PRELIMINARY REPORT ON CONTINENTAL ICHNOFOSSIL DIVERSITY FROM THE SILURIAN BLOOMSBURG FORMATION, EASTERN PENNSYLVANIA

The timing of the invertebrate invasion into continental settings is problematic, with the Ordovician advent mired in controversy. Few unequivocal Silurian examples of continental ichnofossils have been reported. In eastern Pennsylvania, the Silurian Bloomsburg Formation consists of fluvial deposits that have yielded a low-diversity ichnofauna described here. The rarity of reported Silurian continental ichnofauna makes this discovery significant by expanding our understanding of continental ichnodiversity.

The Bloomsburg Formation in eastern Pennsylvanian is characterized by erosion based, fining-up sequences from medium-grained sandstone to mudstones. Channel fills are dominated by ripple strata and thinner graded beds. Overbank deposits are characterized by thinly laminated graded beds of fine-grained siltstones to mudstones. Thicker, laminated mudstones contain reduction halos around ribosome traces, mudcracks and some well-developed Bk horizons. The Bloomsburg fluvial systems are low gradient and low sinuosity systems. The strata contain a low-diversity ichnofauna that includes: 1) sinuous traces, nonmeniscate (?Planolites) and meniscate(Taenidium), 2) bilobate, oval traces (Rusophycus), 3) slightly sinuous traces with a medial ridge and scratches (Cruziana), 4) two rows of parallel circular depressions (Diplichnites), and 5) low, slightly sinuous ridges (Diplopodichnus). This ichnoassemblage has been recognized by the authors, both vertically in stratigraphic sections as well as at different localities within the Bloomsburg. The ichnofossils are not penetrative into the underlying strata. Planolites/Taenidium is the dominant ichnofossil in the Bloomsburg Formation and appears to be an addition to the previously recognized epifaunal arthropod ichnofauna of the Dilichnites ichnoguild developed on floodplain environments during the Silurian to Devonian.