McDougall, I

University of Tasmania Open Access Repository

5074 GEOLOGY OF TASMANIA DROMEDARY ONE INCI-i 5ERIES_UNIVERSITY OF TASMANI... ...~~~_=;OOF_~~~~~____~~"""-,..~~=....~~"C:'_"""'1r-__...._ . ~ _" """""' OE....'"'"' .....______~~__ Jdl .Jdl Jdl Quaternary System LEGEND _ :_ FAULT WITH DOWNTHROWN !OlD!: INDICATED RECCNr SERIES 1«1' ItiAP SHOWING 1JAGNCnt" OE'CLINA"ONS __ F"ULT- POSI'TION APPROXIMATE IGNEOUS ROCKSOro AL.UNIUt.I SEOJLAR VARIATION 1 IJINS P. ...Tertiary System TriassIC System Tb __ FOAt.lATlON BOUNDARY ik KNOC)l;L.OfTV I'ORMAnoH JurOSSIC' ?l System- - - fORMATION BOUNOAJIV JIO$I'TION ... ""ROIIC ....... TE P~rmton System JdlDolerrte Boundaries ~ FERNTREE MUOSTONECONCORDANT SilL. DISCOROANT INTRUSIVE BOUNDARIES D r .J Rtr.oON SANDSTOf'IIE CompilatIon (rom Aerfal Photo<]rophs VERTICAL JOItn'S I- • f'; j 'fIIOODBR1DGE ~LA.CIAl. FORMATION Trlqonometnc Statcon Control by 'HilI( E AND DIP OF JOINTS courtesy of the Department of ~ GRANGE ....UDSTONE Lands and Surveys Hobart and the Australian Army Survey ServICe Oriqln of co-ordinotes 400 ,000 '((Is ~ BERRIEDALE L.IMESTONE ~ West and l,8oo,oooyds South of the True Orl'3ln of Zone 7 of the IntemoNonol Grid MAPPED AND COMPILED BYl M<:DOUGALL 1956 ~ FAULKNER GROUP ~ BUNDELLA M UOSTONE GEOLOGY OF THE DROMEDARY AREA SHEET 5074 PHYSIOGRAPHY The area has in general a relief of about 800', with rounded, but commonly steep-sided, hills rISing up from relatively broad valleys The physiogrophy has been controlled mainly by the Tertiary faulting striking in a north-westerly direction. The Dromedary Fault is the malor fracture and has a throw of 2,000 feet or more decreaSing in magni tude to the north-west. On the upthrown (south-west) side of thie, fault the relief is in excess of 2,(XX),. In the dawnthrown block, which was tilted towards the fault plane, the main drainage of the area takes place parallel to the fault witi, small consequent streams flowing in at approximately right angles from both blocks. The distribution of the resistant dolerite and the relatively soft sediments has also been an important factor in controlling the physiograp~,y. The Jordan River north of Strathallie Hill, has cut a steep-sided valley and appears to follow major joints within the dyke-like intrusion of dolerite. STRUCTURAL GEOLOGY The major structural features are the Tertiary faults. Apart from the large Dromedary Fault two horsts occur within the southern part of the area with the larger Cobb's Hill horst of Triassic sediments having a throw of the order of 900', and the inner, smaller horst, consisting of Permian sediments, having a throw of similar magnitude. STRATIGRAPHIC TABLE Age Group Formation Lithology Thickness (in feet) Quaternary River gravels ond alluvium Tertiary Brighton Olivine basolt flow 100 Basalt Bosanite plugs Early Tertiary P) Strong Faulting Jurassie (?) Tholeiitic dolerite sills and dykes TriCissic Knocklofty Predominantly quartz sandstone, 650 Sandstone and Shale some shale. Basal conglomerates and granule conglomerates. Permian 	 Ferntree Mudstone Mudstone 620 Risdon Sandstone Sandstone, conglomeratic bands 20 "Woodbridge Glacial Siltstones and fine sandstones­ 260 Formation" Basa I coo rse sondstone jGrange Mudstone Mudstone 