SEDIMENTOLOGÍA E ICNOLOGÍA DE LA FORMACIÓN LOS MOLLES EN LAS LOCALIDADES DE PICÚN LEUFÚ, SIERRA DE CHACAICÓ Y CORDILLERA DEL VIENTO, CUENCA NEUQUINA, ARGENTINA

Abstract

Sedimentological and ichnological studies in the Los Molles Formation (Toarcian- Aalenian), in Picún Leufú, Sierra de Chacaicó and Cordillera del Viento localities allowed to recognize six facies associations and six ichnoassemblages. These facies associations comprise shoreface, offshore, shelf, prodelta and delta front affected by turbidites and storms deposits. The six ichnoassemblages are characterized, in general, by low ichnodiversity and abundance. The most common ichnoassemblage is the one recognized in offshore-shelf environments, consisting of Chondrites intricatus, Trichichichnus appendicus, Trichichnus isp., Phycosiphon incertum. Subordinately, Phymatoderma isp, Cylindrichnus concentricus, Teichichnus rectus and Thalassinoides isp. also occur The ichnoassemblage developed in levee deposits of turbidite systems consist of Chondrites intricatus, Phycosiphon incertum, Trichichichnus isp, Chondrites patulus, and Nereites missouriensis, whereas the ichnoassemblage recognized in storm dominated – shelf deposits includes specimens of Chondrites intricatus, Phycosiphon incertum, Nereites isp., Skolithos linearis, Planolites isp., Arenicolites isp., Ophiomorpha isp., and Gyrochorte comosa. The shoreface, delta front and prodelta deposits were only recognized at Picún Leufú locality, where the transition between Los Molles and Lajas formations is well exposed. In this context, the ichnoassemblage recognized in the distal shoreface deposits consist of Skolithos linearis, Ophiomorpha isp. and Arenicolites isp., the ichnoassemblage developed in the prodelta deposits is composed of specimens assigned to Chondrites intricatus, Phycosiphon incertum and Planolites isp, while the ichnoassemblage recognized in the delta front deposits is characterized by the presence of Chondrites intricatus, Phycosiphon incertum, Nereites isp., Skolithos linearis and Ophiomorpha isp. Integration of the sedimentological and ichnological data, indicates that variations in salinity, turbidity and sedimentation rate were limiting factors that affected the development of benthic communities. However, oxygenation would have been the most important parameter controlling not only the diversity but also the abundance of biogenic structures in these substrates. Moreover, in this analysis it is important to take into account the regional context, since during most of the Jurassic and Cretaceous, the Neuquén basin behaved as a back-arc basin associated with a magmatic arc, which was connected to the proto-Pacific Ocean through narrow passages between the volcanic island chain. This palaeogeographic situation produced an important regulation on the internal circulation of the basin, affecting the stratification of the water column, and its physical-chemical properties, and thus significantly affecting the development and establishment of benthic communities.