Geology

The Geotrail follows rocks exposed on the beaches south of Port Macquarie. These rocks record a

fascinating story involving the migration of an oceanic plate away from a mid-ocean ridge (oceanic

spreading ridge) to a subduction zone about 500 million years ago (Figures 1, 2). Back then, our continent

was part of a supercontinent called Gondwana which was located near the Equator (Figure 3). Since then,

this supercontinent has migrated and broken up, with the Australian continent eventually reaching its current

position (Figure 2S). To imagine this process of breaking up and migration, think of the way ice sheets in

Antarctica crack and float across the ocean carried by ocean currents.

Figure 1 shows the migration of oceanic crust away from a mid-ocean ridge

exuding basalt (mid ocean ridge basalt - MORB; Shelly Beach) and down

the subduction zone (Rocky Beach).

Walking the geotrail allows you to track the migration of tectonic plates, observe how the rocks change, and

learn about the setting in which they formed. At Shelly Beach (Stop 1), are dark rocks called basalt that are

thought to have formed close to a spreading ridge (the boundary between two divergent tectonic plates;

Figures 1S, 4) because their chemical composition is similar to mid-oceanic ridge basalts (Och 2007). The

Mid-Atlantic Ridge that divides the North American plate from the African plate is an example of this type

of plate border (Figure 4).

Figure 2 Geological Time Scale

Figure 3 shows the supercontinent Gondwana and the Australian continent as part of

Gondwana. The Australian continent was at the Equator at this time.

A B

Importantly, mid ocean ridge basalts show pillow structures, indicative of lava extruding out of a volcano

under water (Figures 5A, B).

This is what you might expect in the submarine environment where mid-ocean ridges are located.

Continuous eruption of the volcanoes in the mid - ocean ridge setting leads to a substantial thickness of

basalt building up. With migration, the basalt cools, becomes denser and sediments made up of marine

organisms, collect on top (Figure 6). The consolidated form of this sediment is chert (often called ribbon

chert) that can be observed intermixed or on top of basalt at Stops 2 (Shelly Beach) and 5 (Rocky Beach;

Figure 3SF). Examination of thin sections of these rocks reveals that the organisms are radiolarians and

conodonts (Figure 7) that tell us they are quite ancient rocks that formed approximately 460 million years

ago (Och et al. 2007).

Figure 5 A. Pillow lava formation. B. Newly formed pillows on sea floor.

Figure 6. Accumulation of silica-rich skeletons that form a deposit of siliceous ooze on

the seafloor.

Figure 4. The Mid-Atlantic Ridge divides the North American plate from the African

plate. A rift valley (deep, down-faulted structures) occurs in the axis of the ridge at a

depth of over 2,500 metres.

Figure 7. A. Well preserved conodonts

(extinct marine microfossils). B.

Photomicrograph of conodont (extracted

from cherts at Port Macquarie) whose

features suggests a middle Ordovician age

(460 million years old; Och et al. 2007), an

age confirmed by Buckman et al. 2015).

The scene changes at Stop 3 (Nobbys Beach), where further transport has taken the oceanic basalt and chert

closer to a landmass. We observe sedimentary rocks made up of detritus (fragments of rocks and minerals

of different size) that has come from a continent. We know that the continent is distant because the detritus

is very small and thus could be carried over long distances. Recent studies suggest that this continent may

have been Antarctica. Importantly, the rocks at Stop 3 have undergone further transport, been subducted

and converted to metamorphic rocks. They show features that indicate they have been taken down a

subduction zone (Figure 1), deformed and heated up (~350oC).

During subduction, basalts originally derived from the mid-oceanic ridge undergo alteration and

metamorphism that produces hydrated minerals such as chlorite and actinolite (see Glossary). With further

subduction and increasing temperature, these minerals break down, providing water to the rocks in the

mantle wedge (e.g. harzburgite – see Glossary) directly above the subduction zone (Figure 8) and altering

the harzburgite to serpentinite. In time, the serpentinites are brought back up the subduction zone by faulting

and are strongly deformed. These are seen at Flynns Beach (Stop 4), where folded serpentinite showing

closely spaced planes of parting (cleavage) is beautifully exposed.

A B

Figure 8. Illustration of how serpentinite is developed in the mantle wedge above the subduction zone.

Water released from the dehydration of sediment and altered basalt converts mantle rocks to serpentinite.

At Rocky Beach, Stop 5, rare exotic rocks called blueschist and eclogite crop out. Blueschist has a distinct

blue colour and eclogite a brown colour. This is one of the few sites in Australia where they occur and as a

result they have been studied extensively (Barron et al. 1976; Och 2003; Phillips et al.2015; Tamblyn 2016).

They represent the end of this plate tectonic journey, where the original mid-ocean ridge basalts have been

taken to great depths (104 km), subjected to high temperatures (570oC; Tamblyn 2016) and intense

deformation during subduction. This results in the formation of eclogite that is subsequently brought back up

the subduction zone by faulting and partly replaced by blueschist due to decreases in temperature and depth.

Recent studies of these rocks have revealed that the eclogite formed ~490 million years ago and the

blueschist 460 million years ago (Tamblyn 2016). Thus over this long journey that started over 500 million

years ago these exotic rocks have come to rest at Port Macquarie for everyone to see.

