Northern end of Rocky Beach
Port Macquarie's coastline is one of NSW’s most geologically significant regions. Its rocks were formed from the formation, destruction and uplift of an 400 to 600 million year-old geological subduction zone after an oceanic tectonic plate was formed from undersea volcanoes then subducted under the ancient mega-continent of Gondwana. At this site it is possible to see at first hand the results of plate tectonic and metamorphic processes.
This EarthCache features a location which is Stop 5 on the Port Macquarie Coastal Geotrail opened in May 2018. See Geotrail details on the PMH Council website here and download the NSW GeoTours App available here. You can collect a Coastal Geotrail map at the Sea Acres Visitor Centre.
Please respect this site
This area is freely accessible by the public. It is important geologically, even though it is of small extent. Do not remove samples [ref 1]; take only photos and leave nothing behind. If you see plastic rubbish, please do the ocean and its inhabitants a favour and bring it up for disposal.
What you will see
I will take you to this mélange unit and show you two of its many features. On the way down the track you will walk over some exposed and highly weathered serpentenite. This is greenish, slippery and devoid of vegetation.
- The black rock (basalt) platforms you see on the beach formed when a mid oceanic ridge, made up of deep sea volcanoes, created new ocean floor and a new oceanic tectonic plate through continuous eruptions of lava.
- Lighter cream, grey and red rocks with/without banding in them are chert and represent the microskeletons made of silica that have deposited and accumulated on the moving oceanic plate and have been subject to pressure.
- Brown/grey layered sedimentary rock, called turbidites, are rarer here and formed when sediments from nearby continents and volcanic slopes collect in the mouth of the trench.
- Green fibrous serpentinite rock forms as the moving plate enters the subduction zone. Pressure and chemical reactions act on the basalt plate and the earth’s mantle near the top of the subduction zone and form green fibrous rock often with veins.
- The big blue rocks (blueschists) and the pale grey/olive green rocks (eclogite) have formed deep in the earth at the bottom of the subduction zone. Some blueschists include patches of the grey/olive green with small weathered red garnets. This is the retrograde blueschist that was originally formed as eclogite further down the trench. When the pressure was released as the trench was uplifted, some of the eclogite (but not all) turned back (retrograded) into blueschist.
Your tasks
Visit the listed coordinates and one waypoint close by. You can then complete the requirements of this EarthCache by answering the questions below. Everything you need to know you can see at GZ and WP2, or read in the following notes.
Do not put your answers in your log. Log is for general comments only; answers by email or via Message Centre to LHBfans.
Q1: At GZ you will see an example of a lawsonite-eclogite exposed under a small overhang. What are its approximate dimensions? Describe the appearance of this exposure eg colour, intrusions, surface texture.
Q2: The notes define “prograde metamorphism” and “retrograde metamorphism” and also tell you some things about lawsonite-eclogite. Is this lawsonite-eclogite prograde or retrograde?
Q3: At WP Serpentenite face you will see an interesting face of serpentenite intruded by quartz veins. Measure or estimate the angle at which these quartz veins are lying. Describe the appearance of this exposure eg colour, surface texture.
Q4: On the beach there are many different colours,sizes and types of rock - a real mixture. Why do you think this might be so in this particular small area?
Q5: Take a photo of yourself standing on the black basalt rock platform and post it (optional). It was once about 4,000 m below today's sea level.
Q6: Survey question: Did you see any plastic waste on your visit today?
You are permitted to log this cache immediately (with general comments only) if you wish to keep it in your correct sequence (e.g. for personal or Travel Bug statistics). You must follow up as soon as possible with your brief answers to the questions above. Send these to me by email or use the GeoCache messaging system.
Logs not verified by correct answers sent within a few days to LHBfans will be deleted. Logs containing the answers, or specific photos of GZ or WP, will also be deleted.
The site
The site you will be visiting is known as the Rocky Beach Metamorphic Mélange ("mélange" meaning mixture). This small area of only a hectare or so forms part of the Port Macquarie Block. This Block is the south-eastern edge of the vast New England Fold Belt, and north of the Lorne Basin, an ancient and highly faulted region of great geological interest.
Rocky Beach is limited by the Tasman Sea to its east and a steep, densely vegetated slope to its west. Oxley Beach and Flynns Beach are respectively north and south of Rocky Beach, separated from it by rugged headlands.
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The Rocky Beach Metamorphic Mélange unit consists of two lenses separated by a narrow strip of Port Macquarie Serpentinite. [2] |
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The relative accessibility of this site, and other coastal outcrops around Port Macquarie, attracts numerous geological visitors including university groups, conference participants, research scientists and now EarthCachers.
Go to a small track-head leading off the Coastal Walk. The track will take you to a rough and rocky shoreline just north of the points of interest to this EarthCache.
The track is distinct but quite steep and will be slippery after wet weather. The track and beach are relatively rugged so sturdy shoes are recommended and you should take care of your footing. This track is not suitable for children.
Check the tides and try to visit around low tide.
