One thin section of a sandy interval near the base of Unit I and three thin sections from two sandy beds at the base of Unit II were examined. Petrographic descriptions of these samples are given in Table 1 and photomicrographs are shown in Plate 1. The Unit II sandy beds are separated by a 4- to 5-cm bed of gypsum-bearing micritic silty clay within a 17-cm interval of core directly above the Unit II/Unit III contact (Figs. 2, 3A-B). When Core 161-975B-33X was cut into sections, it was split within this micritic bed so that it occurs at the base of Section 975B-33X-3 and at the top of Section 975B-33X-CC (Fig. 3A-B).
Thin sections were prepared from the base (Section 161-975B-33X-CC, 9-10 cm) and the top (Section 975B-33X-CC, 3-5 cm) of the lower sandy interval (Fig. 3B). Both samples show similar compositional ranges with an upsection decrease in sorting. XRD results (Table 2) indicate variable amounts of celestite (SrSO4) and gypsum in this sandy interval.
A series of thin sections were prepared from Unit III cores from Holes 975B and 975C. These provide the basis for characterizing the spectrum of evaporite lithofacies identified by shipboard studies (see Fig. 2: e.g., pinch-and-swell gypsum [7; referred to as nodular gypsum in the shipboard studies], finely laminated gypsum [8], and gypsiferous chalk [9]). We have further subdivided the finely laminated gypsum into planar laminated (8a), planar-wavy laminated (8b), planar-cumulus (8c), and crenulated (8d) varieties. Unfortunately some lithofacies are not represented in our sample suite. Thin sections were not made of clay (10) interbeds because these intervals were depleted for paleontological studies. One very thin interval of what appeared to be intraclastic gypsum (11), which was composed of thin flakes similar to the laminated gypsum, was depleted in an attempt to make a thin section for shipboard study.
Some of these lithofacies (10 through 7) are organized in two broad cycles (Fig. 2) starting at the base with clay or micrite-rich clay (10), followed by laminated to thinly bedded gypsiferous chalk (9), finely laminated gypsum (8), and topped by pinch-and-swell gypsum (7). Figures 4A-B illustrate these components in a correlative interval recovered at Holes 975B and 975C. Planar laminae are most common at the base of the laminated interval within each cycle, and planar-wavy laminae become more common toward the top of the interval, where they evolve into planar-cumulus laminae and finally pinch-and-swell structures. These are described below in detail.
The gypsiferous chalk (9; Figs. 4A-B) consists predominantly of large euhedral to anhedral gypsum crystals in a micritic matrix. The matrix material consists of micritic carbonate, nannofossils, and clay minerals(?) with a few percent foraminifers (Pl. 2, fig. 1) and foraminifer fragments, and traces of altered volcanic glass. The large gypsum crystals range up to 5 mm in length and often have fine inclusions of matrix material, including foraminifers and nannofossils. They exhibit no distinct zoning and may be intergrown. The long axes of gypsum crystals are often subhorizontal to horizontal (Pl. 2, fig. 2), but can show a range of orientation in some intervals (Pl. 2, fig. 1). Where crystals are in contact, they have undergone pressure solution and exhibit stylolitic contacts (Pl. 2, fig. 2). Locally, euhedral crystals exhibit irregular, matrix-rich overgrowths that extend out into the surrounding matrix. In the core photographs (Fig. 4A-B) subtle internal lamination and bedding features can be seen within the gypsiferous chalk intervals. These layers are defined by changes in the relative abundances of gypsum and matrix, by changes in gypsum crystal size, or by the presence of stellate clusters and subvertical bands of intergrown gypsum crystals.
The textures of laminated gypsum (8) vary as a function of the morphology of the upper surface of the laminae from planar (Fig. 5A), to planar-wavy (Fig. 5A), to planar-cumulus (Fig. 5B-C). We introduce the term cumulus because these structures resemble cumulus clouds. Alternative descriptive terms do not accurately describe these features: for example, "grass-like" and "cauliflower" have been applied to more pointed, vertical arrangements of selenitic crystals, and "cavoli," is applied to more rounded fan-like arrangements of crystals (e.g., Richter-Bernburg, 1973). Garrison et al. (1978) described similar laminae at Site 372 as simply "irregular shaped gypsum crusts."
In the instance of planar lamination, where both the upper and lower contacts of the laminae are planar, the gypsum usually consists of silt-sized elongate crystals aligned horizontally. Visible millimeter-scale lamination in the core reflects subtle variations in intercrystalline porosity, grain size and percent of nongypsum components in thin section. The nongypsum components include silt-sized quartz, biotite flakes, and whole and fragmented foraminifers. The clastic nature of these laminae is particularly evident in micrite- and clay-rich laminae, where gypsum grains are outlined. Gypsum grains include euhedral and broken prismatic to lenticular crystals showing various degrees of rounding. Because of the horizontal alignment of crystals where the gypsum is relatively pure, it takes on a felted appearance. Very thin, matrix-rich laminae resemble stylolites and may be zones of enhanced pressure solution (Pl. 2, fig. 3). In thicker, more homogeneous laminae, faint lamination is defined by wispy stringers and pods of matrix material. These matrix "blebs" are disseminated, but show horizontal alignment.