100-0 Cascades Berriedale Limestone Limestone, minor co Icareous mud­ 150-250 stonelNassau Siltstone Siltstone 60 Rayner Sandstone Sandstone and Siltstone 35 Faulkner Sandstone and Siltstone 40 Bundella Mudstone Sandstones, siltstones, mudstones 250+ IGNEOUS ROCKS In the Jurassic P) Period there was widespread injection by tholeiitic dolerite magma into the Permo·Triassic sediments as'large, commonly irregular, sill-like and dyke-like bodies. The sills in most cases have been unroofed by erosion. Over I,OCO feet at dolerite is erxposed in a sill copping Mt. Dromedary in the south-west ·corner of the area. There are twa occurrences of basanite in the form of plugs and these are considered to be Tertiary in age. A flow of olivine basalt of Tertiary age fills portion of the pre-basaltic Jordan valley. POINTS OF SPECIAL INTEREST Basani te plug (S053E 7468N). Horst fault (5060E 7415N). Dromedary Fault (5047E 7413N). Fault breccia (5010E 7402N). Steep intrusive contact of dolerite with sediments (S050E 7466N). Bosal Triassic grits (5070E 7472N). Risdon Sandstone (5067E 7405N). REFERENCES Banks, M. R, and Hale, G. E. A, 19S7-A type section of the Permian System In the Hobart Area. Pap. Proc. Roy. Soc. Tasm., vol. 91, pp 41-64. Lewis, A. N., 1946--Geology of the Hobart District. Mercury Press, Hobart. McDougall, I, 1959-The Brighton Basalts, Tasmania. Pap. Proc. Roy. Soc. Tasm., val. 93. McDougall, I, 1959-The Geology of the Pontville-Dromedary Area, Tasmania. Pap. Proc. Roy. Soc. Tasm., vol. 93. Nye, P B, 1922-The underground water resources of the Jericho-Richmond-Bridgewater Area. Tas. Dept. Mines Underground Water Supply Paper, No.2. GEOLOGY OF TASMANIA 5174PONTVILLE ok o· \k LEGEND _ ,_ FAULT ""trw DOWNTHItOWN SID£ INDICATED o Quoternary System ___ • FAULT CONCEALED RECENT SERIes Compilation from Aerl<Jl Pnotoqropfu 1(£1' UAI' !IHOWI~ UA(;N£T1C DCCLINArJONS 19S7 tnqonometnc StatIOn Control by SECIILAR liAR/AnON ,'pc" """N(/I'I~ ALLUVIUM _ 1 _ FAULT INFERRED courtesy of the Depottl1'l@;nt of __ FOFUrolATION BOUNON!Y Tertiary Syst4!m Lands and Surveys . Hobart, and thern CLAYS AND SANDS Australian Army Survey Se:rvice Triassic System ___ fORMATION BOUNDARY - POSITIOH APPROXIt.lATE Onqln of co -ardmotes -400,000 yds Doleute Boundar les WI!St and 1,100,000 yds South ofI(IIIOCKLOFTY fORMATION True Origm of Zone 7 of the DISCORDANT INTRUSIVE IOUNDAAIES Permian System International GridC!IJ FEIUfTlt£E MUOIITONEVERTICAL JOIN"TS STRIKE AHO OIP OF .K)INTS ~ RISDON SANDSTONE ~ WOClC*IIIIDCOE GLACIAL fOflhVorCN MAPPED AND COMPILED BY I. ~OOUGALL lIS. SCALEJurassic ? System Jdl :F==~==r=~===r==~.~ooo~'===============;'O~OOO~:M::TI .GEOLOGY OF THE PONTVI LLE AREA SHEET 5174 PHYSIOGRAPHY Rounded but steep-sided hills, usually composed of doierite, rise up to some 800' above the brood open volleys which are generally eroded in the relatively soft Triassic sediments. Faulting has also been a controlling factor, particularly the Bagdad Fault, which the Bagdad Rivulet follows for some distance. Commonly dolerite and basalt oct as local bose levels for the streams because at their resistance to erosion. Above these bars the drainage system has sediments as bed rock and wide volleys have been eroded in which the