Ichnological record and its relation with fluvial and wave processes in deltaic deposits from the upper Quintuco Formation, central Neuquén, Argentina

OLIVO, MARIANA S.; CERECEDA, ABRIL; GRIFFIN, MARÍA

Colonia de Sacramento

Simposio; SLIC 2015 - Tercer Simposio Latinoamericano de Icnología; 2015

Deltas represent dynamic systems that are simultaneously influenced by different controls (rivers, waves, storms and tides). As a consequence, integrated detailed sedimentological and ichnological analysis are of major importance in order to determine the relative influence of the environmental processes involved in the construction of deltaic systems. This contribution is based on a combined sedimentological and ichnological analysis of the deltaic front deposits of the upper part of the Quintuco Formation (Early Valanginian) exposed in Sierra de la Vaca Muerta (central Neuquén province). The results of this study allowed to identify and characterize the different physical processes affecting the mouth bar deposition at a deltaic front. The distal parts of the mouth bars consist of heterolithic (sand-mud) intervals with both low and high degrees of bioturbation (Palaeophycus, Bergaueria, Conichnus and Teichichnus) that are intercalated with sandstone beds with a low degree of bioturbation (Gordia and Gyrochorte). These attributes suggest moderate to low energy transitional environments. The proximal parts of the mouth bars on the other hand, are composed of two types of sand bodies. The first type has current ripples, trough cross-bedding and syn-sedimentary load deformation features. They show a low degree of bioturbation and small-sized specimens of the ichnogenera Cylindrichnus and Skolithos, as well as Fugichnia traces. The second type of sand bodies presents wave ripples, low-angle cross-lamination and hummocky cross-stratification. They show a high degree of bioturbation and normal-sized specimens of the ichnogenera Ophiomorpha, Thalassinoides, Gyrochorte, Cylindrichnus, Arenicolites, Skolithos and Palaeophycus. These two types of sand bodies are interpreted to represent moderate to high energy marginal marine environments related to high discharge of sediments events.The energy and the brackish water conditions of the depositional environment are the major controls determining the development of the mixed depauperate Cruziana-Skolithos ichnocoenosis in the mouth bars. However, it is the entry of freshwater into the marine environment caused by fluvial discharge events that is interpreted as the main control in the variability of the ichnological attributes within these mixed ichnoassemblage. The deposits associated to a higher influence of fluvial processes show very low degrees of bioturbation and ichnodiversity represented by small trace fossils with simple excavations. On the contrary, wave-influenced deposits are characterized by high degrees of bioturbation and ichnodiversity and full-sized ichnogenera.