A report I did for university back in 2012
A field trip was undertaken to Cabarita Beach, New South Wales on 27 March 2012. Cabarita Beach is a small Pacific Ocean seaside settlement on the north coast of New South Wales located at 28°33’S 153°57’E (GeoHack, n.d). It is administratively part of the Tweed Shire and the nearest large town is Murwillumbah, approximately 20 kilometres to the west. The town is known as Bogangar on some maps (Google Maps, n.d).
The area of study consists of a variety of rock faces and types. There is lot of layering from different turbidite deposits and many of the rocks show dipping and tilting. Others show extensive faulting and folding with a variety of cracks and joints. An outcropping farther down the study area depicts contact metamorphism with quartz that has altered to quartzite and mudstone to slate and phyllite. The action of hydrothermal fluids has created intrusive quartz veins and oxidation at this area. At site six there was an extensive field of cobbles and pebbles, fractured off the parent rock.
This site has exposed rock with very evident layering of alternating mudstone and sandstone. This was laid down during the Carboniferous Period approximately 300 million years ago (Cabarita Beach Field Notes, 2012). The individual bands or layers can be seen clearly in Figure 1.
Figure 1. Sedimentary rock showing alternating layers of mudstone and sandstone.
These rocks were originally sea beds, composed of successive layers of sediment that were deposited by turbidite currents over a period of many millions of years. There is also a flame structure or injection structure at this site, where one layer of sediment was sucked or forced upward into the next layer to form an intrusion (Whitten, 1986). There were also significant rip-up clasts at this site, where pieces of intrusive stone have freely moved into a higher layer and become embedded. The dip of Figure 1 was approximately 60°E and the strike was approximately 80°N. Note that these are best guess figures as there was no access to a compass.
Site two was an outcropping farther south of the first site. The main feature of interest here was the faulting in the rock. There was an extension or stretching along the fault plane here, which was the primary cause of the fault (Answers to Blackboard Questions, 2012). There is a great offset between the lower layers than the upper layers of the fault. Figure 2 shows the faulting and the sliding of the planes of rock against each other.
Figure 2. The large fault at site two.
There is also some contact metamorphism at this site, where the sedimentary materials, such as mudstone and sandstone have altered into slate. However, as was stated in the Answers to Blackboard Questions (2012), the metamorphism was not enough to alter the rocks into strongly metamorphosed material.
Site three was a near-vertical slate and quartzite rock face that contained a number of faults and prominent dykes. The primary material of the dykes was phenocryst-containing basalt (Answers to Blackboard Questions, 2012). It was noted the dyke in Figure 3 below was lined with a layer of hornfels. Hornfels is a metamorphic rock that was created from sandstone by contact metamorphism when the dyke was formed. This process is called induration (Whitten, 1986). Basalt is an igneous rock that is mafic and extrusive by origin (Smith & Pun, 2010). Activity of the Chillingham volcanics in the Early Miocene period are the most likely origin of this basalt. So it can be assumed that the heat of the magma intrusion through the joints metamorphosed the sandstone into hornfels.
Figure 3. Dyke and jointing in a slate rock face.
In the main, the dyke is younger than the surrounding rock, as it cuts the rock and from the fact that the volcanic process that created it occurred much later. This cutting can be clearly seen in the above figure, however it is interesting to note that the horizontal faulting at the top seems to cut across the dyke, which hints at a more complex geology. Some of the jointing runs parallel to the dyke whereas others cut across at a roughly 45° angle. These appear to have formed before the dyke intrusion.
This site contains a quartzite and slate outcropping on a headland. These rocks were altered by regional metamorphosis but have retained some of their sedimentary character (Answers to Blackboard Questions, 2012). This can be seen in the texture of the rocks, which are grainy and resemble their pre-metamorphic state. During the Chillingham volcanic period in the Early Miocene, hydro thermal fluids were forced through the joints of the parent rock, creating intrusions or veins of quartz. These can be clearly seen below in Figure 4.
Figure 4. Quartz veins in quartzite and slate. Photo courtesy of John Fullerton
Metamorphism and consequent folding had ceased by the time the quartz veins were formed (Answers to Blackboard Questions, 2012). This allowed the joints to relax and widen, permitting the fluids to seep in. There is also a good deal of oxidation in the area – some of which borders the veins and others, as can be seen from Figure 4 at the top left, form fields on the rock surface. This almost “rusty” appearance gives credence to the presence of iron in the parent rock.
At this site there was a large folded outcropping of rock facing east-west. The cause of this was folding by tectonic forces, squashing sediments to the edge of the continent (Stokes, personal communication, 2012). This outcropping also shows extensive jointing and layering, with some of the jointing being pronounced. The colouration differences between the layers is distinct also, with shades of pale orange through to slate greys. This can be seen in Figure 5.
