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A downhole magnetometer log obtained from Hole 801C during Ocean Drilling Program Leg 144 was analyzed to constrain the magnetic character of the Jurassic basement at that location. Hole 801C, drilled during Leg 129, penetrates the uppermost 131 m of Jurassic oceanic crust beneath 463 m of sediments in the Pigafetta Basin of the western Pacific Ocean. Magnetic boundaries, apparent inclination, and polarity of magnetization were determined from variations of the horizontal and vertical magnetic fields in the hole. The magnetized basalts were divided into three principal units based on their magnetization. The first unit, located above 550 meters below seafloor, is characterized by almost no magnetization. The second unit, between 550 and 583 mbsf, mainly has a magnetization that was acquired in the Southern Hemisphere during a reversed polarity chron. In the third unit, below 583 mbsf, magnetization was acquired in the Southern Hemisphere during a normal polarity chron. The intensity of magnetization below 583 mbsf seems to decrease rapidly. A clear change from reversed to normal magnetic polarity was observed between the second and third units. The basalts with a remanent magnetization acquired in the Southern Hemisphere have retained their primary magnetization, and indicate that at least these basalts were formed at a spreading ridge crest in the Southern Hemisphere in the Middle Jurassic. The cause of weak magnetization of the first unit and the sharp decrease in intensity within the third unit is considered to be a result of strong hydrothermal alteration. Several thin magnetic subunits are observed within the second unit. The magnetization of those subunits was acquired in the Northern Hemisphere during a normal polarity chron. These Northern Hemisphere, normal magnetized segments possibly were caused by secondary magnetization induced by hydrothermal effects. This same section has been interpreted previously to have five separate polarity intervals, all acquired during the Jurassic based on paleomagnetic measurements of the discrete core samples. Our results suggest only two Jurassic polarity intervals of primary magnetization, and that the other polarity units were acquired subsequently, possibly a result of hydrothermal alteration.

Magnetic Reversal Stratigraphy of Jurassic Oceanic Crust from Hole 801C Downhole Magnetometer Measurements