Coeval development of Silurian stromatolite reefs in Alaska and the Ural Mountains; implications for paleogeography of the Alexander Terrane

New insights into the paleogeography of the Alexander terrane (Alaska) have been obtained from a comparative study of limestones in southeastern Alaska and the Ural Mountains. Upper Silurian stromatolites preserved in the Heceta Formation of Alaska are remarkably similar in composition, biofabric, and environmental setting to Upper Silurian (Ludlovian) subtidal stromatolites in the Ural Mountains, particularly those discovered near the Ilych River, Northern Urals. The stromatolites were built by an unusual consortium of microbial taxa in association with distinctive sphinctozoan sponges (aphrosalpingids) and share a high degree of similarity with Upper Silurian subtidal stromatolites in the Nixon Fork terrane (southwestern Alaska). That the stromatolites in these three regions are not identical in taxonomic composition but share in common conspecific, reef-dwelling aphrosalpingids and congeneric microbiotas indicates that they evolved separately but in a geographically contiguous area. These fossils establish that the Alexander terrane as an island arc resided at a site that enabled migratory exchange of biotas with northwestern North America or Siberia (Nixon Fork terrane) and northern Baltica (Ural Mountains) in the Late Silurian, a paleogeographic setting that is compatible with Northern Hemisphere options derived from paleomagnetic, isotopic, detrital zircon, and other paleontologic evidence in southeastern Alaska.