Conodont biofacies and biostratigraphic schemes in western North America: A model

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Abstract
Most biostratigraphic schemes for conodonts (and other fossils) are based on a combination of geographically separated stratigraphic sequences that contain abundant fossils. In developing biostratigraphy, generally little attention is given to identification of lithofacies that yield the fossils. Many sections in western North America include a mixture of sediments that represent a variety of marine environments. Such a stratigraphic mix of environment types yields a vertical sequence of fossils that represent a range of environments. Geographically separated but chronologically similar sections containing different environmental sequences may produce different sequences of fossils within a similar stratigraphic framework. Because of this, biostratigraphic schemes for similar age sequences may differ. Lower Triassic biostratigraphy in western North America is based on study of rock sequences that represent a variety of transgressional and progradational sediments. The “standard” Lower Triassic conodont zonation that has emerged is real, even usable, but includes conodont species representing a range of environments from shallow inner shelf to deeper basinal. This ecologic mix has utility simply because most comparable Triassic rock sequences are also a mix of lithofacies, and at least some of the conodont species in the “standard” can be found in any other section. Because certain conodont species probably were more sensitive to a narrow set of environmental parameters than has been realized, application of the ecostratigraphy concept may develop actual conodont biofacies biostratigraphy, a different biostratigraphy for each definable position along an environmental gradient.

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