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coastal walk

Welcome to the Port Macquarie Coastal Geotrail. Here you will find rare and amazing rocks that tell the story of plate tectonics - how the earth’s crust is formed and how it moves. On this Geotrail you can travel the equivalent of 6000 km across the Pacific Ocean of today in just 4 km from Shelly Beach to Rocky Beach! You’ll see rocks made by volcanoes, by microscopic marine animals, and by underwater gravity currents. Some of these rocks formed at high pressures and temperatures over 100 km below the earth’s surface. 

We start the Geotrail by examining the theory of plate tectonics, the most important underlying concept in geology. A  technical resource3MB pdf(PDF, 3MB) is also available looking into what geology is and how the Geotrail was formed - a great resource for school research.

If you experience any issues with viewing the map below you can click to open the application in a new window.

 

At Stop 1, southern Shelly Beach, you are at the start of the Port Macquarie GeoTrail. Here we see rocks formed at the start of their plate tectonic journey across the ocean. Ocean crust is mostly formed from a dark, fine-grained volcanic rock called basalt. This type of rock forms in mostly underwater (or submarine) volcanoes in places like the mid ocean ridge. On southern Shelly Beach the basalt flowed out of submarine volcanoes as molten lava in the shape of pillows. Sometime after the first cooling episode, new molten basalt was injected into the ocean floor crust as long, thin fingers called dykes. In some locations, basalt first cooled slowly deep in the earth forming big shiny feldspar crystals, before later flowing out onto the sea floor and forming the smaller surrounding dark crystals. To see the basalts, pillows and dykes, walk to the right around 100 m SE along the beach. The larger crystals are around 50 m further SE. 

Basalt at the southern end of Shelly Beach. The low area where the man is standing is a dyke, outlined in red. The dyke is split by a fault shown in yellow Four rounded basalt pillows on southern Shelly Beach with two outlined by red dashed line and inset, a modern example of submarine pillow lava formation in Hawaii A thin basalt dyke outlined in red injected into older pillow basalts Large shiny feldspar crystals in a fine grained dark basalt, Shelly Beach

At Stop 2 in the middle of Shelly Beach, we see rocks called chert part way along their plate tectonic journey. After the ocean crust basalt seen at Stop 1 was formed, it travelled away from the mid ocean ridge and began to collect sediment on top. This sediment was mainly formed from the bodies of tiny marine organisms that live near the surface of the ocean. After death, their skeletons rain down on the ocean floor. In the deeper parts of the ocean, (over 4 km), most skeletons dissolved in the acidic water, except for ones made from silica (SiO2) such as radiolarians. They collected in layers above the basalt, each layer formed as changes in productivity and climate occur in the ocean. These layers formed from variations in the type and amount of marine animal skeletons and the amount of external sediment added. When thick layers of sediment build up, pressure and chemical changes convert the fossil skeletons into the rock we call chert. As the chert is transported towards the trench, it often slumps and slides deforming into sinuous folds. Fossils known as conodonts (jaw structures of eel-like animals that lived in the ocean) can be used to date the chert, which at Port Macquarie is around 460 million years old. To see the chert, walk left around 100 m along Shelly Beach to the NE.

Bedded and folded chert (dark and light bands) in the middle of Shelly Beach. Tiny skeletons of modern radiolarians (0.1 mm across) found on the sea floor in water depths over 4 km deep. Radiolarians from 460 million years ago look very similar, and when compressed they form the rock we call chert (source NESTLaboratory, University of Dayton) The basalt/chert boundary at Stop 2, shown by a red dashed line, is where the top of the ocean plate was located 460 million years ago Folded chert at Shelly Beach showing zig-zag folds (outlined by red dashes) formed by slumping on the sea bed Examples of conodont fossils from chert at Port Macquarie used to date these rocks as around 460 million years old (from Och et al., 2007). Each conodont is around half a millimetre in size.

At Stop 3 on the northern side of Nobby Head, we are mid-way along the Port Macquarie Geotrail. The ocean plate is also mid-way along its tectonic journey, nearing a subduction zone and a chain of volcanic islands. Here, sediments are being eroded from the volcanic islands and deposited nearby in the ocean. Rocks made from the smallest grains are named mudstone or siltstone. Sandstone is made from larger grains. At Stop 3 sediments were transported by gravity-driven (or turbidity) currents downslope into a trench where they accumulated with the marine cherts in deposits called turbidites. Trenches up to 11 km deep lie next to volcanic island arcs and are the deepest parts of the ocean. After leaving the trench area, the ocean plate and the sediments riding on it descend into the subduction zone, where they experience increased heat and particularly pressure. There they undergo slumping, sliding, folding and faulting. To see the clastic sediments, walk around 80 m down the path to the headland at the southern end of Nobby Beach.

