TRILOBITE BIOFACIES AND BIOSTRATIGRAPHY OF THE ORDOVICIAN MYERSTOWN FORMATION, LEBANON VALLEY, PENNSYLVANIA

The Ordovician Myerstown Fm of southeastern Pennsylvania consists uniformly of thin- to medium-bedded, medium to dark gray, argillaceous, fine to very fine grained calcisiltite and calcarenite (wackestone, packstone, and grainstone) that weathers medium to dark gray in color. In the Lebanon Valley of southeastern Pennsylvania, the Myerstown Fm crops out in overturned limbs of regionally developed recumbent folds (the Lebanon Valley nappes). Fossils are generally rare or absent in these Myerstown outcrops because they were destroyed by small scale folding, recrystallization, and development of slaty cleavage. The trilobites of this study were discovered during a detailed physical analysis of the Myerstown Fm at Millardsville, where they are rare and occur as molds and poorly silicified forms on weathered rock surfaces. Dissolution of the limestone in dilute acetic acid produced additional silicified fragments of the trilobites. Three trilobite biofacies/zones have been identified. They are a lower (oldest) biofacies/zone containing Tretaspis (n. sp.); a middle biofacies/zone containing Raymondella (n. sp.), Ceraurinella (?), Remopleurides, and Robergia; and an upper (youngest) biofacies/zone containing fragments of Robergia (?). These biofacies/zones correlate, respectively, with trilobite biofacies/zones of the upper part of the Lincolnshire Fm, the Botetourt Mbr of the Edinburg Fm (Robergia-Remopleuridesconcurrent range zone), and the Liberty Hall Mbr of the Edinburg Fm as described by other workers from the Shenandoah Valley of Virginia. This also suggests that the age of the Myerstown Fm in the Lebanon Valley of Pennsylvania is Late Whiterockian to Early Mohawkian (Early Blackriverian).