SEDIMENTOLOGY AND TAPHONOMY OF NEMATOTHALLUS-BEARING BEDS, SILURIAN BLOOMSBURG FORMATION, PORT CLINTON, PENNSYLVANIA, USA

Nematothalloid fossils were widespread during the Upper Silurian and record early colonization of land. In one important locality, Port Clinton Pennsylvania, the Late Silurian Bloomsburg Formation red beds contain Nematothallus phytodebris. This study examines the Nematothallus deposit and integrates sedimentologic, field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) examination to develop a taphonomic model for preservation of the phytodebris.

The deposit consists of three atypical gray beds, each ~ 10 cm with structures consistent with prior documented crevasse splay deposits. Both the lowest and upper beds are composed of medium to fine sandstone with scoured bases and indistinct horizontal and trough stratification. The middle bed is composed of mm-scale normally graded laminae couplets consisting of sand capped by bedding parallel current oriented phytodebris. Laminae thicken upwards to the bed middle followed by thinning upward. Thinner laminae are comprised of very fine sand and cm-scale phytodebris, whereas the thicker laminae consist of fine sand and mmscale phytodebris.

Nematothallus debris is preserved as black, 3-D carbonized remains ranging from isolated hyphae to complete cuticles. Under FESEM, Nematothallus is characterized by abraded interwoven fungal hyphae with an overlying amorphous cortical carbon sheet. The carbon is cracked indicating shrinkage during early diagenesis. Isolated individual and sheets of bacillus-type bacteria are present and probably representing primary consumers in a wet environment. Authigenic minerals associated with the phytodebris include: 1) micron ellipsoidal iron oxide, 2) sub-micron iron oxide cubes, and 3) up to 8 weight % (EDS determined) copper in the carbon of the phytodebris. Microfiberous quartz is associated with some of the phytodebris margins and in surrounding sediment. After transportation and deposition in a crevasse splay deposit, the wet phytodebris was attacked by bacteria in a reduced setting, subject to latter diagenesis by oxidizing iron-rich fluids, and subsequent structural deformation.