The Bermuda Rise
During the 15 years of the Deep Sea Drilling Project, significant basement penetration (> 500 m)
was achieved at several sites in the ocean basins. Although comprehensive borehole geophysical
measurements were successfully conducted at only two sites, both young crustal sites (6.2 Ma),
these measurements are of landmark importance. There is, however, strong evidence that old crust
is profoundly different from young crust and the primary objective of Leg 102 was the acquisition
of a comprehensive baseline suite of borehole geophysical data in old oceanic crust (110 Ma).
Original DSDP Hole 418A was reentered and logged to a depth of 0.5 km into basement during
Leg 102. Acoustic velocity and waveform logs showed relatively high compressional and shear
wave velocities throughout the upper 500 m of layer 2 (the oceanic crust), with slow values
occurring in the predominantly weathered pillow basalt and breccia units and fast values in the
massive basalts. A similar difference was shown by induction and focused electrical resistivity
logs. The upper 500 m below the sea floor was distinguished by unusually high gamma ray
activity and relatively high formation porosities, as determined by density and neutron porosity
logs. The base of this section was marked by a sharp magnetic polarity reversal, followed by a
notable low on the magnetic susceptibility log from 500 to 600 mbsf, then a gradual increase to the
bottom of the log. Temperature measurements indicated conductive rather than convective heat
flow, suggesting a less active hydrothermal regime, a view supported by the water chemistry
which, in contrast to the surrounding rocks, showed borehole water to be enriched in Ca2+ and
depleted in Mg2+ and K+.
Upon completion of Leg 102 operations at DSDP Site 418, it was tentatively concluded that the
properties of old oceanic crust are very different from young crust because of alteration and
infilling of void space with age. Old crust at these sites has been sealed by alteration products, such
as clays and carbonates, within the pillow basalt units and between layers of pillow and massive
basalt, restricting the movement of pore water and causing geophysical layer 2A now to be absent.
This is in contrast to observations at DSDP/ODP Hole 504B in young crust in the Pacific; layer 2A
is still present, the upper basalt is relatively porous and permeable, and observed underpressures
and flow of seawater into the upper permeable zone suggest weak hydrothermal convection within
the pillow basalts.