CONTINENTAL ICHNOFOSSIL DIVERSITY FROM THE SILURIAN BLOOMSBURG FORMATION, SCHUYLKILL GAP, PENNSYLVANIA

The Ordovician invertebrate invasion into continental settings is problematic, with few noncontroversial examples reported. Silurian examples of continental ichnofossils are rare. In eastern Pennsylvania, the Silurian Bloomsburg Formation consists of fluvial deposits with a low-diversity ichnofauna. The rarity of reported Silurian continental ichnoassemblages makes this documentation significant by expanding our understanding of continental ichnodiversity as recorded in the Bloomsburg Formation.

The Bloomsburg Formation exposed at Schuylkill Gap 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), and 5) complex bilaterally arcuate walking traces (Palmichnium). This ichnoassemblage is consistent with that previously reported from the Bloomsburg Formation with the addition of Palmichnium and less abundant, ?Planolites and Taenidium. In 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 Devonian.