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2. Holes 396A and 396B

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2. Holes 396A and 396B 2. HOLES 396A AND 396B Shipboard Scientific Party1 SITE DATA, HOLE 396A SITE DATA, HOLE 396B Site Summary Sheet, Hole 396A Date Occupied: 2000 hours, 6 February 1976 Date Occupied: 0120 hours, 5 February 1976 Date Departed: 1845 hours, 1 March 1976 Date Departed: 2000 hours, 6 February 1976 Time on Hole: 23.9 days Time on Hole: 42.7 hours Position: 22°59.14'N, 43°30.90'W Position: 22°59.14'N, 43°30.90'W Water Depth (sea level): 4459 corrected meters, echo sounding Water Depth (sea level): 4459 corrected meters, echo sounding Water Depth (rig floor): 4463 corrected meters, echo sounding Water Depth (rig floor): 4463 corrected meters, echo sounding Bottom Felt at: 4465 meters, drill pipe Bottom Felt at: 4465 meters, drill pipe Penetration: 405.5 meters Penetration: 123.5 meters Number of Cores: 33 Number of Cores: 2 Total Length of Cored Section: 270.0 meters Total Length of Cored Section: 13 meters Total Core Recovered: 63.56 meters Total Core Recovered: 0.64 meters Percentage Core Recovery: 23.3% Percentage Core Recovery: 4.9% Oldest Sediment Cored Oldest Sediment Cored Depth sub-bottom: 150.5 meters Depth sub-bottom: 120 meters Nature: nannofossil foraminifer ooze Nature: nannofossil foraminifer ooze Age: Middle Miocene Age: Pliocene Measured velocity: 1.5 km/sec Measured velocity: 1.5 to 1.6 km/sec Basement Basement Depth sub-bottom: 405.5 meters Depth sub-bottom: basement not penetrated Nature: basalt Nature: probably basalt (not recovered) Velocity range: 3.4 to 6.0 km/sec Velocity range: 1.5 to 1.6 km/sec Principal Results: The basalts recovered are typical mid-ocean ridge tholeiites with relatively narrow limits in chemical Principal Results: The purpose of drilling this hole was to composition, especially for MgO, while Tiθ2 and FeO* determine exact sediment thickness so proper casing length contents are relatively high. Basalts at the top are sparsely could be hung below cone on deep hole attempt. This hole was phyric; below this, they are porphyritic and similar to ones washed down to near basement. Two sediment cores were from Hole 396. The last 90 meters is a basaltic detritus. taken before drilling indicated basalt was reached. Magnetic units from top to bottom had inclinations of +18°, -67°, -7°, and +31°, with the last being very poorly defined. 3 3 'Leonid Dmitriev (Co-chief scientist), Institute of Geochemistry, Intensities varied from 1.03 to 3.45 × 10~ emu/cm ' Academy of Sciences of the USSR, 47A Vorobiovscoe Shosse, Moscow Downhole logs were run for density, sonic velocity, porosity, W334, USSR; James Heirtzler (Co-chief scientist), Woods Hole electrical resistivity, and natural gamma-ray activity. These Oceanographic Institution, Woods Hole, Massachusetts; James logs were correlated with many of the studies of recovered Kirkpatrick, Scripps Institution of Oceanography, University of California, samples. San Diego, La Jolla, California; Drummond Matthews, Department of Geodesy and Geophysics, Cambridge University, Cambridge, England; Nikolai Petersen, Ludwig-Maximilians-Universitat Munchen Inst. f. Allg. INTRODUCTION und Angew, Geophysik, 8 Munchen 2, Theresienstr. 41/IV; Pierre Cambon, CNEXO, Boite Postale 337, 29.273 Brest Cedex, France; Jose Honnorez, Division of Marine Geology and Geophysics, Rosenstiel School Holes 3 96A and 396B were drilled during Leg 46 in a of Marine and Atmospheric Science, Miami, Florida; Daniel Ohnenstetter, sediment pond about 150 km east of the Mid-Atlantic Ridge Laboratoire de Petrologie CO. 140, 54.037 Nancy, Cedex, France; at 22°59.14'N, 43°30.90'W. Hole 396 had been drilled Hiroaki Sato, Department of Earth Sciences, Kanazawa University, previously (Leg 45) to about 100 meters into basement in the Ishikawa 920, Japan; Hans Schmincke, Institut für Mineralogic Ruhr-Universitat Bochum, D-463 Bochum, W. Germany; Henry Dick, sediment pond. Hole 396A was a mudline test with only 0.64 Department of Geology and Geophysics, Woods Hole Oceanographic meters recovered from the two cores taken at the top of the Institution, Woods Hole, Massachusetts; Michael Dungan, Mail Code sediment column. Hole 396B was a multiple re-entry hole TN6, NASA - Lyndon B. Johnson Space Center, Houston, Texas; Floyd which penetrated 150.5 meters of sediment and 255 meters of N. Hodges, State University of New York at Stony Brook, Department of basaltic basement. The regional setting and the stratigraphic Earth and Space Sciences, Stony Brook, New York; Al Erickson, Department of Geology, University of Georgia, Athens, Georgia; Rudy columns for the holes drilled on Legs 45 and 46 are Aguilar, Schlumberger Well Services, P.O. Box 7458, Long Beach, presented in Initial Report, Volume 45 (Plate 1). This plate California. and the other results of the site survey of the area 15 HOLES 396A AND 396B investigated by these