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View from Rocky Beach at Port Macquarie

Figure 1(a) Location of the New England Fold Belt, eastern Australia; (b) Major Late Palaeozoic-Triassic components of southern New England Fold Belt in northeast New South Wales (cf. Och 2007)
Figure 2 locality geological map of the Rocky Beach area (Och, 2007).
Figure 3 'Exotic' tectonic blocks of eclogite and blueschist (greeny-blue blocks) in the foreground with large chert blocks of the Watonga Formation in the background associated with rockfall from the cliff line behind.
Figure 4 Multiply deformed block of eclogite containing layers of lilac blue mineral called glaucophane, garnet, phengite mica, green layers of omphacite and microscopic lawsonite.
Figure 5 (a) large omphacitite block with other contact with other rocks of the melange.
Figure 5 (b) large tremolite marble block lying in the sand.

Port Macquarie Geological Block

Latitude -31.434278, Longitude 152.925959

Located in the Mid North Coast Region of NSW, nearest town Port Macquarie

Source: David J. Och; Parsons Brinckerhoff Australia Pty Ltd, PO Box 5394 Sydney NSW 2001 ( School of Biological, Earth and Environmental Sciences, UNSW Sydney, NSW 2052

Link to Detailed Map

Explanation of subduction zones

The Rocky Beach Metamorphic Melange is located at Rocky Beach, Port Macquarie on the mid north coast of New South Wales, approximately 400 km north of Sydney. The rock group forms part of the Port Macquarie block (Och et al. 2005 and Och, 2007), located in the eastern New England Fold Belt (Figure 1), is notable for the presence of well-exposed high-pressure metamorphic rocks and a wide variety of igneous rocks, including pillow basalts of MORB (Mid Ocean Ridge Basalt) affinity, and minor mafic and intermediate calc- alkaline intrusions, embedded in lenses of serpentinite melange. It covers an area of just under 1 hectare and is locally bounded by the Tasman Sea to the east, Oxley Beach to the north, Flynns Point headland to the south, and steep densely vegetated slope up to the sealed road which provides a densely vegetated buffer from the surrounding residential development on the western side of the road.

The Rocky Beach Metamorphic Melange (Figure 2) forms part of the Port Macquarie Block (Och, 2007), which is located in the eastern part of the southern New England Fold Belt. It is notable for the presence of Early Cambrian to mid-Silurian high- pressure - low-temperature metamorphic rocks that are tectonically faulted against Late Ordovician ribbon chert, siltstone, and sandstone and pillow basalts of the Watonga Formation.

Near the north end of Rocky Beach a 'melange-in-melange' structure preserving serpentinite masses that have invaded the Watonga Formation, which has also enveloped two bodies of chlorite- actinolite schist, surrounded by highly schistose Port Macquarie Serpentinite, contain inclusions of lawsonite eclogite, omphacitite, blueschist, and tremolite marble. Although these exotic high-pressure metamorphic rocks are known from serpentinite bodies elsewhere across eastern Australia their occurrence is unique because some of these tectonic blocks, namely eclogite, contain a mineral called lawsonite, which have only been identified in 10 localities worldwide {Tsujimori et al. 2006}. The Rocky Beach Metamorphic Melange type area occurs at the north end of Rocky Beach. The unit comprises blocks of multiply deformed polymetamorphosed high-pressure metamorphic rocks (Och et al. 2003) of mafic igneous parentage low-grade blocks of sedimentary and basaltic rocks derived from the Watonga Formation, and a narrow slice of antigorite serpentinite, set in a multiply deformed chlorite- actinolite schist matrix.

The two lenses are separated by a narrow septum of Port Macquarie Serpentinite and originally may have been a continuous northeast trending body (Figure 2). Contacts between the Rocky Beach Metamorphic Melange are discrete faults, although the presence of several small blueschist blocks within the Port Macquarie Serpentinite immediately adjacent to the northwest margin of the larger lens of melange suggests local mixing of material adjacent to this contact.

These tectonic blocks comprise about 20% of the Rocky Beach Metamorphic Melange. Blueschist blocks range up to at least 6 m in longest dimension and have a strong foliation, in some planar and in others intensely folded. Rounded eclogite and omphacitite inclusions range up to at least 2 m in diameter with the former characterised by the presence of almandine crystals (obvious in outcrop) set in a green lawsonite-omphacite bands that have been extensively replaced by glaucophane and phengite (Figure 4). Rare omphacitite forms monomineralogic non-layered extremely high-strength emerald green masses comprising a pyroxene mineral called omphacite, which is the only occurrence in Australia (Figure 5a). Discoid tremolite marble blocks attain 1.5 m in longest dimension (Figure 5b) and are locally concentrated in areas where disrupted calcite veins occur in the matrix. Watonga Formation blocks that closely resemble the formation as found elsewhere along the coastal tract range up to 20 m in longest dimension and are of irregular shape with contacts that are abrupt and angular (Figure 2).

