Limitations of Ichnofossil Approach for Evaluating Paleo-Bottom-Water Redox Conditions

Charles E. Savrda, David J. Bottjer

Information on trace fossil diversity, burrow size, and depth of burrow penetration has been incorporated into an ichnofossil tiering model designed for the evaluation of paleo-bottom-water redox conditions. Previous applications to a variety of Phanerozoic strata indicate that this trace fossil approach is more effective than previously emphasized criteria for reconstructing relative degree of oxygenation as well as rates and magnitudes of temporal change in oxygenation. However, the trace fossil model is not without limitations. Presently, we recognize four factors that could limit the accuracy of ichnology-based redox reconstructions: (1) information loss--increases in penetration depth (associated with reoxygenation) that occur at rates in excess of sedimentation rate will destroy a portion of the previous record of oxygenation whereas extended residence times of deep-tier organisms may mask the record through excessive overprinting; (2) inadequate structure visibility--required information on trace fossil assemblage composition and cross cutting relationships is unobtainable for strata where original or diagenetically enhanced contrast between burrows and surrounding sediments is lacking; (3) complexity introduced by rapid deposition--periodic turbidite deposition can mask the desired oxygen signal through substrate modification, temporary modification of faunal composition and behavior, and/or temporary water chemistry changes; and (4) changes in substrate consistency--temporal variation in substrate stiffness (e.g., development of firmgrounds) may esult in intrastratal variation in trace fossil tiering structure. The success of future applications of the ichnologic approach depends upon thorough consideration of these factors.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.