Ichnology of Late Cretaceous Turbidites from the Rosario Formation, Baja California, Mexico

Abstract

The Late Cretaceous Canyon San Fernando channel-levee system in the Rosario Formation of Mexico contains a diverse assemblage of trace fossils that is fully documented for the first time. The aim of this study is to describe the trace fossils present and document their palaeoenvironmental range across a turbidite-channel-levee system, from the coarse-grained channel axes within the channel belt to the most distal parts of the external levee. Trace fossil distributions are strongly controlled by palaeoenvironments and physical and chemical conditions within the turbidite system. Large, deep-tier deposit-feeding macrofauna are most abundant in and close to the channel axes (i.e., organisms forming Ophiomorpha rudis and Scolicia), and smaller, deposit-feeding and gardening organisms are more abundant in more distal terrace and external levee environments. The distribution of shallow-tier ichnotaxa (e.g., graphoglyptids) is restricted to levee and terrace/overbank settings. It is not known whether this represents the true ecological range of the trace-making organisms, or the limit of the suitable taphonomic conditions required for their preservation. Further studies of this kind will help shed further light on the biological, physical, and chemical controls on trace fossil assemblages across a variety of turbidite depositional systems throughout the Phanerozoic. Integrated ichnological and palaeoenvironmental analysis of deep marine clastic sediments has an important potential role to play in the analysis and characterization of hydrocarbon reservoirs in the subsurface.