ABIOLOGICAL ORIGIN OF DESCRIBED STROMATOLITES OLDER THAN 3.2 GA

The three well-documented occurrences of three-dimensional stromatolit es older than 3.2 Ga meet most criteria for biogenicity except the pre sence of fossil bacteria. However, they also show features more consis tent with nonbiological origins. Small conical structures in the Strel ley Pool chert in the upper part of the Warrawoona Group (3.5-3.2 Ga), Western Australia, lack the structure typical of stromatolites and pr obably formed mainly through evaporitic precipitation. A domal structu re from the North Pole chert, Warrawoona Group, formed by soft-sedimen t deformation of originally flat layers. Laminated chert containing do mal and pseudocolumnar structures in the Onverwacht Group (3.5-3.3 Ga) , Barberton Greenstone Belt, South Africa, extends downward into veins and cavities, where it formed through inorganic precipitation. Althou gh bacterial communities were widespread on Earth prior to 3.2 Ga, the se particular three-dimensional structures are probably abiotic in ori gin and do not provide information on the paleobiology or paleoecology of early organisms. The paucity of Archean stromatolites older than 3 .2 Ga probably reflects the paucity of known and possibly extant carbo nate deposits of this age.