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ABSTRACT: What is the Significance of the Condensed Interval? Oxygen Isotope Data from the Flexure Trend

JOYCE, J. E., and J. M. PRUTZMAN, Unocal, Paleontology and Biostratigraphy Group, Houston, TX, and L. R. C. TJALSMA, Unocal Science and Technology, Brea, CA

Many Gulf Coast explorationists assume that condensed intervals found in Pleistocene and Neogene sediments in the Gulf of Mexico were deposited during eustatic highstands and provide correlation horizons. However, oxygen isotope results from the flexure trend raise serious doubts concerning the eustatic and stratigraphic significance of condensed intervals as used in the Gulf of Mexico. The occurrence of a particular fossil event within a shaley interval, with a certain E-log response, does not necessarily indicate that all shales containing that fossil event and E-log signature are time synchronous. Furthermore, different mechanisms may be responsible for the deposition of such shaley units.

These observations raise the possibility that condensed intervals, as now recognized in Gulf Coast exploration to define the maximum flooding surface, can result from local geologic processes. Because condensed intervals are deposited during periods of decreased terrigenous dilution and subsequent microfossil enrichment, it stands to reason that most planktic datums would occur within relatively condensed intervals, regardless of the mechanism responsible for their deposition. Furthermore, oxygen isotope results indicate that condensed intervals are not necessarily restricted to eustatic highstands.

Rather, condensed intervals commonly are made up of either a eustatic highstand, lowstand, or both.


AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)