Ocean Drilling Program, United States

Drilling a complete section of oceanic crust has been an unfulfilled ambition since the inception of scientific ocean drilling. Recovery of in situ oceanic crust is imperative to understand igneous accretion and the complex interplay between magmatic, hydrothermal, and tectonic processes, as well as a means for calibrating remote geophysical observations, particularly seismic and magnetic data. Only by drilling a complete section of upper crust formed away from fracture zones can the processes operating at normal mid-ocean ridges be understood. There is an observed relationship between the depth to axial low-velocity zones imaged at active mid-ocean ridges and spreading rate. Recent recognition of an episode of superfast spreading (200-220 mm/yr) on the East Pacific Rise approximately 11-20 m.y. ago presents an opportunity to drill through the upper oceanic crust into the gabbroic rocks in minimal time. Even allowing for significant burial by lavas that have flowed off axis ( approximately 300 m), the upper gabbros, thought to be the frozen axial melt lens, are predicted at approximately 1100-1300 meters below seafloor (mbsf). During Leg 206, we completed the initial phase of a planned two-leg project to drill a complete in situ section of the upper oceanic crust that will eventually extend through the extrusive lavas and sheeted dikes and into gabbros. Drilling was conducted at Ocean Drilling Program (ODP) Site 1256 (6.736 degrees N, 91.934 degrees W) on approximately 15-Ma oceanic lithosphere of the Cocos plate, which was formed by superfast spreading (>200 mm/yr) at the East Pacific Rise. To fully characterize the sedimentary overburden and establish depths for the casing strings, three pilot holes were cored, recovering a nearly complete section of the 250.7 m of sediment overlying basement and penetrating 88.5 m into basement with very good recovery (61.3%). The sediments can be subdivided into two main lithologies. Unit I (0-40.6 mbsf) is clay rich with a few carbonate-rich intervals, whereas Unit II (40.6-250.7 mbsf) is predominantly biogenic carbonate. More than 500 m of young Pacific extrusive lavas were cored with moderate to high rates of recovery following the installation of a reentry cone with a 16-in-diameter casing string that extended 20 m into basement in Hole 1256D. Axial sheet flows with subordinate pillow lavas, hyaloclastites, and rare dikes are capped by a more evolved massive flow >74 m thick and other sheet flows that probably ponded in small faulted depressions several kilometers off axis.