In-Situ Structure and Properties of 110-Ma Crust from Geophysical Logs in DSDP Hole 418A

Abstract

DSDP Hole 418A, drilled through 324 m of sediments and 544 m of basalts during DSDP Legs 51-53 at the southern end of the Bermuda Rise, was reoccupied during ODP Leg 102 for the purpose of obtaining a comprehensive suite of geophysical logs. Although a hole restriction at 469 m below seafloor (145 m into basement) prevented the recording of the complete log suite throughout the open-hole interval, two passes of the first Schlumberger tool combination, consisting of resistivity-sonic-gamma-ray-caliper, were recorded from just below the sediment/basalt interface at 324 to 788 mbsf. The remaining 80 m was not logged owing to the suspected presence of a logging tool and cable lost during DSDP Leg 53. The second Schlumberger lowering, consisting of compensated neutron porosity, natural gamma spectroscopy, and lithodensity, and the third lowering, dual resistivity laterolog and gamma ray, were both recorded in open hole from 464 to 788 mbsf. The second combination was also recorded through the pipe to the mud line. Careful calibration of these measurements in fresh flow basalts, corrections for known borehole effects, and comparison of gamma-ray results to core mineralogy result in an excellent set of physical-property measurements throughout the logged interval.Differences in gamma-ray activity, mostly due to the potassium-40 decay series, separate the basement interval into three zones on the basis of smectite content. The uppermost zone 1 consists of a series of relatively unaltered, low-porosity (<15%), high \ p (>5 km/s) massive basalts and more porous lower velocity pillows capped by a 9-m-thick, more highly altered pillow unit, the total extending from 324 to 388 mbsf (log lithologic Units 1 to 4). In zone 2, from 388 to 514 mbsf (log lithologic Unit 5 and Subunit 6A), the pillow basalts contain large amounts of smectite within fractures and voids; porosities generally range from 15% to 25%, and velocities vary inversely with smectite content and porosity from 3.5 to 5 km/s. The bottom of the Subunit 6A breccia at 514 mbsf marks a geophysical boundary separating these altered pillows from the much fresher pillows and flows of Subunit 6B to Subunit 13C in the remainder of the logged interval, which we refer to as zone 3. In this zone velocities are generally above 4 km/s and reach 6 km/s in the massive flows. Porosities are generally below 20%; smectite is generally less than 5%. The density log is more sensitive to well-bore rugosity than the other logs, but aside from this effect, density ranges from 2.4 to 2.95 g/cm 3 . After correction for the presence of smectite, density is generally inversely proportional to porosity, as expected in a two-phase (seawater-basalt) system.Although velocities within the altered section (zone 2) are somewhat lower than in the remainder of the hole, nowhere are they as low as in seismic Layer 2A. In the uppermost 190 m of the basement where smectite is replaced by void space, the average velocity can be reduced to less than 3.5 km/s. Thus, alteration and smectite infilling over ...

Geophysical Properties of Oceanic Crust at Sites 768 and 770

“…7 and 8). Equations (2) and (3) indicate slightly less sensitivity of velocity to porosity change than the similar velocity/porosity relationships determined from Hole 395A basalt cores by Hyndman et al (1984) and from Hole 418A basalt logs by Broglia and Moos (1988). Also, our relationships indicate much less sensitivity of velocity to porosity change than the core analyses of Christensen et al (1979).…”

“…Consequently, Broglia and Moos (1988) found that both the potassium log and total gamma-ray log at Hole 418 A were highly correlated with qualitative ratings of core alteration, making these logs good proxy indicators of variation in the content of potassium-rich alteration minerals at Hole 418A. Low-temperature alteration such as this does not always lead to such a simple pattern.…”

“…For example, Broglia and Moos (1988) found from logs that FF = 11.5Φ" 1 -85 in the uppermost altered portion of Hole 418A, and FF = 29.5(p~1• 16 in the lower portion of that site. Similarly, Pezard (1990) cores at Hole 504B that FF= lOcp l .…”

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

“…[17] Petrophysical properties determined from core samples and by wireline logging commonly differ for a variety of reasons, several of which are specific to fractured igneous rocks of the upper oceanic crust [e.g., Broglia and Moos, 1988;Jarrad et al, 2003]. First, core recovery in the upper oceanic crust is notoriously low and biased; recovery for massive units is favored, and regions having the most intense fracturing tend to be underrepresented.…”

Site 895

“…Only one other such measurement was found in the ocean drilling literature (Hyndman and Drury, 1977). Previous workers (Hyndman and Drury, 1977;Kirkpatrick, 1979;Cann and Von Herzen, 1983;Hyndman et al, 1984;Smith and Vine, 1987;Broglia and Moos, 1988;Pezard et al, 1991) have examined the relationship between porosity and resistivity in oceanic basalts, gabbros, and one peridotite using Archie's Law:…”