Figure 5. Folding and jointing of metamorphic rocks. There are also prominent faults here.
Of interest is the large unjointed block of rock at the top left. This may well be a later addition to the strata caused by tectonic movement of sediment from elsewhere. Although it is curved, matching the layering alongside it, it shows no jointing or lamination and the surface patina is of a different colour and composition to the surrounding quartzite and slate.
This site was adjacent to site five. Along the ground was a field of bluish-grey pebbles and cobbles, fractured off from the parent rock. The cobbles showed the same layering and composition, though weather and the action of sea water has rounded them to a size ranging from 10 to 20cm is diameter. All were fairly oblate in shape, and none were spherical or spheroid. This oblate shape gives clues that the cobbles have rolled along an abrasive surface, possibly due to wave and/or water action. Over time, the cobbles will degrade into sand, a process that is likely to take millions of years.
All six sites give both a composite and comprehensive picture of the geological processes that occurred. There is regional and contact metamorphism, uplifting, weathering, jointing, folding, faulting and erosion. The geology of Cabarita Beach consists of Quaternary beach and dune sand, Quaternary alluvial gravels, sand and clay, and Silurian sand- and mudstones, mainly in the form of greywacke. The entire area is underlain with Carboniferous Neranleigh-Fernvale sedimentary greywacke, and this extends inland as far as the region between Casino and Lismore, New South Wales (NSW Department of Primary Industries, n.d). There are also Tertiary volcanic intrusions, originating from the Mt. Warning/Chillingham volcano system (NSW Department of Primary Industries, n.d). These occurred in the Early Miocene Epoch approximately 20 million years ago when the Tweed Volcano was at its most active.
Eastern Australia in the Carboniferous Period was under deep water (Cabarita Beach Field Notes, 2012). Successive periods of turbidity currents laid down beds of quartz and feldspar sediment, which can be seen in the graded layering of the rocks. The NSW Department of Primary Industries map classifies this rock as greywacke, but for simplicity’s sake, it has been discussed here as either sandstone or mudstone. Accordingly, these beds became sandstone or mudstone depending upon the grain size of the sediment involved. Uneven sedimentary action resulted in the rip-up clasts visible at site one. There is also folding and jointing, caused by tectonic convergent plate activity that occurred in the late Permian, early Triassic Periods (Roberts & Engel, 2007). Farther along there is evidence of igneous intrusion with a number of basalt dykes, and contact metamorphism as evidenced by quartz veins and oxidation. This occurred, as previously mentioned, in the Miocene Epoch. For the entire geological history, there has been erosion and weathering of all exposed rock, including fracturing and breaking apart of the larger masses into pebbles and cobbles. With this, a timeline can be created, that extends from the Silurian Period of approximately 420 million years ago to the present day.
In summary, the geology of the Cabarita Beach can be divided into two major phases. The first occurred in the Carboniferous Period and involved the laying down of turbidity current deposits in a deep sea environment. These later formed into sedimentary rocks by the process of lithification. As Australia broke further apart from New Zealand, there was deformation and metamorphism in the rocks. At a much later date, in the Early Miocene Epoch, the activities of the Chillingham Volcanics caused further contact metamorphism, with the intrusion of mafic dykes and quartz veins along joints. Since this time, the region has been subject to weather and erosion to the present day.
Answers to Blackboard Questions. (2012). Lismore, NSW: Southern Cross University.
Cabarita Beach Field Notes. (2012). Lismore, NSW: Southern Cross University.
GeoHack – Bogangar, New South Wales. (n.d). Retrieved 7 May, 2012 from http://toolserver.org/~geohack/geohack.php?pagename=Bogangar,_New_South_Wales¶ms=28_20_S_153_33.5_E_region:AU
Google Maps. (n.d). Retrieved 7 May, 2012 from http://maps.google.com/maps?
New South Wales Department of Primary Industries. (n.d). Tweed Heads 1:250 000 Geological Map. Retrieved 7 May, 2012 from http://www.dpi.nsw.gov.au/minerals/geological/geological-maps/1-250-001/tweed-heads-250k-geological-map
Roberts, J & Engel, B. (2007). Depositional and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences, (34)1, 1-20. DOI: 10.1080/08120098708729391.
Smith, G & Pun, A. (2010). How Does Earth Work? (2nd ed.). New York City, USA: Prentice Hall Books.
Stokes, D. (2012, 8 May). Site 5. Personal communication. Retrieved 8 May, 2012 from http://learn.scu.edu.au/webapps/portal/frameset.jsp?tab_tab_group_id=_2_1&url=%2Fwebapps%2Fblackboard%2Fexecute%2Flauncher%3Ftype%3DCourse%26id%3D_121067_1%26url%3D
Whitten, D. (1986). The Penguin Dictionary of Geology. London, UK: Penguin Books.