~;. of 150 to 160 feet. the formation consists of Towards the base of the the members are non-Iissile to fissile siltstone. The rocks are mostly essentially coarse to quartz yellow to brown and rich in l·ossils. Erratics are massive in nature with very minor amounts common. Another small outcrop of Grange Mud­ shale beds, the sandstones exhibiting pronounced stone is exposed a mile and a half south of Wester­ cross-bedding and slump structures. These, where way on the first bench of the dolerite hill. exposed, form a distinct. group of beds forming' cliffs with salt efflorescence and can be recognised as a formation. McKellar (1957, p. 4) has described H 'Woodbridge GladaI Formation" 650 feet of massive quartz sandstone from the This formation attains a thickness of 280 feet Great Lake area which he has grouped as Ross and is composed of rhythmic aiternation of sand­ Sandstone. Further work on the Triassic strati­ stone and siltstone. The rocks are grey when graphy is needed to decide the stratigraphic status freshly broken and weather to yellow and yellowish of these, either as a separate formation, or as a brown. Erratics are more profuse than in the basal member of the Knocklofty Sandstone and M. A. AN!l.N!l ALWAR 15 Shale. Since these form a recognizable unit they Plant fossils are common in the shaly beds. deserve to be considered separately and in this Dicroirliurn odontopteroides is found east of Hose area SUcfl rocks have been mapped as :Ross Sand~ Garland: worm casts are also found in the shale stone.