ICHNOLOGY OF THE LOWER MIOCENE CHENQUE FORMATION, PATAGONIA, ARGENTINA: ANIMAL - SUBSTRATE INTERACTIONS AND THE MODERN EVOLUTIONARY FAUNA

Abstract

The lower Miocene Chenque Formation of Patagonia contains superbly preserved and diverse ichnofaunas, including the ichnogenera Asterosoma, Balanoglossites, Chondrites, Gastrochaenolites, Gyrolithes, Helicodromites, Macaronichnus, Nereites, Ophiomorpha, Palaeophycus, Phycosiphon, Planolites, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Teichichnus, and Thalassinoides. Wave-influenced open-marine deposits are characterized by intense bioturbation, complex tiering structures and a very high diversity. Restricted, brackish-water, tide-dominated deposits display lesser degrees of bioturbation, less complex tiering structures and lower ichnodiversity. Both marginal- and open-marine deposits are commonly punctuated by discontinuity surfaces delineated by firmground ichnofaunas. The complex tiering structure of open marine ichnofaunas shows the development of a finely partitioned infaunal niche and an increment in bioturbation by the Neogene, which is consistent with trends revealed by body fossils. However, further expansion of the Cenozoic ichnologic database is necessary in order to evaluate the role of evolutionary and latitudinal controls.