South Sister St. Marys, Tasmania

South Sister


A Division of
Department of Infrastructure, Energy and Resource

07 January, 2005

Dear Mr. Clement,

Thank you for your E-mail of 24 December 2004, requesting available geological and hydrological information related to the current water supply debate about St. Marys town water supply and groundwater regime around South Sister area.


Folded quartzwacke of the Mathinna Beds (slate and sandstone) intruded by the Devonian Granitic intrusions and overlain in places by Devonian pyroclastic rocks form the basement upon which sub horizontal Permian marine sandstone, limestone and mudstone were deposited. These are conformably overlain by middle Triassic quartz sandstone followed by Upper Triassic coal measures, composed of lithic sandstone with interbedded siltstone, mudstone and coal. Basalt of Triassic age is locally present at the base of the Upper Triassic sequence. The Permo-Triassic contact generally lies slightly above the level of the floor of the Fingal Valley, and coal measures form the bulk of the surrounding highlands (Mount Nicholas range) which are capped by Jurassic dolerite. In the area east of the St. Marys towards Mount Elephant contact between Permo-Triassic rocks is slightly higher in the landscape above the valley floor. The lower contact of the dolerite is generally conformable with the intruded sediments but locally complex relationships may be developed. Thick Pleistocene dolerite talus deposits are extensively developed on the slopes of the Nicholas Range, Fingal Tier and Mt. Elephant.

Details of the local geology are shown on the relevant Tasmania Department of Mines Geological Atlas 1:50000 map (St Marys) and described in the accompanying explanatory notes. Further information on geology, groundwater and land stability for this area may be available in some of Mineral Resources Tasmania's (MRT's) unpublished reports. On-line searching, viewing and ordering of MRT's reports (map not available on the web) is available by visiting our Web-Site (

The following reports available from our web site may give you additional information on the geology and land stability of the area around South Sister:

  1. UR1980_13 Baillie, P. W., Calver, C. R. 'Geology of the Mt. Nicholas, Fingal Valley and Mt. St. John Areas.
  2. UR1976_21 Cromer, W. C., Sloane, D. J. Investigation of Proposed Forestry Commission Roads near St. Marys.
  3. UR1976_40 Sloane, D. J. Further Investigation of Proposed Roads near St. Marys.
  4. UR1976_47 Sloane, D. J. Further Inspection of Road Routes near St. Marys.


As you are aware from your previous telephone contacts with our hydrogeologists Mineral Resources Tasmania has not conducted detailed hydrogeological investigations in the area of your interest that would enable us to give you a definite and accurate answer to your specific questions.

Currently we have records for 20 groundwater boreholes in our database that were mostly drilled for the Break O'Day Council by private drilling contractors and consultants (only 2 holes were drilled by our drilling section). Information provided to us by drillers contains basic borehole construction details, borehole logs and estimated borehole yield and in few cases water quality information upon borehole completion.

The location of these boreholes is shown on the attached map and borehole information can be accessed on our web site.

A recent survey of groundwater resources in the northern Midlands and Fingal Valley areas was undertaken as part of the Tasmanian Regional Drought Initiative. The report Groundwater Resources of the Northern Midlands and Fingal Valley Regions by Kris Taylor (Tasmanian Geological survey Record 2000/04) provides an overview of the groundwater resources, including likely yields and chemistry of water derived from local aquifers, and regional groundwater maps to aid in assessing of the potential development of resources. The available results of the previous groundwater drilling in the area by private contractors were incorporated in the study. A detailed investigation of the extent and recharge mechanism of the aquifers in the investigated area was not carried out at the time as it was not the main aim of the study.

