Mineral Resources Tasmania Tasmanian Geological Survey Tasmania DEPARTMENT of INFRASTRUCTURE, Record 2003/08 ENERGY and RESOURCES Geology of the Mount Koonya area by S. M. Forsyth CONTENTS SUMMARY ……………………………………………………………………………………… 3 INTRODUCTION ……………………………………………………………………………… 5 Acknowledgements ………………………………………………………………………… 5 GEOLOGY ……………………………………………………………………………………… 6 Introduction ………………………………………………………………………………… 6 Previous geological maps and investigations …………………………………………………… 6 Stratigraphy ………………………………………………………………………………… 7 Lower Parmeener Supergroup ……………………………………………………………… 7 Upper Parmeener Supergroup ……………………………………………………………… 7 Cygnet Coal Measures correlate — Permian? ………………………………………………… 7 Dominantly quartz sandstone sequence (Rqph) — Early Triassic ………………………………… 8 Interbedded siltstone, fine-grained sandstone and mudstone sequence (Rqm) — Early Triassic ………… 8 Quartz sandstone unit with granules (Rvvp) — Middle? Triassic………………………………… 9 Undifferentiated quartz rich lithic sandstone, quartz sandstone and mudstone (Rvv)— Middle Triassic … 10 Quaternary deposits ……………………………………………………………………… 10 Slope deposits …………………………………………………………………………… 10 Other Quaternary deposits ………………………………………………………………… 11 Igneous rocks ………………………………………………………………………………… 11 Jurassic dolerite …………………………………………………………………………… 11 Metamorphic effects of the dolerite …………………………………………………………… 12 Structure …………………………………………………………………………………… 13 Attitude of Upper Parmeener Supergroup …………………………………………………… 13 Dolerite structure ………………………………………………………………………… 13 Faults …………………………………………………………………………………… 20 Joints ……………………………………………………………………………………… 20 Measured joints and fractures ……………………………………………………………… 20 Photo Lineaments ………………………………………………………………………… 21 Comparison of structural trends …………………………………………………………… 21 COMMENTS ON HYDROGEOLOGY …………………………………………………………… 22 Recharge …………………………………………………………………………………… 23 Comments related to previous aquifer concepts ………………………………………………… 25 Generalised concept of the dolerite fracture aquifer …………………………………………… 25 Comments on the Koonya B spring system …………………………………………………… 25 Flow direction ……………………………………………………………………………… 26 Flow direction near the Koonya divide in the vicinity of the Koonya B spring system ………………… 28 CONCLUSIONS ………………………………………………………………………………… 29 REFERENCES …………………………………………………………………………………… 30 APPENDIX 1: Some properties of dolerite and dolerite talus aquifers from various sources ……………… 31 Tasmanian Geological Survey Record 2003/08 1 Figures 1. Geology of the Koonya area …………………………………………………………………… 32 2. North-south geological section, AMG grid line 567 140 mE ……………………………………… 16 3. East-west geological section, AMG grid line 5 227 870 mE ………………………………………… 16 4. Geological section, Mount Koonya through Kingstons Pinnacle ………………………………… 17 5. Geological section, Plummers Creek to Heywoods Road ………………………………………… 17 6. East-west geological section, Grooms Hill–Kingstons Pinnacle–Newmans Creek ………………… 18 7. Location of section lines in Figures 2 to 6 ……………………………………………………… 19 8. Poles to measured joints in dolerite …………………………………………………………… 20 9. Frequency plot of the horizontal azimuths of joint surfaces in dolerite ……………………………… 21 10. Frequency plot of photo interpreted photo lineaments in dolerite terrain …………………………… 21 11. Frequency plot of the trends of various mapped and photo interpreted features …………………… 22 12. Geological cross section through the Koonya B spring…………………………………………… 25 While every care has been taken in the preparation of this report, no warranty is given as to the correctness of the information and no liability is accepted for any statement or opinion or for any error or omission. No reader should act or fail to act on the basis of any material contained herein. Readers should consult professional advisers. As a result the Crown in Right of the State of Tasmania and its employees, contractors and agents expressly disclaim all and any liability (including all liability from or attributable to any negligent or wrongful act or omission) to any persons whatsoever in respect of anything done or omitted to be done by any such person in reliance whether in whole or in part upon any of the material in this report. Tasmanian Geological Survey Record 2003/08 2 Summary Several residents in the catchment of Cascade Rivulet, two sheets may intersect. In the valley of a tributary or near Koonya on the Tasman Peninsula, utilise spring branch of Newman Creek, Upper Parmeener water that is derived from areas of State Forest at Supergroup rocks are topographically higher than the Mount Koonya and Mount Clark. Two geological dolerite which intrudes them. This may be due to an reports (Sloane, 1987; Weldon, 1991) have dealt with upward transgression of the lower sheet or a the potential effects of forestry operations on