Chris Sharples B.Sc. (Hons), M.Sc. Specialising in geoconservation and earth Geologist science aspects of environmental management ABN: 68 034 834 434 Description and Geoconservation Assessment of a Glacio-fluvial Terrace at the Southwood Development Site, and possible equivalent terraces in the Huon River Valley, Southern Tasmania Report to John Holland Pty Ltd by Chris Sharples November 2002 GPO Box 1941, Hobart, Tasmania, 7001, Australia Phone: (03) 6239 6669 Email: [email protected] Website: sharples.com.au Front Cover Photo: The flat main terrace surface at the Southwood site, looking north from site 6 towards site 13 (at the far end of the visible road section). This part of the Southwood project site is mantled by an unlithified granular quartz sand facies, and is interpreted to be a mostly intact remnant fragment of an original glacial outwash plain surface, that formerly filled to Huon Valley to the same level, preserved much as it was at the time sedimentation ceased. ii CONTENTS Page no. SUMMARY 1 1.0 INTRODUCTION 5 1.1 Purpose of this work 5 1.2 Work undertaken 6 1.3 Limitations and caveats 6 1.4 Conventions used 6 2.0 GLACIAL HISTORY OF THE MIDDLE TO LOWER HUON VALLEY 7 2.1 Introduction 7 2.2 Glaciation and glacio-fluvial processes in the middle to lower Huon Valley 7 3.0 GLACIO-FLUVIAL LANDFORMS AND SEDIMENTS AT THE SOUTHWOOD DEVELOPMENT SITE 13 3.1 Site geology and topography 13 3.2 Quaternary stratigraphy and sedimentology of the Southwood site 16 1. Basal alluvium with boulders 16 2. Silty-clay pebble/cobble alluvium 18 3. Sandy pebble/cobble alluvium 20 4. Granular quartz sand facies 22 Lower level alluvium 24 Quartz pebble lag and colluvium 25 Interpretation: nature, origin and age of unlithified sediments at the Southwood site 28 Origin and depositional environment of the unlithified Southwood sediments 28 Sediment composition and implications 30 Age of the Southwood sediments 32 3.3 Glacio-fluvial terrace landform morphology and preservation at the Southwood site 34 3.4 Conclusions: Interpretation of the Quaternary geomorphic development and history of the Southwood site 40 iii 4.0 POTENTIAL GLACIO-FLUVIAL TERRACES EQUIVALENT TO THE SOUTHWOOD TERRACE ELSEWHERE IN THE MIDDLE – LOWER HUON VALLEY 43 4.1 Introduction – the purpose of this section 43 4.2 Prediction of equivalent glacio-fluvial terraces elsewhere in the Huon Valley 44 Search criteria 49 4.3 Assessment of potential equivalent glacio-fluvial terraces elsewhere in the Huon Valley 59 Judbury 59 Huon / Weld confluence 63 Other sites inspected 68 Priorities for further inspections 69 5.0 GEOCONSERVATION SIGNIFICANCE ASSESSMENT OF THE SOUTHWOOD DEVELOPMENT SITE 71 6.0 RECOMMENDATIONS 74 REFERENCES 76 APPENDIX 1: CONSULTANTS BRIEF 79 APPENDIX 2: SITE DATA 81 APPENDIX 3: SAMPLE CATALOGUE 83 iv Summary SUMMARY Background and Aims The Southwood Project construction site is situated on the broad flat top of a spur on the north side of the Huon River, immediately opposite the Arve River confluence, in southern Tasmania. The Huon Valley and surrounding mountains are known to have been glaciated on at least two (and probably more) occasions during the Quaternary Period (Late Cainozoic), and the spur crest is mantled by unlithified Quaternary sediments whose flat upper surface is interpreted as a remnant fragment of a former valley-filling glacio-fluvial outwash plain surface deposited during an older and more extensive glaciation than the last. Because evidence for glaciations preceding the last is limited in the Huon Valley, and no other well-preserved high-level depositional outwash terrace surfaces relating to pre- Last Glacial phases have previously been documented in that region, the terrace at the Southwood site has been considered to have geoconservation significance for the scientific information it contains about an early phase of southern Tasmania's Cainozoic glacial history, and for its intrinsic value as a remnant landform (not just a sediment deposit) produced by that glacial phase. The excavations, earthworks and constructions associated with the Southwood project will disturb a large proportion of the terrace surface and its contents. This is likely to result in the loss of at least some of the scientific information contained in the terrace landform, and hence, the loss of some of its geoconservation significance. The purposes of the investigation described in this report were to: • document the geomorphic and sedimentary characteristics of the Southwood glacio-fluvial terrace site prior to substantial disturbance and to assess the geoconservation value of the feature; and: • to assess the likelihood of other equivalent features (i.e., other outwash terrace remnants of the same age and likely to provide equivalent scientific