Thursday 12 September 2013

A history of unstable North Coast sea levels?

Last summer much of the northern rivers area had been hit hard by summer storms. These storms often caused erosion on the fore-dune systems behind some beaches. For example, at Kingscliff this has become a major problem. At other locations this erosion has revealed some hidden features.

In the last few months I had a trip to Coffs Harbour where I was able to walk along some of the lovely beaches. On Diggers Beach I noticed a strange looking band through the exposed face of a dune system that had been recently been eroded away by stormy seas. Upon closer inspection the band was a layer of fine gravel and shell fragments. Underlying this layer of gravel and shell was sand with some isolated gravel which graded into the previous layer. The top of the layer was distinct and comprised of fine well-sorted sand, typical of a dune system. I noted another exposed gravel layer about 50 metres further south along the beach at roughly the same height.

Evidence of a palaeo-beach on present day Diggers Beach.
What struck me about the layer below the dune sand was the similarity of the materials when compared with the deposits of fine gravel and shells that exist on Diggers Beach. The gravel and shells have in places been deposited in the berm by the action of wave swash. I could not help think that what I was looking at was an old berm, some of the remnants of a palaeo-beach (an old preserved beach). The sand on beaches is dynamic. Sand moves inland or seaward because of storms and sediment supply (amongst other things). The difference between this old beach was approximately 1.1-1.2 metres above the present high tide mark.

The height of the palaeo-beach seems to indicate that maybe it was formed by a higher sea level, or a lower ground level. Tectonically eastern Australia has been very stable for millions of years so I think it unlikely that the earth has been uplifted. The most likely explanation in my mind is that the sea level was higher.

Thom & Roy (1983) suggested that Holocene sea levels have been very stable. However, sea levels varied in the time period before the Holocene. The Pleistocene sea levels were much higher and much lower than today. In the Pleistocene on north coast NSW sea level variations were first documented in detail by authors including Den Exter (1974) and Drury (1982). The apparent Holocene sea level low-fluctuation and high-stability of Thom & Roy (1983), if true, would be an aberration.

Baker et al (2001b) used fixed biological indicators to attempt to reconstruct Holocene sea levels. Baker et al (2001b) dated the remnants of tubeworms, barnacles and oysters that occurred above their natural ecological limit (i.e. above the intertidal zone). These indicators can be used to trace sea level changes. Baker et al (2001a & 2001b) undertook this work up and down eastern Australia and compared them with other sites including those in Brazil. The resulting information showed that Holocene sea levels have not been as stable as first thought. The sea level changes have been shown by earlier authors (e.g. Thom & Roy 1983) to occur during periods of known palaeo-climate change.

According to Baker et al (2001a & 2001b) the last time the sea level was 1 metre higher than present was around 2400-1800 years ago. Maybe, the layer is a preserved berm from a beach that existed at the time of the Roman Empire (sometimes referred to as the Roman Warm Period). I don’t know for sure, but to my thinking it seems quite plausible.


*Baker. R.G.V, Haworth, R.J. & Flood, P.G. 2001a. Inter-tidal fixed indicators of former Holocene sea levels in Australia: a summary of sites and a review of methods and models. Quaternary International v83-85 p247-273.
*Baker. R.G.V, Haworth, R.J. & Flood, P.G. 2001b. Warmer or cooler late Holocene marine palaeoenvironments? Interpreting southeast Australian and Brazilian sea-level changes using fixed biological indicators and their d18O composition. Palaeogeography, Palaeoclimatology, Palaeoecology v168 p249-272.
*Den Exter, P. 1974. The coastal morphology and Late Quaternary evolution of the Camden Haven district, NSW. Australia. PhD Thesis, University of New England, Armidale.
*Drury, L.W. 1982. Hydrogeology and Quaternary stratigraphy of the Richmond River valley, New South Wales. PhD Thesis. University of New South Wales. Kensington.
*Thom, B.G. & Roy, P.S. 1983. Sea Level Change in New South Wales over the past 15 000 years. In: Hopley, D. Australian Sea Levels in the Last 15,000 Years: a review. James Cook University, Townsville.


  1. Hi Rod, you may want to check out the paper "Post-glacial sea-level changes around the Australian margin : a review " Lewis et al 2013, which draws on a lot of work including your refs.

    A paper cited and discussed within that meta-study,Sloss et al 2007 is focussed on NSW,and suggests that a Holocene high-stand of +1 to 1.5m was reached about 7500y back and pretty much persisted to around 2000y ago,before falling to current levels [which we have improved upon reasonably smartly ;)]

    1. Hi Nick,

      Thanks for steering me towards the paper by Lewis et al. I will have to read it in detail and I'm in luck as there is an in press version on the good ol' interweb.

      From a cursory reading of Lewis et al I can already see that Baker et al may have used some indicators that are a little unreliable.

      Sadly I don't have easy access to the journal Holocene... though this weekend I might have a look at the State Library or National Library and see whether their e-resources include the journal.

  2. Fascinating entry Rod. I have walked along beaches much further south and seen similar indicators of previous beach levels. The coastal areas of continents are certainly not fixed, but fluctuate with major climate changes.
    Another great post.

    1. Thanks again for the encouragement Wangiwriter. I'd like to have a look around the Wangi area again one day. There is actually a petrified forest that is on the banks of lake Macquarie, though I don't know exactly where it is or how old it is.

  3. The evidence of paleo sea levels is very obvious in the Northern Rivers. If you hop onto Google earth or Google maps (in satellite mode), examine the area between Evans Head and Ballina. You will see a series of old berms extending back from the current day beach for several km inland, reaching as far as the town of Broadwater at one end and Doonbar at the other.

    The Tucki swamp area (further inland) must have been a large lake during the holocene high stand.

    1. You have wonderful timing Anonymous. I actually have a post on the Woodburn Sands scheduled for the 1st of October. In that future post I discuss the extent of that unit which can be found in a very widespread area. In fact if you drill a hole in the Tucki Swamp you can find the sand is actually covered by the recent holocene swamp sediments.

      I should say though that care should be taken with interpreting all of the google maps features in the Broadwater National Park area as being Pleistocene. This is because much of the Pleistocene beach sands have been remobilized into active Holocene dune systems.

    2. A pleasant coincidence for me. I have been fascinated by coastal paleo climatic features since I was introduced to the subject 43 years ago when studying geomorphology. As things would have it I became even more fascinated by the mainframe system that ran my chi-squaresa analyses from field trips, and so ended up spending the last 40 years or so debugging software :(

      I don't profess to know a lot about the geomorphology of the area so coming across your blog was a delight for me. I do trample around the scrub in the area whenever I get the chance because the very recent geological past is imprinted into the landscape so well. I find it very interesting.

      The escarpment behind New Italy (west of Reardons Lane) looks to me like it might have been a coastal headland at some stage. I could be totally wrong however.

      The local Bundjalung people may well have folk stories originating from the Holocene.