Results of Downhole Geophysical Logging Hole 396B, DSDP Leg 46

Kirkpatrick, R. James

doi:10.2973/dsdp.proc.46.131.1979

Cited by 11 publications

(18 citation statements)

References 8 publications

(15 reference statements)

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“…The average formation velocity derived from both the oblique seismic experiment and integrated logging transit times is 4.8 km/s, a value consistent with those reported for Layer 2B by Houtz and Ewing (1976). This is considerably higher than the Layer 2A formation velocity of 3.6 km/s reported by Kirkpatrick (1978) for the basement section drilled in Hole 396B in young crust along the same flow line. It is consistent with the observation that Layer 2A does not commonly persist in the Atlantic beyond the 60-m.y.…”

Section: Discussionsupporting

confidence: 85%

“…During the first such experiment (Kirkpatrick, 1978), a 200-meter section of basement was logged in Hole 396B in 10-m.y.-old crust at 23°N on the Mid-Atlantic Ridge. Although the operating conditions were not perfect, the experiment nonetheless clearly demonstrated that in situ compressional wave velocities in young crust are consistent with those of Layer 2A (Houtz and Ewing, 1976) and that the porosity and fluid permeability of the uppermost levels of the basement are extremely high owing to the presence of numerous water-filled cracks and voids.…”

Section: Introductionmentioning

confidence: 99%

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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

Salisbury¹,

Stephen²,

Christensen³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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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.

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Section: Discussionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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

Salisbury¹,

Stephen²,

Christensen³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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show abstract

“…This is consistent with the low seismic velocities reported for young crust near the ridge crest by Houtz and Ewing (1976) and was interpreted as resulting from the presence of numerous water-filled joints, fractures, and interpillow voids near the top of the section (Kirkpatrick, 1979). Lower, but still surprisingly high porosities (10%-15%), were subsequently deduced from logs run in the uppermost levels of the basement at Site 417 in 110 m.y.…”

Section: Introductionsupporting

confidence: 84%

Basement Logs from the Mouth of the Gulf of California, Deep Sea Drilling Project Leg 65

Salisbury¹

1983

Initial Reports of the Deep Sea Drilling Project

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Geophysical logs run in Holes 482C, 483, and 485A on Deep Sea Drilling Project (DSDP) Leg 65 demonstrate that iiiterlayered sediments represent a substantial portion (up to 63%) of the upper levels of the basement in the mouth of the Gulf of California. The rest of the sections consist of massive basalts and minor pillow basalts. The thickness of the interlayered sediments in Hole 483, together with the sediment accumulation rate, imply that late-stage volcanism persisted at Site 483 for at least 490,000 years after most of the crust had formed.The lack of washouts in the sediments immediately overlying and underlying several of the basalt units in Holes 483 and 485A implies hardening resulting from baking or diagenesis. High natural gamma-ray counts in the uppermost basalts in all three holes are attributed to neutron activation of elements introduced by alteration.

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“…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:…”

Section: Electrical Resistivitymentioning

confidence: 99%

Site 895

1993

Proceedings of the Ocean Drilling Program, 147 Initial Reports

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Core recovery (%): 13.8 Oldest sediment cored:Depth (mbsf): 1. Principal results: Site 895 is located along the slope south of the intrarift ridge, at the position of 2°16.7'N, 101°26.7'W, in an area where ultramafic rocks were recovered during a Nautile dive program (Francheteau et al., 1990). The aim of drilling at this site was to recover a section of the shallow mantle. Six holes (895A to 895F) penetrated a total, of 272.9 m, and 64.56 m was recovered, with an average recovery of 27.5%. Among the six holes, only Holes 895D and 895E had substantial penetration (93.7 and 87.60 m, respectively) and recovery. The igneous and metamorphic rocks recovered consist predominantly of ultramafic rocks (dunite and harzburgite) and less abundant mafic rocks (gabbro, olivine gabbro, and troctolite). Although all rock types are present in the holes with significant penetration, harzburgites predominate in Hole 895D, whereas dunites and gabbroic rocks are more abundant in Hole 894E. Gradational changes, from dunite to sparsely plagioclase-bearing dunite, to interconnected veins of Plagioclase and clinopyroxene separated by patches of dunite, occur in continuous sections of core at Holes 895C and 895E and may suggest that these rocks were formed by melt migration and impregnation.Despite pervasive serpentinization, harzburgites retain Porphyroclastic textures. Spinel shape fabric in both dunites and harzburgites defines a foliation attributed to high-temperature solid-state flow. In Holes 895C, 895D, and 895E, the spinel foliation shows an increasing amount of dip with depth. In dunite and troctolite, traces of plastic deformation of olivine are observed in thin section.The relatively small amount (less than 2%) of modal clinopyroxene in the ultramafic rocks indicates that they are depleted abyssal peridotites. The dunites are probably the result of melt-wallrock interaction between host harzburgite and invading basaltic melt. It is also possible that the dunites are simple cumulus products of melt crystallization. The associa-

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Results of Downhole Geophysical Logging Hole 396B, DSDP Leg 46

Cited by 11 publications

References 8 publications

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

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

Basement Logs from the Mouth of the Gulf of California, Deep Sea Drilling Project Leg 65

Site 895

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