Tamar report land Capability Survey of Tasmania k e noble 1992 Tamar Report

35

Photo 2:

Landscape showing alluvial basin (foreground), with Triassic sandstone (lower slopes), capped

with dolerite above.  Tamar map 952175*.  Bridgenorth Road.

Photo 3:

Landscape showing mosaic of soils formed from basalt (dark colours) and Tertiary sediments

(light colours).  Tamar map 586325.  Valleyfield Road.

* Grid references based on the 1:100 000 Tasmap Series

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Areas of dolerite talus occur around the margins of these dolerite bodies, and often overlie

the older basement rocks through which the dolerite has intruded (e.g. slopes of Christmas

Hill, Stephens Hill, Black Sugarloaf, Notley Hills).  The orientation of the dolerite ridges

follow a north west - south east trend, which has been determined by the faulting pattern

within the Graben.  Other areas of dolerite occur at: Rubicon Hills, Wurra Wurra Hills,

Brushy Rivulet, Dalgarth Hill, Drys Sugarloaf, The Tump, Grassy Hut Tier, The Long Hill,

West Head, Sidmouth, Blackwall, Point Sorell and east of George Town.

Tertiary age deposits of sands, clays and gravels are also extensive throughout the mapped

area.  Localities are along the Tamar River, Selbourne - Westwood area, around Parkham

and West Frankford, and in the East Sassafras - Port Sorell area.  These deposits are the

result of Tertiary sedimentation into the Tamar and Port Sorell troughs, and associated

depressions.

There are also areas of basalt occurring throughout the map which have been extruded as

volcanic eruptions in Tertiary times.  In some areas (e.g. Moltema and Dunorlan), the basalt

flows completely overlie and protect the associated underlying Tertiary sediments as

plateau cappings, while in other areas (e.g. East Sassafras), the situation is more complex

with at least two basalt flows interlayered between the Tertiary sediments (refer to Figure 7

and Photo 3.)  In some areas these sediments were partly eroded before further eruptions

took place.  The landscape, soil and land capability patterns form a dissected mosaic,

complicated by the sporadicity of basalt outcrops and subsequent erosion.

It is probable that the basalt flows in the East Sassafras area are a continuation of the

Thirlstane and Moriarty Basalts, as identified further west in the Devonport area, and the

Tertiary sediments between them correlate to the Wesley Vale Sand (refer to Figure 7).

The Thirlstane Basalt overlies the Harford Beds.  These basalt flows are mainly valley fills

from a number of small eruptive centres along the floors of Pre-Tertiary river systems.

Other areas of basalt occur in the Selbourne - Westwood area, and either side of the Tamar

River near George Town.  Basanitic dolerite (a coarse grained basalt) occurs in the Rowella

- Hillwood - Windermere areas.

Some of the basalt flows have influenced or changed the direction of flow of some rivers,

in particular the Tamar River.

The youngest deposits consist of Quaternary age alluvium, swamp deposits, and windblown

sands.  The alluvial deposits occur along river flats and terraces, and range from confined

river valleys to broad depositional infill basins.  The major areas of alluvium are found in

the Exeter Basin, Meander River flats, Reedy Marsh, around Elizabeth Town and The

Avenue Plains.

Windblown sands occur predominantly along the coastal areas.  Quaternary talus deposits

occur around the margins of some of the basalt and dolerite bodies.

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Figure 7:

Cross section showing the complex landscape, geological and associated soil patterns in

Tertiary sediments and interbedded basalt flows.

References for Further Reading:

Australian Bureau of Statistics, 1988, Tasmanian Year Book, No. 21:  Pages 30-32.

Banks, M. R., 1965, Geology and Mineral Deposits, in Atlas of Tasmania. Lands and

Surveys Department, Hobart.

Blake, F., 1961, Landslips at Beauty Point.  Tasmania Department of Mines, Technical

Report 5: 194-196.