evidence) being present in an undisturbed state elsewhere in the Huon Valley, and to assess their likely geoconservation significance in relation to the Southwood site terrace. Description of the Southwood site glacio-fluvial terrace Previous geological investigations (including drilling) at the Southwood site were reviewed, and 3 days were spent in the field documenting the site and collecting samples. A variable thickness (5 to over 12 metres) of unlithified Quaternary-age glacio-fluvial outwash sediments mantle the upper surface of the Southwood Spur, approximately 70 metres above the present Huon River channel, and rest on Permian siltstone/sandstone bedrock whose upper surface slopes gently southwards towards the Huon River. The bedrock is itself deeply weathered to a soft clay regolith for a depth of 14 to 20 metres beneath the Quaternary sediments. Three main distinctive sedimentary facies are recognisable in the glacio-fluvial sediment sequence, characterised by differing clast and matrix compositions and textures, and separated by scoured surfaces representing episodes of erosion (glacio-fluvial "degradation") during the glacio-fluvial deposition process. The sediments contain 1 Glacio-fluvial Terraces at the Southwood Site and elsewhere in the Huon Valley predominantly Precambrian quartzite and Permian mudstone, siltstone and sandstone clasts. Characteristics of the sediments including their lack of any dolerite clasts and their position high in the landscape relative to other glacio-fluvial sediments, are interpreted as implying that the preserved sediments were deposited late in the glacial episode when the glacier had retreated significantly back up the Huon and Manuka Creek valley's from its maximum extent, and when the outwash plain had aggraded (accumulated) to nearly its maximum thickness. Changes in composition and texture upwards through the three main sedimentary facies may represent distinctive phases in the waning of the glacier and glacial outwash system, or may be partly related to more localised changes in the braided outwash channel system. All mudstone and siltstone cobbles in the glacio-fluvial sediments are thoroughly weathered to soft clay which, together with the deep weathering of the underlying bedrock and a number of other criteria, are interpreted as implying an age of deposition significantly greater than the Last Glaciation (circa 19,000 years BP), potentially as old as the most extensive known Cainozoic glaciation in Tasmania (>730,000 years BP, possibly circa 1,000,000 years BP), but probably no older than a few million years. Sediment samples have been obtained from the Southwood site, but no absolute age determinations on these have been attempted yet due to lack of funds. The highest (northern-most) portions of the glacio-fluvial sediments at the Southwood site were partly incised by fluvial erosion (degradation) prior to the final phase of glacio-fluvial deposition at the site. The youngest (final) preserved sediments were deposited on a slightly lower, flat terrace lying between 100 – 105m ASL, and the available evidence suggests that a substantial portion of this terrace surface has not been further incised or eroded subsequent to the end of the phase of glacio-fluvial sedimentation. Thus, this part of the Southwood terrace surface appears to represent a preserved fragment of the original glacio-fluvial outwash plain depositional surface as it was when the glaciation ceased. Search for equivalent terraces elsewhere in the Huon Valley In order to identify target areas to search for equivalent high-level terrace surfaces elsewhere in the Huon Valley, two predictive models of the possible form and gradient of the original outwash plain surface were considered. Based on these models, GIS software was used to analyse the 25 metre Digital Elevation Model of Tasmania in order to identify flat surfaces in the Huon Valley at altitudes where the predictive modelling suggested that remnant outwash terrace fragments equivalent to that at the Southwood site were most likely to be found. Two days were spent visiting potential equivalent terrace sites identified by this method. Several of the visited sites proved to be flat surfaces of other unrelated types, however two sites (one near Judbury and one north of the Huon – Weld Rivers confluence) exhibit remnant alluvial sediments that are potentially equivalent to those at the Southwood terrace site (although no absolute or relative age tests have yet been attempted). However, these sites preserve the alluvial sediments only as residual lags, not as un-modified sedimentary deposits with original surfaces intact, and it is evident that the original glacio-fluvial deposits and terrace surfaces are somewhat degraded and not preserved intact