the downward transgression of the Mount Koonya sheet. springs. These investigations related the common This ambiguity imposes limitations on the occurrence of seepages near the lower boundary of a interpretation of dolerite structure in this area and sill or sheet of Jurassic dolerite where it intrudes a most of the study area further south. succession of Triassic sandstone and mudstone. Some The distribution of the marker sandstone unit (Rvvp) seepages were noted to disappear downslope beneath indicates that equivalent stratigraphic horizons and dolerite talus deposits and reappear at lower the base of the Mount Koonya dolerite sheet are both elevations. considerably lower in the east compared to the west. The current work has enabled a stratigraphic This fall in elevation is equivalent to an average subdivision of the Triassic rocks that has assisted in the easterly dip of 5°, but is probably a result of both structural interpretation of the area. The oldest Upper faulting and variable dip on different faulted blocks. Parmeener Supergroup rocks are of Early Triassic age The position of possible faults can be inferred, but the and consist predominantly of cross-bedded quartz faults have not been accurately located. Although the sandstone (Rqph) and subordinate beds and thin base of the Mount Koonya sheet has been mapped on intervals of siltstone and mudstone, some of which are the escarpment slopes, the possibility of faults, steep mappable (Rqpc) at Norfolk Bay. The sandstone is well transgressive segments and intersection of separate exposed at coastal sections and extends inland to form sheets limits the accuracy of predictions of the sheet the foothills of the Mount Koonya area. base altitude beneath the dolerite plateau area. The overlying unit (Rqm) consists predominantly of The Koonya B spring system and similar springs on the siltstone and mudstone with less and finer grained western side of Mount Koonya, between the fire tower quartz sandstone compared to unit Rqph. The strata site and Kingstons Pinnacle, are probably fed by a include a thin bed of characteristic silicified and combination of surface or near-surface water flow and strongly bioturbated fine-grained sandstone. fractured aquifer groundwater derived from local Correlation with similar Early Triassic units north of infiltration zones largely west, but in some cases Hobart is indicated. Contrasting with these slightly east, of the surface water divide between predominantly fine-grained rocks is a unit of Cascades Rivulet and Newman Creek. Individual predominantly medium-grained to coarse-grained fracture zones, such as faults, could convey water to quartz sandstone (Rvvp) that usually contains quartz seepages on the western or northern side of Mount granules and, in some places, quartz pebbles. This unit Koonya, possibly from infiltration areas east of the is probably little more than three metres thick and has divide. proved to be a useful marker horizon that has been Based on the current limited information, the location observed discontinuously to wrap around Mount of the springs, the concept of topographically driven Koonya from the headwaters of Cascades Rivulet into flow cells, consideration of symmetry and potential the catchment of Newman Creek. The marker horizon preferred pathways probably provide the best means is well exposed on the foreshore near Premaydena of crudely estimating the flow paths and the location of where its relationship to the next two overlying units recharge areas for groups of springs within flow cells. can be observed, but at Mount Koonya all succeeding The recharge areas for individual springs are more strata up to the dolerite base have been grouped difficult to define. together as one undifferentiated unit (Rvv). This unit consists of interbedded siltstone, mudstone and In the vicinity of the Koonya divide the piezometric quartz-rich lithic sandstone with minor quartz surface crest is probably east of the divide when the sandstone near the base and common grey shale about forest water yield is symmetric about the surface 60 m above the base. A Middle Triassic age for the unit divide. Forestry operations have the potential to (Rvv) can be inferred based on correlation with rocks displace the piezometric crest and the boundaries near Oatlands. between the inferred flow cells and hence alter the areas of recharge for individual springs. The Jurassic dolerite sheet that caps Mount Koonya intrudes transgressively within the unit Rvv. North of Forestry operations within a recharge area may both the study area, dolerite (probably part of a different increase and decrease water yield (precipitation less sheet) extends inland from Shelly Beach and intrudes water loss by evapotranspiration) and spring base the older