Chapter 2.2 Geodiversity and Geological Resources


Geodiversity is defined in the Australian Natural Heritage Charter as meaning the natural range (diversity) of geological (bedrock), geomorphological (landform) and soil features, assemblages, systems and processes. Geodiversity includes evidence of the past life, ecosystems and environments in the history of the earth as well as a range of atmospheric, hydrological and biological processes currently acting on rocks, landforms and soils.

Geology

The rock underlying the whole Merri catchment is yellow to brown marine siltstones and sandstones from the Silurian period. These are often heavily folded and faulted. The early Silurian siltstones and sandstones are called the Andersons Creek Formation and the late Silurian siltstones and sandstones are called the Dargile Formation.

During the Tertiary period, volcanoes deposited basalts on top of the Silurian rock (the Older Basalts). Sands were deposited over the top of the Silurian rock when the area was submerged again under the sea during the Tertiary period. These sands have been eroded away by the creeks in the last few million years, leaving deposits of Tertiary sands (the Brighton Group sands) on the ridges. Since that time the spread of lava southwards from twelve separate eruption points in the area between Wallan and Craigieburn formed the basalt plains characteristic of the catchment. These basalts are called the Newer Basalts. The youngest of these lava flows was from either Mount Fraser or Hayes Hill some 800,000 years ago[84]. These flows followed and filled former valleys of the Merri and Darebin Creeks extending south to the valley of the Yarra River and then westward along the Yarra valley to the vicinity of Spencer Street in Melbourne.

With the lava surface sloping down towards the east and south from these eruption points, the course of the ancestral Merri Creek came to be determined by natural depressions and ridges in the lava surface and by the major fractures and joints in the volcanic rock.

In places the lava flows blocked the ancestral Merri Creek or its tributaries, creating lakes or swamps that filled with sediment forming flat plains that are characteristic of the area immediately to the south of Wallan. Other such ancient swamps are scattered along the Creek.

Today parts of the beds of the Creek and its tributaries have cut through the basalt to the underlying Silurian rock. An example of the Silurian rock outcropping is in the cliffs of the Edgars Creek valley in the vicinity of Tilley Street Coburg North.

Significance

The assigning or reviewing of geological significance is undertaken in Victoria by the Heritage subcommittee of the Geological Society of Australia Inc (Victoria Division)[85]. The GSA is a volunteer society of qualified geologists. The GSA has developed a methodology and protocol for assigning or reviewing geological significance which has been accepted as reliable and repeatable by organizations such as the Australian Heritage Commission (now Australian Heritage Council).

It is important to note that geological, including geomorphological, significance may not necessarily relate to the aesthetics of a landscape. Some sites of very high significance may not be at all aesthetic, e.g. quarry faces or road cuttings whereas aesthetically pleasing views may not always be assigned a high geological significance.

Merri Catchment Geological Sites

A study by Rosengren (1993a) identified 38 sites of geological or geomorphological significance on Merri Creek between the Yarra River and the Wallan-Woodstock Road. A detailed listing of the sites is available on the MCMC website.

Unfortunately the Merri Creeks natural geomorphology has been extensively disturbed since European settlement and many opportunities to protect and preserve the natural interactive processes between the stream and its floodplain have been lost. Rosengren detailed the nature of human activities over the past 150 years which have significantly altered Merri Creek. Extensive channel modification by Councils and then the Melbourne Metropolitan Board of Works (MMBW) over many decades was principally responsible for the changes to the Creek from Campbellfield to the Yarra River up to the mid 1980s.

Since Rosengrens 1993 study a number of the sites he identified have been destroyed by channel modification and other works[86].

Mitchell et al (2000) documented known geological sites in the Melbourne 1:250 000 Map sheet area.

According to the Geological Society of Australia (Victorian Branch), geological significance assessments older than 10 years need to be re-assessed (i.e. the assessments by Rosengren which have not been revised in Mitchell et. al.).

No sites of National significance are known from the Merri Catchment.

Four sites of State significance are known, Barry Road Gorge, OHerns Road Merri Creek Cliff, Mount Fraser, and Dights Falls.

Nineteen sites of Regional significance, and 21 sites of local significance are known and shown on the map.