The rock along the Rocky Beach coastal area contain a fragmentary Early Cambrian to Early Palaeozoic orogenic* history, that includes subduction of mafic rocks to depths of 60+km (Och et al. 2003), undergoing multiple stages of deformation and metamorphism during subduction c. 536 Ma (Fanning et al., 2002) with subsequent exhumation during middle Ordovician and tectonic juxtapositioning with hydrating ultramafic cumulates (c.530 Ma) of possible forearc origin Aitchison et al. (1994) that formed serpentinites, which reacted with the high-pressure metamorphic rocks to produce the remnant eclogite omphacitite and blueschist blocks embedded in a chlorite-actinolite schist matrix (Och et al. 2010).

The Watonga Formation comprises rock types including ribbon chert, mudstone, siltstone, tuffaceous sandstone, tuff, conglomerate, and basalt that are inferred to have accumulated on an oceanic plate during its passage from spreading ridge to trench. The cherts were deposited during the Middle-Late Ordovician, based on the occurrence of conodont microfossils (Och et al. 2007). The deformation and accretion of the Watonga Formation commenced prior to consolidation of the clastic rocks but was most intense during frontal accretion of the formation during which they underwent stratal disruption and several episodes of deformation. The broken formation character of these sedimentary and associated mafic igneous rocks was established during this accretionary deformation. The age of the inferred trench-fill Watonga Formation clastics is unestablished and hence the age of accretion of the Watonga rocks can only be very broadly constrained between Middle Ordovician and Late Carboniferous (Och 2007).

Therefore, the Rocky beach exposure has excellent formational contacts that demonstrate age relationships that are not observable for similar rock types elsewhere in Australia. Och and Graham (2011) summarised the significance of the Rocky Beach area:

Geological significance of the Rocky Beach Metamorphic Melange:

  • There are only 10 recorded occurrences of lawsonite eclogite in Phanerozoic orogenic belts, globally.
  • This is possibly the oldest known occurrence (early Cambrian) along with a similar occurrence in Spitsbergen in the Arctic.
  • Formation and preservation requires cold subduction to mantle depths and rapid exhumation.
  • Lawsonite eclogite and omphacitite are the only occurrences documented in Australia.

Social Significance of the Rocky Beach Metamorphic Melange:

  • The variety of ancient rocks and excellent exposures along this coastline provide a scientific interest for visitors.
  • Ready accessibility of the coastal outcrops have attracted numerous geological visitors;
  1. University undergraduate groups
  2. Geological conference excursions
  3. Research scientists from around the world

They also indicated that the exposure should be allocated a geoheritage reserve status (possibly as an extension of the Sea Acres National Park Reserve) as it would help preserve the rocks for future geology students, researchers and geotourists, but also potentially stop the destruction of a very rare geological site. Och and Graham (2011) also suggested that access via a platform and staircase as part of the Port Macquarie coastal walk to the exposure with a continuation through to the south with an exit via possible staircase at Flynns Point, also, along the coastal walk, Plaques describing distinctive geology of key geological sites along the tourist trail (i.e. Town beach, Rocky Beach, Miners Beach and Tacking Point) would also help to promote geotourism in the Port Macquarie region and thus having people frequenting this section of coastal exposure could potentially help prevent the destruction of these rocks.

References/other reading

AITCHISON, J. C., BLAKE, M. C., JR., FLOOD, P. G. and JAYKO, A. S. 1994. Palaeozoic ophiolitic assemblages within the southern New England Orogen of eastern Australia: Implications for growth of the Gondwana margin. Tectonics 13, 1135-1149.

Fanning, C. M., E. C. Leitch and T. Watanabe: (2002): An updated assessment of the SHRIMP U-Pb Zircon dating of the Attunga eclogite in New South Wales, Australia. Relevance to the Pacific margin of Gondwana. Int. Symp. On the amalgamation of Precambrian blocks and the role of the Paleozoic orogens in Asia. Abst. V. Supp. Data.

Och, D.J., 2007. Eclogite, serpentinite, melange and mafic intrusive rocks: manifestation of long-lived Palaeozoic convergent margin activity, Port Macquarie, eastern Australia. PhD Thesis, University of Technology, Sydney.

Och, D.J. and Graham, I.T., 2011. Preservation of the Rocky Beach Blueschist-Eclogite Outcrop, Port Macquarie, NSW as a Geoheritage Reserve. Proceedings of the Linnean Society of New South Wales, 132: 109-114. Och, D.J., Leitch, E.C., Caprarelli, G. and Watanabe, T., 2003. Blueschist and eclogite in tectonic melange, Port Macquarie, New South Wales, Australia. Mineralogical Magazine, 67:609-624.

Och, D.J., Leitch, E.C., Graham, I.T. and Caprarelli, G. (Editors), 2005. The geology of the Port Macquarie-Tacking Point coastal tract. SGGMP-Port Macquarie 2005, First biennial conference, 76. Specialist Group in Geochemistry, Mineralogy and Petrology, Geological Society of Australia, Port Macquarie, 95-99 pp.

Och, D.J., Leitch, E.C. and Caprarelli, G., 2007. Geological units of the Port Macquarie- Tacking Point tract, north-eastern Port Macquarie Block, Mid North Coast region of New South Wales. Quarterly Note - geological Survey of New South Wales, 126: 20.

Tsujimori, T., Sisson, V.B., Liou, J.G., Harlow, G.E. and Sorensen, S.S., 2006. Very-low- temperature record of the subduction process: A review of worldwide lawsonite eclogites. Lithos, 92 (3-4):609-624.

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