BUATOIS, Luis A., Mangano, M.Gabriela, and MAPLES, Christopher G., Kansas Geological Survey, 1930 Constant Ave, Lawrence KS 66047, and cmaples@nsf.gov; LANIER, William P., department of Earth Sciences, Emporia State University, Emporia KS 66081, lanierw@esumail.emporia.edu
Tidal rhythmites of the Pennsylvanian Tonganoxie Sandstone Member (Stranger Formation, Douglas Group) at buildex Quarry (Eastern Kansas) contain a relatively diverse ichnofauna, dominated by arthropod trackways, grazing traces, subsurface feeding structures, insect resting and feeding traces, fish trails, tetrapod trackways, and root marks. Trace fossils occur in Planar-laminated coarse-grained siltstones that exhibit recurrent thickness fluctuations. Such thickness variations suggest the strong influence of tidal processes and give evidence that these deposits are tidal rhythmites, with thicker strata representing spring tides and thinner ones recording neap tides.
However, in contrast to trace fossil assemblages typically recorded from brackish-water estuarine settings, the Buildex ichnofauna is characterized by: (1) a moderate to relatively high ichnodiversity, (2) ichnotaxa commonly present in terrestrial/freshwater environments, (3) a dominance of surface trails and absence of burrows, (4) a dominance of temporary structures produced by a mobile deposit feeder fauna, (5) a mixture of traces belonging to the nonmarine Scouyenia and Mermia ichnofacies, (6) a moderate density of individual ichnotaxa, and (7) an absence of monospecific suites. This ichnofauna most likely records the activity of a freshwater/terrestrial biota.
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 salinity limit further towards the sea. Although facies distribution in estuarine valleys is mainly salinity-independent, the distribution of benthos is not. Accordingly, ichnologic studies of estuarine systems have the potential to provide a high resolution delineation of fluvio-estuarine transitions in incised valley systems. Similar trace fossil assemblages have been recorded in other Carboniferous rhythmites of the USA Midcontinent; their recognition and characterization may be of use in sedimentologic analysis of marginal marine deposystems.
Kansas Geological Survey
Updated April 17, 1997
URL=http://www.kgs.ku.edu/Conferences/GSA96/buatois.html
Send comments and/or suggestions to webadmin@kgs.ku.edu