Abstract:
Deep Sea Drilling Project/Ocean Drilling Program Holes 504B and 896A were drilled into the upper oceanic crust of the Costa Rica Rift, only 1 km apart. Thus they provide an excellent opportunity to study lateral variation in the volcanic-rock section of the oceanic crust. Recovery of cores from these holes was poor. Therefore, comparison of one hole with the other is based on continuous information from geophysical well-logging data. Continuous lithologic profiles were created by calibration of logging data to cores, followed by statistical analysis. Four rock types can be distinguished as electrofacies from sets of log responses: massive units of basalt, thin flow-basalts, pillow basalts, and fractured or brecciated basalts. Massive units, thin flow-basalts, and pillow basalts are in both holes. These terms describe morphologies of lava flows. The morphologies can be distinguished by using physical information from logs; the evidence results from characteristic differences in fractured and altered rock. Fractured or brecciated basalts were classified only in Hole 896A; they are restricted to the depth between 350 m and 380 m below seafloor (mbsf). They are related genetically to a fault zone. Comparison of records of the two holes shows that average P-wave velocities and total gamma radiation are similar. In all log responses from Hole 504B, scattering is larger. This is especially true of electrical resistivities and P-wave velocities. Differences among physical properties measured in the two holes can be related to variation in thicknesses of lava flows and variation in fracturing and alteration. Massive units of basalt are much thicker in Hole 504B than in Hole 896A, which explains the higher electrical resistivity and P-wave velocity recorded in this hole. Fractures are more numerous and altered rock more abundant in Hole 896A.