Burns, K.L., 1964, Geological Survey Explanatory Report to accompany Geological Atlas

1 Mile Series, Sheet No. 29, Devonport.  Tasmania Department of Mines, Hobart.

Carey, S.W., 1947,  Geology of the Launceston District, Tasmania.  Records of the Queen

Victoria Museum 11.1.

Cromer, W.C. & Sloane, D.J., 1976, Geology and Hydrology of the Tertiary and

Quaternary sediments near Greens Beach, Northern Tasmania.  Unpub. Rep. Dep. Mines

Tasmania, 1976/24.

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38

Cromer, W.C., in press, Geology and Groundwater Resources of the Devonport - Port

Sorell - Sassafras Tertiary Basin. Geol. Survey Bull. Division of Mines & Mineral

Resources, Tasmania.

Davies, J.L., 1961, Tasmanian Beach Ridge Systems in Relation to Sea Level change.  Pap.

and Proc. Royal Society of Tasmania, Vol 95: 35-41.

Davies, J.L., 1965, Landforms, in Atlas of Tasmania.  Lands and Surveys Department,

Hobart.

Department of Resources and Energy, 1970-1992, Various reports on slope stability,

landslip hazard site inspections, Tamar Valley area.  Div. Mines and Min. Resources,

Unpublished Reports.

Edwards, A.B., 1941, The North-West Coast of Tasmania.  Proc. Royal Society Victoria,

53, Pt II:  233-267.

Gee, R.D. & Legge, P.J., 1971, Geological Atlas 1 Mile Series, Sheet No. 30, Beaconsfield.

Tasmania Department of Mines, Hobart.

Gee, R.D. & Legge, P.J., 1979, Geological Survey Explanatory Report to accompany

Geological Atlas 1 Mile Series, Sheet No. 30, Beaconsfield. Tasmania Department of

Mines, Hobart.

Green, D.H., 1959, Geology of the Beaconsfield District, including the Anderson's Creek

Ultrabasic Complex.  Records of the Queen Victoria Museum, Launceston, New Series No.

10.

Gulline, A.B., 1981, Geological Survey Explanatory Report to accompany Geological Atlas

1 Mile Series, Sheet No. 38, Frankford.  Tasmania Department of Mines, Hobart.

Gulline, A.B., Bravo, A.P. & Naqvi., 1973, Geological Atlas 1 Mile Series, Sheet No. 38,

Frankford.  Tasmania Department of Mines, Hobart.

Hughes, T.D., 1954, Geological Report on Country east of Port Sorell.  Unpub. Rep. Dep.

Mines Tasmania, 1954: 12-18.

Hughes, T.D., 1957, Limestones in Tasmania.  Geological Survey Mineral Resources No.

10.  Tasmania Department of Mines, Hobart.

Jennings, I., 1963, Slope stability at Beauty Point.  Tasmania Department of Mines

Technical Report No. 8.

Kershaw, R.C., 1955, Geological Observations on the West Tamar.  The Victorian

Naturalist, Vol 71: 138-144, 153-156, 175-179.

Kershaw, R.C., 1958, Further Observations on the Geology of the Tamar River.  The

Victorian Naturalist, Vol 74: 179-188.

Knights, C.J. & Matthews, W.L., 1976,  A Landslip Study in Tertiary Sediments, Northern

Tasmania.  Bulletin of the International Association of Engineering Geology No. 14.

Nicolls, K.D., 1960, Erosion Surfaces, River Terraces, and River Capture in the Launceston

Tertiary Basin.  Pap. and Proc. Royal Society of Tasmania, Vol 94.

Noakes, L.C., Burton, G.M. & Randal, M.A., 1954, The Flowery Gully Limestone Deposit,

Tasmania. Rec. Bur. Min. Resour. Geol. Geophys. Aust. 1954/55.

Nye, P.B., 1934, Physiography of Tasmania.  Typed Report, Geological Survey, Tasmania.

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