Based on our geological knowledge and available groundwater information it appears that the main aquifers from which the town water supply is obtained are Permian and Middle Triassic fracture aquifers located at the valley floor. Permian marine sandstone, limestone and mudstone are conformably overlain by middle Triassic quartz sandstone and their combined thickness at St. Marys is about 200 m. Depths of water bores are within a range from 5 to 55 m with water struck in the range from 4-46 m. Reported estimated yield for 11 holes were variable with majority of the hole in a range from 0.19 to 1.51 litres per second (l/s), one hole yielded 3.5 l/s and only three holes exceeded yield of 10 l/s (10-12.63 l/s). The high yielding holes include town water supply holes from which one borehole has encountered water in cavities developed in the Permian limestone. These aquifers are overlain by the shallow unconfined alluvial gravel aquifer formed in the Quaternary alluvial sediments of the Break O'Day River. The recharge area for these aquifers probably lies between the St Marys and Mt. Elephant, St. Patricks Head and Mount Nicholas range with South Sister being a minor part of the recharge area.

Upper Triassic (Coal Measures) sedimentary rocks form the bulk of the surrounding highlands and overlie lower Permo-Triassic aquifers used for town water supply. Their thickness in the South Sister area is about 250 m. There is no available groundwater borehole information for this area. In the other parts of the state under the same climatic conditions upper Triassic rocks (Coal measures) are usually less prospective for ground water than quartz sandstone rich middle and lower Triassic rocks and usually contain water of worse quality. In this area this unit from a regional prospective represents a confining layer for the lower Permo-Triassic aquifers. Usually the Coal measures contain local aquifers that in the situation like on the slopes of the South Sister support a majority of the permanent springs and streams. The groundwater prospectivity of the overlying Dolerite (up to 100 m thick) is generally low but it probably contains localised fracture aquifers that are dependent on the level of fracturing and recharge from rainfall. These local aquifers in the South Sister area probably play an important role in the maintenance of the spring and stream flow in the area. They are overlain by thick talus sediments that generally have low groundwater yields and storage capacity from which on the steep slopes water is usually discharged during the wet season, but during the summer, spring flow lessens or ceases.

The Mathinna Beds (slate and sandstone) in this area have been affected by the granite. A couple of dry holes (15 and 45 m deep) were drilled in this area (one yielded a small unreported quantity of the water). In the Scottsdale area there are some suggestions that water quality may be poorer in sequence altered by metamorphism near the granite. Mathinna beds together with granitic rocks underlie the Permo-Triassic aquifers used for the town water supply.

The location of Gould Cornwall fault is mainly based on surface geological mapping and as far as we are aware there is no available information on its hydrogeological properties (transmissivity and hydraulic conductivity) and ability to transfer water between different hydrogeological units in the area.

In summary, based on the available geological and limited groundwater information the available quality of the groundwater for the town water supply is regarded as having low risk of being affected by the proposed forestry activity in the vicinity of the South Sister. This conclusion is also based on the hypothesis that South Sister area is only a minor part of the greater catchment's recharge area for Middle Triassic and upper Permian aquifers that provide water for the town water supply.

However it is possible that local aquifers in the South Sister area which supply water to local springs and streams may be affected by the proposed forestry operations if not properly monitored and managed. Over period of the time landslides may develop, surface water turbidity and spring flow variation may occur in the absence of active management of the coupe.

Monitoring of groundwater water level, water quality and water usage (surface and groundwater) in both areas (Town water supply and Forestry coupe) followed with regular reporting is regarded as a desired course of action in order to achieve better understanding of hydrogeological conditions in the area. Only then can an informed assessment of the risk to the water resources caused by all water users in the area be made.

Our 80 m deep regional monitoring borehole installed about 4 km west of St. Marys has encountered water bearing zones in Permian sedimentary rocks at depths of 12 and 22 m. Seasonal water level variation in this bore were in a range of 7.8-9.0 m from the surface in the period from 1990-2004. The output was recorded at 0.64 l/s and chemical analysis since installation have shown total dissolved content ranging from 778 to 1540 mg/l with a general trend of improving quality.

Please note that the water bore information used in the text is provided for indication only. It is derived from historical records provided mostly by drillers and should be interpreted with this in mind. Well yields indicated are generally derived from short-term well yield tests and may not be representative of the long term sustainable yield of aquifers. You should also note that the data might not accurately reflect present conditions.

Yours sincerely

Miladin Latinovic

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