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Cainozoic Volcanism in and Around Great Lake, Central Tasmania

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Cainozoic Volcanism in and Around Great Lake, Central Tasmania PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 104 [Manuscript received 30 October 1969 CAINOZOIC VOLCANISM IN AND AROUND GREAT LAKE, CENTRAL TASMANIA By F. L. SUTHERLAND (Tasmanian Museum, Hobart) and G. E. A. HALE (Hydro-Electric Commission, Hobart) (With five text figures and four plates) ABSTRACT INTRODUCTION Upper Cainozoic basaltic volcanism about Great Drought conditions in Central Tasmanian catch­ Lake involved the eruption of a succession of ments during 1967-68 lowered the level of Great mineralised entrail breccias, 215+ feet (65 m), Lake from above 3,378 feet to a low at 3,346 feet aquagene tUffs and agglomerates, 40+ feet (12 m), on 29 March 1968 (fig. 1), almost returning the unmineralised entrail breccias, 160 + feet (48 m), lake to its original size and configuration prior to and massive flows and dykes, individually up to damming in 1923 (3,333 feet; Legge, 1904; Lewis, 200+ feet (60 m) thick with sequences up to four 1933). Large peripheral areas of the lake bottom flows and 300+ feet (90m) thick. Associated with became exposed, providing some excellent, cleanly the volcanics are some lacustrine and fluviatile sedi­ washed outcrops of basaltic rocks. A study of ments, up to 88+ feet (27 m) thick. these combined with detailed mapping of basalt outcrops inland, has deciphered much of the The aquagene pyroclastics and entrail breccias volcanic history of the region, and is reported in are confined within the present Great Lake depres­ this paper. sion, and closely resemble hyaloclastites and bedded breccias in the upper parts of Icelandic intraglacial pillow lava piles. They probably represent Level emergent elongate fissure volcanoes that erupted 40 I0' into past high water levels in Great Lake. The massive sub-aerial lavas erupted from centres both within and outlying the Great Lake depression; those within probably erupted during low or drained water levels. Over twenty eruptive centres can be inferred on structural and petrological grounds and most are aligned along intersecting NW, NNW, N, NNE and ENE lineaments. There is some evidence of late or post-volcanic local tilting and jointing and 1968 of recent adjustment movements on lineaments. Low Level 3346' The bulk of the volcanic rock.<=; are tholeiitic Area Exposed --··J!:10~'?/ olivine-basalt, but some tholeiite and alkali olivine­ 1968 basalt occurs amongst the massive lavas. The Great Lake volcanic association is a typical example FIG. l.-Great Lake locality and levels. of the tholeiitic associations of Tasmania and falls within a general belt of such rocks extending from Following initial examinations of the new far NW Tasmania to the Derwent Valley. The exposures by G. E. A. Hale in February 1968, Great Lake rocks resemble to some extent basalts detailed geological mapping, sampling and micro­ of the Hawaiian province, and the known strati­ scopic petrographic studies were commenced by graphy suggests a somewhat similar pattern of F. L. Sutherland in March and extended inland by magmatic evolution and eruption. 1969 to cover all the basalt outcrops in the Great RS.-3 17 ..... 0:> Nasa --:-4 1 1[;.... 2i 0 N ....0 0 <l 0 I:"' 0 :>- ....2i [f1 is: H 2i :>- 2i t:! :>- :;d 0 q 2i t:! :;d "l'j :>- t'l I:"' :>- ~ }"l 0 l'j 2i t'l :;d :>- t" t'l :>- [f1 is: :>- 2i.... :>- UPPER CAINOZOiC MESOZOIC ---- UnminerQLised EntrQ[L t-1.cssive Olivine 8Qsolts Bosett Breccia JlIrassrc Dolerit.e III II 1/ II III Massive Olivine ThoLeiite UDII Aquogene Basalt Tuffs (with cover) QUATERNARY li&:'~ Massive Tholeiite and Ag.9lomerates Pe.rmo - Triass lC Alluvial, Lunette Gnd Glacial Hinerolised Entrai L Sub-Basaltic Sediments ~ strota Deposits 8asalt Breccia - 20 CAINOZOIC VOLCANISM IN AND AROUND GREAT LAKE, CENTRAL TASMANIA L1-L4 Liawencc Flows UPPER CAINOZOlC 51-53 Skittle boll Plains Flow t-::>:\:':--.I ALLuvium & GLaciaL Deposits M1-M2 Murderers Hill FLows £....-.:.::.:] Basqlt Tatus ReynoLds Island Flow Canal Bay Flow I«~~;~;'/';-:;::l Massive Alkali Olivine-Basalt lods Corner Flow Mdssive ThoLeiitic Olivine-Basalt --:::- ReynoLds Island Dyke5 ~ Ma~sive Tholeiite FLow Top in Mussive Flows Dip of Basalt Breccia 1:··..;·:1 Baked Sub-Basaltic Sediments ED Striated Dolerite Pavement UnrnjneraLiscd Entreil Basalt Breccias Aq~Clgene BasaLt Tuffs & AggLomerates Mineralised Entrail Basalt Breccias MAJOR EROSiONAL BREAK Jurassic DoLerite (with cover) Permo -Triasslc strata FIG. 3.