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Textures, microstructures, and major lithologies of samples from 750 to 1081 mbsf of Hole 799B, Leg 128, Japan Sea, were studied in 40 thin sections. The study provided evidence for the sequence of diagenetic mineralization for the lower portion where the sediments become well-lithified. The diagenetic sequence for the lower portion of Hole 799B revealed from crosscutting relationships in textures and structures is (1) hard, round, "D-phosphate" nodules and glauconite grains transported and deposited with the sediments; (2) "F-phosphate" streaks and pyrite framboids; (3) clay minerals; (4) rotation, flattening, and compaction; and (5) irregular glauconite grains. Fabric that has been defined by the subparallel to parallel optical orientation of clay minerals becomes more pronounced with increasing depth. Compaction enhances clay mineral fabric by flattening soft F-phosphate nodules, burrows, and microfossils (like the sponge Sagarites makiyama or Makiyama titanni). INTRODUCTION Forty samples collected from Hole 799B (Figs. 1 and 2) were examined petrographically for textures and microstructures that might bear on the origin and history of Japan Sea sediments. Samples were collected as hard chips where sediments were lithified; otherwise, small plastic boxes were used to collect an undisturbed cube of sediment. The emphasis of this petrographic study is the sedimentary record of the lower portion of Hole 799B from 750 to 1081 mbsf (meters below sea floor). Sediments have undergone a change from poorly lithified to well-lithified in the lower interval of Hole 799B. The petrographic study provides observations of diagenetic sequence as well as of development of early structural features. Samples used (Fig. 2) are from the lower portion of lithologic Units III, IV, and V, which were defined in the lithostratigraphy portion of the Site 799 chapter, p. 260-265, Leg 128 Initial Reports (Shipboard Scientific Party, 1990b). The samples studied here range in age from middle to early Miocene (Shipboard Scientific Party, 1990b, p. 260-265). Preparation of Thin Sections Lithified sediments were cut into thin sections using standard techniques. Partially lithified sediments were impregnated with epoxy and allowed to harden before being cut into thin sections. Impermeable unlithified sediments were hardened with a low-viscosity resin, using the techniques described by Swartz and Lindsley-Griffin (1990) and then were cut into thin sections 40 to 50 μm thick. A greater thickness of sections was necessary because soft sediments have a tendency to pluck and grind unevenly, even when impregnated. Although blue-dyed epoxy was used to impregnate many of the thin sections in an attempt to determine original porosity, results were ambiguous because the extremely low permeability of the rocks resulted in uneven impregnation. Most thin sections also were treated with Alizarin-red stain to distinguish between calcite and dolomite. LITHOLOGIES OF HOLE 799B Most of the samples we studied are siliceous claystones and mudstones containing variable amounts of silt (Appendix). Minor lithologies studied include Porcellanite, carbonate, and sandstone. All of the Pisciotto, K. A., Ingle, J. C , Jr., von Breymann, M. T., Barron, J., et al., 1992. Proc. ODP, Sci. Results, 127/128, Pt. 1: College Station, TX (Ocean Drilling Program). 2 Department of Geology, 214 Bessey Hall, University of Nebraska, Lincoln, NE 68588-0340, U.S.A. samples, except a few without clay minerals, exhibit a fabric defined by optically subparallel orientations of clay minerals and laminae of phosphate nodules and pyrite framboids. Many of the samples also display bedding that has been defined by concentrations of silt and sand grains, or concentrations of diagenetic silica or carbonate. Siliceous Pelites and Porcellanites Most of the lithologies in the interval studied are siliceous pelites (PI. 1, Fig. 1). These include clay-rich pelite, silica-rich pelite, and Porcellanite. Clay-rich pelite consists dominantly of isotropic to subisotropic diagenetic silica and microcrystalline clay in roughly equal proportions. Silica-rich pelite is composed of isotropic to subisotropic diagenetic silica, with minor microcrystalline clay. Porcellanites, defined as well-indurated rocks that consist of 50% to 75% diagenetic silica (Shipboard Scientific Party, 1990a, p. 49), occur as finely laminated to thinly bedded small concretions within the siliceous claystones and mudstones. These porcellanites, like the silicarich pelites, consist of isotropic to subisotropic diagenetic silica, with minor microcrystalline clay; however, they are much more indurated than the silica-rich pelites and cannot be scratched by a steel needle. One of the most interesting porcellanites in Hole 799B (PI. 1, Fig. 2) is composed of silica pellets in a siliceous claystone matrix that is associated with phosphatic pelagic debris. Authigenic minerals in the pelites include framboidal pyrite and phosphate in both nodular and dispersed forms. Silt-size and rare sand-size clastic grains are ubiquitous in the siliceous pelites, but they typically make up less than 10% of the volume (PI. 1, Fig. 1). Clastic grains are angular to subangular, rarely subround, and consist mainly of quartz. Minor clastic components include feldspar, glauconite, rock fragments (claystone, siltstone), calcareous and siliceous microfossils, and D-phosphate nodules. Silty Pelites As depth increases within the studied interval of Hole 799B, the amount of diagenetic silica appears to decrease along with an increase in proportion of clay minerals. Other components of these rocks are similar to those of the siliceous pelites, except that the amount of carbonate increases. These rocks are best described as silty claystones and mudstones. Sandstones Among the samples studied, several consist of fineto mediumgrained, subangular to angular, poorly sorted sandstone and silty sand-

Textures, Microstructures, and Lithologies of Japan Sea Sediments from the Lower Portion of Hole 799B, Leg 128