Narrawallee Beach
This area looks like a typical Australian east coast beach with a long curve of gold sand and a distinct peninsula at the Northern end, surrounded by bush and a small inland tidal lake. On its own it is a great place to visit, walk and relax. The peninsula is an impressive stand of layered rock with a thick basaltic base. The all too common sight of layered rocks at the coast is easy to dismiss, I found this isolated point to be an impressive example of the layering, erosion and continental movement encapsulated in one place. The layers are formed as part of the Sydney Basin.
Geological history:
In the Cambrian and earlier period only the far west of New South Wales existed in its present form. It was joined to what is now Western North America in the Rodinia supercontinent. North America was detached opening up the Pacific Ocean. In the Ordovician period, sediment deposited on the newly formed sea floor, and in the Silurian a back arc basin behind a chain of volcanoes was formed and then accreted back onto the east coast of Australia to form the Lachlan Fold Belt. The main structure in this is north-south, and this is reflected nowadays with the orientation of rivers and mountain ranges.
The Sydney Basin:
The Sydney Basin consists of Permian and Triassic sedimentary rocks stretching from Newcastle to Batemans Bay. The land component covers 44000 km2, but there is another 5000 km2 underwater on the continental shelf. The basement consists of the Lachlan Fold Belt, and the New England Fold Belt is found to the north across the Hunter Thrust. It is a foreland basin, likely to be formed by compression loading.
The first rocks formed in the Permian were the Dalwood Group and lower Shoalhaven Group. Material for this came from basaltic volcanoes to the north in the Hunter region. The Greta Coal Measures then formed in the north side in an alluvial fan from the mountains of the New England Orogen. The basin sank lower and sea water came much further inland, covering the Permian sediments and the basement further west, and the Maitland Group and upper Shoalhaven Group were deposited. The Muree/Nowra Sandstone formed in the centre of the basin.
In the next stage the Hunter – Bowen Orogeny cause faulting and folding on the north side of the basin. In the Late Permian river and delta deposition formed the Tomago and Whittingham Coal Measures in the north. But deepening seawater formed the Kulnura Marine Tongue and Bulga Formation. In the southern part of the basin a delta formed the lower Illawarra Coal Measures. The Erins Vale Formation interrupts this. Marine transgression formed Dempsey Formation, Denman Formation, Bargo Claystone, and Ball Bone Formation. Beach deposition then formed Waratah/Watts Sandstone and the Darkes Forest Sandstone and Angus Place Sandstone. Over the beach land formed again with the Newcastle and Wollombi Coal Measures in the north, and upper Illawarra Coal Measures in the south. Early in the Triassic the New England Fold Belt was uplifted and an alluvial flood plain formed the Narrabeen group. The Bald Hill Claystone, which is a redbed containing laterite was formed by weathering of the Gerringong Volcanics.
The Hawkesbury sandstone is the prominent rock found in Sydney, and also forming the top of escarpments in the area, such as the Illawarra escarpment. This is overlain by the fine sandstone of the Mittagong Formation and the Wianamatta shale.
The basin was later buried under 1 to 4 km of Jurassic and Cretaceous sediments. The Tasman Sea formed by rifting and underplating of the basin may have happened in Aptian times. This resulted in the upper sediment being completely eroded off…..and here we find ourselves standing on a beach!
Questions:
Area 1
- What is the thickness of the basaltic layer?
- There are three distinct layers of colour present, albeit comprised of hundreds of layers of silt, stone and rock. What are the colours and in what order are they layered?
Area 2
- What are the sizes of quartz pebbles visible at this point?
Please submit your answers as soon as you can. It's ok to post prior, I will check your findings and respond if there are issues. Feel free to add a photo of yourself at the site if you want too. Group visits are encouraged. I hope you enjoy the experience, this is a very special area.