Ichnological research in Lower Cretaceous marginal-marine facies from Patagonia: outcrop studies, SEM examinations and paleontological/sedimentological integration

摘要

In-depth ichnological studies of Lower Cretaceous, marginal-marine facies from the Agrio Formation (Patagonia, Argentina) have been carried out during the past few years. Several ichnofossils were identified and described for the first time in this unit, including a new ichnospecies (Hillichnus agrioensis) produced by tellinid bivalves, xiphosurid trackways (the first record from the Early Cretaceous worldwide) and dinosaur tracks (one of the few examples from Early Cretaceous carbonates in Gondwana). The sedimentological information showed the presence of marginal-marine intervals previously interpreted as open marine. The outcrop studies resulted in the finding of indirect evidence of microbial mats, while SEM examinations resulted in the direct evidence of their presence in certain levels; this has taphonomical implications. Both ichnological and sedimentological results allowed to challenge previous facies model belts that suggested permanent submerged littoral facies for this part of the basin during the Early Cretaceous. The inferred paleoenvironment for the top of the unit is shallower and more tidally-influenced than previously thought.