Lithologic Cycles And Paleo Fluid Flow Channels In Old Oceanic Crust From Geophysical Logs At ODP Site 418A

Abstract

Using some new processing of the multichannel sonic (MCS) log data from Site 418A, the resulting P, S, and Stoneley wave velocity estimates and apparent attenuation were integrated with the natural gamma, spectral gamma, resistivity, neutron, density, and caliper logs and core lithology information for interpretation of lithologic cycles and possible paleo fluid flow intervals. These data indicate the presence of multiple breccia or rubble zones in the lower portions of the borehole. These zones are interpreted as the last stages of eruptive cycles as described by Hyndman and Salisbury (1984). The results of permeability (packer) tests and temperature gradient measurements at Sites 395A and 504B indicate that fluid flow in the crust at those sites is localized to brecciated zones which occur below massive flow basalts. By analogy, the breccia zones interpreted at Site 418A may have acted as fluid flow channels at an earlier time. Six major paleo fluid flow zones are interpreted at Site 418A. These breccia zones have low velocities (P, S, and Stoneley), increased apparent attenuation, and an increase in gamma activity. These intervals are interpreted as permeable pathways which may have been altered by the second stage of oxidizing alteration as described by Holmes (1988). Breccia units occur just below massive basalt flow units. The massive basalt flow units are also easily identified in the MCS data. The resistivity log data suggest that each major eruptive cycle trend is made up of several smaller sub-cycles. The MCS data provides much insight into the variations in lithology in ODP boreholes. The trace energy provides a stable measure of apparent attenuation which may be related to alteration, fracturing, or permeability (if there are open fractures). Velocity estimates for P, S, and Stoneley waves provide useful information about lithologic variability if interpreted in detail.