Analysis of the Rock Formations at Cabarita Beach, Bogangar, New South Wales, Australia.

In the first year of my degree, I undertook a unit called Earth Systems 1: The Lithosphere. The subjects aim was to develop an understanding in the earth’s geological systems on which all life relies. The major assessment task was a consultancies report  (as opposed to a scientific style report which we would normally write) regarding the rock formations at Cabarita Beach at far north New South Wales. I’ve decided to revisit this paper and to share it with you. It’s been a funny reflection for me, because I find this paper to be quite simple compared to what I write these days. I guess three years of education has taunt me one thing: how to write a good paper. I hope you will enjoy it.

Introduction

Location

Boganbar is a beach side town in the Northern Rivers region of New South Wales, Australia. All six sites for this exercise are located on the local beach, Cabarita Beach(Figure 1). Road access is via tarred roads, and access to the beach sites is available at the end of Pandanus Parade via a public access ramp.

Figure 1 Google Map showing the location of all six sites and public access ramp. (Photo Credit: Google)

Evidence of History

The geography and geology of this area was exceedingly different 400 million years ago during the Late Paleozoic Era. The coastline laid further west then it is currently located and was in close proximity to a chain of volcanic mountains that included the Tweed Volcano. Wollumbin -Mount Warningis the central volcanic remnant of this ancient shield volcano. Beyond this mountain chain, the then active continental margin extended seaward and the present coastline was under more than 200m of water (CBEH, 2010). It was during this era that the layers of sands and muds were deposited that would later form the graded sedimentary rock bedding sequences that is visible at this location.

The sedimentary basin experienced some tectonic activities as ancient ocean flooring was subducted beneath the continental plate approximately 360 – 280 million years ago (Willmott, 1986). At such times, more sediment was scraped onto the continental plate causing folding and crumpling of existing bedding during some low-grade regional metamorphism as the area was uplifted (CBEH, 2010).

It was during the late Mesozoic Era approximately 210 million years ago (Willmott, 1986), that the area experienced an onset of volcanic activity. Dykes that remain intact and visible give evidence to a changed landscape one of heavy volcanic activity. The faulted landscape surrounding the site is further indicators. It is estimated that the Tweed Volcano erupted just 20 million years ago (Seach,ND; NA, 2009), during the early Cainozoic Era, layering the area with further sedimentation.

Site Description

Site One

Site one introduces us to all the sedimentary rock types present at all the sites. The area has been exposed to gentle and medium currents allowing for the formation of light coloured medium grained sandstone in the former, and dark coloured fine-grained mudstone in the later. It is presented today as a classic bouma sequence of graded bedding. The lack of traditional layer cake formations indicates that did not commence horizontally, and at some point in the past has “fallen” to its current position. Some of the other structures visible at this site include:

• Rip Up Clasts – largely eroded out of the surrounding rock, leaving enough material to tell what once filled it.
• Flame Structures – wavy mud structures resembling flames projected onto the surrounding sandstone.
• Cross Laminations – formed in the lee side of bedrocks as ripples due to sediment movement by tidal water.

 

Figure 2 Site one demonstrates a primary sedimentary graded bedding with several features including Rip up Clasts and Flame Structures. (Photo Credit: Joseph Sparks)

 

Figure 3 Cross Lamination as seen at Site One. (Photo Credit: Joseph Sparks)

Site Two

Site two consists of light coloured medium grained sandstone and dark coloured fine-grained mudstones that form a sedimentary rock structure.  The fault that is present at site two must be viewed from the top of the rock structure (figure 4). This structure is only 1 – 2 meters high and is easily climbed. The fault off set is approximately 50cms. Other smaller faults may be seen at some of the other sites. A small fault of 4cms was found in a boulder at site five.

 

Figure 4 The fault at Site Two is difficult to see unless standing over the structure. (Photo Credit: Joseph Sparks)

Site Three

Site three consists of sedimentary rock structure formed from light coloured medium grained sandstones and dark coloured mudstones.  The dominant dyke at this rock face has been offset at the top by a slump by approximately 20cms, and is intercepted at several points by joints such as that in figure 5. The dykes all contained mafic igneous rock formed from cooled magma, and was identified as basalt. The basalt contained minerals such as feldspars and quartz. Further analysis is needed to determine if other minerals are present in the basalt (figure 6) as basalt formed at 600^oC can also contain muscovite and orthoclase (Scheffers, 2009).

 

Figure 5 One of the three dykes found at Site Three. (Photo Credit: Joseph Sparks)

 

Figure 6 A close up of the basalt within the dyke containing Feldspar and Quartz. (Photo Credit: Joseph Sparks)

Site Four

Site four is located close to Norries Head just off of the main beach. This location can be difficult to gain access and is obtained via the rocky headlands. The eroded rock surface shows signs of chemical and physical weathering, and features many quartz veins, known as Cherts, such as those in figure 7. These veins form when aqueous solutions carry minerals ooze through fractures in the mudstone (Scheffers, 2009).

