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The Case for Erecting Two New Seilacherian Ichnofacies for Deltaic Successions: The Rosselia Ichnofacies and the Phycosiphon Ichnofacies

Abstract

Seilacherian Ichnofacies have been established, to date, for characterizing relatively stable depositional settings. Environments characterized by temporally and spatially variable physico-chemical stresses, however, have languished and been described ichnologically in the context of their “departures” from the archetypal expressions of ambient environments. Correspondingly, discrimination between shoreface and deltaic deposits have been addressed mainly by identifying variations in the individual trace fossil suites without an over-arching ichnological model. Based on trace fossil suites reported from globally distributed strata throughout the Phanerozoic, the case can now be made for erecting two new temporally and geographically recurring Seilacherian ichnofacies - the Rosselia Ichnofacies for sandy delta-front settings, and the Phycosiphon Ichnofacies for muddy prodelta environments.

The Rosselia Ichnofacies is characterized by generally low (BI 0-3) bioturbation intensities and sporadically distributed burrows. Most ichnogenera record deposit-feeding behaviours, many associated with vertically and horizontally oriented permanent dwelling structures. Dwellings commonly record re-equilibration, associated with elevated sedimentation rates and sporadic deposition. While many of the trace fossils constitute facies-crossing elements, fully marine (ambient) forms also may occur in the suite. Associated mudstone interbeds display low BI values and top-down bioturbation, also consistent with rapid deposition.

The Phycosiphon Ichnofacies is characterized by variable bioturbation intensities (BI 0-5), although many sand and mud beds may show very low bioturbation intensities (BI 0-1), pointing to their rapid, event-style sedimentation. Most biogenic structures record grazing or deposit-feeding behaviours, with subordinate horizontal dwellings for deposit feeding or carnivory. The ichnofacies typically shows beds characterized by diverse, fully marine suites intercalated with discrete beds dominated by low-diversity, facies-crossing elements. Such bed-scale juxtaposition points to short-term but recurring alternations between stable marine conditions and physico-chemically stressed conditions.

The two ichnofacies may pass gradationally into one another vertically, associated with lobe progradation or autogenic lobe abandonment. Further, the two ichnofacies may pass along depositional strike into their non-deltaic counterparts - the Rosselia Ichnofacies passing into the Skolithos Ichnofacies in shallow-water positions, and the Phycosiphon Ichnofacies passing into the Cruziana Ichnofacies in more distal positions. The creation of these two ichnofacies will greatly improve the ability of workers to differentiate classical shoreface deposits from those associated with deltas.