Departures from the Archetypal Ichnofacies: Effective Recognition of Physico-Chemical Stresses in the Rock Record

Many depositional settings are characterized by temporally and spatially variable physico-chemical stresses, leading to trace fossil suites that depart from those attributable to the archetypal ichnofacies. These departures impart critical information about the depositional setting that could not be derived using physical sedimentology alone. Organisms are sensitive to subtle changes in the environment and hence, are particularly adept at highlighting a range of physico-chemical stresses. Recurring departures from the archetypal ichnofacies have been identified from numerous depositional settings, including estuarine incised valleys, open bays and lagoons, storm-dominated shorelines, deltaic complexes, stagnant or stratified water bodies, and oxygen-depleted shelves and slopes.

Settings prone to physiological stress are characterized by ichnological suites that are dominated by facies-crossing elements showing high degrees of infaunal opportunism. Suites display general reductions in diversity and commonly, by decreases in the range of ethological groupings. Stressed settings, as well, generally result in a greater proportion of simple structures generated by inferred trophic generalists. Environmental stresses occur along a continuum from toxic to ambient, and hence, familiarity with unstressed suites is essential to recognizing their presence and magnitude in the rock record. The most common ichnologically delineated environmental stresses are salinity reductions, increased depositional rates, episodic deposition, heightened water turbidity, and reduced oxygenation.

The fundamental characteristics of the brackish-water ichnological model are 1) suites characterized by reductions in the numbers and diversities of ichnogenera, corresponding to an impoverished marine suite; 2) traces that are generally diminutive compared to their fully marine counterparts; 3) a predominance of simple opportunistic structures of inferred trophic generalists; 4) Suites comprising elements that record variations in substrate consistency and depositional rates and 5) successions showing locally high degrees of bioturbation, though locally with monogeneric suites.

Rapid deposition rates are reflected by 1) overall decreases in bioturbation intensity and 2) a paucity of elaborate, specialized-feeding structures, in favor of more mobile or temporary (sessile) deposit-feeding structures. Dwellings that facilitate readjustment or re- equilibration are locally common. Where settings record sporadic deposition, partitioning of fair-weather infaunal communities and post-event communities are apparent. Turbidites and tempestites, in particular, show juxtaposition of substrate types in much of the shallow marine realm. Successions record partial to complete extermination of fair-weather communities, organism escape through the event bed, initial recolonization of the new substrate and (depending upon the magnitude of environmental contrast between the ambient and event bed conditions) a replacement of the event suite with the fair-weather suite. Juxtaposed suites may range from Skolithos Ichnofacies elements juxtaposed with Nereites Ichnofacies elements (e.g., deep-sea turbidites), to suites attributable to the Skolithos Ichnofacies juxtaposed against other suites of the Skolithos Ichnofacies (e.g., lower and middle shoreface settings). In all cases, the alternation of such suites record in loco changes in energy and depositional rates. Mud turbidites, contourites, phytodetrital pulses, and freshet-induced, hyperpycnal flood deposits are common to river- and/or tide-dominated deltaic lobes, tidal shelves, continental slopes, and more rarely, large restricted bays or central basins adjacent to bay-head deltas in estuarine incised valleys, but are less well-studied. The depositional positions of these event beds, their substrate types, and the nature of food resources contained therein yield markedly different organism responses. Biogenic structures tend to be overwhelmingly those of facies-crossing deposit-feeders, locomotion and resting structures, or deep-tier structures that exploit these anomalous layers after burial.

Increased water turbidity is commonly associated with persistent, suspended sediment-laden distributary discharge on deltas and brackish-water bays, or mixing at the turbidity maximum zone of tidal-fluvial channels. Ichnological suites are characterized by reduced bioturbation intensities, reduced ichnogenera assemblage diversities, and the limitation of ethological categories to those of locomotion, resting, deposit-feeding, and grazing behaviors. Turbid water reduces primary productivity, clogs the feeding apparatus of endobenthic filter feeders, and increases the clastic content to be processed by suspension feeders and some carnivores. Consequently, highly turbid settings display a pronounced impoverishment of ichnogenera normally attributed to the Skolithos Ichnofacies, and consist, instead, of facies-crossing elements attributable to the Cruziana Ichnofacies.

Settings characterized by fluid muds include estuarine channel systems prone to development of a turbidity maximum zone, or to delta distributaries and delta front complexes, particularly in river- and tide-dominated systems. Initial deposition of the flocculated clay yields soupground conditions. Ichnological characteristics include surface pascichnia, indistinct mottling, and rare, open, mucous-lined tubes that collapse readily during compaction. Few of these structures are likely to survive into the rock record, Biogenic structures generated after compaction are more commonly preserved, and include surface trails and resting structures, intrastratal deposit-feeding structures, and deep-tier dwelling and/or deposit-feeding structures from higher levels.

Reduced oxygenation models yield oxygen-restricted ichnocoenoses (ORI). Reduced oxygen settings range from the deep sea, slope, and shelf, as well as stratified lagoons, bays, estuaries, and abandoned tidal and distributary channels. Ichnological responses to dysoxic to anoxic conditions are reflected by 1) reductions in ichnogenera diversities; 2) reduced trace fossil abundances; 3) decreasing burrow diameters; and 4) decreasing depth of burrow penetration into the substrate. Ethological distributions or discrete ichno