Links to sites of interest.

https://www.youtube.com/watch?v=g_iEWvtKcuQ

http://jan.ucc.nau.edu/~rcb7/Camb.jpg

http://deeptimemaps.com/wp-content/uploads/2016/05/460_Ma_Ord_GPT-1.png

Figure 10 shows where different rock types are produced in the

subduction zone. Eclogite forms at very great depths (>100 km).

http://jan.ucc.nau.edu/~rcb7/480_1st.jpg

https://australianmuseum.net.au/image/reconstruction-of-the-early-ordovician-biogeography

References

Barron, B.J., Scheibner, E. & Slansky, E. 1976. A dismembered ophiolite suite at Port Macquarie, New

South Wales. Records of the Geological Survey of NSW. 18, Pt 1, 69-102.

Buckman, S., Nutman, A.P., Aitchison, J.C., Parker, J., Bembrick, S., Line, T., Hidaka, H. & Kamiichi, T.

2015. The Watonga Formation and Tacking Point Gabbro, Port Macquarie, Australia: crustal growth

mechanisms on the eastern margin of Gondwana. Gondwana Research 28, 133-151.

Nutman, A.P., Buckman, S., Hidaka, H., Kamiichi, T., Belousova, E. & Aitchison, J, 2013. Middle

Carboniferous-Early Triassic eclogite-blueschist blocks within a serpentinite melange at port Macquarie,

eastern Australia: Implications for the evolution of Gondwana’s eastern margin. Gondwana Research 24,

1038-1050.

Och, D.J., Percival, I.G. & Leitch, E.C. 2007. Ordovician conodonts from the Watonga Formation, Port

Macquarie, northeast New South Wales. Proceedings of the Linnean Society of New South Wales 128, 209-

216.

Och, D.J., Leitch, E.C., Caprarelli, G. & Watanabe, T.2003. Blueschist and eclogite in tectonic melange,

Port Macquarie, New South Wales, Australia. Mineralogical Magazine, 67, 609-624.

Phillips, G., Offler, R., Rubatto, D. & Phillips, D. 2015, High-pressure metamorphism in the southern New

England Orogen: Implications for long-lived accretionary orogenesis in eastern Australia. Tectonics, 34,

doi:10.1002/2015TC003920.

Tamblyn, R.J. 2016. Eclogite and blueschist in the southern New England Fold Belt: P-T conditions and

long lived subduction on the Gondwanan Eastern margin. BSc(Hons) Thesis (unpubl.) University of

Adelaide.

Glossary

Actinolite is a silicate mineral that belongs to the Amphibole Group. It has the chemical formula Ca2(Mg4.5-

2.5 Fe2+

0.5-2.5)Si8O22(OH)2.

Bastite forms when single crystals of pyroxene in mantle rocks (e.g. harzburgite) are replaced by serpentine

minerals.

Blueschist is a metamorphic rock characterised by a blue, Na-rich amphibole called glaucophane.

Muscovite (mica), albite (feldspar) and epidote (hydrated Ca Al silicate) may also be present.

Chlorite belongs to a group of usually green, platy silicate minerals. It is a hydrated Fe Al Mg

Silicate.

Conodonts are a group of extinct microfossils known from the Late Cambrian (approximately 500 million

years ago) to the Late Triassic (about 200 million years ago). They are the only known hard parts of an

extinct group of animals believed to be distantly related to the living hagfish.

Eclogite is a metamorphic rock consisting of pyroxene and garnet

Harzburgite is an ultramafic, igneous rock and a variety of peridotite consisting mostly of olivine and low-

calcium (Ca) pyroxene.

Peridotite is a dense, coarse-grained ultramafic igneous rock that consists mostly of olivine and pyroxene

and is high in magnesium (Mg).

Phacoid refers to ellipsoidal, doubly tapered, deformed clasts, which may have asymmetric shapes and are

generally elongate, parallel to cleavage. In serpentinite, the clast is enclosed by serpentinite showing well

developed cleavage. Commonly, the clast is harzburgite that has been replaced partly or completely by

serpentine minerals.

Serpentinite is a metamorphic rock composed of one or more serpentine group minerals.

Turbidite is a sedimentary rock composed of layered particles that grade upward from coarse to fine sizes

and that were deposited by turbidity currents in the oceans.

Turbidity current is an underwater current of usually rapidly moving, sediment-laden water moving down

a slope. See video at: https://www.youtube.com/watch?v=8gYJJjxY8g0.

Figure 1S. Mid-ocean ridges with different spreading rates in the Pacific, Indian and Atlantic Oceans.

Figure 2S. Location of Tectonic

plates today.

Figure 3SA. Pillow structures in basalt. Stop 1. Shelly

Beach.

Figure 3SB. Basalt boulder showing phenocrysts (large

crystals) of feldspar; arrow). Stop 1. Shelly Beach.

Figure 3SC. Dyke cutting basalt. Stop 1. Shelly Beach. Figure 3SD. Folded layering (arrow) in chert. Stop 3.

Figure 3SE. Phacoids (P) of massive serpentinite in

serpentinite showing well developed cleavage (Cl).

Stop 4.

P

Cl

Figure 3SF. Basalt-chert contact (arrow). Rocky

Beach. Stop 5.

Supplementary figures

Figure 4SA. Bastite (see Glossary) glinting in the sun (arrow) in

massive serpentinite. Stop 4.

Figure 4SB. Well - defined bedding in turbidites,

Windmill Hill Beach.

Figure 4SD. Blueschist cut by numerous quartz veins

(arrow). Rocky Beach. Stop 5.

Figure 4SC. Turbidites. Nobbys Beach. Stop 3.

Figure 4SE. Blueschist showing well - developed

layering and folding (arrow). Stop 5.

Figure 4SF. Eclogite (Eg) boulder. Rocky Beach. Stop

5.

Eg