gradient of the streams is low. Within the bosalt and dolerite 1he rivers have developed gorges. The surface of the basalt, except where the streams breach it, is markedly planar. STRUCTURAL GEOLOGY The Bagdad Fault is the main structural feature of the area and has an estimated throw of about 800', downthrowing to the north-east. The south side of Jew's Hill is also faulted, with a minimum throw of 150'. STRATIGRAPHIC TABLE Age Formation Lithology 	 Thickness (in feet) Quaternary Alluvium and river grovels Tertiary Mangalore Cloy and sand 5+ Brighton Basalt 	 Olivine basalt flow 150 + Plug of olivine basalt Early Tertiary (?) Strong Faulting Jurassic m Tholeiitic dolerite sills and dykes Triassic Knocklofty Sandstone and Shale Sandstones and sholes HXXl + Permian Ferntree Mudstone Mudstone 200 + Risdon Sandstone Sandstone 20 "Woodbridge Glacial Formation" Siltstones and fine sandstones 250 IGNEOUS ROCKS Jurassic m dolerite was intruded as rather irregular sheet-like and dyke-like bodies. Almost invariably the sedimentary roof rocks of the intrusions have been removed by erosion along with some of the dolerite, sa that the maximum thickness remaining in a sill-like bady is abaut 800 feet. The complex outcrop pattern of dolerite and sediment in the north-eastern port of the area is possibly due to the interfingering of two sheets of dolerite originating from different centres of irruption. The Brighton Basalt of Tertiary age has flowed down the pre-existing Jordan volley, flooding bock up t~e valleys of the Bagdad and Strathallern Rivulets. Only one flow is present which in the sauth exceeds ISO feet in thickness. The basalt is a massive to vesicular and amygdaloidal, olivine-bearing type normatively saturated in silica. Depeneing on the pOSition within the flow the texture varies from porphyritic with a glossy groundmass to typically ophitic. Polygonal joint columns are usually developed and in some cases groups of such columns present most extra­ordinary forms, varying from fan-shaped groups to "synclines" up to 120 yards across. A plug of olivine bosalt, probably also of Tertiary age, occurs north-west of Maiden Early. POINTS OF SPECIAL INTEREST "Syncline" of basalt columns (5115E 74ooNl. Fan shaped group of basalt columns (S135E 7'\38NI. Section through basalt flow showing the strong textural variation (5142E 7438N). Intrusive contact of dolerite with sandstone (S139E 7438N). Triassic sandstones showing current bedding (5187E 7439N) and slump structures 	 (5133E 7446N). REFERENCES Edwards, A. B, 1950-The petrology of the Cainozoic basaltic rocks of Tasmania. Pap. Proc. Roy. Soc. Vic., vol. 62 (New Series), pp.97-120. Lewis, A. N, 1946-Geology of the Hobart District. Mercury' Press, Hobart. McDougall, I., 1959-The Geology. of the Pontville-Dromedary Area, Tasmania. Pap. Proc. Roy. Soc. Ta5m., Vol. 93. McDougall, I., 1959-The Brighton Basalts, Tasmania. Pap. Proc. Roy. Soc. Tasm., Vol. 93. Nye, P. B., 1922-The underground water resaurces of the Jericho-Richmond-Bridgewater Area. Tas. Dept. Mines Underground Water Supply Paper, No.2. 

The geology of the Pontville-Dromedary area, Tasmania

Abstract

Similar works

Full text

Available Versions

University of Tasmania Open Access Repository