COMPOSITE ICHNOFABRICS: CATEGORIZATION BASED ON NUMBER OF ICHNOCOENOSES AND THEIR TEMPORAL INCONGRUENCE

Ichnofabric refers to all aspects of a sediment that result from bioturbation at all scales. Composite ichnofabrics are those that record sequential overprinting of trace fossils within the same sediment host through time. With the exception of ichnofabrics in which all biogenic structures were emplaced in a geologically instantaneous period of time (e.g., traces observed within tidal laminite sequences), most ichnofabrics arguably are composite to varying degrees—they record continuous to episodic occupation of substrates by one or more communities of organisms over periods of as little as a few days to as many as tens or hundreds of millions of years. Three general types of composite ichnofabrics can be distinguished based on mechanisms responsible for trace-fossil overprinting, number of ichnocoenoses present, and relative timing of ichnocoenosis emplacement. Autocomposite ichnofabrics, which comprise a single ichnocoenosis, form by gradual upward migration of a vertically-partitioned benthic community in pace with sedimentation. Such ichnofabrics, best exemplified in chalks, are presumed to reflect paleoenvironmental stasis and, hence, they are most useful in assessing aspects of benthic paleoecology (e.g., endobenthic tiering). Heterocomposite ichnofabrics comprise two or more ichnocoenoses produced in response to short- to long-term environmental perturbations within a depositional system; they reflect depositional process (e.g., event sedimentation) and longer-term paleoenvironmental dynamics (e.g., sea-level change). Examples include ichnofabrics produced in storm beds and at sequence stratigraphic boundaries (e.g., transgressive surfaces). Ultracomposite ichnofabrics form by overprinting of ichnocoenoses that are significantly temporally incongruous and reflect bioturbation in disparate depositional systems. Examples from the Gulf coastal plain include Cretaceous continental and Eocene marine ichnofabrics overprinted by biogenic structures produced by modern insect larvae, bees, and/or crayfish. Recognition of ultracomposite ichnofabrics is vital for paleoichnologic studies, particularly those that target surface exposures of old but relatively poorly indurated strata that host or may have hosted significantly younger assemblages of organisms.