Tasmanian Geological Survey Record 2007/02 SSoommee nnootteess oonn tthhee ggeeoollooggyy ooff KKiinngg IIssllaanndd Department of In frastructure, Energy and Resources Mineral Resources Tasmania Tasmanian Geological Survey Record 2007/02 Some notes on the geology of King Island by C . R. Calver Department of Infrastructure, EnergTasmaniany and R Geologicalesourc eSurveys Record 2007/02 1 Mineral Resources Tasmania Mineral Resources Tasmania PO Box 56 Rosny Park Tasmania 7018 Phone: (03) 6233 8377 l Fax: (03) 6233 8338 Email: [email protected] l Internet: www.mrt.tas.gov.au 2 Tasmanian Geological Survey Record 2007/02 Contents Summary ………………………………………………………………………………… 3 Western King Island ……………………………………………………………………… 5 Surprise Bay Formation ………………………………………………………………… 5 Regional deformation and metamorphism………………………………………………… 5 Granitoid intrusion and local deformation………………………………………………… 6 Eastern King Island ……………………………………………………………………… 6 Naracoopa Formation …………………………………………………………………… 6 Grassy Group…………………………………………………………………………… 6 Base of the Ediacaran ……………………………………………………………………… 9 Devonian granites and contact metamorphism and metasomatism …………………………… 9 Scheelite deposits ………………………………………………………………………… 9 Tertiary–Quaternary ……………………………………………………………………… 12 Detailed location descriptions ……………………………………………………………… 13 Western King Island …………………………………………………………………… 13 Cape Wickham………………………………………………………………………… 13 Ettrick Beach ………………………………………………………………………… 14 Seal Rocks State Reserve ……………………………………………………………… 14 City of Melbourne Bay area ……………………………………………………………… 15 South of City of Melbourne Bay ………………………………………………………… 15 North of City of Melbourne Bay ………………………………………………………… 18 Acknowledgements ……………………………………………………………………… 19 References………………………………………………………………………………… 19 Figures 1. Geological map of King Island …………………………………………………………… 4 2. Detrital zircon age profiles: Jacob Quartzite, Detention Subgroup, Naracoopa Formation and Surprise Bay Formation ………………………………………………………………… 5 3. Time-space diagram for Proterozoic of King Island and other Tasmanian terranes …………… 8 4. Stratigraphic succession, No. 1 Open Cut and Dolphin mine ……………………………… 10 5. Aerial view of Grassy open cut showing extent of planned open cut and proposed sea wall … 11 6. Geological maps of southeast coast, King Island (four parts) ……………………………… 16 Summary King Island consists mainly of Proterozoic rocks and some Devonian granite, with an extensive inland cover of Pleistocene to Recent windblown sand (fig. 1). The Proterozoic geology of the western half of the island is different to that of the eastern half, and the relationship of the two areas is problematic. The geology is unusual in the Southeast Australian context, and is well-exposed around the coast. The western half of the island consists of Mesoproterozoic (1300 Ma) amphibolite-grade metasediments, regionally deformed and metamorphosed at c. 1290 Ma and intruded by 760 Ma granite. The metasediments strike N–S and the granite is regionally concordant. The eastern half of the island mainly consists of a thick succession of relatively unmetamorphosed siltstone, probably a correlate of the lower Neoproterozoic (c. 1000–750 Ma) Cowrie Siltstone of northwest Tasmania. The contact between the metasediments and the siltstone is concealed by surficial sediments, and its nature (whether a fault or unconformity) is unknown. Along the southeast coast, the siltstone succession is overlain by an upper Cryogenian to Ediacaran, east-dipping succession of diamictite, cap dolostone, shale and mafic volcanic rocks (basalt and picrite). Along the east coast, three small, early Carboniferous granite stocks intrude the Neoproterozoic sedimentary rocks. Scheelite orebodies, which were mined up until 1990, occur in the contact aureoles of the two southern granite stocks. The world-class Grassy scheelite mine is planned to be re-opened in 2008. Most of the interior of the island is covered by Quaternary surficial deposits — mainly windblown sand and stabilised dunes. A number of small Tertiary basalt plugs are known, and Tertiary limestone crops out on the east coast. Cover: Coastal outcrop, Quarantine Bay, west coast of King Island. [Photo: Brett Stewart] Tasmanian Geological Survey Record 2007/02 3 Cape Wickham Disappointment Bay KING ISLAND GEOLOGICAL MAP Phoques Bay Lavinia Point 5600000 mN Whistler Point The Blowhole Naracoopa Fraser Bluff Currie Qh ‘New Dunes’ Qu Qps ‘Old Dunes’ City of Melbourne Bay Tb Tertiary basalt Fitzmaurice Bold Head Tm Tertiary limestone Bay Grassy Cataraqui Point Psv Upper Neoproterozoic basaltic volcanic rocks Upper Neoproterozoic diamictite, Pss dolostone and shale Prp Lower Neoproterozoic siltstone Surprise Bay Gulchway 5550000 mN Ptp Proterozoic metasedimentary rocks Stokes Point E m Dg Devonian granite 0 0 0 0 5 Pg Proterozoic granite 2 5 0 5 10 kilometres Pa Proterozoic amphibolite Figure 1 Geological map of King Island, from current Tasmanian Geological Survey 1:250 000 scale digital geological coverage. 