Trazas fósiles de trilobites de la Formación Mojotoro (Ordovícico Inferior-Medio de Salta, Argentina): implicancias paleoecológicas, paleobiológicas y bioestratigráficas

The Mojotoro Formation (Lower to Middle Ordovician, Salta, northwest Argentina) comprises thick packages of quartzites interbedded with fine-grained heterolithic facies, recording deposition in a tide-dominated shallow­ marine setting. Depositional environments include subtidal sandwave complexes, interbars, bar margins, and tidal flats. Heterolithic facies hosts an abundant trilobite ichnofauna. The ichnogenus Cruziana, represented by ichnospecies of the rugosa group (C. rugosa, C. furcifera, and C. goldfussi), is the dominant form, though Dimorphichnus has been recorded also. Trilobite traces of the Cruziana ichnofacies are cross-cut by vertical elements of the Skolithos ichnofacies representing palimpsest bedding surfaces. Thick packages of quartzites typically lack bioturbation due to strong tidal currents and highly mobile bedforms in the most active zones of subtidal sandwave complexes. Thinner beds representing deposition in smaller sandwavesand bar margins are dominated by Skolithos linearis, recording the activity of infaunal suspension feeders. Trilobite traces are abundant at the base of sandstone interbedded with mudstone layers or partings, recording outer zones of sandwave complexes, interbars, and tidal flats.The high density of trilobite traces in heterolitc biofacies with sedimentary structures indicative of periodic emersion (e.g.des iccation cracks) is thought to record the feeding activities of trilobites in tidal flats. These periodic landward incursions were probably controlled by tidal cycles. The presence of the rugosa group and the absence of C. semiplicata indicate an Arenig-Llandeil age for the Mojotoro Formation, which is consistent with the age suggested by the faunal content of the underlying and overlying units.