Equilibrium Responses Reflected in a Large Conichnus (Upper Cretaceous Eutaw Formation,Alabama, USA)

A vertically extensive (1.4 m) specimen of Conichnus in the Cretaceous Eutaw Formation (Alabama) exhibits marginal and internal fabrics that reflect equilibrium behavioral responses to changes in sedimentation rate and erosion events associated with tidal sandwave dynamics. Distinct fabrics can be linked to: (1) initial substrate penetration by the tracemaker, which followed stoss-side erosion of sandwave foresets; (2) gradual retrusive movement in response to slow sediment aggradation (<1 cm/day) in sandwave bottomsets; (3) intermediate to high rates of retrusive migration in response to moderate to rapid sediment aggradation (～1 cm/day) in sandwave toes; (4) tracemaker escape associated with extremely rapid (>5 cm/day) foreset deposition; and (5) protrusive equilibrium movement in response to stoss-side erosion of foresets. Morphological observations in the context of tidal cyclicity suggest that the trace, presumably produced by an anemone, was a semipermanent domicile occupied for at least two to three months. Relationships established between burrow fabric and sedimentary conditions in the Eutaw specimen may aid in the interpretation of conichnian burrows in other high-energy, shallow marine sequences.