 

Figure 7 Quartz veins as seen at all sites including Site Four. (Photo Credit: Joseph Sparks)

Site Five

The dramatic folds present at site five are showing signs of physical and chemical weathering caused by the tidal sea waters. The rock face display faults and joints amongst the sedimentary layering, however, the sediments at this end of the beach display mineralogical and textural changes resulting from a regional metamorphism. This is evident by the less distinct layering indicating that recrystallisation of the rock structures has occurred.

 

Figure 8 Folds such as this one were seen at a few of the sites.  (Photo Credit: Joseph Sparks)

 

Figure 9 Folding in sedimentary rock as seen at Site Five. (Photo Credit: Joseph Sparks)

Site Six

Mass wasting is evident at several sites, and includes wasting events such as slumps, falls and slides. Site six shows evidence of a fall, with much of the debris loose at the bottom of the slope originating from this rock face. The boulders at the bottom are showing signs of erosion as they are moved backwards and forwards short distances on the tidal waves. Eventually the boulders will be reduced to sand through such actions, having been at the mercy of physical erosion (Grotzinger, et al, 2007).

 

Figure 10 Mass Wasting events were seen at multiple sites, featuring slumps and falls as indicated here. (Photo Credit: Joseph Sparks)

Conclusion

The geology of this area commenced formation around 400 million years ago (Willmott, 1986) when the geography was vastly different then it is today. The current coastline was miles inland from this location. This site was under vast pressure as it was 200m underwater (CBEH, 2010). This helped form the geology that we see today in this area.

The rock sediment that is visible through out the sites was accumulated over more than 200 million years. The joints, faults, and folds that are found throughout all sites are evidence of tectonic movement through uplifting and crumpling between plate movements (Willmott, 1986).  The continental margin that produced this movement through its activities has since ceased to exist, making the margin passive (Clarke, 2010).

The dykes and basalt found is a constant reminder of the volcanic era of this area ranging over the last 200 million years that ended with the eruption of the Tweed Volcano 20 million years ago. The Wollumbin – Mount Warning is the massive remnant of this shield volcano that dominates the surrounding environment, and is one of the largest of this type in the world (NA, 2009; Seach, 2010).

The mass wastage events we see today result from both chemical and physical erosion indicate the decay of the locations history. The rip up clasts allows us to see deeper into the formations without destroying structure. The boulders present at Norries Head were once a part of this greater rock face but today are at the mercy of the waves. Ultimately, they will be reduced to sand, eroding with them the visible history of this locations formation (Grotzinger, et al, 2007).

A concise sequence of events describing the formation of this landscape can be described thus:

• Turbidite bedding deposited in a deep-sea environment. Each deposit adds to the graded bedding.
• The sediment is buried and is lithified.
• The sedimentary rock is exposed to metamorphism associated by mountain building along the active plate margin. This results in folding and faulting of the rock bedding.
• Dykes form along existing join structures crossing folds and other metamorphism.
• Mineral rich aqueous solutions run thru jointing forming quartz veins.
• Further joining occurs.
• Major faulting offsets some dykes.
• Uplifting from within the earths crust brings the rock beds to the surface and exposes it to the elements of weathering to reveal what is the current landscape (CBEH, 2010).

Acknowledgements

I would like to thank Malcolm Clarke for his contagious enthusiasm in geology. The time you have given us will not be forgotten.

I would like to thank Jason and Simon who help me to understand and forgave my questioning and panic.

And I would like to thank Joe Sparks, who helps me in all ways.

References

Clarke, M. (2010): Coastlines and Ocean Basins

Grotzinger, J., Jordan, T. H., Press, F., Siever, R., (2007): “Understanding Earth.” Fifth Edition. Published by W.H. Freeman and Company.

NA, (2009): Big Volcano Tourism Marketing and Media: “The Caldera of the Tweed Volcano.” (http://www.bigvolcano.com.au/natural/wollum.htm) Retrieved May 2, 2010

NA, (2001): “Cabarita Beach Excursion Handout 2010” (CBEH).

NA, (ND): Tweed Shire, Jewel in the crown of the Northern Rivers. “Mount Warning in the Tweed Coast Hinterland. Far North Coast of NSW, Australia. Australia’s Green Cauldron.” (http://www.mtwarning.com/) Retrieved May 3, 2010.

Scheffers, A. Dr, (2009) “Earth System 1: The Lithosphere. Study Guide.” Third Edition. Published by Southern Cross University.

Seach, J. (n.d): Volcano Live: “Mount Warning Volcano – John Seach” (http://www.volcanolive.com/tweed.html) retrieved May 2, 2010

Willmott, W. F., (1986) Rocks and Landscapes of the Gold Coast Hinterland. Published by Geological Society of Australia.