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Field Excursion Guide Notes for the Roger, Sumac and Dempster Digital 1:25 000 Scale Geological Map Sheets, Northwestern Tasmania

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Field Excursion Guide Notes for the Roger, Sumac and Dempster Digital 1:25 000 Scale Geological Map Sheets, Northwestern Tasmania DEPARTMENT of INFRASTRUCTURE, ENERGY and RESOURCES MINERAL RESOURCES TASMANIA Tasmania Tasmanian Geological Survey Record 2000/02 FieldField excursion excursion guide guide notes notes for for the the Roger,Roger, Sumac Sumac and and Dempster Dempster digital digital 1:251:25 000 000 scale scale geological geological map map sheets,sheets, northwestern northwestern Tasmania Tasmania by J. L. Everard, A. R. Reed, M. P. McClenaghan and D. B. Seymour Tasmanian Geological Survey Tasmania Record 2000/02 Field excursion guide notes for the Roger, Sumac and Dempster digital 1:25 000 scale geological map sheets, northwestern Tasmania by J. L. Everard, A. R. Reed, M. P. McClenaghan and D. B. Seymour The Roger, Sumac and Dempster 1:25 000 scale map recorded locally is about 22°. The contact is also sheets cover an area between Balfour and Trowutta in strongly erosional at other localities around the margin northwestern Tasmania (fig. 1). This report is a brief of the Smithton Synclinorium, but is commonly field guide to some of the more important exposures in concordant or with a maximum discordance of less this area. than 10°. Regional geological mapping since the early 1980s has confirmed that the basal units of the Togari The regional geological setting of the area is given in Group rest on different stratigraphic levels of the Figure 1, with a composite stratigraphic column given Rocky Cape Group basement at various points along in Figure 2. the contact. 1: Black River The unconformity may represent a period of gentle regional folding of the Rocky Cape Group prior to Unconformity between Rocky Cape Group and Togari Group deposition. If so, this early Togari Group (357 240 mE, 5 476 740 mN) compressional deformation may be a distal effect of This introductory location is at the northeastern the major tectonothermal event at c. 760 Ma which margin of the Smithton Synclinorium, outside the affected ?Mesoproterozoic rocks on King Island Roger–Sumac–Dempster map sheets. This location (Wickham Orogeny, Turner et al., 1998). However, if contains the best-known exposure of the unconformity normal faulting accompanied Neoproterozoic between the basal unit (Forest Conglomerate and extension and the onset of Togari Group Quartzite) of the late Neoproterozoic Togari Group sedimentation, then deposition into a graben was and the underlying Cowrie Siltstone of the likely to be accompanied by erosion of topographically ?Mesoproterozoic Rocky Cape Group (Plate 1). upstanding blocks of Rocky Cape Group rocks. The Forest Conglomerate and Quartzite is not always present, but here it comprises a siliceous boulder conglomerate containing commonly angular clasts of laminated or massive quartzite up to 0.5 m in diameter, fining upwards into a sequence of quartzarenite with common cross-bedding and minor conglomerate. Considerable erosion is evident at the contact, but the maximum discordance Plate 1 Unconformity between the Rocky Cape Group and Togari Group, Black River Tasmanian Geological Survey Record 2000/02 1 300 000 mE 400 000 mE 5 500 000 mN BASS Montagu Smithton Rocky Cape STRAIT SMITHTON Marrawah SYNCLINORIUM Wynyard Fault Trowutta River Burnie Roger Housetop Granite Lineament Balfour 6 Kara 4 6 4 4 2 2 Cleveland Sandy Cape Waratah Savage 5 400 000 mN 2 River SOUTHERN Meredith Interview Granite Granite Corinna Arthur Renison Bell OCEAN Heemskirk Granite Zeehan 0 20 40 km LATE CARBONIFEROUS – RECENT NEOPROTEROZOIC Clastic-carbonate shelf, and basalt/volcaniclastic rift Cover sequences sequences. Basalts indicated. (Includes Togari Group, Ahrberg Group, Crimson Creek Formation, Success Creek Group). LATE DEVONIAN – EARLY CARBONIFEROUS Quartzwacke turbidite sequences (Oonah and Burnie Formations). Granitoids ?MESOPROTEROZOIC – ?NEOPROTEROZOIC (Protolith age) MIDDLE CAMBRIAN – EARLY DEVONIAN Arthur Metamorphic Complex Sedimentary and volcanic rocks ?MESOPROTEROZOIC ?EARLY CAMBRIAN Sandstone-siltstone shelf sequences (Rocky Cape Group ?Allochthonous units: ultramafic rocks; basalts; & correlates) clastic sedimentary rocks NEOPROTEROZOIC – CAMBRIAN Faults: undifferentiated; known or inferred thrusts, teeth on upper plate; wrench, relative offset indicated Undifferentiated rock assemblages 2 Contours of depth (in km) to granite around Interview Granite and its northward continuation (from gravity interpretation) Figure 1 Regional setting of the Roger–Sumac–Dempster 1:25 000 scale geological map sheets. Geology adapted from Seymour and Calver (1995), with modifications from Leaman and Richardson (1992), Everard et al. (1996) and McClenaghan and Seymour (1996). Tasmanian Geological