MICROBIAL CONTRIBUTIONS TO PHANEROZOIC REEF DEVELOPMENT

Recent discoveries of stromatolite components in ancient and modern reefs reveal that microbial communities, despite their decline in the Late Proterozoic, continued to play an important role in Phanerozoic marine environments. Substantial evidence from many sites suggests that stromatolites had a persistent presence in many reef (and level-bottom) habitats throughout the Phanerozoic and were not restricted to the intertidal zone nor did they function primarily as "disaster" taxa following times of global ecologic crisis.

During the Late Silurian, for example, stromatolites of subtidal origin formed impressive reefs along the margins of the Uralian Seaway. A stromatolite reef complex in Alaska’s Alexander terrane contains paleogeographically significant microbial-sponge biotas. Stromatolites and early marine cementation at the platform margin produced a reef-fringed rim, which influenced platform sedimentation patterns by protecting peritidal and lagoonal habitats in backreef sites. Despite sea-level fluctuations and tectonic instability in the Alexander terrane, the stromatolite reefs were rebuilt periodically following episodes in which debris from the platform was shed to the foreslope. Demise of the stromatolite reefs occurred during the Klakas orogeny, which induced catastrophic collapse of the platform and progradation of a clastic wedge in the Early Devonian.

These and other post-Cryptozoic stromatolites demonstrate the importance of microbial components in many marine environments past and present, and they contribute to a paleobiogeographic database for elucidating the conditions that were conducive (or detrimental) to Phanerozoic stromatolite growth. For example, Silurian stromatolites in the Urals and Siberia that are similar to those in Alaska indicate that the Uralian Seaway provided an important migratory route for invertebrate and microbial organisms. The depauperate suites of metazoans associated with the stromatolites suggest that the subequatorial seaway was a partially enclosed, narrow marine corridor. Compressional activity along the northern margins of Laurentia and Baltica in the Late Silurian produced fluctuating environmental conditions favorable for the diversification of microbial communities.