Studies of Holocene stromatolitic sediments indicate that the biologic makeup of surficial blue-green algal mats controls the microstructure (features less than several centimeters in size) of stromatolites. Microstructural features include relief along a single growth surface, grain-to-grain relations within laminae, and distribution of organic matter. Recent stromatolitic sediments are basically an intertidal phenomenon. Within the intertidal zone, blue-green algal species are organized into distinct biologic communities. Each community occupies the sediment surface within a specific flooding-frequency range, and each community produces a distinctive microstructure. Recent algal communities have a wide geographic range. Similar communities and zonations may be hundreds nd thousands of miles apart, but only where the areas are connected by rapidly flowing open water or wind currents (e.g., areas connected by the Indian Ocean North Equatorial Current). Therefore, major stromatolite-forming algal communities do not have a worldwide distribution.

These data may be applied to the assemblages of stromatolitic microstructures which characterize 100-300 m. y. intervals in the late Proterozoic (Riphean)

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of the USSR. The nonrepetitive changes in microstructural assemblages through the Riphean may be understood in terms of the community organization of blue-green algae. The presence of distinctive microstructures which survived for long periods of time implies the presence of very highly biologically accommodated communities of blue-green algae, i.e., of assemblages of algal species with high levels of physiologic and biochemical accommodation among species. The time range of each community and microstructure was determined by the timing of major regressions or of major reorganizations in the distribution of shallow seas. Biologically accommodated communities were broken down during these regressions or reorganizations. Microstructural assemblages had broad areal distributions within t e USSR, as well as specific time ranges. Paleogeographic reconstructions of the USSR indicate that areas with similar microstructural assemblages were in open-water contact during the corresponding time period. Open-water connection accounts for the synchroneity of microstructural assemblages in the USSR, but similarity of hydrographic conditions throughout the Precambrian world ocean is improbable. Therefore, time-stratigraphic microstructural units defined in the USSR should not be used for worldwide correlation until paleogeographic relations are established firmly.

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