Abstract

A quantitative biostratigraphic analysis of the Paleocene to lower Miocene of the Taranaki Basin has enabled high precision in correlation, zonation, and assessment of depositional history. Biostratigraphic range-end events, based on 493 taxa in cuttings samples from eight wells, representing foraminifera, nannofossils, dinoflagellates, and miospores, were culled to 87 range-top events that were then analyzed by deterministic (constrained optimization [CONOP]) and probabilistic (ranking and scaling [RASC]) techniques. All except 16 of the events are found to have relatively good biostratigraphic reliability. The RASC probable sequence and probabilistic zonation give the best estimate of the sequence of events and zones to be encountered in any new well in the basin and a precise biostratigraphic scale for future exploration. The CONOP composite section, which matches well with that derived by conventional graphic correlation (GRAPHCOR), is readily related to previous zonations based on maximum ranges of taxa but gives an order-of-magnitude greater precision. CONOP provides a precise correlation framework and reveals marked variation in thickness of stages across the basin. When the composite section is calibrated against the time scale, basinwide changes in depositional rate are revealed. The upper Eocene and Oligocene mark an interval of slow deposition, whereas the Miocene marks a sharp increase in deposition. The time-calibrated composite section enables unconformities and changes in depositional rate found in individual wells to be precisely estimated. Many new unconformities are indicated, particularly in the Paleocene and Eocene.

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