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Abstract

Biostratigraphic signals have been utilized in allostratigraphic studies in conjunction with wire-line log analysis and seismic stratigraphy. In particular, systematic variations in abundance and diversity (simple species diversity and the Shannon–Weiner information function (H) of planktonic and benthonic foraminifera have been shown to characterize critical surfaces and individual systems tracts. Species diversity is a measure of both species richness and evenness. However, diversity values can vary greatly when evenness is constant and species richness varies. Diversity is, therefore, not necessarily an easy tool to interpret. The inability to partition diversity into its two components, richness and evenness, was addressed by Buzas and Hayek (1996), who demonstrated the utility of their new diversity measure (SHE) for biofacies identification along a depth transect in the Gulf of Mexico (Buzas and Hayek, 1998). SHE analysis should, therefore, be of use in bio-sequence stratigraphic studies, a hypothesis that is explored. The SHE plot used shows species richness ln(S) and H on the same plot. Evenness ln(E) is represented by the envelope formed by the H and ln(S) curves. Variations in the SHE plot are used to characterize systems tracts and critical surfaces. Within a depositional sequence the decrease to a minimum of ln(E) that coincides with decreasing ln(S) and H characterizes the candidate sequence boundary. An increase in ln(E) to a maximum in conjunction with increasing ln(S) and H characterizes flooding surfaces, and the maximum flooding surface in particular.

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