Continental ichnofossil assemblage from the upper Silurian of Laurentia: The Bloomsburg Formation of eastern Pennsylvania
Abstract Few unequivocal Silurian examples of continental ichnofossils have been reported. In eastern Pennsylvania, the upper Silurian Bloomsburg Formation consists of fluvial deposits that have yielded a low-diversity ichnofauna described here. The rarity of reported Silurian continental ichnofauna, the quality of the recovered ichnofossil specimens, as well as the richly descriptive sedimentary and palaeontological context, makes this discovery significant by expanding our understanding of continental ichnodiversity and affirming ichnotaxa reported elsewhere. The Bloomsburg Formation ichnofauna now suggests the possibility that there was a cosmopolitan, global ichnofauna of relatively low ichnodiversity in continental (especially fluvial) paleoenvironnments during the late Silurian. The fluvially deposited Bloomsburg Formation is characterized by erosively based, fining-up sequences from medium-grained sandstone to mudstones. Channel fills are dominated by ripple-laminated strata and thinner, graded beds. Floodplain deposits are characterized by thinly laminated graded beds consisting of fine-grained siltstones to mudstones. Thicker, laminated mudstones contain reduction halos around ribosome traces, mudcracks and some well-developed Bk horizons of calcisols. The Bloomsburg fluvial systems thus were low gradient systems that built extensive floodplain deposits and were transitional between low and high sinuosity fluvial systems. The strata contain a low-diversity invertebrate ichnofauna that includes: 1) sinuous traces (Palaeophycus), 2) bilobate, oval traces (Rusophycus), 3) slightly sinuous traces with a medial ridge and scratches (Cruziana), 4) two rows of parallel circular or ellipsoid depressions (Diplichnites), 5) substrate penetrating burrows (not identified), and 6) traces of fossil plant roots. Palaeophycus and substrate penetrating burrows are the dominant ichnofossils in the Bloomsburg Formation. The Bloomsburg Formation ichnofauna adds an important datapoint in western Laurentia to the distribution of the biota of the freshwater ecosystem of the Late Silurian. It also well represents the epifaunal arthropod ichnofauna of the Diplichnites ichnoguild of the Scoyenia ichnofacies developed in fluvial environments during the Silurian to Devonian.