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The rock formations at Bexhill quarry are sedimentary. The rock formations are made up of a base layer of coal Walloon coal measures with Kangaroo creek sandstone on top (Figure 1). This can be observed in the flow banding and layering of the rocks. The coal measures formed approximately 150 million years ago during the Jurassic period (Brunker, 1969). The land scape at the time was most likely like an everglades type environment that was rich in plants and vegetation. Over millions of years these plants were compacted and fossilised forming layers of coal measures, shale, sandstone and ironstone.
The landscape changed over time, high energy systems such as turbulent river systems most likely came through the area. These high energy systems were able to transport sediments over distances that eventually settled in the area allowing sandstones to form on top of the coal measures during the Jurassic-crustaceous period (Brunker, 1969). Although the coal measures formed first during the Jurassic period and the sandstone formed later in the Jurassic-crustaceous period, both rock types formed during the Mesozoic epoch (Brunker, 1969).
A change in landscape has occurred at some point between the end of the Mesozoic era and now as the banding in the rocks has an approximate 7° tilt which could be caused by uplift (Press, Siever, Grotzinger, & Jordan, 2004). The Tweed heads geological map claims that these rocks are Lismore basalt (Brunker, 1969). This shows that it is important to observe geological formations properly and use geological maps as a reference only. However, the map does show sections of Walloon coal measures close by within the region. This can assist in correctly identifying rock structures and geological formations.
Figure 1. Bexhill Quarry. Observe the flow banding on the base layer of Walloon coal measure and layering of kangaroo creek sand stone on top. The banding tilts at approximately 7° suggesting a change in landscape over time.
The geologic features present at the Byron Bay coastline were formed through processes occurring during the Quaternary period and Holocene epoch at the latter part of the Cenezoic era (Fox, 2016). The quaternary period began approximately 2.6 million years ago with the Holocene epoch starting approximately 15,000 years ago (Gibbard, Head, Walker, & Stratigraphy, 2010; Press et al., 2004). These periods saw an instability in sea levels due to the last of the great glaciations 18,000 years ago (Blewett & Blewett, 2012). The warming of the climate brought stability to sea levels approximately 3,500 years ago (Fox, 2016).
This instability of sea level and climatic changes can be seen in the Byron Bay Coastline. Rises in sea level left marine organism fossils up to 3.5km inland from current shorelines (Fox, 2016). These deposits were left during high sea levels up to 20,000 years ago during the Pleistocene epoch at the first half of the Cenezoic period (Fox, 2016; Press et al., 2004). These fossil deposits have been converted to sedimentary rock known as coffee rock (Fox, 2016). These dark coloured rocks get their colour from decaying vegetation that leaches into soil and stains sandstone (Fox, 2016).
Ian Fox recounted an indigenous story told to European settlers in 1938 by Charlotte Williams. She told the story of two brothers who walked to shark bay in western Australia and back over seven years. Upon their return they spoke of a cave with a cleft in the rock formation (Fox, 2019). This cave and cleft exist but are under the ocean and favoured by grey nurse sharks. This tells us that when these two brothers observed this cave, it was at a point in time where sea levels were much lower than they are now. The last time sea levels were low enough for this cave to be on dry land was approximately 10,000 years ago (Figure 2). Indigenous knowledge is important in studying geological areas as the stories told have been passed down for tens of thousands of years and help to give an idea of how landscapes may have changed over time.
Figure 2. Graph showing fluctuation in sea levels and CO2 levels over the past 400,000 years.
The columnar rock formations found at Fingal Headland are examples of igneous rocks formed by aa lava. Aa is a Hawaiian word that describes lava that has lost gasses, making it viscous and blocky, forming a thick skin (Press et al., 2004). The volcanic activity can be observed from the north to south on the Australian east coast chronologically through mantle convection currents due to continental drift north (Fox, 2019). The area is comprised of mafic basalts that have formed columns as well as basalt containing iron minerals that have oxidised through weathering, giving them a red appearance (Fox, 2019). These Lamington volcanic rocks were formed during the tertiary epoch of the Cainozoic period (Brunker, 1969). These columnar rocks form from the cooling of molten lava in the same shape as the atomic structure of the minerals contained within (Fox, 2016). The lava cools from the outside inwards, cooling at right angles to the surface (Fox, 2016). This creates a cracking pattern on the surface which run deeper as the inside cools, forming columns (Fox, 2016).
