Site 1277 is located on the top of a basement high (Mauzy Ridge) close to interpreted magnetic Anomaly M1. The basement high is part of a margin-parallel ridge, which is apparent from the observed structure in two seismic reflection profiles that cross Site 1277, SCREECH line 2MCS and SCREECH line 206 (Fig. F20). There are no logging data or other concrete tie points between the core and seismic data, and linking seismic data to core data at Site 1277 is difficult. Coring began below an unexpectedly shallow basement surface, so a definitive sediment/basement contact cannot be used as a reference. Even if this boundary had been cored, correlating it to the reflection record would be complicated; the uppermost crust in the seismic section has a "layered" appearance (Fig. F21), which makes identifying the top of basement difficult.
Prior to drilling, basement depth was estimated to be 136 mbsf, based on MCS stacking velocities obtained from the SCREECH survey. During drilling with a wash core to 103.9 mbsf, hard layers were encountered at 85–89 and 97.5–100 mbsf and 2.29 m of basalt, gabbro, and volcaniclastic breccia were recovered in the core. Similar rocks were recovered beginning at 103.9 mbsf in the first rotary core (Core 210-1277A-2R), and serpentinized peridotite was recovered in deeper cores. The shallowest "basement" reflection in the MCS record could be generated by either the layer at 85 mbsf or the layer at 97.5 mbsf. The shallowest irregular "basement-type" reflection beneath onlapping sediments is ~110 ms below the seafloor (Fig. F21). If this correlates to the 85-mbsf hard layer, the seafloor-to-basement interval velocity would be 1545 m/s; if it correlates to the 97.5-mbsf hard layer, the interval velocity would be 1772 m/s.
Below 103.9 mbsf, coring continued for another 76.4 m. To estimate the depth of basement penetration on the seismic record, we used velocities based on laboratory measurements of the cored rocks. The average vertical z-velocity for Site 1277 samples is ~4480 m/s (see "Physical Properties"). It is likely that this average is a high-end estimate because velocities were not measured on sections of core that were more delicate, including breccias and shear zones, that most likely would have slower velocities. Assuming an average basement velocity of 4000 m/s, our total basement penetration would be a two-way traveltime of 41–48 ms, depending on whether basement is picked at 97.5 or 85 mbsf, respectively. At an average basement velocity of 4450 m/s, total basement penetration would be 37–43 ms. All of these estimates place the bottom of the hole within the zone of irregularly layered reflections observed at the top of basement (Figs. F20, F21).