Preliminary report of microstructures of probable biologic origin from the Mescal Formation (Proterozoic) of central Arizona

Microstructures of probable biologic origin have been described within early diagenetic chert from near the top of the Mescal Formation of Proterozo c (1.2–1.4 b.y.) age, from exposures in the vicinity of Horse Camp, McFadden Peak Quadrangle, Arizona. The structures, nearly all hematite in composition, are interpreted as replacements of microorganisms and have been divided into three categories: Type A, filamentous chains; Type B, discrete and grouped spherules; and Type C, globular aggregates. The Type A structures are interpreted as replacements of filamentous blue-green algae. Type B structures are thought to be replacements of blue-green algae, although the possibility exists that some may represent chlorophycean algae. Type C structures are tentatively interpreted as pyrite framboids subsequently altered to hematite.