Sequence Stratigraphy, Biostratigraphy, and Taphonomy in Shallow Marine Environments

Sequence stratigraphy provides an integrated framework within which to examine historical patterns of paleontological phenomena. Depositional sequences are the stratigraphic record of fluctuations of sea level and sedimentation, environmental parameters that also exert critical controls on the distribution of shallow marine organisms, and the preservation and accumulation of their skeletal remains. As such, stratigraphic paleontology and sequence stratigraphy share multiple associations. Biostratigraphy is a critical tool for relative age-dating and correlation of depositional sequences; in turn, sedimentological and depth-related variables exert a primary control on the occurrence of zonally significant fossils. The most readily dated portions of many sequences are offshore condensed facies. Correlation of a hierarchy of discontinuity surfaces, however, permits extension of biostratigraphic dating into less fossiliferous facies. In turn, mingling of fossils of distinct zones provides a key to recognition of condensed sections, and truncation of zones points to significant unconformities such as sequence boundaries. The combination of refined graphic biostratigraphy, cycle-based ecostratigraphy, and sequence stratigraphy, sequence biostratigraphy, will ultimately permit extremely prrecise stratigraphic correlation and dating of marine strata. Taphonomic attributes of fossil assemblages also relate closely to sequence stratigraphy. Taphofacies vary predictably in depositional sequences because of the dependence of fossil preservation upon rates of burial and environmental energy. A spectrum of preservation should exist from highly corroded, fragmented remains, typical of erosive lowstand or early transgressive conditions, to intact multielement skeletons, characteristic of rapid background sedimentation during mid-highstand progradation; geochemically stable lags of corroded fossils (e.g., bone beds) typify many highly condensed sections. Skeletal accumulations develop during intervals of low sediment input. Sediment-starved accumulations overlying marine flooding surfaces occur at bases of parasequences, winnowed shell beds at parasequence tops. At a larger scale, thin, sheet-like skeletal deposits may form transgressive systems tracts of third order sequences. Fossil Lagerstätten (typically involving rapid burial and/or anoxia) are characteristic of the transgressive to early highstand interval. Although higher background sedimentation rates typify the later highstand, fossil event beds are commonly less recognizable due to dilution and bioturbation effects. Close integration of paleontologic and sequence stratigraphicdata should foster a greatly improved understanding of biases and relationships of biotic and abiotic processes in the accumulation of the stratigraphic record.