MICROBIAL FRAMEWORK BUILDERS OF A CRYOGENIAN REEF: THE PERSISTANCE OF EVOLUTION TOWARD THE CAMBRIAN

The reef building communities of the Precambrian were dominated by stromatolites with contributions from calcified microfossils (calcimicrobes). Reefs dominated by calcimicrobes began to occur in the mid to late Proterozoic. However, they display similar ecological homogeneity to stromatolite reefs. Studies of Cryogenian carbonates of the northern Flinders Ranges in Australia reveal the presence of a unique and diverse community of non-stromatolitic microbes. It is clear that these microbes interacted to form a rigid framework that ultimately produced a large biological reef.

These framework building microbial textures have affinities with calcimicrobes. They consist of clots or films of carbonate (dolomite or calcite) micrite and have a variety of morphologies. In order of volumetric importance, they include: a dendritic clotted micrite structure; a columnar network structure; an encrusting lobate structure; and bulbous textures consisting of aragonite pseudomorphs. Each of these textures is morphologically diverse but have a common elementary structure. They occur together in different combinations to form a biological framework with abundant cavities lined with marine cements. Stromatolite dominated facies occur in this reef but were not essential to the development of the reef.

This study reveals that microbes were able to form skeletal structures and complex ecosystems in the Cryogenian. These microbes potentially represent a link between Proterozoic and Cambrian reef building biota. This suggests that there was a level of continuity in the evolution of reef ecologies toward the Cambrian.