(Table 3) Physical properties of DSDP Hole 72-515A

Hole 515A was drilled in the southern Brazil Basin by the Glomar Challenger on DSDP Leg 72, using the newly developed hydraulic piston corer (HPC). A total of 108 m of undisturbed terrigenous mud (T1) and sandy mud (T2) was obtained (see site chapter, Site 515, this volume). The core was sampled at levels that corresponded to 50-kPa increments of overburden pressure to determine if a pattern of clay fabric development could be discerned. After criticalpoint drying and gold-palladium coating, 15 samples were analyzed with a scanning electron microscope (SEM).
Clay fabric consists of randomly oriented, large and small domains of face-to-face (FF) stacked clay particles, frequently linked together by short "chains" in an edge-to-face (EF) orientation. As overburden pressure increases, linkages decrease, and the "chains" become incorporated into the domains. Distortion of platelike domains into "cornflake" structures (well-crystallized montmorillonite?) and spherical aggregates occurs at overburden pressures of 400 kPa. Orientation parallel with bedding occurs at 500 kPa as the "cornflake" structures become flattened and compressed.
Maximum fabric development occurs at 590 kPa. Large plates and coalesced domains form a pervasive linear FF pattern, interrupted by a few large and small EF bridging structures. Deformation of the large EF-oriented plates occurs, forming circular, pitlike voids.
Fabric development in cores from this deep-sea site follows well the developmental model suggested by Bennett and others (1981) for Mississippi Delta clays with high to medium void ratios.