Basement Logging on the Mid-Atlantic Ridge, Deep Sea Drilling Project Hole 395A

Abstract

On Leg 78B, the Glomar Challenger returned to Hole 395A on the Mid-Atlantic Ridge and logged the upper 0.5 km of the crust, obtaining excellent resistivity, natural-gamma, and caliper logs throughout the section and reasonable density and velocity logs near the base. The logs show that the crust may be divided into two distinct geophysical units. The upper 400 m of the crust displays high porosities and low velocities, densities and resistivities. Below this, between 400 and 500 m sub-basement, the crust displays a low crack porosity (1-2%), high resistivities (up to 1000 öm), and high velocities (up to 6.0 km/s). For the lower unit, a value of 2.2 was obtained for the exponent m in Archie's Law, which is consistent with low permeabilities measured in the same interval with a packer (2 to 9 µdarcies). Since the boundary between these two units is marked by a sharp increase in alteration products and the lower unit behaves as if it were sealed, we conclude that Hole 395A penetrated a major geophysical discontinuity.

Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

“…Natural gamma ray and resistivity data are consistent with results from earlier logging (e.g., Bartetzko et al, 2001;Matthews et al, 1984;Moos, 1990), defining individual eruptive and flow units. The upper 300 m of basement comprises lithologic units having thicknesses of approximately 10-70 m, within which there is generally a bottom-to-top decrease in electrical resistivity, bulk density, and compressional velocity, and an increase in natural gamma ray emissions.…”

“…The pillow units are typically tens of meters thick and separated by a sedimentary breccia unit that contains cobbles of gabbro and serpentinized peridotite derived from the surrounding basement peaks (Bartetzko, Pezard, Goldberg, Sun, & Becker, 2001;Matthews, Salisbury, & Hyndman, 1984). Nearby Hole 395 also contains a peridotite-gabbro complex that is several meters thick with brecciated contacts (Shipboard Scientific Party, 1979).…”

Channelized fluid flow in oceanic crust reconciles heat-flow and permeability data

“…It does not preclude local thermal homogenization of upper basement 28 , either by organized or chaotic ow 29 , but local convection could also be channelized. Unlike a homogeneous permeability model for the upper crust, our model is consistent with observed lithostratigraphic, hydrologic, and alteration heterogeneity 5,7,17,30 . There would still be chemically and biologically signi®cant¯uid¯ow within much of the upper crust, but uid¯uxes away from the primary channels would be small relative to those in the channels.…”

Mapping of seismic layer 2A/2B boundary above the sheeted dike unit at intermediate spreading crust exposed near the Blanco Transform

“…At DSDP Site 395A (∼8 Ma crust formed at slow spreading Mid‐Atlantic Ridge [ Shipboard Scientific Party , 1978]) a decrease in porosity from ∼8.7% to ∼4.7% is measured ∼425–450 m subbasement [ Hyndman and Salisbury , 1984], and is associated with abundant fracture filling with saponite [ Lawrence et al , 1984]. Mathews et al [1984] argue that this represents a major geophysical boundary in the crust. The layer 2A/2B boundary at our seismic survey area is located at an average depth of 485 m below the seafloor (Figure 10b), which is close to the depth of the observed change in fracture filling and porosity, and the interpreted major geophysical boundary, at DSDP Site 395A.…”