ICHNOLOGIC FIDELITY AND TEMPORAL RESOLUTION OF ICHNOFABRICS: FROM ONE EXTREME TO THE OTHER

The potential of ichnofabrics to yield valuable information on endobenthic communities, organism behaviors, and paleoenvironmental conditions is generally widely recognized among sedimentary geologists. However, the extreme range in both ichnologic fidelity—i.e., the extent to which preserved ichnofabrics reflect the complete range of biogenic activities that occurred in a body of sediment—and temporal resolution of ichnofabrics is commonly not fully appreciated. The potential variation in the fidelity and temporal resolution of ichnofabrics is illustrated by near end-member ichnofabrics observed in shelf deposits of the Eocene Tallahatta Formation and Cretaceous Demopolis Chalk in the Alabama coastal plain. In eastern Alabama, the Tallahatta Formation includes packages of siliceous claystone with subordinate, thin storm-event sands and sandstones. Ichnofabrics developed within and immediately beneath storm beds are characterized by high ichnologic fidelity and provide high temporal resolution; they preserve a virtually complete record of the work accomplished in a matter of only a few hours to days by short-lived assemblages of storm-transported tracemakers. In contrast, ichnofabrics in the Demopolis Chalk (western Alabama), preeminently expressed at transitions between slowly deposited (3-4 cm/ka) marl and chalks, are composites that, at best, provide significantly time-averaged records of biogenic activity that occurred over periods of ~10 ka. Ichnologic fidelity is low; distinct biogenic structures reflect only the work of elite deep-tier tracemakers. Moreover, preliminary modeling that employs densities of distinct burrows or burrow systems, sedimentation rates, and conservative estimates of tracemaker longevities indicate that preserved ichnofossils represent far less than 10% of the time involved in the development of these composite ichnofabrics. The limited temporal resolution provided by these and comparable ichnofabrics calls for caution in paleoenvironmental and paleoecologic interpretations.