Applications of ichnology in lacustrine sequence stratigraphy: Potential and limitations

In comparison with their marine counterparts, lacustrine ichnofaunas have not been extensively used in sequence stratigraphy. Application of trace fossils in continental sequence stratigraphy cannot be based simply on the extrapolation of marine sequence stratigraphy. Marine substrate-controlled ichnofacies, in particular the firmground Glossifungites ichnofacies, develop in stable and cohesive substrates, reflecting erosive exhumation of the sediment. In contrast, in lacustrine sequences, substrate-controlled ichnofossils only rarely indicate erosional exhumation because they are commonly related to desiccation of water bodies. The tripartite division of overfilled, balanced-fill, and underfilled lakes provides a comprehensive framework to evaluate the potential and limitations of lacustrine ichnofaunas in sequence stratigraphy. Based on a number of case studies spanning the three lake-basin types, a model of trace-fossil distribution in overfilled, balanced-fill, and underfilled basins is proposed.Overfilled lake basins contain well-developed softground trace fossils (Mermia and Skolithos ichnofacies, and the softground suite of the Scoyenia ichnofacies) that are useful to delineate parasequences and parasequence sets. Fluvial discharge into overfilled lakes usually generates density currents that oxygenate lake bottoms, allowing the establishment of epifaunal and infaunal communities. Additionally, these are freshwater lakes where no stress due to hypersalinity occurs, leading to the development of a relatively diverse benthos. Land–plant derived organic matter is the prime source of nutrients, favoring the development of a deposit-feeding benthic fauna in permanently subaqueous, low-energy zones. Firmground suites are rare because such large lakes usually do not experience desiccation.Abundant firmground trace-fossil suites of the Scoyenia ichnofacies occur in balanced-fill lakes, but softground assemblages are usually depauperate. Lowstand deposits contain abundant and widespread ichnofaunas, commonly meniscate trace fossils and arthropod trackways of the Scoyenia ichnofacies. Lake hydrology is closed during lowstands and salinity usually increases, imposing a stress factor on the lake biota and, therefore, softground ichnofaunas are depauperate. Ichnofaunas from turbidite systems in balanced-fill lakes are less abundant and diverse than those from overfilled lacustrine turbidites. Freshwater conditions are common during transgression, but dysaerobic conditions may prevail, imparting a stress factor on the benthic biota. In hydrologically closed lakes, the Scoyenia ichnofacies is widespread, but the Mermia ichnofacies is commonly absent or impoverished.The Scoyenia ichnofacies is associated with lowstand desiccated substrates in underfilled lakes. The density of arthropod trackways may be high, forming tracked omission surfaces. Some of these omission surfaces may represent sequence boundaries expressed by co-planar surfaces of lowstand and subsequent flooding. Rapid changes in depositional conditions reflecting desiccation during vertical aggradation can lead to the formation of composite ichnofabrics that reflect successive bioturbation events.