Biostratigraphy and depositional environment of algal stromatolites from the Mescal Limestone (Proterozoic) of central Arizona

The Apache Group of central Arizona is subdivided into, from base upward, the Pioneer Formation, the Dripping Spring Quartzite and the Mescal Limestone. Radiometric age determinations by Silver, and Livingston and Damon indicate an age of 1.2–1.4 billion years. Within the Mescal Limestone, algal stromatolites form a conspicuous biostrome, commonly 20–25 m thick. The basal 1–5 m of the biostrome consists of up to three zones of digitate stromatolites, which often form discrete bush-like bioherms. These forms are interpreted as Baicalia baicalica, Parmites sp. and Tungussia sp.; the latter form previously reported by Cloud and Semikhatov (1969). The form Parmites is interpreted as a modification of digitate stromatolites probably by decrease in current velocity within the shallow marine environment, which allowed discrete heads to coalesce. Basal digitate forms are replaced upward in the biostrome by domal and undulatory laminated (stratiform) stromatolites, interpreted to represent gradual upward shoaling, with lower intertidal and subtidal forms (digitate morphology) being replaced by upper intertidal and possibly supratidal forms (stratiform types).The digitate form B. baicalica is suggested by Soviet workers to be indicative of Middle Riphean time (1350-950 m.y.). While many empirical data suggest the possibility of gross subdivision of Late Proterozoic time on the basis of algal stromatolite “zones”, the intercontinental applicability and the ultimate validity of this concept in unresolved.