The Physical State of the Upper Levels of Cretaceous Oceanic Crust from the Results of Logging, Laboratory Studies, and the Oblique Seismic Experiment at Deep Sea Drilling Project Sites 417 and 418

Abstract

The combined results of logging, physical properties studies, and the oblique seismic experiment conducted during DSDP Legs 51 through 53 at Sites 417 and 418 in Cretaceous crust at the southern end of the Bermuda Rise make possible the first detailed evaluation of the physical state of the upper levels of old oceanic crust. From an analysis of the results of the oblique seismic experiment, it appears that the P-wave velocity increases linearly from 4.8 ±0.2 km/s at the top of Layer 2 to 6.4 ±0.2 km/s at a sub-basement depth of 1.3 km. The P-wave velocity of Layer 3 at approximately 1.5 km is 6.7 ±0.2 km/s. The 5-wave velocity is 2.6 ±0.1 km/s at the top of Layer 2 and 3.7 ±0.1 km/s at the top of Layer 3. The average value of Vp (4.8 km/s) measured by logging in the uppermost basement in Hole 417D is in excellent agreement with the value obtained from the oblique seismic experiment, but is lower than the formation velocity (5.3 to 5.6 km/s) reconstructed for the site from laboratory measurements of velocity through recovered core material. This requires that the formation contains cracks on a scale finer than the resolution of the logging and oblique seismic experiments, but greater than that of laboratory samples. On the basis of these results and petrologic constraints imposed by the core, the upper crust in Hole 417D consists of 90 per cent basalt with an average grain boundary porosity of 8 per cent, less than 1 per cent interpiUow limestone, 5 per cent smectite consisting of about 50 per cent water, and 5 per cent open cracks filled with standing water. The formation porosity thus resides in two domains, grain boundaries and open cracks, and totals 13 to 14 per cent. This value is confirmed by electrical resistivity logs which indicate, in addition, that the cracks are interconnected, giving the formation an average permeability in the thousands of darcies, with lower values in the less fractured massive basalts. Comparison of these results with logging data obtained in young crust in Hole 396B on the Mid-Atlantic Ridge indicates that, although the porosity and permeability of the upper levels of the crust at Site 417 are much lower than at the ridge crest, the formation is not entirely sealed. Although water circulation is thus still possible in old crust, it may be limited by the presence of massive basalts and the absence of shallow sources of heat. Deep Sea Drilling Project and Geological Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California. Department of Geodesy and Geophysics, University of Cambridge, Cambridge, England (now at the Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts). Department of Geological Sciences and Graduate Program in Geophysics, University of Washington, Seattle, Washington. Centre Oceanologique de Bretagne, Brest, France,. Geophysical Institute, University of Tokyo, Tokyo, Japan. Lamont-Doherty Geological Observatory, Palisades, New York. Program in Geosciences, University of Texas at Dallas, Richardson, Texas (now at the Graduate Program in Geophysics, University of Washington, Seattle, Washington). INTRODUCTION During the past several decades, our perception of the structure of the oceanic crust (Table 1) has evolved from simple layered models derived from surface refraction data (e.g., Raitt, 1963), through multiple layered models based on ocean bottom seismometer and Sonobuoy data (e.g., Hussong, 1972; Peterson et al., 1974; Houtz and Ewing, 1976), to detailed velocity gradient models derived from velocity and amplitude analysis of reflection and refraction data (e.g., Helmberger and Morris, 1970; Orcutt et al., 1976; Spudich et al., 1978). Although the velocity structure of the crust can now be determined in considerable detail,