ICHNOLOGY OF THE MARINE MEMBER OF THE MIOCENE-PLIOCENE RIO NEGRO FORMATION OF ARGENTINA: AN UPDATE

CARMONA, N.B.; PONCE, J.J.; WETZEL, A.; BOURNOD C.N.; CUADRADO D. G.

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

IANIGLA

Mio-Pliocene Rio Negro Formation consists of three members; a lower continental member, the middle marine member and an upper continental member. In this study we focus on the middle marine member which is strongly affected by bioturbation. Sedimentologic and ichnologic analyses of this member allowed the recognition of tidal flat, tidal-bar, upper, middle and lower shoreface-offshore transition deposits, recording a complete transgressive¨Cregressive cycle. The tidal-flat facies that occurs at the base of the marine member is reworked by abundant Arenicolites and Skolithos, and also presents extensive microbially induced sedimentary structures (MISS). Large Ophiomorpha colonized the sandy strata in the tidal-bar facies, being a typical representative of the Skolithos ichnofacies. These shallow deposits pass upwards into lower-shoreface-offshore transition deposits with representatives of the archetypal Cruziana ichnofacies. These deposits are fully bioturbated by deposit©feeder structures, such as Asterosoma, Helicodromites, Nereites, Scolicia, Siponichnus, Teichichnus, and Thalassinoides and burrows of chemosymbiotic organisms such as Chondrites. Some relict tempestites can also be observed in this part of the member, having low bioturbation intensity, with specimens of Teichichnus, Cylindrichnus, Scalichnus and other unidentifiable equilibrium/escape structures. Upwards there is a shallowing trend, with upper shoreface deposits and tidal flats at the top. In the upper shoreface, bioclastic levels erode the finer deposits below, producing firm ground surfaces that are colonized by Thalassinoides©producers. The upper shoreface deposits are colonized mainly by crustaceans, which produce large Ophiomorpha burrows that are associated to brooding structures (e.g. Maiakarichnus). Finally, on top of this member, tidal flat deposits with MISS, bird tracks and large desiccation cracks occur. The development of the Modern Evolutionary Fauna from the Jurassic onwards, led to important ecological changes in the marine communities, with the dominance of mollusks, crustaceans and echinoids in the benthic communities. Interestingly, these are the same groups that were identified as the dominant trace-makers in the studied Mio-Pliocene, both in the shallower and in the deeper marine settings. These results agree with previous studies in other Cenozoic successions from Patagonia, which suggest that the establishment of the Modern Evolutionary Fauna is clearly reflected by the ichnologic record.