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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 (Tl) 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. INTRODUCTION The process whereby newly deposited sediment particles gradually become lithified under increasing overburden pressure is called "gravitational compaction" (Hedberg, 1936). The process consists of three stages: (1) the mechanical rearrangement and dewatering stage in which porosity is reduced from 90% to 35%; (2) the mechanical deformation stage in which porosity is reduced from 35% to 10%; and (3) the recrystallization stage, where porosity is less than 10% and the main mechanism is recrystallization under high pressures. Weller (1959) proposed a similar compaction model for sediments having an initial porosity from 85% to 45%. Under increasing overburden pressure, interstitial fluids are expelled, with porosity decreasing to 10%. Associated with this is a rearrangement of mineral grains and a development of closer packing, related to the yielding of the clay minerals between the more resistant grains. In this contribution, we are concerned with the response of a deep-sea sedimentary section composed of terrigenous clay to the process of reduction of the initial porosity from 85% to 58% under overburden pressures gradually increasing to 590 kPa. In particular, we examine the microstructural changes of the clay particles, including particle rearrangement and particle-packing relationships. In addition, we are interested in determining if early diagenesis may be occurring at porosities greater than 10%. STUDY MATERIAL AND ANALYTICAL PROCEDURE DSDP Leg 72 provided us with an opportunity to study the early aspects of the compaction process. During that voyage, the newly deBarker, P. F., Carlson, R. L., Johnson, D. A., et al., Init. Repts. DSDP, 72: Washington (U.S. Govt. Printing Office). veloped hydraulic piston corer (HPC) was used and a number of high quality, essentially undisturbed cores were obtained from the southern Brazil Basin, about 200 km north of the northern exit of the Vema Channel (Fig. 1). At Hole 515A, the HPC recovered 108 m of undisturbed sediment, providing the samples for our study. The sediment is a grayish brown terrigenous mud with occasional nannofossil-rich layers and some foraminifer-rich layers. Clay content is high (55%), with quartz plus feldspar (10%), pyrite and micronodules (2%), heavy minerals (2%), and mica (1%). The average texture is 3% sand, 20% silt, and 77% clay. Onboard X-ray diffractograms indicated that montmorillonite, illite, and phillipsite are the prevailing clay minerals (see Coulbourn, this volume). Our intention was to study development of the clay fabric under increasing increments of overburden pressure using the ISI Super IIIA scanning electron microscope at Lafayette College. This instrument is equipped with a Princeton Gamma Tech System III microanalysis system enabling us to identify semiquantitatively the elements present in each sample. Cores from Hole 515A were sampled at 50-kPa intervals where possible (Samples 515A-1-1, 105-107 cm, and 515A-2-2, 31-33 cm, were taken at 10 and 25 kPa, respectively). A 30-cm cylinder was pressed into the split half of the core, extracted, and sealed in a polyethelene container until analyzed. All samples were oriented according to the long axis of the core. A sample was trimmed from the cylinder with a sharp knife producing a wafer of approximately 2-mm thickness (Fig. 2). The wafer was scored parallel to the horizontal plane, then pulled apart along this plane and trimmed. The trimmed sample was carefully transferred to an evaporating dish maintaining its orientation. It was then washed successively with 70%, 80%, 90%, and then absolute ethanol, testing for complete removal of Cl~ using silver nitrate solution. A portion of the sample was then carefully cemented on an aluminum stub using carbon paint, critical-point dried using liquid carbon dioxide (Bennett et al., 1979), and lightly gold-coated. This provided an undisturbed surface of a plane parallel to the horizontal (see Fig. 2). BACKGROUND Over the past decade, interest has been shown in the configuration and orientation of clay particles of sediments accumulating in various types of sedimentary environments. Earlier studies using the transmission electron microscope (TEM) showed that marine clay parti-

Clay Fabric Development in a Deep-Sea Core: Site 515, Deep Sea Drilling Project Leg 72