Way back in 1972, Don Wright, Jim Coleman and I wrote a short paper on “Emerged tidal flats in the Ord River Estuary, Western Australia” (Search, 3, 339-341). It is a pity that a journal like Search no longer exists to take short notes on Australian science issues. In the paper we noted that the possibility of progressive modification and redistribution of process intensities leading to the development of “emerged” phenomena within estuaries had received little attention in Australia.
A relict tidal flat was examined at 2m elevation above present-day high tide limit covered by Holocene alluvium. Our understanding of tidal dynamics of the Ord estuary led us to explain this elevated tidal flat not as the result of Holocene sea level fall, but more likely due to progressive accumulation of sediments in the estuary involving change to the tidal range at the site. We suggested that prior to the development of the modern Ord Delta at the time sea level was reaching its present position around 6000-7000years ago, frictional attenuation would have been less and tide wavelength would have been several times greater that today. Under such circumstances tidal amplitude in the vicinity of the relict exposure would have been higher than today. We concluded that “The possible role of similar modifications of the dynamic regime should be seriously considered in evaluating the cause of emergence in other estuaries”.
I have kept this thought in mind over the years as I examined other estuaries and read the many papers on Holocene relative sea level (RSL) change around the Australian coast, noting along the way how infrequently the Wright et al. 1972 paper had been cited. The driving forces for Quaternary relative sea-level changes are well covered the excellent book by Colin Murray-Wallace and Colin Woodroffe (2014, Cambridge University Press). They document evidence for changes in many locations which can be attributed to different forces. It is often very difficult to attribute cause as different forces may be operating at the same place over time to elevate or lower RSL. I have found it difficult to determine just what the dominant forces are and what are less dominant since eustatic sea levels reached around present position. Over the years I have grown more cautious not just on attribution but also on accuracy of position, indicator and dating uncertainty. This has led me to adopt the conservative envelope approach.
The Gippsland Lakes offers a field laboratory for examining the role of different factors that affect RSL change. Edmund Gill, Eric Bird and others have published on the subject. My drilling and dating project in the 1980s led me to think further about RSL here especially from a morphodynamic perspective based on previous work in the Ord and along the east coast.
Last Interglacial sand barrier (Inner Barrier) at Gippsland offers similarities with what we studied in Port Stephens-Myall lakes area. Similarly the Outer Barrier in both areas can be dated as mid-to late Holocene in age. One stark similarity for me was the closure of two relict flood tidal deltas around 4000-5000 years ago, the Broadwater (part of Myall Lakes) and at Ocean Grange, Gippsland. Furthermore, both areas display extensive washover fan deposits behind Outer Barrier foredunes ridges; they are more obvious at Gippsland bordering Lake Reeve. Radiocarbon dating suggests an age pre-5000 to less than 7000 although more dating is needed.
This morphostratigraphic background indicates the interplay of several forces in building the Outer Barrier, in blocking a large mid-Holocene tidal entrance, and in modifying the tidal regime in the lakes. The large foredune dominated Outer Barrier appears to have been mostly constructed in the mid-Holocene. Sand swept landward from an inherited transgressed disequilibrium profile took about 2000 years leading to offshore conditions more adjusted in terms of profile geometry to prevailing wave conditions. But before that condition was reached a more energetic shoreline prevailed with waves sweeping across young barrier to form the washover fans. In one sense an energy window was closed as the barrier grew and wave surges became less effective. During this time sand was building the massive flood tide delta at Ocean Grange.
My conceptual model involves a lake system that was fully flushed with tides and hence experiencing ocean tide levels around 6000 years ago. Tidal regime became quickly modified as frictional attenuation took effect across a growing flood tide delta until around 4000-5000 years ago the entrance blocked as a result of foreshore-beach-dune sand overwhelming the entrance with its reduced tidal outflows. The tide range dropped to zero around the larger lakes such as King, Victoria and Wellington leaving stranded as “emerged “any high tide deposits. The entrance shifted towards Cunningham Arm and may have periodically been blocked until artificially opened at Lakes Entrance in the 1880s.
The morphodynamic approach offers another way to look at RSL change in estuaries. Undoubtedly other factors may be at work where “emerged” deposits occur. The reasons why deposits occur at particular levels can be quite complex but it often pays to look at how sediment movements over time can influence estuarine dynamics and sedimentation.
Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2017 for correspondence about this blog post please email firstname.lastname@example.org