ICHNOLOGY OF RHYTHMICALLY BEDDED DEMOPOLIS CHALK (UPPER CRETACEOUS, ALABAMA) - IMPLICATIONS FOR PALEOENVIRONMENT, DEPOSITIONAL CYCLE ORIGINS, AND TRACEMAKER BEHAVIOR

Ichnofossils typically are well expressed at bed transitions within rh ythmically bedded marine sequences, owing to high contrast between amb ient sediments and burrow fills derived from overlying Layers. These p iped zones can provide important information on paleoenvironments, pal eoceanographic mechanisms responsible for depositional cyclicity, and tracemaker behavior. This is illustrated via quantitative analysis of piped-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, C hondrites, Planolites, Taenidium, Teichichnus, Thalassinoides, and Zoo phycos. This ichnocoenosis reflects deposition in a quiet, well-oxygen ated outer-shelf setting and indicates that redox and scour cycles wer e not important controls of carbonate cyclicity. A statistically signi ficant positive relationship is observed between piped-zone burrow den sities and carbonate contents of overlying strata. This relationship, when modeled in the context of fluctuations in, sedimentation rate and associated changes in the residence time of sediment in the zone of a ctive bioturbation, is consistent with a elastic-dilution mechanism fo r carbonate cyclicity. The sediment fills of most piped-zone ichnofoss ils can be readily linked to ambient and/or overlying sediments. These linkages, combined with relationships among burrow densities, carbona te contents, and, indirectly, organic carbon contents of associated se diments, provide evidence for animal behavior. Such relationships in t he Demopolis Chalk support the surface detritus feeder model for Zooph ycos and invalidate previous interpretations of Taenidium and Teichich nus, at least for some pelagic carbonate substrates.