Where planar-wavy laminae are present, they alternate with intervals of planar laminae (e.g., Fig. 5A). A typical planar-wavy lamina begins with a few millimeters of relatively pure, silt to fine sand size (0.05-0.125 mm) rectangular gypsum crystals that are aligned parallel to bedding. Up-section in the top few millimeters of the lamina, they become coarser, more randomly oriented, and equigranular, but where rectangular or prismatic, take on a more subvertical orientation. In some cases the change to coarser crystals is marked by a planar-discontinuous, matrix-rich seam that may be slightly stylolitized. Intracrystalline and intercrystalline porosity is present throughout the lamina. The coarse, interlocking gypsum is abruptly overlain by a darker (in thin section) matrix-rich zone (Pl. 3, fig. 1). This overlying "matrix" material is a mixture of clay minerals(?), silt-sized carbonate including nannofossils, and rectangular to anhedral gypsum crystals. As in the planar laminations, the gypsum grains include euhedral and broken, prismatic to lenticular crystals showing various degrees of rounding. The matrix also contains traces of quartz silt, biotite flakes, and whole and fragmented foraminifers. This matrix material has a definite detrital appearance, with horizontal alignment of the long axes of gypsum grains, and its contact with the underlying coarse gypsum appears to be erosional. This matrix-rich zone passes upward gradationally into relatively pure gypsum.
In the case of planar-cumulus laminae (Fig. 5B-C) the coarsely crystalline cap observed in the planar-wavy laminae is better developed and thicker (up to 1-2 cm). Again, crystal size and degree of vertical orientation increase up through the structure. The equigranular to rectangular crystals are interlocking and can range up to 0.5 mm in length (Pl. 3, fig. 2). The crystals are locally replaced by irregular patches of fibroradiating quartz. The coarsely crystalline, massive gypsum within individual "clouds" is abruptly overlain by a laminated matrix. The matrix onlaps and drapes the coarsely crystalline, irregular "cumulus" topography at depositional to erosional contacts (Pl. 3, fig. 2). The matrix is essentially the same composition as that described above and includes detrital gypsum. In many instances the planar-cumulus laminae are actually compound features directly built upon thin, planar-wavy laminae. They grade upward into pinch-and-swell structures, and locally form keel-like structures (miniature depression cones) where they apparently sank into the surrounding sediment.
One thin interval of crenulated gypsum (8d), where laminae boundaries are irregular in a parallel fashion, is present near the base of Core 161-975B-34X (Fig. 6). These laminae differ from those described above in the following ways: (1) the matrix is composed of granular carbonate (slightly irregular and circular in shape) with high relief (probably dolomite, see Table 3), (2) gypsum crystals (both detrital and possible in situ precipitates) are predominantly lenticular in shape (Pl. 2, fig. 4), and (3) the development of stylolitization is more extreme.
Pinch-and-swell gypsum (7) consists of thin beds or laminae of massive gypsum separated by finely laminated matrix. The massive gypsum layers exhibit pinch-and-swell structure with matrix concentrated in the "pinch" zones (Fig. 7). These pinch-and-swell structures are often offset and nested, resulting in vertically alternating "pinch" zones and "swell" zones. The matrix appears to drape and onlap massive gypsum "swells."
The massive gypsum consists of interlocking coarse crystals of gypsum with high (15%-20%) intercrystalline and intracrystalline porosity (Pl. 3, fig. 3). The gypsum crystals are equant to rectangular in shape and where elongate, have a subvertical orientation. The nature of intercrystalline contacts is hard to evaluate because they may have been modified by pressure solution.
The silty matrix is laminated to massive and exhibits clastic textures modified by compaction and pressure solution. Silt-sized, rectangular gypsum crystals are the dominant component with rare, small foraminifers (whole and fragmental), biotite flakes, and quartz grains. Carbonate is rare to common, and occurs as fine micrite (associated with clay minerals?), isolated rhombs, and "ring"-shaped crystals, which are interpreted to be recrystallized nannofossils. Flattened pods of micrite are present locally. In thin section, matrix can be seen to interfinger with massive gypsum on the margins of some "swell" structures (Pl. 3, fig. 4). Thinning of matrix intervals across "swells" appears to be a depositional artifact that may have been enhanced by subsequent pressure solution and differential compaction. The surfaces of "swells" appear eroded and larger clasts of gypsum are scattered in the matrix along this contact.