Technical Tour

Mornington Peninsula Technical Tour
Sunday 15 July 2012
8.30am – 7.00pm
Cost: AUD $200 per person

The geological structure of the Mornington Peninsular is typical of horst and graben landforms; an uplifted block (Mornington Peninsular) bound by a depressed graben to the west (Port Phillip Bay) and lesser depression to the east (Western Port Bay). This structure is most evident along the coast with the coastal cliffs and steep slopes comprising the edges of the uplifted block. The geomorphic development of the region is due to the erosion and weathering of this uplifted block compounded by Pleistocene aged dune formation and coastal erosion. The tour covers a number of interesting sites along the Mornington Peninsula that demonstrate both the highly variable geological conditions and issues associated with ongoing urbanisation, potential sea level rise and resulting instability that could affect existing and future developments.

Site 1 – Fossil Beach, Mornington

This site features a number of geological units of Tertiary age including:

   -   Weathered columnar basalt of the Older Volcanics -;
   -   Baxter Sandstone;
   -   Marina Cove Sand;
   -   Mt Martha Sand Beds; and
   -   Balcombe Clay (Fyansford Formation).

At the southern end of Fossil Beach, the weathered basalt is exposed with siderite veins and is overlain by up to 4.5m of non-marine cross-bedded fine sandy gravels and quartz grits, often with logs of wood and carbonaceous silts. (Visibility of these units is tide dependent)

Overlying these sediments are sands and coarse sands of marine and fluviatile origin that are early stages of the marine transgression leading to the overlying deep water marine Balcombe Clay phase.

The northern end of Fossil beach is the best known area for viewing the Balcombe Clay, which is here poorly sorted grey high plasticity calcareous silt. Up to one quarter of the clay may consist of calcium carbonate, but in places, where exposed on the cliffs, some of this mineral may be leached out. Where weathered, the clay is in a reduced state and pyrite is present in aggregates and infilling worm tubes and other fossils which are abundant and well preserved (and unfortunately, not available for collection).

There is slumping present in several areas along the beach demonstrating the effects of weather and coastal actions on these materials.

Fissures, jointing and other relict structure are also clearly visible in this unit. This site demonstrates potential issues associated with slope instability both within the Balcombe Clay and generally within the region.

Site 2 – Diamond Bay

A series of Pleistocene and Recent calcareous sand dunes are exposed in the cliffs. The lower dunes are calcified into aeolianite. Strong cross-bedding is evident in these exposures.

Numerous palaeosols are also present, some up to 1m thick.

Nine Pleistocene soils are represented, eight being terra rossas and one a redzina. (Sorry, no vineyard at this particular site).

Blowouts are present in the unconsolidated upper parts of the cliffs and various sections of the lower cliffs have failed due to wave action and undercutting.

This is a particularly interesting site geologically and well demonstrates the issues associated with sea level rise.

Site 3 – Cape Schanck Coastal Park

The cliffs in this area consist of basalt in the lower part capped by cross-bedded aeolianite. The basalt at the base of Cape Schanck is distinctly columnar and narrow, rough surfaced shore platforms have formed. The basalt section of the cliff consists of a number of flows in which the joints are predominantly vertical.

There are various sections to the basalt visible including aragonite veins, spheroidal weathering, buckshot gravels, and a basaltic dyke.

Angel Cave was formed by the removal of a lower strength band between two lava flows.

Catering

Lunch will take place at the historic Rye Hotel with a sit down meal being provided together with drinks.

On our return to Melbourne, we will be stopping at Darling Wines to taste some of their wines together with platters of cheese, biscuits and some local produce.

Bottled water and snack packs will be provided on the bus.