SKOLITHOS LIFE ASSEMBLAGES AS SEDIMENTATION RATE AND “STRATIGRAPHIC UP” INDICATORS: PSEDUO-SPREITEN IN EARLY CAMBRIAN TRACE FOSSILS

Vertical, cylindrical burrows of the ichnogenus Skolithos (Haldeman, 1840) are iconic in many quartz sandstones of the early Cambrian of the Appalachians. The unbranched, smooth-walled tubes (3-5 mm), with structureless fill can be so abundant that they dominate the fabric of the sediment. When complete, Skolithos specimens contain a vertical, funnel-like aperture (2-3 cm) with a single vertical tube through the center. The trace makers of this ichnogenus are unknown from the Cambrian, but interpreted as either suspension feeding or carnivorous marine “worms”. In the early Paleozoic, another common, vertical, tube dwelling trace is the U-shaped Diplocraterion (Torell, 1870), which serves as a well-known “stratigraphic up” indicator (apertures – up, closed U – down). Recently, at several localities in the southern Appalachians of the Erwin Fm (lower Antietam Fm), in beds with abundant Skolithos, common U-shaped structures are present that are strongly reminiscent of spriten (sediment fill-lines of Diplocraterion). Actually, these U-shaped traces result from a life assemblage of Skolithos organisms (sublitoral, sandy sea floor of closely spaced burrows, with funnel surface – similar in appearance to a modern antlion colony), adjusting to the episodic aggradation of fine sediment. The position of the funnels across the assemblage moved upward in synchrony. As a result of close spacing of dwelling, the result of two adjacent funnels and the small space between results in a pattern of parallel beds in an inverted U-shape, defined here as “pseduo-spreiten” – closed part of the U indicating the stratigraphic-up direction. Under these special conditions, inverted U-beds can be confused with Diplocraterion, but when recognized may serve as a useful tool in the study of Skolithos bearing sandstones.