The Glossifungites Ichnofacies: Modern examples from the Georgia coast Modern examples from the Georgia coast

The Glossifungites ichnofacies, albeit areally restricted, constitutes a distinct intermediary between freely shifting, particulate substrates of the Skolithos ichnofacies and fully consolidated substrates of the Trypanites ichnofacies. All three typically occur within, but are not restricted to, littoral to shallow sublittoral, relatively high-energy zones. Substrates of the Glossifungites ichnofacies ordinarily are palimpsest—altered by contemporary physical and biogenic processes—and may exhibit relict features indicative of conditions prior to semiconsolidation of sediments. Characteristic lebensspuren consist of unlined domiciles of suspension-feeding or foraging animals, such as certain bivalves, crustaceans, and polychaetes, and these crosscut relict physical and biogenic sedimentary structures.

On the Georgia coast, the ichnofacies occurs most commonly among coherent, partially dewatered, Holocene salt marsh muds formerly buried underneath transgressive beach ridges but now being exhumed by beach or tidal stream erosion. Relict insitu biotic features include roots of the grass Spartina, burrows of the crab ilea, and valves of the snail Littorina, the mussel Geukensia, and the oyster Crassostrea. On St. Catherines and Petit Chou Islands, the substrates now are occupied by three intergradational ichnocoenoses: (1) a petricolid assemblage, (2) a petricolid-pholad-crustacean assemblage, or (3) a petricolid-crustacean-polydoran assemblage. Distributions of these traces seem to be governed mainly by incidence of wave versus current energy, duration of subaerial exposure, density of relict root mats, and variations In sand content, cohesiveness, and related geotechnical properties of the substrate. In the rock record, diagnostic biogenic structures among the assemblages would include: firm- (as opposed to hard-) substrate Gastrochaenolites and (or) closely related ichnogenera, made by petricolid and pholad bivalves; small Diplocraterion, made by the polychaete Polydora; essentially unlined Palaeophycus and Thalassinoides, made by polychaetes and shrimp, respectively; and Psilonichnus, or unlined crab burrows. Of the last three, some forms occur in relict (pre-semiconsolidation) assemblages as well.