Silurian microbial associations from the Alexander terrane, Alaska

Abstract

Silurian calcareous algae, cyanobacteria, and microproblematica are abundantly preserved in the Alexander terrane of southeastern Alaska. They represent a diverse population of calcified microbes that contributed to the formation of a variety of shallow- and deep-water carbonate deposits. Five associations are recognized on the basis of recurring groups of microbial taxa. These include aGirvanella-Tuxekanellaassociation that formed oncoids and thick encrustations on skeletal grains in shelf environments. ARenalcisassociation predominated in a stromatoporoid-coral reef that developed at the incipient shelf margin on a crinoid-solenoporid shoal (“Solenopora” association). Other organic buildups are characterized by aLudloviaassociation, which constructed skeletal stromatolite reefs, and by anEpiphyton-Sphaerinaassociation that contributed to the formation of a stromatolitic mud mound. A mixed microbial assemblage reflects transport and mixing of shallow-water microbial biotas that were deposited by turbidity currents, debris flows, and slumps in a slope environment.