Ichnology and Sedimentology of Estuarine deposits from Mata Amarilla Formation, Austral Basin, Argentina

Abstract

The Mata Amarilla Formation is part of the infilling of the Austral Basin, located in Southern Patagonia, Argentina. This unit is mainly composed of grey to black mudstones interbedded with white fine- to coarse-grained sandstones, deposited in littoral and continental environments. The unit has been dated between the lower Cenomanian to the Santonian and has been divided into three informal sections: lower (littoral), middle (continental) and upper (littoral). The lower and upper sections constitute the main object of this study and they are interpreted as estuary, bay-head delta and coastal lagoon deposits. The intervals contain Arenicolites, Chondrites, Cylindrichnus, Ophiomorpha, Palaeophycus, Skolithos, Thalassinoides, Teredolites and fugichnia. The sedimentological and ichnological analyses allowed the interpretation of an overall estuarine palaeoenvironment, which is composed of three facies associations (FA) characterized by discrete associations of trace fossils. The more proximal FA is the bay-head delta, comprising delta-front bars of coarse-grained sandstone, containing Arenicolites, Skolithos and allochthonous petrified wood containing Teredolites. The trace fossils recorded in this FA indicate high-energy conditions. The second unit is interpreted as estuarine bars. This second FA is composed of fine- to medium-grained sandstone containing herringbone cross-stratification and mud drapes with Cylindrichnus, Ophiomorpha nodosa, Skolithos, Thalassinoides and fugichnia. This reflects a setting with a high sedimentation rates, moderate energy, unstable substrates, and normal salinity. Finally, the fine-grained estuarine deposits are characterized by heterolithic composite bedsets (e.g., flaser, wavy and lenticular bedding), locally with synaeresis cracks. Such units are bioturbated with Arenicolites, Chondrites, Cylindrichnus, Palaeophycus, Thalassinoides and Skolithos. These ichnogenera are generally characterized by their small sizes, consistent with a stressed palaeoenvironment, possibly a function of fluctuations in salinity. The distribution, size and abundance of the trace fossils were controlled by several palaeoenvironmental factors including energy, the sedimentation rate, and the fluctuations of salinity contents. This variability of palaeoenvironmental factors is directly related to the fresh water and sediment input from rivers flood, which took place during the wet seasons of a warm, temperate climate.