PYRITIZED MICROSPHERES FROM THE TULLY FORMATION (GIVETIAN), LOCK HAVEN, PA

Smooth surfaced and spinose spherical microfossils occur in the lower part of the Middle Devonian (Givetian) Tully Formation near Lock Haven, PA. They are confined to a single 20 cm thick bed of hard, dark-colored, micritic limestone containing abundant disseminated pyrite grains. The microspheres are about 100 to 150 um in diameter and show signs of originally containing a thin, probably flexible surface membrane composed of at least two layers. The spinose forms have spines that are up to 20 um in length, expand toward the base, and are circular in cross-section. They are arranged in a regular pattern on the surface of a microsphere. Some microspheres are composed of authigenic calcite and pyrite crystals 5-10 um in length, but most are completely pyritized. Phosphatic material is not present. Microspheres are surrounded by a halo of larger, prismatic calcite crystals adhering radially to the microsphere surface. The microspheres occur with microscopic bivalves, gastropods, dacryoconarids, and other small organisms. The size, shape, and preservational features of these microfossils indicate that they belong to a group of palynomorphs, called calcispheres, of unresolved affinities, but fairly common in Paleozoic rocks. Following Kazmierczak & Kremer (2005) and Kremer (2005), it is probably most reasonable to consider calcispheres as representing preservational products of algae, dinoflagellates, or acritarchs. Pyritization of undistorted calcispheres indicates rapid, bacterially mediated preservational processes operating in an anaerobic sedimentary microenvironment. The Tully material shows that pyritization can be added to calcification and phosphatization as an alternative pathway for the preservation of organic-walled microorganisms. Thus, calcispheres, radiosphaerids, mazuelloids, Muellerisphaerids, and pyritized calcispheres are best seen, not as different organisms, but rather as end-products of different syndepositional diagenetic preservational processes. The presence of pyritized calcispheres in the Tully Formation suggests deposition in a shallow back-reef or lagoonal environment. Although calcispheres are known to occur in Paleozoic rocks of the United States, the Lock Haven specimens are the first pyritized forms to be reported from this country