4 Tasmanian Geological Survey Record 2007/02 Western King Island Re gional de for ma tion and meta mor phism, c. 1290 Ma Sur prise Bay For ma tion: Electron microprobe dating of metamorphic monazite Meta sedi ments c. 1300 Ma from metasediments at Fitzmaurice Bay and Surprise The western half of King Island is dominated by Bay (southwest coast of King Island), away from the metasediments (Surprise Bay Formation) and minor granite aureoles, gives ages of around 1270 Ma, and mafic intrusive rock, intruded by c. 760 Ma granitoids this is interpreted as the age of the regional, (Gresham, 1972; Cox, 1973; Blackney, 1982; Cox, 1989; amphibolite-grade metamorphism that affects these Turner et al., 1998; Holm, 2002). The Surprise Bay rocks (Berry et al., 2005). The first major deformation Formation consists of at least 1000 m of dominantly phase (D1) produced large, tight to isoclinal folds, quartzofeldspathic schist with minor quartzite, pelitic associated minor folds, and a penetrative axial surface schist and rare calcareous lenses. Schists typically cleavage defined by muscovite. Prograde consist of quartz + muscovite + biotite (+ plagioclase). metamorphism appears to have commenced during Locally, pelite contains garnet and minor potash D1, and S1 microfabrics are defined by amphibolite feldspar, and andalusite is present in alumina-rich facies mineral assemblages. rocks. Coastal outcrops between Ettrick River and Fitzmaurice Bay preserve graded bedding and appear This early Grenvillian-aged metamorphic event is to be fine-grained, quartz-rich sandy turbidites. The unknown in northwest Tasmania, where the oldest metasediments at Cape Wickham include relatively succession (Rocky Cape Group) is thought to be pure quartzite with rare cross bedding (Cox, 1973). significantly younger (<1000 Ma) (fig. 3). However, a 1300 Ma event is recorded in monazite in the cores of The mafic intrusive rocks are amphibolites and are garnets in the Franklin Metamorphic Complex in chemically similar to tholeiitic basalt. Extrusive rocks were reported at Gulchway by Blackney (1982), but the central Tasmania, and this constitutes the only known 2004 Australian Geological Convention excursion visit occurrence of possible Mesoproterozoic basement on to this locality failed to substantiate an extrusive mainland Tasmania (R. F. Berry, pers. comm., in origin. Seymour et al., 2007). A maximum constraint for the depositional age of the The position of Tasmania in Rodinia reconstructions is Surprise Bay Formation is given by the age of the highly uncertain. The presence of a Grenville-aged youngest detrital zircons (c. 1350 Ma) in a sample of the orogeny suggests it may have been close to the turbidites north of Fitzmaurice Bay (Black et al., 2004). Musgrave Block, or alternatively near the recently The zircons in this sample show a predominance of identified Grenville-aged terrane in the Transantarctic ages at around 1450 Ma and 1600–1850 Ma (fig. 2). Mountains (Berry et al., 2005). Figure 2 Jacob Quartzite (Rocky Cape Group) Detrital zircon age profiles: 1.0 N=76 Jacob Quartzite and Detention 0.5 Subgroup (both of the Rocky Cape Group of northwest 0 Tasmania), and Naracoopa Formation and Surprise Bay Detention Subgroup (Rocky Cape Group) Formation (eastern and western y 4.0 t i l i N=78 King Island respectively). b a 2.0 b o r p 0 e v i Naracoopa Fm. (eastern King Island) t 4.0 a l N = 84 e R 2.0 0 Surprise Bay Fm. (western King Island) N = 64 1.0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 5 0 5 0 5 1 1 2 2 3 3 Age (Ma) Tasmanian Geological Survey Record 2007/02 5 Granitoid in tru sion and lo cal de for ma tion: Eastern King Island Wickham Orog eny, c. 760 Ma Naracoopa Forma tion (c. 1000–750 Ma) Turner et al. (1998), in defining the Wickham Orogeny, followed Cox (1989) in believing that the early regional The eastern half of King Island consists mainly of a phase of metamorphism and deformation was related thick (6–7 km?) succession of relatively to the c. 760 Ma granitoids, and included all these unmetamorphosed shale, siltstone and fine-grained events in the Wickham Orogeny. Berry et al. (2005) muscovitic quartz sandstone, which dips and faces dated the early regional phase at 1270 Ma (see above), east. This unit, the Naracoopa Formation, so the Wickham Orogeny, as presently understood, lithologically resembles the Cowrie Siltstone of only encompasses the granitoid intrusion, and northwest Tasmania, which is part of the Rocky Cape localised deformation (D2–D4) in the aureoles of the Group whose depositional age lies between 1000 Ma