Site 1256
Priority:
1
Position:
6°44.19'N, 91°56.06'W
Water depth (m):
3655
Sediment thickness (m):
252
Target drilling depth (mbsf):
As deep as possible
Approved maximum penetration (mbsf):
1700 (although EPSP notes also report approved as proposed and proposal requests >1700 m total penetration)
Survey coverage:
Acquired during Ewing cruise EW9903, March–April, 1999:
• MCS with 480 channels, 10 air guns
• Refraction shooting 20 air guns recorded on ocean bottom hydrophones (OBHs)
• 3.5 kHz reflection
• Hydrosweep multibeam bathymetry
Site 1256 is at the crossing of EW9903 Lines 22 and 27 (see Figures F8, F9)
Objective:
• Provide a reference with in situ samples through the extrusive volcanic and feeder dike section and into gabbroic rock normal, intact oceanic crust formed at very fast spreading rate at 15 Ma to constrain models of formation and alteration of oceanic crust
• Maintain a "legacy hole" available for future deepening
Drilling and logging program:
Continuous coring as deep as possible
Logging program:
• WSTP on arrival
• Triple combo to determine borehole condition. If borehole has notably deteriorated we may elect to conduct more logging runs with additional tools
• Full logging suites planned at the end of each expedition include
• Triple combo
• FMS/sonic
• BGR borehole magnetometer: oriented magnetic field measurements
• UBI: acoustic borehole images
• WST: normal-incidence vertical seismic profiling
• Incremental logging runs may be implemented if hole conditions warrant
Nature of rock anticipated:
Basalt, diabase, and gabbro