CONICET | Buscador de Institutos y Recursos Humanos

Depositional processes operating in deltaic shorelines create a variety of physico-chemical stressful conditionsthat impact directly on benthic fauna. The ichnological signatures of deltaic successions reflect these physicochemicalstresses, such as rapid sediment influx, elevated water turbidity, periodically reduced oxygenation,salinity fluctuations, and wave- and tidal-energy. In the Campanian La Anita Formation of the Austral-Magallanes Basin, two examples of deltaic successions, each showing different relative dominance of wave andfluvial processes, are described in detail, allowing the recognition of contrasting stresses that acted differentiallyin a wave-dominated, fluvial-influenced delta and a fluvial-dominated delta. Variations in the burrowing intensityand ichnodiversity allowed us to discriminate the different physico-chemical stresses.In the wave-dominated, fluvial-influenced deltaic deposits, the alternation between fair-weather waves andstorm events acted as the main controlling factor on benthic fauna in the facies of the prodelta and distal deltafront, displaying facies-crossing and horizontal elements attributable to the Skolithos and Cruziana Ichnofacies,such as Arenicolites isp., Chondrites isp., Cylindrichnus isp., Diplocraterion isp., Gyrochorte isp., Ophiomorpha isp.,Palaeophycus isp., Planolites isp., Rhizocorallium isp., Rossellia isp., Schaubcylindrichnus isp. and Skolithos isp.Water turbidity and periodically reduced bottom water oxygenation conditions also played an important butsubordinate role. The proximal delta front, and distributary and terminal distributary channel deposits show lowburrowing intensities and ichnodiversities with structures attributable to the depauperate Skolithos Ichnofacieswith few Diplocraterion isp., Gordia isp., Lockeia isp., Macaronichnus isp., Ophiomorpha isp., Schaubcylindrichnuscoronus and Siphonichnus isp., showing stress conditions associated with rapid sediment influx, high energy andreduced salinity conditions related to fluvial discharges as the most important controlling factors. Periodically,fluctuations in salinity conditions due to river discharges and channel avulsion may have also played importantroles.The fluvial-dominated deltaic deposits show that the controlling factors were associated with fluvial dischargeand river-sediment influx with no clear influence of wave or tidal action. The facies of the distal deltafront were controlled by episodic sedimentation, elevated water turbidity, periodically reduced oxygenation andsalinity fluctuations, the associated trace fossil suite consists of a monospecific assemblage of Ophiomorpha isp.and sparse Palaeophycus isp. The proximal delta front and the distributary channel deposits are mostly unburrowed,locally displaying a few Arenicolites isp., Lockeia isp., Ophiomorpha isp., Psilonichnus isp. andTeredolites isp. The development of these suites was controlled mainly by heightened sedimentation rates influxtriggered by high fluvial discharges, elevated water turbidity due to heightened suspended load and buoyantplumes, periodic fluctuations in salinity conditions caused by river discharges and channel avulsion.Interdistributary areas are characterized by facies preserving evidence that reduced oxygenation was the mostimportant controlling factor in this subenvironment.Fluvial processes generate the most extreme stressful conditions for benthic colonization by tracemakers.Wave processes tend to buffer the stresses produced by river discharge. The effect of waves on fluvial stress isreflected in the higher bioturbation intensities and ichnodiversities that the wave-dominated deltaic deposits ofthe La Anita Formation delta show in relation to the fluvial-dominated ones.