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Gulf of Mexico Neogene Astronomically-Tuned Biostratigraphic Time Scale

Abstract

With the publication of the BP Gulf of Mexico Neogene Astronomically Tuned Time Scale” (BP GNATTS) (GSA Bulletin in Review), BP made public the integrated Neogene microfossil biostratigraphic chart developed within post-merger BP for the Gulf of Mexico Basin and published the first industrial framework “fully-tuned” to orbital periodicities. This innovative framework evolved by merging heritage frameworks derived from the analysis of hundreds of Gulf of Mexico wells over many decades by multiple fossil disciplines. Its publication represents the preservation of stratigraphic concepts with hope of replication and furthering the application and advancement of biostratigraphy to understanding the subsurface. This framework extends from the Late Oligocene (25.05 Ma) to Recent at an average Chart Horizon resolution for the Neogene of 144 k.y., approximately double that of published Gulf of Mexico biostratigraphic charts and a five-fold increase over the highest resolution global calcareous microfossil biozonation. Such resolution approximates that of 4th to 5th order parasequences and is a critical component in the verification of seismic correlations between mini-basins in the deep-water Gulf of Mexico and for sand to sand correlation within reservoir intervals. The ability to establish correlations at ~130 k.y. resolution could impact future calibration and correlation of interpreted eustatic records and it may now be possible that calcareous microfossil biostratigraphy be utilized to evaluate and correlate Neogene cyclostratigraphic and magnetostratigraphic records, the latter previously used to assess the synchronicity of biostratigraphic events. Astronomical-tuning was accomplished through a 15-year research program on ODP Leg 154 sediments (offshore NE Brazil) with sampling resolution for calcareous nannofossils and planktonic foraminifera about 20 k.y. and 40 k.y. (thousand year), respectively. Its utility in global time-scale construction and correlation has been proven, in part, by application of the scheme in full to internal research for the Oligocene-Miocene boundary interval on the Global Boundary Stratotype Section and Point (GSSP) in northern Italy and offshore wells in the eastern Mediterranean Sea. This step change in Neogene resolution, now at the level of cyclostratigraphy (the orbital periodicity of eccentricity) and the magnetostratigraphic chron, demonstrates the potential for calcareous microfossil biostratigraphy to more consistently reinforce correlations of these time scale parameters. The integration of microfossil disciplines, consistent taxonomies, and rigorous analytical methodologies are all critical to obtaining and reproducing this new level of biostratigraphic resolution.