Journal of Conference Abstracts

The Ecological Setting of Hamelin Pool Stromatolites and the Holocene History of Shark Bay

Robert Burne (burne@interact.net.au)

Dept of Geology, Australian National University, Canberra, ACT 0200, Australia

The ecological significance and evolution of the Stromatolites of Hamelin Pool have remained the subject of considerable controversy since their discovery. Detailed study of the coastal regions of Hamelin Pool, Shark Bay, Western Australia, combined with analysis of vibrocores from the sea-floor, have enabled reconstruction of the Holocene History of the area and the growth of its stromatolites. During the last interglacial Hamelin Pool was contained marine waters of normal salinity, but the glacial regression left Hamelin Pool exposed. About 7000 years B.P. a barrier at the site of the Fauré Sill protected Hamelin Pool from rising sea level, but the deeper part of the Hamelin Pool Basin contained a lake with seasonally fluctuating salinity. About 6000 years B.P. sea water of normal salinity flooded the area, and sea-grass grew around the Pool margins, and on the Fauré Sill. Transgression continued until about 4000 years B.P. when sea level reached about 2 metres above its present relative level. Sedimentation on the sea grass banks raised the level of the Fauré Sill to near sea level. This restricted exchange of water between Hamelin Pool and the open sea, and salinity increased to 1.5 that of normal sea water. Sea grasses no longer grew within Hamelin Pool, and other organisms became restricted. However, large, though stunted, populations of the bivalve Fragum erugatum flourished, and their shells accumulated as extensive beach ridges. Over the past 400 years relative sea level has fallen, and the Fauré Sill has eroded to its present form of a bank traversed by tidal channels. The further restriction of Hamelin Pool has resulted in a gradual increase in salinity to its present level of about twice that of normal sea water. Because of the restriction of competitive benthic organisms, stromatolites started to develop about 2-300 years B.P. They colonised waters less than 4 metres deep, and the older ones have now become stranded in the intertidal zone by falling sea level.

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