Thin section descriptions (Table T1) indicate that the samples range from chert to porcellanite to limestone/chalk, with variable amounts of volcanic and biogenic components. Radiolarians and nannofossils are most important, with lesser foraminifers, ostracodes, and fish debris. Fine matrix mineral proportions are also estimated. Nannofossil chalk/limestone, porcellanite, and chert are similar in appearance to the samples described from underlying and overlying units in the Leg 198 Initial Reports (see descriptions and photomicrographs in Bralower, Premoli Silva, Malone, et al., 2002). These shipboard samples were not impregnated with blue epoxy, however, so porosity relationships are better defined here. Primary interparticle, intraparticle, and matrix porosity are present, but these have been affected by recrystallization of the matrix or by precipitation of authigenic phases (e.g., infilled). The distribution of porosity can be seen in the blue hues of the scanned thin sections in Figure F4. This series of thin sections spans the OAE1a interval in Hole 1207B. The altered tuff beds are particularly porous (Fig. F5) in contrast to the organic-rich shales (Figs. F6, F7).
The main authigenic phases are various forms of silica (opal-CT and chalcedony), carbonate, zeolites, pyrite, and gypsum; these occur as cements, grain replacements, and matrix replacements. Secondary porosity arises from the dissolution of biogenic debris, particularly radiolarians and volcanic debris (glass and feldspar). None of the volcanic glass remains; it has either been dissolved (more common) or replaced by authigenic phases such as zeolites (Figs. F5, F6). Locally, opal-CT is also dissolved, forming secondary porosity. Textural relationships suggest that the opal-CT and pyrite routinely preceded precipitation of carbonate (less common), all of which preceded precipitation of chalcedony. Zeolite cements in volcaniclastic intervals appear to postdate all of the above (Figs. F5, F6). Most samples exhibit slight to moderate bioturbation (Figs. F4, F7), but it should be noted that homogeneous samples may be structureless because of thorough bioturbation. Many of the samples are laminated. Thin sections of the organic-rich intervals show what appear to be intact fish remains, as well as fragments (Fig. F6). Foraminifers, even silicified or ghost (dissolved) specimens, are notably absent in the organic-rich intervals. Authigenic calcite is concentrated just below the organic-rich intervals where there are spectacular thin beds of calcite-cemented radiolarite (Fig. F7).
There is no evidence for terrestrial organic matter in the thin sections produced from the OAE1a interval at Sites 1207, 1213, and 1214; however, the thin sections show a variety of textures for the organic matter. The organic matter locally has a globular (Fig. F6) texture (sapropellic algal matter). Where laminae are developed they are first irregular and discontinuous (wispy) (Fig. F7) and grade upward into more continuous laminae, particularly at the top of the OAE1a interval in Hole 1207B (Fig. F6).
DSDP Site 463 was selected for comparison because it was described as containing volcanic ash and organic-rich intervals in the lower Aptian section (Dean et al., 1984). Although a nearly intact section of the OAE1a interval was recovered at Site 463, the working half of the core has been heavily sampled for shipboard and postcruise studies. Seventeen samples were taken spanning the OAE1a (Table T1): five above the organic horizons in Sections 62-463-69R-1 and 70R-1, two within the organic horizons from Sections 70R-2 and 70R-3, and nine below the organic horizons from Sections 70R-7 and 70R-CC and Core 71R. From shipboard descriptions, all had the potential to be ash bearing. However, thin sections produced from these samples indicate most of the samples (11 out of 17) (Table T1) are porcellanite, chalk, and shale, with a few organic-rich shales. Only four samples were tuffaceous, two from above and two from directly below the main organic-rich interval. The textures in the volcanic debris include blocky to moderately vesicular glass altered to zeolites and microlitic volcanic fragments (Fig. F7).
Mineral percentages for the 19 samples analyzed are presented in Table T2. The mineral assemblage is dominated by silica phases (quartz and opal-CT), calcite, zeolite (clinoptilolite/heulandite), and phyllosilicates according to the lithology or mix of lithologies present in the sample (e.g., limestone, shale, tuff, porcellanite, and chert). Other important minerals are plagioclase, pyrite, and marcasite, with rare to trace amounts of K-feldspar, halite, and gypsum. The phyllosilicate fraction is mainly randomly ordered mixed-layered illite-smectite with 80%–90% smectite layers and illite/mica. This mineral group includes the petrographically defined glauconite minerals. Traces of kaolinite and chlorite are also present. Although what appeared to be unaltered (isotropic) radiolarians are present, opal-A was not detected in the diffractograms. Clinoptilolite and heulandite are virtually indistinguishable using X-ray diffraction techniques, but the morphology of the zeolite crystals in thin sections of the Leg 198 samples suggests that they are clinoptilolite.
None of the samples from Site 463 were subjected to XRD analysis in this study. However, XRD analyses by several workers (Vallier and Jefferson, 1981; Hein and Vanek, 1981; Mélières et al., 1981; Nagel and Schumann, 1981; Rateev et al., 1981) showed the ash intervals at Site 463 to be mainly composed of montmorillonitic/smectitic and illitic clay minerals, with some siderite. Nagel and Schumann (1981) define tuffaceous intervals by the presence of pyrite, clinoptilolite, and opal-CT.