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Sea Level variation and the Zonation of Stromatolites in Hamelin Pool, Shark Bay

Robert V Burne1 and Ken Johnson2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Fenner School of Environment and Society, Australian National University, Canberra, ACT 0200, Australia

The zonation of stromatolites of Hamelin Pool, Shark Bay, Western Australia (see Fig. 1), has, since their discovery in 1954, been related to tidal variations of sea-level.  Initially these stromatolites were considered to be entirely intertidal ecosystems (Logan 1961), though they were later recognised also in shallow subtidal waters (Logan et al 1974). Data from various sources were drawn together into a notional cartoon by Playford (1980) that summarised his understanding of the tidal zonation of stromatolite types in Hamelin Pool.  This cartoon has been frequently cited since to account for stromatolite zonation in Hamelin Pool.  Detailed surveys of stromatolite types by John Bauld (pers. comm) confirm the conclusion of Burne and James (1986) that Hamelin Pool stromatolite principally grow in shallow subtidal  environments where they are colonised by colloform microbial mats, with some being later exposed by falling sea levels .  The intermittently exposed stromatolites are re-colonised by smooth or pustular microbial mats. 

The application of modern methods of time-series analysis to  a record of sea-level variation at Flint Cliff, Hamelin Pool between October 1983 and April 1985 has shown that tidal variation accounts for only a minor component of sea level variation in Hamelin Pool (Fig. 2). We have identified five key components of the variation of sea level in Hamelin Pool; a seasonal oceanic cycle; a short term irregular cycle; the complex tidal system in the Pool ;  isolated major events; and less marked variations, still able to defeat the tide in the short term, probably by wind stress. Clearly it is not valid to conceptualize the zonation of stromatolite types in terms of tidal variation alone. The dominance of a seasonal cycle is the fundamental determinant of the timing of immersion and exposure. We conclude that the zonation of of microbial communities in Hamelin Pool (Fig. 1) is controlled by duration of periods of inundation and exposure in the littoral zone, but that tidal variation is not the major cause of this variation.  The stromatolite forming colloform mat is virtually never exposed.  The exposed stromatolites have been stranded by relative fall of sea level, and are colonised by intermittently submerged microbial communities capable of modifying but not creating the club-shaped stromatolites characteristic of this locality.

References

Burne, R.V., and James, N.P., (1986) - Subtidal origin of club-shaped stromatolites, Hamelin Pool, Shark Bay. 12th IAS International Sedimentological Congress, Canberra. Abstract Volume, Page 49.

Logan B.W., (1961). Cryptozoon and associated stromatolites from the Recent, Shark Bay, Western Australia. Journal of Geology, Vol.69, No. 5, pp. 517- 533

Logan B.W., Hoffman P. & Gebelein C.D., (1974): Algal Mats, Cryptalgal Fabrics, and Structures, Hamelin Pool, Western Australia.. American Association of Petroleum Geologists, Tulsa, Oklahoma, Memoir 22, p. 140 – 194.

Playford, P.E., (1980):  Environmental controls on the morphology of modern stromatolites at Hamelin Pool, Western Australia. West. Aust. Geol. Suvey Annual Report (for 1979), Pages 73 - 77.


Figure 2. The first pass of the time series analysis identified the varying amplitude characteristic of tidal activity, the seasonal cycle of sea level, and an irregular 11 day cycle.