Ichnology of the Alfarcito Member (Santa Rosita Formation) of northwestern Argentina: animal-substrate interactions in a lower Paleozoic wave-dominated shallow sea

The lower fine-grained interval of the Alfarcito Member of the Upper Cambrian-Tremadocian Santa Rosita Formation in northwestern Argentina represents an overall transgressive-regressive cycle, recording the establishment of an open-marine, shallow, low-gradient platform. Four main sedimentary facies have been recognized, environmentally ranging from the lower offshore to the lower/middle shoreface. Alternating and contrasting energy conditions due to repeated storm events superimposed on fairweather sediments were among the key controlling factors for trace fossil distribution and preservation. Fairweather and storm-related trace fossil assemblages are recognized. The first is the most diverse and includes the more varied behavioral strategies, such as locomotion (Archaeonassa fossulata, Cruziana semiplicata, C. problematica, Cruziana isp. and Diplichnites isp.); resting (Rusophycus moyensis, R. carbonarius, Rusophycus issp. and Bergaueria aff. B. hemispherica); pascichnia (Dimorphichnus aff. D. quadrifidus); feeding ( Arthrophycus minimus, ? Gyrolithes isp., Gyrophyllites isp., ? Phycodes isp. and Planolites reinecki); and dwelling (Palaeophycus tubularis, P. striatus). This assemblage represents the Cruziana ichnofacies. The storm-related assemblage is monospecific and includes vertical dwelling traces (Skolithos linearis), and is attributed to the Skolithos ichnofacies. Integration of ichnologic and sedimentologic data allows proximal-distal trends in shallow-marine trace fossil assemblages along a nearshore-offshore transect to be reconstructed. High-energy conditions prevailed in lower and middle shoreface environments and bioturbation is restricted to vertical burrows (Skolithos linearis), recording colonization after storm events. Environmental conditions in the offshore transition and the upper offshore are more variable and reflect the alternation of high-energy storm events and low-energy fairweather mudstone deposition. The storm-related Skolithos ichnofacies is present, but alternates with the faiweather assemblage (Cruziana ichnofacies) that reaches a diversity maximum in the upper offshore. Trace fossils are scarce in lower offshore deposits, being restricted to a few non-descript burrows, mostly Palaeophycus tubularis. Shoreface deposits from the Alfarcito Member compare favorably with the strongly storm dominated type. All these deposits are stacked to form regional coarsening-upward parasequences that record short-term progradational episodes separated by drowning surfaces. Vertical distribution of trace fossils parallels changes in stratal stacking patterns. Several ichnotaxa (especially Cruziana semiplicata, Rusophycus moyensis and Arthrophycus minimus) have biostratigraphic implications.