Ichnology and sedimentology of estuarina deposits, Mata Amarilla Formation, Austral Basin, Argentina

Abstract

The Mata Amarilla Formation (Upper Cretaceous) is part of the infilling of the Austral Basin (Patagonia, Argentina). This unit is mainly composed of grey to black mudstones interbedded with fine-to coarse-grained sandstones, deposited in littoral and continental environments. Three informal sections have been defined; the littoral deposits of the lower and upper sections constitute the object of this study. The intervals contain Arenicolites, Chondrites, Cylindrichnus, Ophiomorpha, Palaeophycus, Skolithos, Thalassinoides, Teredolites and fugichnia. The sedimentological and ichnological analyses allowed the distinction of three facies associations (FA) developed in an estuarine palaeoenvironment The more proximal FA1 is the bay-head delta, comprising delta-front bars of course-grained sandstone, containing Arenicolites, Skolithos and allochthonous petrified wood with Teredolites. This trace fossils association indicates high-energy conditions. The FA2, interpreted as estuarine bars, is composed of fine-to medium-grained containing herringbone cross-stratification and mud drapes with Cylindrichnus, Ophiomorpha, Skolithos, Thalassinoides and fugichnia. This reflects a setting with a high sedimentation rates, moderate energy, unstable substrates, and normal salinity. Finally, the FA3 (fine-grained estuarine deposits) are characterized by heterolithic rocks with synaeresis cracks and Arenicolites, Chondrites, Cylindrichnus, Palaeophycus, Thalassinoides and Skolithos. These ichnogenera are generally characterized by small sizes, consistent with a Salinity stressed paleoenvironment. The distribution, size and abundance of the trace fossils were controlled by several environmental factors, e.g. energy, sedimentation rate and salinity contents. The palaeoenvironmental factors variability is directly related to the fresh water and sediment input from rivers flood, which took place during the wet season of a warm, temperate climate