Animal-substrate interactions in freshwater environments: applications of ichnology in facies and sequence stratigraphic analysis of fluvio-lacustrine successions

Abstract

Abstract At present, three continental archetypal ichnofacies are widely accepted: the Scoyenia, Mermia and Coprinisphaera ichnofacies. The last is present in palaeosols, and the first two occur in fluvio-lacustrine environments. Additionally, the Skolithos ichnofacies may be present in relatively high-energy fluvio-lacustrine deposits. The ichnofauna from active fluvial channels is characterised by low-diversity assemblages of simple vertical burrows and escape traces, referred to the Skolithos ichnofacies. Abandoned or inactive channel deposits characteristically contain low-diversity assemblages dominated by meniscate traces. Floodplain water bodies that experienced progressive drying (desiccated overbank deposits) may contain abundant arthropod and vertebrate trackways, backfilled meniscate traces, ornamented burrows and bilobate traces with scratch marks, which allow recognition of the Scoyenia ichnofacies. Floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (overfilled overbank deposits) include simple grazing trails, locomotion trails and horizontal dwelling burrows, representing impoverished occurrences of the Mermia ichnofacies. Hydrologically closed lakes are very stressful environments in which subaqueous ichnofaunas are rare. The richest ichnofaunas in closed lakes are present at the lake margins, and record the activity of terrestrial rather than aquatic faunas (Scoyenia ichnofacies). Hydrologically open lakes host relatively diverse and abundant ichnofaunas, comprising the Scoyenia ichnofacies in low-energy, lake-margin areas, and the Mermia ichnofacies in permanent subaqueous lacustrine zones. Sediments deposited in relatively high-energy lacustrine environments, such as wave-dominated shorelines and delta mouth-bars, commonly are represented by the Skolithos ichnofacies. Although continental trace fossils have not been extensively used in sequence stratigraphy, they have potential for future integrated study. Softground trace fossils are commonly well developed in overfilled lake basins and are useful to delineate parasequences and parasequence sets. In balanced-fill and underfilled lake basins, softground ichnofaunas are poorly developed because of stressful conditions. In contrast, firmground suites are rare in overfilled lake basins, but widespread in lowstand deposits of balanced-fill and underfilled lake basins. Early lowstand amalgamated incised fluvial channels are usually unbioturbated, but palaeosol ichnofaunas (e.g. the Coprinisphaera ichnofacies) may delineate sequence boundaries in interfluve areas. Increasingly isolated fluvial channels encased in overbank deposits develop during late lowstand, and lacustrine deposits accumulate during transgressions, favouring preservation of biogenic structures.