DIVERSITY OF SILURIAN CONTINENTAL ICHNOFOSSILS FROM THE BLOOMSBURG FORMATION OF EASTERN PENNSYLVANIA AT THE SCHUYLKILL GAP

The Ordovician invasion of nonmarine ecosystems by invertebrate animals remains problematic, with few noncontroversial examples reported. Silurian examples of nonmarine ichnofossils are rare. In eastern Pennsylvania, the Upper Silurian (Wenlockian-Ludlowian) Bloomsburg Formation consists of continental deposits (siliciclastic red beds) with a low-diversity ichnofauna. The rarity of reported Silurian continental ichnoassemblages makes documentation of this ichnofauna significant by expanding our understanding of early nonmarine ichnodiversity. The Bloomsburg Formation consists of erosionally based channel sandstones, composing fining-up sequences from medium-grained sandstones to overbank mudstones. Channel fills are dominated by medium-scale trough cross beds, ripple stratification and thinner graded beds. Laminated graded beds of fine-grained siltstones pass vertically into thick overbank mudstone deposits. Reduction halos around rhizome traces, mud cracks, microbial mats and some well-developed Bk horizons characterize the overbank deposits. The Bloomsburg fluvial systems are low gradient and relatively low sinuosity systems. The strata contain a low-diversity ichnofauna that includes: 1) sinuous burrows, nonmeniscate (?Planolites) and meniscate (Taenidium), 2) bilobate, oval resting traces (Rusophycus), 3) slightly sinuous trails with a medial ridge and perpendicular scratches (Cruziana), 4) walking traces that consist of two rows of parallel circular depressions (Diplichnites), 5) complex, bilaterally arcuate walking traces (Palmichnium), and 6) two unbranched, parallel convex ridges (Diplopodichnus). This ichnoassemblage is consistent with that previously reported from the Bloomsburg Formation with the addition of Palmichnium and less abundant ?Planolites and Taenidium. In the Bloomsburg Formation at the Schuylkill Gap, epifaunal arthropod traces are the dominant ichnofossils, characteristic of the well-known Diplichnites ichnoguild reported from overbank environments from the Silurian to Permian (and younger).