Distribution & Morphological Variability of Hamelin Pool Stromatolites

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The Distribution and Morphological Variability of Hamelin Pool Stromatolites, Shark Bay, Western Australia – Shedding New Light on a Fifty-Year Old Riddle Gumpei Izuno 1 , Robert Burne 2 & Murray Batchelor 3 1 (g_izuno@coastal.t.u-tokyo.ac.jp) 2 ( robert.burne@anu.edu ) 3 (murray.batchelor@anu.edu.au)

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In 1954 the first modern analogues of the “ Stromatoliths ” described by Kalkowsky were recognised by Richard Chase fringing the shores of Hamelin Pool, Western Australia. Heeseberg Hamelin Pool Photo:Josef Paul

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In 1961 it was thought that the Hamelin Pool Stromatolites formed only in the intertidal zone

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By 1974 it was realised that stromatolites extended from the high intertidal zone to subtidal depths of about 2 metres in Hamelin Pool.

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By 1979 several authors had attempted to relate variations in stromatolite structure to differences in microbial communities at different elevations .

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Burne & James (1986) proposed an alternative model in which - stromatolite growth initiates only in subtidal environments the present intertidal distribution is a result of falling sea levels and modification of relict forms by intertidal microbial communities.

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Meischner [1994] agreed that Holocene stromatolites grow subtidally, however a careful review of the localities described by Prof. Meischner has found no evidence to support his conclusion that some of the Hamelin Pool stromatolites are of Pleistocene age. From Meischner (1994) Re-examination shows Holocene stromatolite growth over Pleistocene limestone

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In this study we have precisely surveyed the elevation of stromatolites and related this to records of the short term sea level variations due to tidal and wind effects Surveyed Elevations Tidal Variation

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Shallow Deep Flat mat or flat bed Eroded flat mat Column shape cut off from flat mat by erosion Column shape, Club shape, Cauliflower shape etc. Arrayed column shape Dome shape Flat shape Column shape Cut off by current-flow mechanisms such as tidal activity, currents, and wave action Weak sunlight Strong sunlight Intertidal Sub-tidal Constituent materials are the same, and consist of shell fragments, foraminifers, ooids and fragments of stromatolites Method of growth at the microbial mat colony level is similar, but macro shape is different Accept weak erosion Accept strong erosion Accept weak erosion Belt shape and Carpet shape Wave base Results in strong elongation above wave base Results in rounded shape in intertidal areas Results in dome shape in sub-tidal areas HTL LTL -3.5m Stromatolite shape varies consistently with position relative to present sea-level

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VARIATION IN INTERTIDAL STROMATOLITE SHAPE Flat forms dominate the high intertidal zone Domical & Cauliflower-shape d stromatolites are found in the lower intertidal zone ,

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VARIATION IN SUBTIDAL STROMATOLITE SHAPE Columnar-shape s dominate in shallow subtidal environments Domical forms occur in deeper water The deepest examples are all small do mes .

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Stromatolites at similar depths all have similar relief.

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We have modelled stromatolite distribution under different conditions of - depth limits for stromatolite growth stromatolite growth rates rate and direction of sea-level change periods suitable for stromatolite growth

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Preferred Model principal growth occurs between mean sea level and a depth of 2 metres growth rate is 5 mm/decade growth commenced 1,500 years bp relative sea level has fallen by 2 metres in the past 4000 years.

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These conditions correspond to our analysis of variation in stromatolite height relative to mean sea-level. They explain why - the tallest structures occur in the shallow subtidal zone the stromatolite relief decreases toward both the upper intertidal zone and toward the deeper subtidal limit of occurrence. stromatolites at similar depths all have similar relief.

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References Burne R.V., ( 1991-92 ): Lilliput ’ s Castles: Stromatolites of Hamelin Pool. Landscope Vol.7 No.2 Summer ed. p. 34-40. 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. Golubic S., (1992): Stromatolites of Shark Bay. – In Margulis L. & Olendzenski L., (eds) Environmental Evolution: Effects of the Origin and Evolution of Life on Planet earth, p. 131-147, Cambridge (MIT Press). Kalkowsky, E., 1908. Oolith und Stromatolith im nord-deutschen Buntsandstein. Z. dt. geol. Ges. 60, p. 68–125. 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. – In Logan B.W., Read J.F., Hagan G.M., Hoffman P., Brown R.G., Woods P.J. & Gebelein C.D., (eds) Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia. American Association of Petroleum Geologists, Tulsa, Oklahoma, Memoir 22, p. 140 – 194. Meischner D., (1994): Stromatolites in Shark Bay, Western Australia: Interglacial and Subtidal Origins. – In : Awramik S.W., (ed): Death Valley International Stromatolite Symposium, Laughlin, Nevada, U.S.A., 15-17 Oct 1994, Abstract with Program, page S.47. 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. Playford P.E., (1990): Geology of the Shark Bay Area, Western Australia. – In Berry P.F., Bradshaw S.D. & Wilson B.R., (eds) Research in Shark Bay, p. 13 – 31, Western Australian Museum.