The Narooma Accretionary Complex and Chevron Folds in rocks

Latitude -36.230276, Longitude 150.143137

Located in the Eurobodalla Region of NSW, nearest town Narooma

Link to Detailed Map

Geology

On Narooma Beach, in the Eurobodalla region of NSW, there are some very impressive rock formations, including the amazing "chevron" folded rocks which outcrop midway between the beach headlands. The remarkable folding pattern exhibited in these rocks is the result of tremendous pressure coming from the Pacific Tectonic Plate's collision with the land: this collision has effectively foreshortened the existing layered rock into a zig zag pattern.

The two adjacent images show interesting mylonite rock formations lying on the beach just south of the chevron folded rock formation and before Glasshouse Rock (the large vertical rock in the background). The bands of stone circling the rock are evidence of weathering resistant material. There are many examples of this type of formation on the beach, some so powerfully weathered that the spirals are all that remain.

Further south, off-shore from the headland, stands a large vertical formation known as Glasshouse Rocks. These consist of Narooma Chert deposited on the Pacific Ocean floor over a period of 50 million years (from Late Cambrian to Ordovician period) and moved westward to its present location when the Pacific plate collided with Gondwana. (a supercontinent of the Southern Hemisphere made up of the landmasses that currently correspond to India, Australia, Antarctica, and South America).

Another interesting configuration on Narooma Beach is this band of rock (adjacent photo) which is thought to consist of iron oxides concretions that have been pushed up at a 45 degree angle and then have been weathered back to the most durable components.

At the northern end of Narooma beach, "pillow lava" of inferred Cambrian age can be viewed. This rock shape is most often the result of undersea volcano eruptions. When lava from these eruptions was cooled quickly by the ocean it formed pillow shapes: as the outside hardened and then filled and burst, the fresh molten lava within then spilled over to form another pillow.

How did these formations occur?

The continental crust of eastern Australia formed along the margin of the supercontinent of Gondwana during the Palaeozoic and early Mesozoic ages. It resulted from an increase in size by gradual external addition of oceanic crust, recycled continent-derived turbidite (material which has been subject to ocean currents), and volcanic activity.

"Orogenic belts" are associated with subduction zones, which consume crust, produce volcanos, and build island arcs. Eastern Australia is composed of distinct orogenic belts, collectively referred to as the Tasmanides. These long tracts of highly deformed rock were created as a result of tectonic plate movement and are younger than the Gondwana western portion of the continent. When tectonic plates collide something has to give; usually one continent dives beneath the other (subduction zone) and is melded into the earth's mantle. This collision results in huge pressure and deformation of rocks and landscapes.

The Narooma Accretionary Complex is a geological structural region that is the remains of a subduction zone. It attached itself to the Lachlan Fold Belt in the early Silurian period and was moved by the Pacific Plate westwards until it encountered the east coast of Gondwana. The accretionary prism complex at Narooma Beach is in the transition zone between the Narooma Chert and the Bogolo Formation. The Narooma Chert is thought to be part of an old sea floor. The Bogolo Formation contains highly deformed sedimentary rocks and is thought to be Late Cambrian to early Ordovician.

Getting There

The section of Narooma Beach mentioned here can be accessed by driving south through Narooma, turning left (east) down Glasshouse Rocks Road and following it until you reach the cemetery. Go to the rear of the south- western part of the cemetery and you will see a path leading down towards the beach; follow this until you see another rough track leading east (left) off the main track. Be careful here, as the path is steep and eroded in places. When you reach the beach turn right (make sure you note where the track entrance is from the beach, it can be hard to find on the return journey). About mid-beach, between the headlands and on the land (western) side, you will find the chevron folded rock outcrop. Just south of that are the mylonite rock formations and further south, towards Glasshouse Rocks is a fine place to examine the distorted nature of the beach rocks.