Trace-fossil model for reconstructing oxygenation histories of ancient marine bottom waters: Application to Upper Cretaceous Niobrara formation, Colorado

Trends of decreasing trace-fossil diversity, burrow size, and penetration depth with diminishing oxygen availability in contemporary marine settings have been incorporated with the concept of trace-fossil tiering to develop a model for reconstructing the oxygenation history of ancient basin bottom waters. More sensitive than traditionally-employed criteria, the model permits detailed evaluation of relative degree of oxygenation through the recognition of oxygen-related ichnocoenosis (ORI) units, or units of strata deposited under similar levels of bottom-water oxygenation. When employed in detailed vertical sequence analyses, the model may also be used to construct interpreted oxygenation curves that reflect rates and magnitudes of temporal change in redox conditions. The utility of the model is demonstrated by its application to rhythmically-bedded, calcareous strata of the Niobrara Formation (Upper Cretaceous) exposed in Colorado. Non-laminated strata of the Niobrara are characterized by four recurring trace-fossil suites referred to as the (1) Chondrites, (2) Zoophycos/Teichichnus, (3) Planolites, and (4) Thalassinoides assemblages. On the basis of trends in trace-fossil diversity, burrow diameter and penetration depth, these assemblages are recognized as oxygen-related ichnocoenoses. Interpreted oxygenation curves for the basal Fort Hays Member reflect high-magnitude redox cycles characterized by rapid deoxygenation events followed by gradual reoxygenation phases. Curves for the overlying Smoky Hill Member reflect an overall lower degree of oxygenation with more irregular, lower magnitude redox fluctuations. Significant correlation between interpreted levels of paleooxygenation and organic-carbon content illustrate the model's potential for evaluating the distribution of organic-rich fine-grained strata. Co-interpretation of these variables with carbonate content supports the premise that deposition was controlled by paleoceanographic perturbations modulated by Milankovitch-like climatic cycles. Comparison with traditionally-emphasized macroinvertebrate body-fossil criteria clearly demonstrates the greater utility of the trace-fossil approach for evaluating paleo-oxygenation.