Lower Miocene quantitative calcareous nannofossil biostratigraphy from the Tropical Atlantic

Quantitative analysis of the lower Miocene of Ocean Drilling Program Hole 959A from the West African margin was performed to document all the calcareous nannofossil biostratigraphic events present. Combined with data from previous investigations of the lower Miocene from the tropical Atlantic, this research identifies and tests the viability of markers used in current zonation schemes, recognizes alternative markers for age boundaries, and examines statistically the most probable order of events in the lower Miocene using the Ranking and Scaling method (RASC). In Hole 959A, all major zonal and subzonal boundaries from CN1 to CN4 were identified, except for the boundary between Subzones CN1a and CN1b, using primary and secondary markers from the Okada and Bukry (1980) zonation. In addition, all age boundaries from the Chattian to Langhian stages were recognized or closely estimated using the calcareous nannoplankton markers. The resultant list of events extracted from Hole 959Aalong with events from other seven sites were examined biostratigraphically using RASC. A well threshold of four was selected as an appropriate control parameter, resulting in 22 events in the optimum sequence, 13 of which had a low standard deviation. Furthermore, interpolation of ages of events using Age/Depth model for Hole 959A was examined. The extracted ages provided a reasonable preliminary age estimate for the secondary events.