Ocean Drilling Program (ODP) Leg 206 is the first of a multileg campaign to sample a complete crustal section from the extrusive lavas, through the sheeted dike complex, and into the uppermost gabbros within in situ ocean crust, by drilling a deep hole into basement formed at a superfast spreading rate (Shipboard Scientific Party, 2003). Site 1256 (6°44.2´N, 91°56.1´W; Fig. F1) lies in 3635 m of water in the Guatemala Basin on Cocos plate crust that formed ~15 m.y. ago on the eastern flank of the East Pacific Rise (EPR) when the site experienced superfast spreading (Wilson, 1996). This site formed at an equatorial latitude (Fig. F1) within the equatorial high-productivity zone and initially endured very high sedimentation rates (>35 m/m.y.) (e.g., Shipboard Scientific Party, 2003).
Three holes were drilled at Site 1256 with the intention of retrieving sediments. Hole 1256A produced a single mudline core that recovered the uppermost 2.37 m of the sedimentary section. This interval was re-sampled using the advanced piston corer (APC) with a second mudline core in Hole 1256B, where coring continued to a depth of 250.70 meters below seafloor (mbsf), at which point basement was contacted. Roughly 1 m more penetration recovered ~5 cm of the uppermost basalt. The lower portion of the sedimentary section was resampled in the rotary-cored Hole 1256C from a depth of 220.10 to 245 mbsf. Sediments in Hole 1256B provide a complete sampling of the sedimentary sequence overlying the oceanic basement. The sediments are subdivided into two principal lithologic units. Unit I (0–40.6 mbsf) is clay rich with a few carbonate-rich intervals and becomes increasingly calcareous with depth. Unit II (40.6–250.7 mbsf) is predominantly biogenic carbonate with some minor more siliceous and diatom-rich intervals. Bioturbation is common throughout the whole sedimentary sequence.
Calcareous nannofossils are generally abundant and moderately to well preserved in all samples from Site 1256. Preservation and individual nannofossil species abundance are recorded in Table T1. All species considered in this report are listed in alphabetical order in the distribution chart (Table T1) and the "Appendix." Descriptions of species and genera noted in this study and their bibliographic references are given in Perch-Nielsen (1985), Bown (1998), and Wise et al. (2004).
This chapter examines the nannofossil biostratigraphy of Hole 1256B and some associated postdepositional events from the middle Miocene to the Quaternary. The biostratigraphy for the sedimentary sequence immediately above the crust provides a useful tool for dating the underlying oceanic crust because direct dating of usually altered oceanic basalts has seldom been possible. Correlation of nannofossil datums in eastern equatorial Pacific (EEP) drill holes would indicate intra- and interplate variations in geologic and tectonic history.