SMALL OPEN SLENDER-CORAL (TETRADIUM) THICKETS IN THE MID-ORDOVICIAN OF CENTRAL PENNSYLVANIA

Bryozoan reefs are rare enough that any reported occurrence – as by Harper ('04 Pittsburgh Assoc. Petrol. Geol. Gdbk. p. 60) -- warrants examination. Analysis of what phylum was principal bioherm builder is especially important for understanding paleoecologic differences among the various major colonial groups then.

The reported bryoherms are in the middle of the long roadcut on Pa. Hwy. 453 4.2 mi (6.7 km) northwest of its junction with U.S. Hwy. 22 in Water Street; the cut is in Blair County, just west of Union Furnace and Spruce Creek villages. These structures are in the middle and lower Snyder Limestone (especially base of unit 13, also 11/12 contact and basal 9, of Sevon '86 Ann. Fld. Conf. Pa. Geol. Gdbk. p.. 113; middle Blackriveran).

The bioherms examined appear as ill-defined clusters of dark-gray, very slender branch fragments scattered through light-gray micrite in well-defined planar beds. The branch fragments are fallen, horizontal or oblique, rather than upright and in-place. Many are completely recrystallized to coarse clear spar, but others range from vague ghosts to fairly well-preserved tubes.

In transverse/cross-section, the best-preserved are square, with four prongs extending inward from the centers of the straight side walls. Many (measuring 1.0-2.2 mm on each side overall) exhibit four or more of these square cells, identifiable as the schizocoral Tetradium cellulosum. Many others (0.5-0.8 mm on each side of the branch fragment) consist of only one or two square corallites, recognizable as Tetradium syringoporoides. Some Tetradium fragments have recrystallized into more rounded cross-sections superficially resembling very thin bryozoan branches.

A few rock sections reveal ghostly remnants of the tabulate coral Lichenaria cf. cloudi. Also occurring are very rare fragments of poorly preserved bryozoans (Batostoma? aff. varium, B.? aff. gallowayi, Hallopora? aff. dumalis, Championodictya? aff. pleasantensis) plus Solenopora algae.

The bioherms studied are interpreted as low-standing, open stands (thickets) of erect, widely separated, slender stick-like Tetradium corals, probably baffling/trapping some suspended lime-mud but not forming interlocking frameworks. They would have grown as scattered clump-like patches on shallow muddy shoals. They were coral build-ups, fitting within the vast array of coelenterate/cnidarian reefs, instead of the far fewer bryozoan bioherms known so far.