two Legs are presented in the Leg 45 Mg/(Mg+Fe) = 0.57 to 0.66%, TiO2 = 0.9 to 1.7%, CaO Initial Report (Purdy et al., 1978). = 10.8 to 12.8%, AI2O3 = 15 to 18%, total Fe as FeO = For Hole 396B the top 122 meters of sediment were 7.4 to 10.4%, K2O - 0.1 to 0.35%, Zr = 60 to 130 ppm, Sr washed to set the casing for the re-entry cone, and there = 110 to 170 ppm, Cr = 250 to 370 ppm, and Ni = 110 to were no cores taken in this interval. Cores 1, 2, and 3 160 ppm. The step-like chemical changes between units and recovered marly nannofossil ooze and foraminiferal the relative chemical homogeneity that exists within many nannofossil ooze. Core 4 had 7 cm of unmetamorphosed of the groups may indicate that the chemically defined units marly nannofossil ooze just above basement, which gives a represent discrete magma batches. date of middle Miocene (about 13 m.y., D. Bukry, personal The basaltic rocks from Hole 396B are mostly slightly communication). The interpretation of the magnetic data weathered to almost fresh. Six zones of more severe (Purdy, et al., 1978) indicates that Site 369 is located in alteration occur near interlayers of nannofossil oozes, magnetic anomaly 5, about 9 m.y. The nannofossil age of cemented pillow-rind breccias, and/or filled open fractures 13 m.y. has been used throughout this volume. and voids in the basalt. Four of these zones coincide with the upper part of four lithological units, including the Summary of Basement Drilling uppermost basalts of the basement just under the pond We have identified eight lithologic units in the basaltic sediments. basement in Hole 396B (Figure 1). Units 1, 2, and 3 are The alteration is accompanied by fissure and vesicle composed primarily of sparsely olivine and fillings of secondary minerals such as calcite, zeolites, plagioclase-phyric basalt. Units 1 and 2 are pillow smectites, and Mn oxides/hydroxides. Strontium, "loss on sequences with limestone-cemented (lithified nannofossil ignition," and (to a lesser degree) K2O increase ooze) palagonite breccias in the upper part. Unit 1 is significantly during alteration. A correlation also seems to separated from Unit 2 by 20 cm of limestone. Unit 3 is a exist between the presence of smectites in slightly flow or sill 8.5 meters thick, although neither its top nor weathered samples and their K2O content, and between CO2 bottom could be identified with certainty. Unit 4 is a pillow content and the observed presence of calcite. sequence composed of porphyritic basalt with 15 to 25 per A satisfactory correlation was noticed between the zones cent olivine and plagioclase phenocrysts. The average of maximum alteration and the porosity, density, and sound phenocryst plagioclase to olivine ratio is about 6:1. Unit 5 is velocity logs. composed of sparsely olivine and plagioclase-phyric basalt The well-known relationships among wet bulk density, pillows and carbonate-cemented breccia. Recovery in this porosity, and velocity were found. It was somewhat of a unit was poor, and the bottom of the unit was chosen on the surprise that the grain densities could be put into three basis of downhole logs. Unit 6 also has poor recovery but groups (di, d2, dβ) that have densities greater than 2.86, appears to be primarily basaltic sand, gravel, and sparsely 2.81, and 2.87 g/cm3, respectively. phyric basalt pillows. Unit 7 is a moderately olivine and A unique aspect of the Leg 46 program was the ability to plagioclase phyric basalt pillow sequence, again with poor make continuous logs of density, porosity, sonic velocity, recovery. Unit 8 is basaltic gravel or sand. natural gamma-ray activity, and electrical resistivity. This Magnetic Unit I has a mean inclination of +18°, but the facility was provided by Schlumberger Well Services of sediment in Section 396B-13-2 has a significantly different Long Beach, California. Other special equipment included inclination of -35°. Unit II has a mean inclination of -67°, an a.c. washer and spinner magnetometer provided by excluding one sample in Section 396B-13-3 which has an Dalhousie University, and an X-ray fluorescence unit inclination of +55°. That exception is probably the result of provided by CNEXO. All this equipment greatly enhanced misorientation. Unit III has a mean inclination of —7° with shipboard analysis. very little scatter around this value. Unit IV has a mean The four downhole logs that were made required a total of inclination of +31°, but is poorly defined since there were 34 hours of ship's time. The logs were extremely valuable, few oriented samples available for magnetic study. Mean especially in correlating the properties of recovered values for intensity of remanent magnetization for Units I samples. For example, the unique zone of coarse basaltic 3 3 through IV are, respectively, 1.03 × 10~ , 2.37 × 10~ , sand which was found below about 310 meters 3 3 3.45 × 10-3, and 1.85 × 10 - emu/cm .
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