Criteria for significance of Geological Sites

Geological sites should possess at least one of the following attributes to be considered for assessment on their significance:

  • a type section of a geological unit,
  • a fossil locality,
  • exposures of a range of features characteristic of the rock unit, or exposures of features which are unusual in the rock unit,
  • an unusual occurrence of a particular feature or mineral,
  • an illustration of tectonic and/or volcanic processes,
  • features which enable palaeoclimatic reconstruction,
  • demonstration of the effects of weathering, erosion and/or deposition on landform evolution. This geomorphic process may be active or relict,
  • a representative example of a landform type.

The criteria for significance is related to whether a site can be regarded as important with regards to it being representative or outstanding. A choice often has to be made between the most outstanding or unusual example and an excellent representative example from a group of very similar ones.

The representative approach has been found to be the most appropriate in assessing significance but outstanding examples must be considered. Criteria used in such assessment include:

  • How representative is the feature?
  • How adequately is each type of feature represented over a particular scale?
  • Which feature is the most appropriate to represent a particular type?
  • How many representatives are justified?
  • How far would you travel with an interstate or international visitor with an interest in that type of site?
  • How does it compare with other similar sites - regionally, in the state, in Australia, Internationally?
  • Is it under threat and if so, what is the nature of that threat?
  • How common or rare is the feature?
  • Is it a particularly good example?
  • Is it a type section or type example (for landforms).

Other aspects of the site such as present and past land use, diversity of features present, access, and vulnerability to damage are also considered. Features or areas are also described according to size, physical and/or geological type and age.

The level of geological significance is classified at local, regional, state, national or international level by documentation, assessment and comparison. The significance rating assigned to a site is periodically reassessed in the light of new information and/or site condition.

The criteria for classification are:

  • International Significance: These sites are landforms, structures, rock formations or fossils which are rare in the world, and/or by the nature of their scale, state of preservation or display, are comparable with examples known internationally. They may be global type examples and are widely known as reference sites by the international geological community. A site could be included in an international register of sites of scientific significance and would rate listing on the Register of the National Estate by the Australian Heritage Commission (now Council). Forty-five features of international significance have been recognised, documented and assessed in Victoria. Tower Hill is an example of an internationally significant site due to the well-preserved evidence of phreatic volcanic processes.
  • National Significance: Sites that are rare in Australia or are important nationally by virtue of their scale or state of preservation are assigned national significance. Widely used as reference sites by the Australian geological community, they should be included in a national register of sites of scientific significance and would be considered for listing on the Register of the National Estate by the Australian Heritage Commission (now Council). Seventy sites of national significance have been identified in Victoria.
  • State Significance: These sites are important in defining the geology and geomorphology of Victoria and may be reference sites or type examples. There are over 200 sites of state significance identified and documented in Victoria.
  • Regional Significance: These sites include landforms or geological features representative of regions of about 60km radius..
  • Local Significance: These are features representative of smaller areas in a region. Such sites are usually related to an area of a local municipality or an area with a radius of 20km.
  • Unknown Significance: Sites are assigned this rating if there is insufficient data to allow a complete assessment to be made. Typically these sites are either under investigation or subject to continual change e.g. active quarry faces.

Sites which are of value for teaching geology may or may not count as sites of significance.

Criteria for the significance of Geological sites
From the Heritage subcommittee of the Geological Society of Australia Inc
Victoria Division (White 2007)

Significant geological sites in the Merri Creek catchment

Map 12 - Geology and geological sites of the Merri catchment

Site Protection

Little statutory protection is available for sites of geological significance.

The Merri Creek ESO includes an objective to protect natural landforms and geological features in Moreland, Darebin, Yarra, Whittlesea, and Hume. There is no similar objective In the Mitchell Shires Watercourse Protection ESO. The ESO in Whittlesea and Hume does not cover all the sites associated with the Merri Creek as identified by Rosengren. Where the ESO does cover sites, it has not always been effective in protecting them. For example the Silurian quarry face at Capp Reserve, West Preston, was identified as regionally significant by Rosengren but was buried as part of the development of an adjacent industrial site for housing.

Use of the Heritage Overlay to specifically identify geological sites and their values could be investigated.

Another option for site protection is inclusion in conservation reserves. Dights Falls is in Yarra Bend Park, and the proposed Merri Creek Park would include the Barry Road Gorge and OHerns Road Cliff sites. This leaves the only other site of state significance, Mount Fraser, as unreserved.

