HAMELIN POOL (Shark Bay, Western Australia) —AN ESTUARY?

In writing the blog on estuary health and management, I was prompted to think about the diversity of estuary types around the Australian coast. Many have written on this topic. I am reminded of the work done under the auspices of the former Coastal CRC and its input into “OzEstuaries” database with support of Geosciences Australia. Then there is the great work by Eric Wolanski in his book on Estuarine Ecohydrology (2007, Amsterdam, Elsevier). The very detailed study of Broad Sound Estuary in Queensland by Cook and Mayo (BMR, Bulletin 170, 1978), and the much-cited paper of Peter Roy and others in Estuarine and Coastal Shelf Science (2001, 53, 351-384) are two other studies that I call upon from time to time. But the first paper that enlightened me on the geomorphology of Australian estuaries was by Joe Jennings and Eric Bird presented at the Conference on Estuaries held on Sapelo Island, Georgia, in 1964. It appeared in a volume edited by George Lauff in 1967 entitled Estuaries (Publication No.83, American Association for the Advancement of Science, Washington,755pp). The Jennings-Bird paper highlighted diversity in geomorphological type, climatic and tidal conditions, and what they termed “biological factors which vary regionally around the margins of the continent” (p.121).

The Lauff volume covered many aspects of estuary science. Papers on estuary classification, physical factors, geomorphology, sedimentation, biota, nutrients, ecology, fisheries, and human influences cover a lot of research up to the mid-1960s. These studies formed a basis for much research since then as seen in the work of Wolanski and others. Classification of estuary type revolved around approaches based on geomorphology (drowned river valley etc), circulation (well mixed, partially mixed, inverse etc) and biota. But it was interesting to see  varied perspectives on what was the working definition of an estuary provided in the opening chapter of the book by Donald Pritchard: “An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage” (p.3). There were several examples cited where the influence of sea water was minimal (e.g. Amazon), or where freshwater inputs were of less significance than tidal forces (e.g. King Sound, WA, cited in the Jennings and Bird paper, Fig.6, p.125). Recognition that estuaries are more than the tidal mouths of rivers, and that some for much of the time could be fully saline is picked up in the more comprehensive definition of Eric Wolanski in his 2007 book stating that estuaries are: “ semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff; however, the freshwater flow may not be perennial, the connection to the sea may be closed for part of the year and the tidal influence may be negligible”.

This brings me to Hamelin Pool (or Basin) which forms part of the Shark Bay complex of reaches, sounds, passages, basins, and pools in Western Australia. From the outset, I must confess not to have visited Shark Bay except for a brief fly in and out experience. I am very envious of those who have worked there and studied its diverse physical and biological conditions. But I had the pleasure while a graduate student at LSU to attend a set of visiting lectures by Brian Logan of University of WA. One reason for going to LSU was to learn more sedimentology and petrography by taking John Ferm’s sedimentary petrology courses. This was how I got to hear Brian. He expanded on his study of carbonate sequences from different environments around Shark Bay. His work along with colleagues was later brought together in 1974 in a volume published by the American Association of Petroleum Geologists, Memoir 22 (Logan, B., et al., “Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia”).

There are many papers in this memoir which discuss how particular environmental factors have over time influence sedimentation. The one that attracted me for purposes of this blog relates to the formation of stromatolites in Hamelin Pool. These “cryptalgal” structures have long attracted geologists as they are extremely abundant in Proterozoic and lower Palaeozoic carbonate rocks and serve as indicators of intertidal environments. Logan et al. write that “Of modern occurrences, only the blue-green algae in Hamelin Poll rival their ancient counterparts in size and variety of structures” (p.140). There are various types of algal mats occurring mainly in the intertidal-supratidal platform although they also occur in shallow depths on what they term the sublittoral platform. It is the particular sea level history, tidal exchange and climate conditions that allows these microbial communities to flourish over the last 3000 years.

Hamelin Pool is separated from the open waters of Shark Bay by what is called the Faure Sill—a wall of sand built up around seagrass banks since sea level reach around its present position c.7000 years ago. Channels across the sill allow some tidal exchange, but the shallow basin cannot be completely flushed. Low rainfall in the area limits nutrient input and coupled with high salinities (nearly double sea water in summer) allows the microbes that build stromatolites to flourish. Evaporation of sea water helps drive up the salinity which falls only slightly in winter months. The Wooramel River to the north of the basin appears to have little influence on Hamelin biota and sedimentary processes. Adjoining basins such as Freycinet have salinities much closer to that of the open sea.

In this semi-arid area where there are no rivers flowing into the semi-enclosed basin, a very distinctive set of conditions occurs that could eliminate the Hamelin Pool from being classified as an estuary. Yet it may also be seen as an endmember of a type where marine conditions are altered by its geomorphology and climate conditions to create these very peculiar sedimentary environments.

Note: I have a very informative poster on the Stromatolites of Hamelin Pool. If anyone would like it, please email me at bruce.thom@sydney.edu.au and I will send you the poster ( first come, first served!).

Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2020. For correspondence about this blog post please email austcoastsoc@gmail.com 

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