The Paradox of Nonmarine Ichnofaunas in Tidal Rhythmites: Integrating Sedimentologic and Ichnologic Data from the Late Carboniferous of Eastern Kansas, USA

The occurrence of trace fossil assemblages dominated by arthropod trackways and surface grazing trails within Carboniferous tidal rhythmites has puzzled sedimentologists and ichnologists, who interpreted them either as marine or nonmarine. The Virgilian (Stephanian) Tonganoxie Sandstone Member (Stranger Formation) at Buildex Quarry (eastern Kansas) consists, for the most part, of planar-laminated coarse grained siltstones deposited on an upper tidal flat, close to or at the fluvial-estuarine transition of a macrotidal estuarine paleovalley. Recurrent thickness fluctuations demonstrate the strong influence of tidal process and provide evidence that these deposits are tidal rhythmites, with thicker strata representing spring tides and thinner ones recording neap tides. The Buildex sequence hosts a moderately diverse ichnofauna composed of arthropod trackways (Dendroidichnites irregulare, Diplichnites gouldi, Diplopodichnus bifurcus, Kouphichnium isp., Mirandaichnium famatinense, Stiallia pilosa, Stiaria intermedia), grazing traces (Gordia indianaensis, Helminthoidichnites tenuis, Helminthopsis hieroglyphica), subsurface feeding traces (Treptichnus bifurcus, T. pollardi, irregular networks), apterygote insect resting and feeding traces (Tonganoxichnus buildexensis, T. ottawensis), fish traces (Undichna britannica, U. simplicitas), and tetrapod trackways. In contrast to trace fossil assemblages from brackish-water estuarine settings, the Buildex ichnofauna is characterized by moderate to relatively high ichnodiversity, ichnotaxa commonly present in terrestrial/freshwater environments, dominance of surface trails and absence of burrows, dominance of temporary structures produced by a mobile deposit-feeder fauna, a mixture of traces belonging to the Scoyenia and Mermia ichnofacies, moderate density of individual ichnotaxa, and absence of monospecific suites. This ichnofauna is thought to record the activity of a typical freshwater/terrestrial benthos. The presence of this mixed freshwater/terrestrial ichnofauna in tidal rhythmites is regarded as indicative of tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions, more precisely in a zone between the maximum limit of landward tidal currents and the salinity limit further towards the sea. Although lithofacies distribution in estuarine valleys is mainly salinity-independent, the distribution of benthos is not. Accordingly, ichnologic studies have the potential to provide a high-resolution delineation of fluvio-estuarine transitions.