Rhizocorallium and turtle tracks: a late Cretaceous proximal distributary channel trace-fossil assemblage, central Utah

A proximal distributary channel deposit in the Turonian Ferron Sandstone of central Utah contains a trace-fossil assemblage comprised of turtle tracks (Chelonipus), vertebrate swim trace fossils (Characichnos), repichnial traces of mollusks (Archaeonassa), Rhizocorallium, and Teredolites. Mayflies are abundant producers of horizontal, spreiten, U-shaped burrows in modern freshwater channels, but rarely ascribed as producers of Rhizocorallium in the rock record. The in situ trace fossils studied are interpreted as a freshwater assemblage. Teredolites occurs within allochthonous debris reworked from stratigraphically lower brackish-water deposits. This Ferron Sandstone assemblage provides an example of the juxtaposition of freshwater ichnocoenose with exhumed brackish-water clasts. Caution must be exercised in environmental interpretation when taking into consideration both adjacent floodplain and the sedimentary structures surrounding the clasts. Complex trace-fossil relationships such as these may play a role in the scarcity of Rhizocorallium reports from freshwater deposits in the rock record, and may be more common than presently recognized. This study illustrates the importance of addressing the detailed ichnology above and below the base of channel deposits in progradational environments (e.g., delta plain), where reworked, brackish-water deposits (and bored clasts) may be entrained within overlying lower salinity facies.