A MINERALIZED ALGA AND ACRITARCH DOMINATED MICROBIOTA FROM THE TULLY FORMATION (GIVETIAN) OF PENNSYLVANIA

Sphaeromorphic algal cysts, most probably of the prasinophyte Tasmanites, and acanthomorphic acritarch vesicles, most probably Solisphaeridium, occur in a single 20 cm thick bed of hard, dark-colored, micritic limestone in the lower part of the Middle Devonian (Givetian) Tully Formation near Lock Haven, Pennsylvania. Specimens are composed of a mosaic of authigenic calcite and pyrite crystals about 5-10 μm in length. The proportions of these minerals vary widely among individual specimens; some are completely calcitic; some contain both pyrite and calcite; and many are composed totally of pyrite. The microfossils are about 80 to 150 μm in diameter and many show signs of originally containing a thin, probably flexible vesicle wall composed of at least two layers. Some appear to have been enclosed in a mucilaginous sheath or membrane when alive. The acanthomorphic forms have spines that are up to 20 μm in length, expand toward the base, and are circular in cross-section. Spines are spaced about 40 μm apart, and are arranged in a regular pattern over the surface. The microflora occurs with microscopic bivalves, gastropods, cephalopods, and other very small organisms, including the enigmatic Jinonicella, and the oldest zooecia of ctenostome bryozoans known from North America. Macrofossils do not occur in the microalgal horizon although burrows up to about 1 cm in diameter are present. The microalgae and acritarchs have been preserved via a complex preservational process involving rapid post-mortem mineralization of dead cells. Pyritization of distorted and mucilage-covered specimens indicates rapid, bacterially mediated preservational processes operating in a strongly dysaerobic sedimentary microenvironment. The presence of these pyritized microfossils suggests that the bed in which they occur, and possibly much of the lower part of the Tully Formation at Lock Haven with similar lithology, formed in a relatively deep, off-shore basin with reduced oxygen availability in the substrate