--> ABSTRACT: Seismic Reflection Calibration of Foraminiferal Indicators of Paleobathymetry, Taranaki Basin, New Zealand, by G. P. Thrasher, G. S. Scott, M. P. Crundwell; #91020 (1995).

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Seismic Reflection Calibration of Foraminiferal Indicators of Paleobathymetry, Taranaki Basin, New Zealand

G. P. Thrasher, G. S. Scott, M. P. Crundwell

Episodes of continental shelf progradation, a feature of Neogene sedimentation in Taranaki Basin, are clearly imaged on seismic reflection profiles. These unconformity-bounded sequences consisting of well developed sigmoidal-shaped topset, foreset, and bottomset facies, allow for the estimation of paleobathymetry by analyzing the amplitude of foresetting depicted by the seismic reflection data. The paleobathymetry of any point in the foreset facies can be estimated by calculating the depth of that point below the time-equivalent topset facies. The paleobathymetric position of the highest occurrence of a benthic foraminifer in a well which penetrates the foresets can thus be estimated from seismic stratigraphy, independent of paleontologically based inferences. The princip l sources of error are the estimated paleodepth of the reference datum (base of topset deposition), and corrections for post-depositional compaction.

This technique was applied to a seismically identified Pliocene progradational sequence penetrated by Taimana-1 and Taranga-1 petroleum exploration wells, to check the paleobathymetry of potential isobathyal taxa, including species currently used to identify the shelfbreak (nominally 200 m) and the top of the middle bathyal zone (nominally 600 m) in Miocene and younger strata. Results indicate that the upper bathymetric limit of the widely used shelfbreak marker Karreriella cylindrica, needs to be revised downward, to about 250 m below the shelfbreak. The upper bathymetric limit of Haeuslerella parri is proposed as a more reliable indicator of the shelfbreak in post-Miocene sequences. Data for Sigmoilopsis schlumbergeri indicate that its upper bathymetric li it is about 400 m below the shelfbreak, and verify the practice of using it as a middle bathyal marker.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995