Crustal structure of the deepwater west Niger Delta passive margin from the interpretation of seismic reflection data

The interpretation of 2D and 3D seismic reflection data complemented with gravity data allows the crustal architecture of the deepwater west Niger Delta passive margin to be defined. The data show that the area is underlain by oceanic crust that is characterised by a thickness of 5–7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Some of the dipping reflections can be traced up to the top of the basement where they offset it across a series of minor to major thrust faults. Other internal reflections are attributed to extensional shear zones and possibly due to intrusions in the lower crust. The Moho can be correlated as a discrete reflection over >70% of the study area. It is generally smooth, but localised relief of up to 1 km is observed. The southeastern part of the study area is dominated by a zone of SW–NE striking basement thrusts.     

Small-scale fluctuations in acoustic and physical properties in surficial carbonate sediments and their relationship to bioturbation

 Measurements of physical and acoustic properties within the top 40 cm of sediment indicate that there are three classes of parameter variability relevant to acoustic bottom scattering at the Marquesas Keys and Dry Tortugas sites. The three classes of variability, spatially and temporally different in terms of scale, are generated by biological and physical processes acting on sediments. Interplay of bioturbation, trawling and storm events create fine laminations of sand–silt–clay and high gravel-size mollusk shell content at the Marquesas Keys site; the predominance of biological processes creates strong surficial gradients and lateral variability in sediment properties at the Dry Tortugas site.     

TERRA BOOKS PALAEOBIOLOGY A SYNTHESIS

1990, £89.50 (hbk), 583 pp. Blackwell Scientific Publications, Oxford ISBN 0–632–02525–5     

Comparison of the magnetic properties of glass from Luna 20 with similar properties of glass from the Apollo missions

Magnetic susceptibility measurements have been made on four glass spherules and fragments from the Luna 20 fines; two at 300°K and two from 300°K to 4°K. From these data the magnetic susceptibility extrapolated to infinite field, the magnetization at low fields and also the saturation magnetization at high fields, the Curie constant, the Weiss temperature, and the temperature-independent susceptibility were determined. Using a model previously proposed for the Apollo specimens, the Curie constant of the antiferromagnetic inclusions and a zero field splitting parameter were calculated for the same specimens. The data show the relatively low concentration of iron in all forms in these specimens. In addition, the Weiss temperature is lower than that measured for the Apollo specimens, and can be attributed almost entirely to the ligand field distortion about the Fe²⁺ ions in the glassy phase. The data further suggest that the Luna 20 specimens cooled more slowly than those of the Apollo missions, and that some of the antiferromagnetic inclusions in the glass may have crystallized from the glass during cooling.     

Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography

Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological processes within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physical properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the highporosity sediment.