The ichnofacies paradigm: high-resolution paleoenvironmental interpretation of the rock record

The ichnofacies paradigm endures as the elegant, unifying framework within which accurate ichnological observations and their reliable environmental interpretations can be derived from the rock record. These recurring, strongly facies-controlled groupings of trace fossils, reflecting specific combinations of organism behavior (ethology), constitute the benchmark animal-sediment responses to optimum environmental conditions. Seilacherian ichnofacies are therefore distinctive, recurrent, archetypal associations of traces, made most useful when placed into the context of the original suites (i.e. traces that record the activities of coherent, environmentally related infauna). Ichnofacies are part of the total aspect of the rock, and consist of primary features imparted by the organisms inhabiting the depositional environment (biogenic structures). Insights into the depositional environment are derived from the fact that organisms respond in predictable ways to variations in energy conditions, food resource types, substrate consistency, water salinity, oxygenation, subaerial exposure, substrate moisture, temperature and others. Although in the marine realm many of these conditions change progressively with increasing water depth, ichnogenera display, at most, a passive relationship to bathymetry. Additionally, like lithofacies, ichnofacies are subject to Walther's Law. The utility of ichnofacies to paleoenvironmental reconstruction, therefore, also lies in their lateral continuity and predictable vertical succession, leading to mappable constructs. Accurate interpretations of depositional environments favor reliable predictions of laterally adjacent settings and their associated ichnofacies. Like all facies analyses, interpretations of ichnofaunas are improved substantially when they are evaluated in the context of the host rocks and their sedimentologic (i.e., lithofacies) and stratigraphic implications. Archetypal ichnofacies are especially effective for characterizing deep marine through to shallow marine settings, though more recent studies have investigated and expanded their utility in continental regimes as well. Intergradations between the archetypal ichnofacies are also common and demonstrate a continuum of changing depositional conditions. As a result, very high-resolution analyses can be achieved. Departures from the archetypal ichnofacies are common, but their recognition and interpretation are only possible by comparison with these established temporally and globally recurring groupings. By their very nature, such anomalous ichnological suites yield valuable insights into the specific characteristics of the depositional setting, highlighting animal-sediment interactions in response to imposed environmental stresses. In this way, brackish-water environments, anoxic to dysaerobic settings, and areas of fluvio-deltaic deposition can be readily recognized. Thirteen temporally and geographically recurring archetypal ichnofacies that demonstrate temporal and global recurrence have been defined. Most are named for a representative ichnogenus: Scoyenia, Mermia, Coprinisphaera, Trypanites, Entobia, Gnathichnus, Teredolites, Glossifungites, Psilonichnus, Skolithos, Cruziana, Zoophycos, and Nereites. Traces in freshwater (continental) (i.e., Scoyenia, Mermia, Coprinisphaera), and brackish-water settings are in need of further study. The marine softground ichnofacies (i.e., Psilonichnus, Skolithos, Cruziana, Zoophycos, and Nereites) are comparatively better understood and constitute robust models. Current research has demonstrated that the marine softground ichnofacies form a continuum along the depositional profile, adding precision to paleoenvironmental interpretations. Traces in the firmground (Glossifungites), woodground (Teredolites), and hardground (Trypanites, Entobia, and Gnathichnus) ichnofacies are principally distributed on the basis of substrate type and consistency. Ongoing research of these substrate-controlled ichnofacies continues to highlight subtle, previously overlooked complexities, expanding their utility in the rock record.