Survey Record 2000/02 2 Composite stratigraphy, Roger–Sumac–Dempster 1:25 000 scale map sheets Units discussed in this report indicated (J ) Qha Y Alluvium and swamp deposits (Qha). HOLOCENE QUATERNAR Qlc Lag of dominantly oolitic and banded chert, with associated silt and sand (Qlc) Erosional surface CAINOZOIC Tb Basalt (Tb) Y TERTIAR Tsgs Interbedded siliceous gravel, quartz sand and clay (Tsgs) Angular unconformity Reddish-brown weathering, interbedded laminated siltstone, lithic wacke and Óm conglomerate (Ó m) [Scopus Formation] CAMBRIAN LATE MIDDLE - EARLY LATE OZOIC PALAE- Inferred disconformity or low-angle unconformity CAMBRIAN Pale weathering, commonly silicified, thin bedded, laminated quartz siltstone with Õsr subordinate interbedded fissile shale (Õ sr) [Salmon River Siltstone] ?EARLY CAMBRIAN Õsdc Õsd Shallow marine dolomite and minor limestone (Õ sd). Areas of silicification and/or clayey pug indicated (ÕÕÕ sdc). [ sd, sdc: Smithton Dolomite] Interbedded laminated mudstone, siltstone, and lithic wacke with mafic volcanic Õkw Õki detritus (ÕÕ kw). Impure limestone indicated ( kl). Hematitic ironstone indicated ( Õ ki). Õkl Õkb Massive basalt (Õ kb) [Spinks Creek Volcanics] Õki Dominantly mixtite (with clasts of basaltic and felsic volcanic rocks, dolomite, chert and mudstone-siltstone in a fine-grained non-dolomitic matrix), with interbedded T OGARI G ROUP Õkx K ANUNNAH S UBGROUP laminated mudstone, siltstone and calcareous siltstone (Õ kx) [Croles Hill Mixtite] Õbr NEOPROTEROZOIC Dolomitic breccia, with clasts of dolomite, stromatolitic dolomite and oolitic chert in a Õbr dolomitic matrix (Õ br) [Julius River Member] Õbd Interbedded dolomite, chert, siltstone and mudstone (Õ bd). Interbedded, massive or Õbdc banded, black, white and grey chert (oolitic in part) and laminated siltstone, with D OLOMITE minor dolomite (Õ bdc) B LACK R IVER Õfcb Chert breccia & conglomerate (with clasts dominantly of black, grey and white chert, and subordinate orthoquartzite) interbedded with pale grey-weathering distinctly laminated medium-grained orthoquartzite (Õ fcb) [Correlate of Forest Conglomerate Erosional and transgressive surface; low angle unconformity at some localities. PROTEROZOIC Õrcs Interbedded, black, dark grey and green, commonly pyritic, laminated siltstone Õrc and mudstone, with rare sandstone and mud pellet conglomerate (Õ rc). Silicified equivalent of unitÕÕ rc ( rcs). [ ÕÕ rc, rcs: Cowrie Siltstone]. Laminated to thinly bedded, chloritic to siliceous siltstone to fine sandstone, containing variably disseminated porphyroblastic chlorite (Õ rbs). Micaceous Õrbsa Õrbsq Õrbs fine-grained sandstone with subordinate interbedded siltstone and graphitic shale, and containing porphyroblastic chlorite (Õ rbsa). Pale-weathering, thickly Õrbgs Laminated chloritic mudstone to siltstone, containing variably disseminated porphyroblastic chlorite (Õ rbgs). Õrbss Interbedded packages of quartzose sandstone to siliceous siltstone and laminated carbonaceous shale (Õ rbss). R OCKY C APE G ROUP B ALFOUR S UBGROUP ?MESOPROTEROZOIC Laminated to gutter-cast and trough cross-bedded, siliceous to carbonaceous Õrbls slltstone (Õ rbls). Õrlq Medium grained, trough cross-bedded to parallel-bedded quartzose sandstone, and rare angular quartz-pebble conglomerate and shale (Õ rlq). IGNEOUS INTRUSIVE ROCKS Õdd Õdf Dolerite dykes (ÕÕ dd). Foliated variety indicated ( df). Õgb Medium to coarse-grained or pegmatitic dolerite (Õ gb). NEOPROTEROZOIC Figure 2 Tasmanian Geological Survey Record 2000/02 3 Juxtaposition of Togari Group sedimentary rocks with quarry at 319 120 mE, 5 453 220 mN near the eastern different levels in the Rocky Cape Group stratigraphy margin of the Bluff 1:25 000 scale map sheet (west of the might therefore be a consequence of coincident erosion Sumac sheet), where recovered fossils included two of the Rocky Cape Group during Togari Group species of inarticulate brachiopods, a few cruciform deposition. Although the angular unconformity stauract sponge spicules, and elliptical blobs ~20 mm between the Rocky Cape and Togari Groups might be in diameter which may be sponge bodies (J. B. Jago, attributed to compressional deformation, it might also University of South Australia, pers. comm., 1999). be the product of block rotation or ‘roll-over’ of hanging-wall strata above non-planar extensional Away from the quarries and roadcuts, the formation faults. The irregular distribution of the basal has been mapped based on the presence of a pale grey conglomerate, and the predominance of quartzite sandy soil with abundant small angular chips of pale pebbles contained within this unit, are consistent with grey to white silicified siltstone. This soil and float type basin compartmentalisation by normal faulting early contrasts with the rich red-brown soil and during Togari Group deposition. siltstone-sandstone fragments found in the areas of the overlying Scopus Formation. 2:
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