Ichnology of distal overbank deposits of the Santa Cruz Formation (late Early Miocene) : paleohydrologic and paleoclimatic significance

Abstract

This chapter deals with the trace fossils, also called ichnofossils, preserved in the lower Estancia La Costa Member of the Santa Cruz Formation (late Early Miocene) along the Atlantic coast, between the Río Gallegos and Río Coyle, Santa Cruz Province, Argentina. The succession is mostly composed of fluvial deposits. Extensive overbank areas record trace fossils in a variety of deposits such as floodplain water bodies and paleosols that developed under variable climatic conditions. From bottom to top, it records a general trend of paleosol development from humid to drier climatic conditions, with waterlogged areas developed in the middle portion of the succession. Floodplain water bodies record Taenidium barretti and Palaeophycus tubularis which correspond to a “pre-desiccation suite” of the Scoyenia ichnofacies that developed in soft substrates. Also, root traces are preserved when the time between depositional events is long enough to allow colonization by plants but not so much as to obliterate animal traces. Integrated ichnology and sedimentology suggests that paleosols that record abundant cf. Capayanichnus vinchinensis, fine and haloed root traces, and the less common occurrence of Taenidium barretti and Planolites beverleyensis were episodically waterlogged and are considered moderately drained. Other paleosols that record abundant ferric root traces suggest that they were moderately well-drained and developed under more humid climatic conditions. Similar moderately well-drained paleosols record abundant calcareous rhizoconcretions and a dwelling burrow attributed to a mammal. A third type of paleosol contains cells of solitary digging bees (Celliforma isp.) and ferric root traces, and is interpreted as being moderately well-drained and developed under drier climatic conditions.