LAUSR

Abstract The Hartbeesfontein Basin contains the most extensive deposits of Archean lacustrine stromatolites on the Kaapvaal Craton, with stromatolitic facies occurring over ∼100 km 2 in beds up to 7 m thick. Stromatolitic dolostones and cherts both preserve evidence of microbial processes. Dolomitic stromatolites have grumelous microspar textures between organic-rich laminae, suggestive of carbonate precipitation within microbial mats. Stromatolitic laminae within chert preserve detrital material beyond the angle of repose, indicating the trapping and binding of grains by microbial mats. Stromatolitic cherts also preserve fenestral textures and filamentous microfossils. Many fenestrae have rounded shapes surrounded by filamentous laminae and appear to have formed in situ within stromatolite fabrics before lithification. Fenestrae within stromatolitic chert resemble “hourglass-associated fenestrae” noted from recent silica stromatolites from Yellowstone National Park, and are interpreted to originate from gas bubbles forming within stromatolite-building mats. The preservation of delicate structures in Hartbeesfontein stromatolitic chert (e.g., filamentous microfossils and gas-related fenestrae) implies rapid lithification of microbial mats, while the mm to cm scale of fenestrae indicate equally rapid rates of microbial gas production. Textural and mineralogical evidence associated with gas-related fenestrae support the presence of oxygenic photosynthesis, which in turn strengthens previous hypotheses on Archean lakes as potential oxygen oases before the Great Oxidation Event.