Here an oceanic plate made of basalt (blue) and chert (red) approaches a volcanic island arc. Sediments are shed from the islands and move downslope into the trench, depositing sand and mud together with chert Thin beds of mudstone and siltstone (dark and light grey bands) deposited as turbidites by gravity flows into a deep sea trench. Here they are deposited with marine chert and extensively deformed by folds and faults Interbedded chert (brown) and mudstone (grey) that has been strongly folded. Sediments either accumulated from marine animals in the ocean above (chert), or from the land nearby (mudstone), and after deposition they were folded during subduction The dark bands are mudstone, and the lighter bands are siltstone. This rock is formed from many gravity flows transporting fine-grained sediments down slope from volcanic islands into deep water. Each siltstone to mudstone layer is one flow event Siltstone from Nobby Head is composed of dark minerals like chlorite (Chl) and light minerals like quartz (Qtz) as seen here through a microscope. The brighter colours are from a quartz vein containing the mineral prehnite (Prh), an indicator that this clastic rock has been subjected to a temperature of around 260 ˚C, and burial to at least 15 km depth. The image is about 1 cm wide

At Flynn’s Beach, we are nearing the end of the Geotrail. The rocks here are descending into the earth down a subduction zone and affecting the mantle rocks around them. The original mantle material surrounding the basalt is transformed into a rock called serpentinite that has a soft greasy feel, and is often bright green. This colour comes from the original dark mantle minerals transforming into new greenish minerals such as lizardite and antigorite by heat and water produced during subduction. The serpentinite was formed at depths of 1-60 km in the earth where pressures are low, but temperatures are relatively high between 350-600°C. Due to stress in the earth or shearing, the serpentinite has a strong aligned texture called cleavage. It also contains unsheared, large dark fragments of the mantle suspended in it, making it look like green fruit cake on cliff faces. To see the serpentinite, walk right around 200 m to the SE to the southern headland of Flynn’s Beach.

Serpentinite at Flynn’s Beach shows the characteristic green colour of serpentine minerals and their alignment after being deformed. Rocks like this on the beach have a greasy feel, and contain black blocks of mantle material. The scale bar is 10 cm long Under a microscope, blade-like crystals of the mineral antigorite can be seen cutting older crystals of another serpentine mineral lizardite. These minerals help us tell at what temperatures and pressures the rock was formed. The image is about 1 cm across Increasing pressure during subduction has resulted in the serpentinite forming cleavage, a curved alignment of crystals traced out by the dashed red lines. The scale is 10 cm long Serpentinite at the southern end of Flynn’s Beach is a distinctive greenish colour. It has a lumpy texture with large blocks of black mantle rock preserved within it (shown here by the red arrows)

At Flynn’s Beach, we are nearing the end of the Geotrail. The rocks here are descending into the earth down a subduction zone and affecting the mantle rocks around them. The original mantle material surrounding the basalt is transformed into a rock called serpentinite that has a soft greasy feel, and is often bright green. This colour comes from the original dark mantle minerals transforming into new greenish minerals such as lizardite and antigorite by heat and water produced during subduction. The serpentinite was formed at depths of 1-60 km in the earth where pressures are low, but temperatures are relatively high between 350-600°C. Due to stress in the earth or shearing, the serpentinite has a strong aligned texture called cleavage. It also contains unsheared, large dark fragments of the mantle suspended in it, making it look like green fruit cake on cliff faces. To see the serpentinite, walk right around 200 m to the SE to the southern headland of Flynn’s Beach.

Blueschist with white quartz veins and pyrite crystals (dark pits on surface), and greenish eclogite boulder behind Blueschist seen under a microscope showing colourful phengite minerals. The crystals are folded during subduction, as shown by the red dashed lines. The scale bar is 2 mm long. Eclogite seen under a microscope, showing high pressure and high temperature minerals such as brown garnet (G), silver phengite (Ph), and grey glaucophane (Gl). The scale bar is 2 mm long

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To make your walk safe and enjoyable:

  • Remember to take plenty of water and sun protection (sunscreen and hats) 
  • Do not leave valuables in the car
  • Keep to the track, beware of traffic when close to the road and keep away from cliff edges
  • Please take rubbish with your or use bins provided

This page was last updated on: 11 December 2017