Our Presenter:

Allan Donaldson is a retired professor from the Department of Geology, Carleton University in Ottawa. His specialty is sedimentology, with an emphasis on rock strata of Precambrian age.

The Presentation: 

Stromatolites are distinctive layered structures formed by biofilms of cyanobacteria. They provide an impressive record of the only life that existed throughout the first 90% of our planet’s 4.6 billion-year-old history. When more complex life forms evolved at the end of the Precambrian, predation on cyanobacterial precursors was a predictable consequence, resulting in a paucity of stromatolites thereafter. In much of the Ottawa Valley, however, stromatolites are remarkably abundant in Ordovician strata, and some occur in the underlying Cambrian strata. An environmental condition unsuitable for biofilm browsers such as gastropods is the likely reason. Absence of gastropod fossils within the stromatolite-bearing strata, along with associated evidence of evaporitic conditions, suggest that hypersalinity is the likely explanation for abundant stromatolites throughout the Ottawa Valley. Hamelin Pool in Shark Bay, Australia, provides a present-day example of such an environment.

Biography:

Allan Donaldson (BSc Queen’s 1956, PhD Johns Hopkins 1960) spent a decade working for the Geological Survey of Canada, followed by 35 years as a professor at Carleton University. His teaching and research specialty is sedimentology, with an emphasis on studies of strata of Precambrian age. In retirement Allan continues to contribute to geological outreach projects in Eastern Ontario.