Stromatolites are the largest and most impressive biologically-made structures in Precambrian rocks. If they are fossils, they have the longest stratigraphic record of any organism; reaching back 3.5 billion years

Living stromatolites at low tide at Shark Bay, western Australia. (Photo by Paul Hoffman (c))

Shark Bay is a large embayment off the Indian Ocean located a long day's drive north of Perth along the North West Coastal Highway that crosses arid Western Australia. This bay has become somewhat of a sanctum sanctorum for sedimentary geologists because it is the only place in the world where the living algal mounds called stromatolites approach the dimensions and the variety of shapes they achieved during their heyday in the Precambrian. First discovered here in the late 1950s, the best stromatolites in Shark Bay are found in the intertidal zone along an inner lagoon called Hamelin Pool. They flourish here because the high salinity -- twice that of normal sea water -- keeps out algal grazers such as chitons and snails. The shape and size of the Hamelin Pool stromatolites are directly controlled by the immediate environment. Large mushroom-shaped stromatolites grow at headlands where waves and tide scour approach from different directions. Loaf-shaped stromatolites occur in protected bights perpendicular to the shore.

Stromatolites are solid cauliflower-shaped structures built by consortia of microbes. A thin sticky carpet of oxygen-producing photosynthetic cyanobacteria coats the sunlit top surface while microbes that depend on fermentation survive in the dark recesses just below. The cyanobacterial carpet traps sediment and also promotes the precipitation of carbonate to build up solid cauliflower-shaped structures. Because their growth is controlled by tides, temperature and sunlight, stromatolites are rhythmically laminated on the sub-millimetre scale.

Exhumed stromatolites 1.8 billion years old from the East Arm of Great Slave Lake. (Photo by Paul Hoffman (c))

Cross-section of Great Slave Lake stromatolites. (Photo by Paul Hoffman (c))

Paul Hoffman was one of the geologists who travelled to Shark Bay in the 1960s to take a close look at these living stromatolites. Hoffman, a Canadian, was pursuing a Ph.D. thesis at Johns Hopkins University in Baltimore on the stratigraphy of late Precambrian rocks exposed in the East Arm area of Great Slave Lake. The East Arm reaches 300 kilometres northeastward from Great Slave Lake into the granite heart of the Canadian Shield. The islands and the long finger-like peninsulas that choke the East Arm are largely made of sedimentary rocks with superbly preserved and exposed stromatolites of the Pethei Group and of volcanic rocks. Despite their great age (about 1.8 Ga), these rocks are well preserved and they probably have never been deeply buried.

The shallow-water stromatolites of the Pethei are strikingly similar to the living shallow-water stromatolites in Shark Bay. The loaf-shaped fossil forms, arrayed perpendicular to the shoreline, were clearly controlled by the same environmental forces as the living forms. Hoffman showed that quite different stromatolites grew in deep water settings of the Pethei. These are minuscule cones and stunted columns that stood only a couple of centimetres high. Living in perennial darkness, these dwarf stromatolites could not have been made by photosynthetic cyanobacteria. They were probably formed by microbes sustained by fermentation.