William Lowrie

Magnetostratigraphy of the Permo-Triassic boundary section at Shangsi (Guangyuan, Sichuan Province, China)

Continous marine sedimentation characterizes many Late Permian to Early Triassic sections on the Yangtze terrane in South China. The Permo-Triassic (P/Tr) boundary section at Shangsi (Sichuan Province) consists of limestones intercalated with clays and mudstones which belong to the Wuchiapingian and Changxingian (Upper Permian) and the Griesbachian and Dienerian (Lower Triassic) stages. The P/Tr boundary is formed by a clay horizon with an unusually high iridium concentration. The intensity of natural remanent magnetization is very low with a mean of 0.15 mA m−1. About 40% of the samples contain secondary or unstable magnetization components only, whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy. Normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones, the P/Tr boundary being situated very close to the transition from a reversed to a normal polarity zone. The Shangsi polarity sequence represents part of the Illawarra interval of mixed polarity, the exact beginning of which has still to be determined.

On the magnetic susceptibility anisotropy of deep-sea sediment

Abstract Susceptibility anisotropies in the form of vertically prolate ellipsoids have been reported in many deep-sea sediment cores. The results of the present investigation suggest that these anisotropies may not describe the original magnetic fabric of deep-sea sediment, but are more likely due to either a measurement effect or to deformation of the sediment during coring. Anisotropy measurements made on a spinner magnetometer sometimes were found to be greatly affected by the shape of the sample. This apparent “sample-shape effect” was not observed on a low-field torque meter. The anisotropy of samples taken near the base or the top of some piston cores often reflects sediment disturbance during the coring operation. Most samples of deep-sea sediment examined had weak anisotropies that could be interpreted as due to normal depositional processes, including bioturbation. The best-fitting susceptibility ellipsoids were usually oblate with near vertical minimum susceptibility axes.

The origin of the White Beds below the Cretaceous-Tertiary boundary in the Gubbio section, Italy

Abstract Immediately below the Cretaceous-Tertiary (K-T) boundary clay layer in Umbrian pelagic limestone sections is a 0.5 m thick interval of white limestone beds. Geochemical and rock magnetic studies have been carried out to investigate the origin of these beds. Fe/Si ratios suggest that the iron abundance is a constant proportion of the detrital component in the red marlstone above the boundary and the pink limestone below the white beds. The stratigraphic variations of initial susceptibility and the IRM intensities acquired in different fields behave similarly to the Fe/Si ratio, and reflect primarily the changing concentrations of the ferromagnetic minerals caused by sedimentary fluctuations. The Fe/Si ratio, overall Fe abundance and the intensities of the magnetic parameters are all anomalously low in the white beds. The differing magnetic properties of the samples are due mainly to differing proportions and grain-size distributions of pigmentary hematite and detrital magnetite. The coercivity spectra of samples from the white beds are very similar to those of the pink beds, but are clearly distinct from those of the red beds. The white beds were probably deposited under the same conditions as the underlying pink beds. The anomalously low intensities in the white beds result from reduction of hematite in originally pink beds and subsequent removal of the Fe 2+ ions. The reduction may be a consequence of downwards infiltration of reducing waters resulting from the large quantity of organic matter produced by the extinctions at the K-T boundary.