Ichnologic and paleoenvironmental characterization of the Orchesteropus atavus Frenguelli type locality, Huerta de Huachi, San Juan province, Argentina

The ichnofossil Orchesteropus atavus Frenguelli is only known from the Carboniferous of Argentina. Originally described from the locality of Huerta de Huachi, near Jáchal, San Juan Province, no detailed information on the associated ichnofauna and sedimentary facies was available. Orchesteropus atavus occurs in parallel laminated mudstones accumulated from suspension fallout, but interbedded with dilute turbidites. Orchesteropus atavus is associated with simple grazing trails (Cochlichnus anguineus Hitchcock, Gordia marina Emmons, Helminthoidichnites tenuis Fitch, Helminthopsis tenuis Ksiazkiewicz, Mermia carickensis Smith), feeding traces (?Circulichnis montanus Vyalov, ?Treptichnus bifurcus Miller), resting traces (Rusophycus isp.), other arthropod trackways (Diplichnites gouldi Gevers, Maculichna carboniferus (Pazos), ?Umfolozia isp.), and fish trails (Undichna britannica Higgs, Undichna isp.). Dominance of superficial trails rather than infaunal traces, preservation of delicate traces in the absence of bioturbation, presence of plurispecific rather than monospecific suites, relatively high ichnodiversity, presence of ichnotaxa common in continental settings, and absence of ichnotaxa indicative of marine influence collectively argue for subaqueous deposition under freshwater conditions. Palynomorphs from the trace fossil-bearing beds are terrestrially derived with no marine representatives associated. The analyzed ichnofauna is remarkably different from brackish water trace fossil assemblages described from late Paleozoic marginal marine environments of Laurasia and Gondwana. Integration of ichnologic data with stratigraphic, sedimentologic, and palynologic information suggests deposition in fjord settings.