Benthic response to event deposition and environmental disturbance in a shoreface to subaqueous delta system: Ichnology of the Silurian-Devonian Furada Formation of Asturias, Spain

The Furada Formation of Asturias, Spain, represents a clastic shallow-marine unit deposited during the middle Silurian to the Early Devonian. This formation contains abundant evidence of event deposition and non-uniform distribution of bioturbation structures representing a benthic response to multiple stressors. The 170-m-thick succession was measured and described, and the ichnotaxa were recorded and associated with five sedimentary facies. Most shallow-marine environments are characterized by periodic events of environmental disturbance, mainly by episodic deposition, which may be recorded by a change in both the degree of bioturbation and ichnodiversity. The general depositional setting for this formation has been previously interpreted as a wave-dominated and storm-influenced shallow-marine environment. However, sedimentologic features described in this study, such as anomalous heterolithic and mudstone units, representing fluid mud layers, hyperpycnal flow deposits and plume collapse accumulations, suggest the influence of fluvial discharge, making it a more complex depositional setting. The proposed model comprises an inner sandy shoreface belt flanked seawards by a muddy subaqueous delta platform. The trace-fossil assemblages of this formation reflect environmental stress factors introduced by the interplay of storms and fluvial input (e.g., high sedimentation rate, fluctuating hydrodynamic energy, decreased substrate consolidation), resulting in reduced ichnodiversity and low abundance. From an ichnofacies perspective, the shoreface complex is characterized by the Cruziana Ichnofacies, whereas the subaqueous delta platform is represented by the Phycosiphon Ichnofacies. Integration of sedimentologic and ichnologic datasets allows for a refined depositional interpretation of the formation and greater understanding of the environmental diversity of wave-and river-influenced shallow-marine clastic systems, including the responses of the middle Paleozoic shallow-marine benthos to event sedimentation and environmental disturbance. This study is one of the first detailed documentations of the ichnology of subaqueous deltas.