The significance of some sites depends on their visibility. Management of the geological site may therefore conflict with revegetation and screening objectives. Protection of the values of geological sites needs to be taken into account in site design.

Mineral and Stone Resources

The Merri Creek Catchment has been quarried for sand and gravels but is best known for providing bluestone for road metal, building and paving and clays for brick making.

Quarry Work Authorities

There are eleven current quarry work authorities in the catchment:

· four are for quarrying basalt (bluestone)

· one is for scoria,

· five are for clay or clay/shale

· one is sand/gravel and

· one is sedimentary.

Boral Resources has lodged an application for a basalt quarry east of Bald Hill (see Map 13 below). This application has potential major impacts on the Bald Hill Grasslands, and on groundwater in the area.

Mineral Quarrying and Geothermal licences in the Merri Catchment

Map 13 - Mineral, quarrying and geothermal licences in the catchment.

Extractive Industry Interest Areas

One Extractive Industry Interest Area (EIIA) is located in the mid-upper catchment (see Map 13 below). The purpose of EIIAs[87] is to

· Provide a basis for the long term protection of stone resources from sterilisation by inappropriate land uses

· Provide a basis for ensuring the long term availability of stone resources for use by the community at a minimal detriment to the environment,

· Assist in considering extractive industry values in long term strategic planning as well as local strategy plans,

· Ensure that planning or responsible authorities consult with all relevant agencies about land use proposals which may impact on the reduction of stone resources within these areas, and

· Create awareness that extractive industry is a potential land use in these areas.

EIIAs do not:

· Provide statutory protection for sand and stone resources

· Allow extractive industry as-of-right unless specified by planning schemes,

· Imply that future extractive industry will be confined to these areas, or

· Preclude the use and development of the land for other purposes.

Exploration Licences

There is one current exploration licence which covers part of Merri Creeks headwaters around Pretty Sally for Gold/Silver/Platinum and Antimony. An application for a licence has been lodged covering much of the western slopes of the upper catchment for Gold/Silver/Platinum, Antimony, and base Metals (copper, lead, and zinc)

Geothermal Tenements

The entire catchment is covered by three Geothermal Tenements which give different companies the right to develop proposals for geothermal energy projects within the tenement.

Key References

Mitchell, M.M., Cochrane, R.M. & King, R.L. (2000) Sites of Geological Significance in the Melbourne 1:250 000 map sheet area. Geological Survey of Victoria Technical Record 2000/1.

Rosengren, N. (1993a). The Merri Creek: Sites of Geological and Geomorphological Significance, prepared for Merri Creek Management Committee, Melbourne.

Rosengren, N. (1993b). Soils Study, prepared for Melbourne Water and Merri Creek Management Committee, Melbourne.

White S. (2007) GSA Protocol for assigning or reviewing geological significance. Unpublished report.

Issues

1. Sites of geological or geomorphological significance have no formal protection.

2. Sites have been destroyed in recent times with Council and Melbourne Water approval, and continue to be damaged (e.g. Mt Fraser by quarrying).

3. At some sites revegetation and geological site protection objectives may conflict.

4. Much of the catchment has not been surveyed for significant sites.

5. Significance ratings older than 10 years are not reliable

6. Heritage overlay listing may be appropriate for geological sites.

7. The ESO does not protect all identified sites along Merri Creek let alone its tributaries.

8. There are many gaps in the themes illustrated by known geological sites; e.g. gilgai soil formations.

Objectives

1. Sites of geological or geomorphological significance are identified, protected and used for interpretation of the catchments geological history.

Targets

2. Surveys of geological sites on tributaries of Merri Creek and the headwaters undertaken by 2010

3. Conservation and significance of sites in the catchment reassessed by 2010

4. Sites protected in planning schemes with appropriate controls by 2010

Actions

See Section E page 189.



[84] Rosengren (1993a) p10

[85] White (2007)

[86] E.g. Rosengren (1993a) site 14 Edgars Creek Terrace & Meanders was destroyed by channel stabilization works, and site 9 Capp Reserve Preston Melbourne Formation was destroyed by filling.

[87] Melbourne Supply Area Extractive Industry Interest Areas Review