Ichnofossil tiering analysis of a rhythmically bedded chalk-marl sequence in the Upper Cretaceous of Alabama

Ichnofossils are typically well expressed at bed transitions within rhythmically bedded marine sequences. This is because most discrete biogenic structures therein are actively or passively filled with sediment derived wholly or partly from overlying layers; hence they contrast sharply with ambient sediments. These bed transitions, or 'piped zones', provide the opportunity to assess the nature of infaunal tiering using two different approaches: (1) cross-cutting relationships among recurring ichnotaxa; and (2) apparent maximum penetration depths of piped-zone ichnofossils below their intervals of origination (primary strata). Both approaches were applied to piped zones at chalk-marl and marl-chalk transitions within the rhythmically bedded Upper Cretaceous (Campanian) Demopolis Chalk of western Alabama. Quantitative analysis of cross-cutting relationships indicates the existence of a deep Chondrites tier, a shallow Thalassinoides tier, and a diverse intermediate tier characterized by Anconichnus, Planolites, Taenidium, Teichichnus , and Zoophycos. Analyses of penetration depths similarly document the presence of a deep Chondrites tier but indicate that the Thalassinoides producer occupied the same tier as the tracemakers of the remaining five ichnotaxa. The latter approach, which should be broadly applicable to heterolithic sequences of various sorts, appears to yield a more accurate picture of tiering because it provides a more direct reading of the depths of activity of tracemakers within a substrate, and because results are less likely to be influenced by behavioral factors (e.g., phobotaxis and preferential sediment exploitation) or other parameters (e.g., burrow size differences) that can impact cross-cutting relationships. Results of both approaches indicate that the tiering structure in pelagic carbonate substrates may not be as organized or complex as indicated by previous qualitative analyses of cross-cutting relationships in analogous chalk sequences.