| Popis: |
Ichnofossils typically are well expressed at bed transitions within rhythmically bedded marine sequences, owing to high contrast between ambient sediments and burrow fills derived from overlying layers. These piped zones can provide important information on paleoenvironments, paleoceanographic mechanisms responsible for depositional cyclicity, and tracemaker behavior. This is illustrated via quantitative analysis ofpiped-zone ichnofabrics at the transitions between chalk and marl beds within the Campanian Demopolis Chalk, western Alabama. The Demopolis Chalk is characterized by an ichnocoenosis dominated by Anconichnus, Chondrites, Planolites, Taenidium, Teichichnus, Thalassinoides, and Zoophycos. This ichnocoenosis reflects deposition in a quiet, well-oxygenated outer-shelf setting and indicates that redox and scour cycles were not important controls of carbonate cyclicity. A statistically significant positive relationship is observed between piped-zone burrow densities and carbonate contents of overlying strata. This relationship, when modeled in the context offluctuations in sedimentation rate and associated changes in the residence time of sediment in the zone of active bioturbation, is consistent with a clastic-dilution mechanism for carbonate cyclicity. The sediment fills of most piped-zone ichnofossils can be readily linked to ambient andlor overlying sediments. These linkages, combined with relationships among burrow densities, carbonate contents, and, indirectly, organic carbon contents of associated sediments, provide evidence for animal behavior. Such relationships in the Demopolis Chalk support the surface detritus feeder model for Zoophycos and invalidate previous interpretations of Taenidium and Teichichnus, at least for some pelagic carbonate substrates. |