The five sites visited along the 600m stretch from Cabarita beach to Norries headland contain rock formations that give a 23-million-year history of the tweed coasts geologic timeline. The first site contains an outcrop of Neranleigh-Fernvale Metasediments (Fox, 2016). This outcrop contains alternating bands of sandstone and mudstone. This type of layering is known as a turbidite sequence and is achieved by the carrying of sediments from river systems to the ocean, over thousands of years these sediments lithify, becoming rocks (Fox, 2016; Press et al., 2004). Graded bedding is present sorting the coarser sediments to the base of the formations as well as rip up clasts that can be observed. This is a blending of the mudstone and sandstone (Fox, 2016). The layers have been metamorphosized by heat and pressure, hardening the rock, and are best classed as low-grade metamorphic rocks.
The second site contains the same type rocks formed through turbidite sequencing, however, these rocks have undergone further metamorphism through more intense heat and pressure. Concretions are visible in the layers and occur through heat and pressure low-grade metamorphic rocks (Fox, 2016). There is faulting present in the rocks of this site which give an insight into the geologic processes involved in the formation of the coastline. Earthquake activity most likely played a role in this faulting as there is evidence of rip up clasts that are also formed through earthquakes (Press et al., 2004).
The third site is also Neranleigh-Fernvale Metasediments but is viewed from the front which means that the layering is not visible. What is visible is dykes of mafic basalt containing feldspar. The basalt intrusions would have flowed into these cracks during the tertiary period 23 million years ago (Brunker, 1969). These basalts intruded through fissures from below which can be observed as they start at the base and taper at the top (Figure 3). Parallel to the dykes, contact metamorphism is visible. The fracturing in the rock face would not have been achieved by the intrusions and suggest that huge forces were at play to cause these fractures and faulting (Fox, 2016).
At site four the Neranleigh-Fernvale Metasediments have a different structure to the previous sites and have a brown red colour. This is due to a combination of algae and iron minerals (Fox, 2016). The rock formation has veins of quartz throughout the outcrop. This crystallisation of quartz happens at approximately 600°C and most likely occurred through hydro thermal activity (Press et al., 2004). This increase in temperature and pressure has metamorphosised this outcrop from low-grade metamorphic to a medium grade metamorphic rocks through regional metamorphism. This process has changed the sandstone to greywacke or quartz and the mudstone to slate or phyllite(Fox, 2016; Press et al., 2004).
The final site contains formations of quartzite and greywacke rocks. Observing the formation of these rocks, it is obvious that great geological changes have occurred in this area. The curving and buckling of the rocks can be seen in the banding as well high levels of fracturing (Figure 4). Through observing the outcrops at the last four sites it becomes clear that this site is closest to the activity zone of a major geologic process (Fox, 2016). The curves, buckles and fracturing have been caused by uplift during intrusion of volcanic matter from below (Press et al., 2004). This heat and pressure caused contact metamorphism for these rocks have become medium-grade metamorphic rocks.
Photo: Phil Trovato
Figure 3. Dyke of Basalt intrusion into Neranleigh-Fernvale metasediment
Figure 4. Highly curved and buckled turbidite sequence at site five
- Blewett, R., & Blewett, R. (2012). Shaping a nation: A geology of Australia: Geoscience Australia and ANU E-Press.
- Brunker, R. L., Cameron, R.G. (Cartographer). (1969). Tweed Heads
- Fox, I. (2016). The Fragile Edge: a Natural History of the Tweed Coast. In Geology. Bongar NSW.
- Gibbard, P. L., Head, M. J., Walker, M. J., & Stratigraphy, S. o. Q. (2010). Formal ratification of the Quaternary System/Period and the Pleistocene Series/Epoch with a base at 2.58 Ma. Journal of Quaternary Science, 25(2), 96-102.
- Press, F., Siever, R., Grotzinger, J., & Jordan, T. H. (2004). Understanding earth: Macmillan.
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