-Detailed geology, volcanic rocks, Great Lake area. F. L. SUTHERLAND AND G. E. A. HALE 21 2 feet 3 inches, light yellow brown, very fine The agglomerate contains scoriaceous basalt in sandstone, with dips at 35°, and limonitic angular to sub-rounded fragments, up to 5 em and staining near base, on dolerite bedrock. in rarer rounded bombs up to 10 em across, embedded and grading into a lapilli tuff matrix, Mineralised Entrail Breccias weathering reddish yellow. It includes sporadic This series outcrops from Liawenee Canal in the angular to sub-rounded pieces of Jurassic dolerite north, W of Ouse River to Murderers Hill in the and massive basalt, with some larger basalt bombs, south (fig. 3). It is at least 215 feet (65 m) up to 45 em across; similar pieces occasionally thick, marginally overlaps Jurassic dolerite and occur in the other finer beds. extends to unknown depth below Ouse River. The beds are gently folded and warped, and Exposures are poor except in Liawenee Canal and 0 0 g~r;~~.~y strike between 15 .. 150 with dips mostly in cliffs on Ouse River. b , but steeper. are cut Successions of entrail lavas, largely brecciated, steep and embedded in a fragmentary rubbly and tachy- main at 125 0 -130° and HUc matrix are mineralised with amygda- and small dislocations show throws of several cm loidal fillings and cement. Their (plate 2, 2). The prllnt.hlP vent was ap,pa,rently structure is to unmineralised located the NW cnd O~e~~:b~~l~~ entraE breccias to whose internal forms probably also erupted the 0 are unobscured mineralisation and and massive lavas. are described in The secondary minerals (Sutherland, 1965) show a general para- Howells Island genetic inferred from numerous Fine and coarse vitric tuffs, at least 7 feet (2 m) ~.lJ;~,I~~ll,r~:~~~l",.:sequences, in depositional of form small isolated around Howells massive), phillipsite (rare), chaba- from 3,380 to 3,350 resemble the zite Table 1), an association of tach- J:-1,l=YJ:lOlds Island the coarser aranite, and nontronite, followed tuffs and by apophyllite and further calcite. interbed with entrail breccias S'N of to 42° in the breccias suggest a series H()w(~lls Island. and show a strike of 115°_ of di,;se!ctc,d cones, mostly between 1 to 2 miles 120 0 and dips' to 12' Slump and intra- 0.5-4.5 km) across, erupted from about six centres stratal structures are common in aligned NNE which coalesced to form some which have a glassy baked elongate fissure (figs 3 and 4). They are conchoidal fracture (plate a~~"::~>~~i~r~t;h~~~e~l~Oldest extrusions in the area, being north, the beds underlie the d overlain by massive lavas at inverse grading and scour Liawenee overlapped by unmineralised breccias and are more strongly con- E the Ouse River, but their precise age is torted dislocated (plate 2, fig. 4). one unknown. place overturned beds dip SUb-vertically east, with Aql.lagene Tuffs and Agglomerates cross-faults throwing up to 'I feet (2 m), and slide structure 2, fig. 5). Bedded pyroclastics occur around Reynolds indicate and dis- disconformably underlying unmineralised location of unconsolidated beds by emplacement breccias and massive lavas, around Howells of entrall breccias. The general dips suggest an interbedded with and disconformably under­ eruptive source to the south, probably the same lying breccias, and between Maclanachans centre that erupted the entrail breccias. Point and Tods Corner, disconformably underlying massive lava (fig. 3, plate 2). Their base is Maclanachans Point nowhere exposed. They are almost wholly basaltic, Bedded tuffs and agglomerates, weathering with sporadic pieces of country rocks, and show yellowish or reddish ochre in colour, disconform­ structures consistent with shallow water deposition. ably underlie massive basalt, inland from 3,455 Vitric tuffs predominate and in thin sections show feet to below 3,345 feet on Great Lake shore palagonitised basalt glass (Appendix 1) suggesting (plate 2, fig. 6) with a maximum exposed height a sub-aqueous eruptive hyaloclastite origin, but the of almost 25 feet (8 m). The stratigraphy is com­ agglomerates may include some sub-aerial ejecta. plicated by folding and lateral variations, but a Reynolds Island measured sequence from top downwards gave:- Outcrops from 3,385 to 3,345 feet indicate a total 10 (+) feet (3 m)-agglomerate pyroclastic thickness greater than 40 feet (12 m). 10 feet (3 m)-tuffs and agglomerate, Exposures are poor in the NW, but good outcrops grading from up to 3 feet agglomerate at up to 'I feet (2 m) thick lie under basalt cappings base into lapilli tuffs interbedded with and in the SW (plate 2, fig. 1). Here, alternations of dark to light grey, finer compact and coarser passing into fine grained vitric tuffs at top. friable vitrie tuffs, weathering yellow, contain rarer 3 (+) feet (l m) -fine to medium grained lapilli tuffs and agglomerate in the top 3 feet vitric tuffs. (plate 2, fig. 2). Bedding is commonly irregular The vitric tuffs form compact, conchoidally and wavy, and ranges from laminae about 0.5 mm brittle, dark grey to chocolate red, fine grained, thick in the fine tuffs to 30 cm thick for the irregular laminae, 0.5 mm to 1 em thick, alternat­ agglomerate. Small-scale current ripple and ing with coarser, more friable beds up to 10 em tabular cross-bedding is common in some of the thick.
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