Ichnology of a glacially-influenced continental shelf and slope; the Late Cenozoic Gulf of Alaska (Yakataga Formation)
Abstract The study of trace fossils in glaciomarine strata is important because of the low preservation potential of fauna but glaciomarine settings have received little attention from ichnologists. Widespread biological activity is recorded by ichnofossil assemblages in glaciomarine strata of the Late Cenozoic (6 Ma to present) Yakataga Formation, Alaska. Glacial sedimentation began in the Gulf of Alaska in the Miocene along a deep water active margin basin dominated by turbidity currents and debris flows. Later strata record progradation of the continental slope by mass flow processes and the establishment of a broad shelf influenced by repeated glaciation, ice-rafting of coarse debris, changes in water depth and salinity, storms, earthquakes, and high sedimentation rates. Yakataga Formation strata show overall, a limited ichnological diversity and low degree of bioturbation in relation to comparable non-glacial settings although certain beds are highly bioturbated. Twenty-four ichnotaxa are identified and grouped into nine assemblages characteristic of different depositional settings and substrate types. Ichnogenera and body fossil evidence shows that polychaetes, arthropods, molluscs and echinoderms were the dominant infaunal benthic organisms with deposit feeding being the predominant feeding strategy; poorly consolidated muddy sediments excluded suspension feeders and limited the development of the Skolithos ichnofacies typical of non-glacial shelves characterized by clean, shifting sand substrates. Evidence of rapid sedimentation is given by escape structures (fugichnia), the presence of Diplocraterion, and by the mass mortality of bivalves. Firm-ground burrows, typical of the Glossifungites ichnofacies, are associated with erosional surfaces, boulder pavements and coquinas. Diminutive feeding traces may reflect salinities lowered by glacial meltwater inputs. The downslope transport of food by turbidity currents and mass flows may explain the development of a diverse Cruziana ichnofacies in water depths where in non-glacial settings the Nereites ichnofacies is found. An ichnological summary is presented for the Late Cenozoic Gulf of Alaska continental margin; similar ecological conditions obtain in the modern setting and provide valuable supportive data. Ichnofacies identified from the Yakataga Formation are probably widespread in Late Paleozoic and Late Cenozoic glacially-influenced marine strata.