1978 Articles

Paleomagnetism and Rock Magnetism of Upper Jurassic Limestone and Basalt from Site 367

Kent, Dennis V.; Tsai, Lan Ping

Remanent magnetization measurements were made on 49 samples of upper Jurassic limestone varying in color from gray to red and 7 samples of basalt recovered at Site 367. The gray limestones have very weak magnetizations and are of limited use paleomagnetically. The red limestones have stronger remanences and fair to good magnetic stabilities. The mean remanent inclination of the most reliable set of red limestone samples (N=l4) after 200-oe AF demagnetization is 25°±5.1°, near the present axial dipole field inclination (24°) as well as the expected inclination of the Jurassic field (21° to 30°) at the core site. A single stable reversed direction of magnetization (Sample 33-3, 88 cm) may represent the earliest interval of reversed geomagnetic field polarity of the late Jurassic-early Cretaceous magnetic reversal sequence. The basalt samples had very unstable remanent magnetic properties, characterized by median demagnetizing fields less than 100 oe. The samples were able to acquire large viscous remanences (VRM) in the laboratory in a 1.0-oe field. Moreover, the intensity of VRM acquired in the presence of the NRM was at least a factor of five greater than that acquired under similar conditions but after AF demagnetization. The measured magnetic properties of these basalt samples suggest that the magnetic quiet zone in the general region of Site 367 may be due in part to reduction of magnetization contrast in the magnetized basement layer by viscous remagnetization.