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Geology the Geotrail Follows Rocks Exposed on the Beaches South of Port Macquarie

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Geology the Geotrail Follows Rocks Exposed on the Beaches South of Port Macquarie 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). Figure 2 Geological Time Scale 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 3 shows the supercontinent Gondwana and the Australian continent as part of Gondwana. The Australian continent was at the Equator at this time. 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. Importantly, mid ocean ridge basalts show pillow structures, indicative of lava extruding out of a volcano under water (Figures 5A, B). Figure 5 A. Pillow lava formation. B. Newly formed pillows on sea floor. 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). A B Figure 6. Accumulation of silica-rich skeletons that form a deposit of siliceous ooze on the seafloor. A B Figure 7. A. Well preserved conodonts (extinct marine microfossils). B. Photomicrog raph 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. 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. Figure 10 shows where different rock types are produced in the subduction zone. Eclogite forms at very great depths (>100 km). 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 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+ 2.5 Fe 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. Supplementary figures Figure 3SA. Pillow structures in basalt.
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