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Abstract

Much of industry’s perceptions and applications of biostratigraphy in relation to sequence stratigraphy were developed in expanded, siliciclastic-dominated, continental margin deposits. A nearly universal paradigm is that high concentrations of microfossils, especially planktic forms, are indicative of condensed sections, and more specifically, maximum flooding surfaces (MFS). Thus, planktic abundance curves are often used to identify MFS’s in systems tracts interpretations, for correlations, and to identify potential reservoir seals. Although this is a proven technique on continental margins, it is not necessarily applicable to the study of condensed, organic-rich mud rocks deposited within epeiric seas that are often the target of unconventional resource plays.

There are several assumptions about microfossil abundance curves that are implicit when using them to recognize MFS’s: (1) abundances within samples are reflective of the original microfossil assemblage; (2) high abundances directly correlate to a decrease in clastic input and are not due to other processes; and (3) the microfossils are in situ. The accuracy of a microfossil count is dependent on fossil preservation, recovery, and the selection of representative samples. Diagenetic processes, thermal maturity, redox conditions, and compaction can negatively impact fossil preservation, whereas fossil recovery is often poor in brittle mud rocks and carbonates. Examination of thin-sections has found that microfossil occurrences are often concentrated in discrete laminations or lag deposits, making it difficult to accurately estimate average abundances from thin-sections or core plugs. High microfossil abundances are often related to low sedimentation rates, but they may also be a product of plankton productivity and winnowing by bottom-water currents. Likewise, turbidity currents and other downslope transport processes may produce high concentrations of reworked and/or transported microfossils. It is recommended that sequence biostratigraphy in organic-rich mud rocks should focus on identifying hiatal surfaces, depositional environments, estimating rock accumulation rates, and correlating to updip locations where sequence stratigraphic surfaces are more readily identified.

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