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Bacterialike (Filamentous) Structures Associated with Pyritized Burrow Linings, Arnheim Formation (Upper Ordovician), Southeastern Indiana

Will H. Blackwell, Anthony J. Martin

Much naturally occurring pyritization is biologically mediated, with specific types of bacteria (sulfate reducers) promoting the reactions. Among the criteria required for pyritization in a marine environment are the presence of: (1) interstitial iron ions, (2) a primarily anaerobic (reducing) environment, (3) an organic-rich substrate, and (4) sulfate-reducing bacteria (releasing sulfide). However, the direct connection between pyritization and bacteria (microfloral remains) is difficult to visualize in the fossil record.

This study focuses specifically on pyritized burrow linings that occur in strongly bioturbated wackestones from the Arnheim Formation (Cincinnatian Series, Upper Ordovician). Specific reducing microenvironments (i.e., mucoidal burrow linings) were the sites of early diagenetic pyritization in otherwise oxygenated, organic-rich sediments. Material examined under both the light and electron microscopes revealed occasional evidence of pyrite associated with filamentous structures. These structures possess a shape and size consistent with certain types of bacteria. This relationship, bacterialike structures with pyrite, may be more common in the fossil record than previously suspected.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.