PALEOENVIRONMENTAL IMPLICATIONS OF SILURIAN SPONGE-MICROBE ASSOCIATIONS FROM THE NORTHERN URALS, RUSSIA

The Ludlow Ilych Reef Formation of the Northern Ural Mountains, Russia, consists primarily of microbial boundstones that represent stromatolite barrier reefs. These boundstones are interbedded with skeletal packstones that formed in the cavities created by stromatoporoid sponges and microbial mounds. The major metazoan constituents comprise low-diversity assemblages of aphrosalpingid (sphinctozoan) sponges, Fistulella problematic hydroids, stromatoporoids, crinoids, brachiopods, and ostracods. However, relative to other tropical reefs of the Late Silurian, the buildups show an unusual dominance of calcareous microorganisms, which served as the primary reef constructors. Microbial communities were dominated by Ludlovia, Sphaerina, Rothpletzella, Epiphyton, Hecetaphyton, and Renalcis.

The reefs formed along the passive eastern margin of Baltica in the northern tropical latitudes along the Uralian Seaway. The aphrosalpingid sponges, Sphaerina, Hecetaphyton, and Ludlovia, are unique to the Alexander terrane (se Alaska), the Farewell terrane (sw Alaska), and the Urals. This confirms that the Alaskan terranes were located along the Uralian Seaway and that the seaway was conducive to biotic transport between these sites and eastern Baltica. The limited diversity of fossils in these locations further suggests that during the Ludlow, the Uralian Seaway was a restrictive environment because of changing tectonic conditions that inhibited abundant metazoan growth.