Take your time, take care and enjoy your visit to this EarthCache.
Notes: Background information.
Plate tectonics [W]
The Earth’s outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. There are seven or eight major plates and many minor plates. 
Where plates meet, their relative motion determines the type of boundary: convergent (where the plates collide, forcing up mountain ranges), divergent (where the plates separate, giving rise to ocean trenches), or transform (sideways movement of the plates).
The operation of plate tectonics can give rise to metamorphism because the acts of convergence and divergence generate immense forces of pressure and heat, or bring rocks into intensely hot areas in the earth.
Metamorphism [W]
Metamorphism is the change of minerals or geologic texture in pre-existing rocks without them having been melted into liquid magma. 
Metamorphism occurs primarily due to heat, pressure, and the introduction of chemically active fluids. These forces can change the chemical components and crystal structures of the minerals making up the rock, even though the rock remains a solid.
Geologists distinguish between metamorphism produced with increasing pressure and temperature ("prograde") conditions versus that produced with decreasing pressure and temperature ("retrograde") conditions.
Other extremes occur under under low pressure, high temperature conditions ("low-P and high-T") and also at very high pressure but low temperature ("high-P and low-T") conditions which are often found in subducting slabs.
Mélange [W]
A mélange in geology is a body of rock characterised by a lack of continuity and the inclusion of fragments of rock of many sizes and types, contained in a fine-grained matrix.
Before the understanding of plate tectonics in the early 1970s, it was difficult to explain mélanges by the then-known geological mechanisms. Sometimes low temperature and high pressure metamorphic rocks were found in direct contact with rocks that were formed in high temperature environments or deposited in a sedimentary environment. Plate tectonics proved a mechanism for, and an understanding of, processes which could give rise to a mélange of seemingly incompatible combinations.
A mélange can be formed in the accretionary wedge above a subduction zone above diverging plates. Both tectonic and sedimentary processes can form mélange. The outcomes of both processes are evident in the Rocky Beach Mélange.
Eclogite [W]
Eclogite is a mafic metamorphic rock. "Mafic" rocks are very dense, with high proportions of magnesium and iron. So, as an unusually dense rock, eclogite can play an important role in driving convection within the Earth. Eclogite typically results from high-pressure metamorphism of mafic igneous rock (typically basalt or gabbro) as it plunges into the mantle in a subduction zone.
Lawsonite [W]
Lawsonite is a very widespread mineral (a hydrous calcium-aluminium silicate) and is a marker of moderate/high pressure and low temperature (300 – 400°C) metamorphic conditions. Lawsonite is most commonly found in subduction zones where cold oceanic crust subducts down oceanic trenches into the mantle. The initial low temperature of the descending slab, and fluids taken down with it, keeps the it colder than the surrounding mantle, allowing for these unusual high pressure, low temperature conditions.
Lawsonite-eclogites: where things get interesting [W]
Eclogites containing lawsonite are rarely exposed at Earth’s surface, although they are readily formed during cold subduction of oceanic crust to mantle depths between 45 – 300 kilometres ("high-P and low-T") conditions.
The fact that lawsonite eclogites are rare does not so much reflect their formation conditions ("high-P, low-T") but rather their unusual “exhumation” – their relatively rapid return to the surface.
What makes this site unique? [2], [3]
Rocky Beach is notable for its well exposed high-pressure metamorphic rocks and a wide variety of igneous rocks, including pillow basalts of Mid Ocean Ridge Basalts and other intrusions.
The site consists of two lenses separated by a narrow strip of Port Macquarie Serpentinite.
Although high-pressure metamorphic rocks are known from serpentinite bodies elsewhere in the New England Fold Belt, this occurrence at Rocky Beach is unique in the abundance and range of blocks and the nature of the matrix. Thus, in spite of its limited extent, it was given the formal name “Rocky Beach Metamorphic Mélange”.
There are only 10 recorded occurrences of lawsonite-eclogites globally and you are visiting one of them. [4] It is the only one documented in Australia and is possibly the oldest known occurrence (early Cambrian i.e. about 550 million years ago) along with a similar occurrence in Spitsbergen in the Arctic.
Global occurrences of lawsonite-eclogites [4]
References and further reading.
[W] For more details on metamorphism, plate tectonics, mélange, eclogite, lawsonite and lawsonite-eclogites go straight to Wikipedia.
[1] Och, D.J. & Graham, I.T.: “Preservation of the Rocky Beach blueschist-eclogite outcrop, Port Macquarie, NSW as a geoheritage reserve.” Proceedings of the Linnean Society of New South Wales, Vol. 132, 2011: 109-114. Available here
[2] D.J. Och, E.C. Leitch & G. Caprarelli: “Geological units of the Port Macquarie–Tacking Point tract, north-eastern Port Macquarie Block, Mid North Coast region of New South Wales"Available here
[3] “Port Macquarie Geological Block” Available here
[4] T. Tsujimori et al: “Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites.”Available here
[5] Department of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia “Metamorphic Rocks”. Available here